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PODCAST: Central Vs Dedicated Return Ducts (with Dr Allison Bailes)

Dr. Allison Bailes, author of the Energy Vanguard blog joins the show to talk about air return ducts.

They talk about heating and air conditioning systems. Allison discusses that the most affected are the bedroom and it’s important to have a return air pathway to help with better airflow. He then talks about having central return and dedicated return pathways and how to set these systems up. According to him, common methods include door undercuts and transfer grills. 

Allison also shares about Manual J., a protocol for doing calculations to find the heating and cooling load on the house in the summer and winter. They further talk about the proper sizing of these systems for homes.  

E-mail us your comments, questions, and suggestions at


The following is a transcription from an audio recording. Although the transcription is largely accurate, in some cases it may be slightly incomplete or contain minor inaccuracies due to inaudible passages or transcription errors.


Bill Oelrich: Welcome, everyone. You’re listening to Structure Talk a Structure Tech presentation. My name is Bill Oelrich, alongside Tessa Murry and Reuben Saltzman, as always, your three-legged stool coming to you from the Northland, talking all things houses, home inspections and anything else that’s rattling around in our heads. Well, we’re really excited for today’s episode, we have Dr. Allison Bailes on, this is a conversation that we are all delighted to be having. So Reuben, I’m gonna let you do the official introduction because you two have a history together. So I’m gonna hand the baton off at this point.


Reuben Saltzman: Yeah. I don’t know about our history, it’s more, I’ve been reading everything that he’s written for the past decade or so, it’s something closer to that. Allison Bailes has a fantastic blog. If you care anything about the energy sphere, then you’re well aware of his blog, it’s the Energy Vanguard blog. I’ve been reading this forever, it feels like. And he’s just… He’s got a really unique perspectives on a lot of different stuff, a lot of good insight, and the only way I know him is that we had to… Well, we didn’t have to, we had the honor of hiring him to come out and do a training session here in Minnesota for our local home inspector group, the American Society of Home Inspectors, our local chapter. He did some teaching for us many years ago, pre-COVID. We were having a conversation on the podcast not long ago, and we talked about one of the articles that Dr. Bailes had written, talking about return ducts and supply ducts and how you don’t really need returns in every single room. And when we started chatting about this, I thought, “Who better to talk about this than… ” I keep saying Dr. Bailes, is that too formal for you? 


Allison Bailes: No, you can call me Allison. Allison’s fine.


RS: Alright, and I thought, “We gotta get Allison on here to talk about this ’cause he had a really unique perspective.” I remember the first time I came across a house, it was a new reconstruction, and it had a central return and there were no returns in any of the bedrooms. And I just kinda scratched my head and went, “I don’t think this is right. I don’t think this is gonna cool the upper level right.” And I ended up calling up the building official before I made a stink about this. And I said, “How could this be? Is this actually allowable?” ‘Cause I’ve gone through my code book and I don’t see anything that actually says you need a return in a bedroom, but it’s just never done this way, and it feels wrong. And he said, “Aaah, it’s not the best way to do it. It doesn’t work all that well, but it does meet code.” So that’s always kinda been my take on it, but then Allison, you had a great article saying you actually prefer having a central return. So I’m wondering if you can just kinda take us through that whole thing, explain to the listeners what I’m trying to muddle through.


AB: Sure. The issue is your heating and air conditioning system, your forced air heating and air conditioning system moves air through the equipment that heats it or cools it. And sometimes it doesn’t heat it or cool it, if you’ve got a fan in the on position and it’s just moving air, it’s still pumping air through the supply ducts. So sending air out through the supply ducts, then it comes out in all of the rooms where you have a vent. Bedrooms are the main issue here because they often get closed at night, the bedroom doors get closed at night. If you’re pumping air into the room with the closed door, what happens to that air? You think of blowing up a balloon.


RS: Yeah.


AB: Except the bedroom can’t expand. What happens is the air finds ways out and it will build up some pressure, and so this is an easy measurement that anybody with a manometer or pressure gauge can do. You can turn on the air handler, close the bedroom doors and then put a tube under the door and measure the pressure difference between the hallway and the bedroom and see how pressurized it gets with the air handler running and the doors closed. That gives you a very good indication of the return air pathways.


RS: Mm-hmm.


AB: That term is very important. It’s not a return vent that you need for every bedroom, what you need for every bedroom is a return air pathway. You’ve gotta have a way for that air that’s being pumped into the bedrooms to get back to the return, if you have a central return or multiple returns but they’re not in the bedrooms.


[automated voice]


Siri: Sorry. I couldn’t hear what you said.


AB: Oh shut up Siri. I didn’t ask you anything again.




AB: Second time she’s done that to me. Maybe I’ll just take my watch off and put it…


RS: You’re being watched and you probably don’t even know it.




AB: Yeah. Okay. So the air gets pumped into the bedrooms, it needs to find a pathway back to the return or returns, as I was just saying, there could be multiple returns. We do HVAC design for homes all over North America, and sometimes we have multiple central returns because… Oh, we’ll get into this in a little bit, but one reason we like the central returns is because we like to put the filter in a filter grill at the return and we can talk about that later. But you’ve gotta have a return air pathway, that return air pathway could be a dedicated return. This is what happens in a lot of homes. And I’ve heard contractors say that… Builders and contractors and others say that, “Well, I wouldn’t want a house, or I wouldn’t wanna build a house that didn’t have a return in every bedroom because that makes it look cheap, it’s not a high quality house.”


RS: Yeah.


AB: “If it doesn’t have a return in every bedroom.” Well, that’s not the case, because you can have a return in every bedroom that has more problems than having a well-designed house without returns in the bedrooms, but have return air pathways for every bedroom. Because here’s the issue with having a return vent in every bedroom, you’ve gotta have a duct going to all those return vents and where are those ducts? Now, you guys are in Minnesota, and so up there, you probably don’t put ducts in the attic I bet. Have you seen ever any ducts in the attic? 


RS: It happens, but it’s not the standard.


AB: Right. I hear that from people in the North all the time, even Canada, sometimes. They say every once in a while they’ll see a system in an attic but it’s not useful there. Here in the South, it’s common to put ducts in the attic.


AB: And an example that would be appropriate for people in cold climates is maybe the ducts are going through an unconditioned basement. In winter time that basement gets a lot colder than the house, and the more duct area you have, the more heat loss you’re gonna have in the winter time, or heat gain in the summer time when you’re air conditioning. The more duct area, the more duct you have, the more duct sealing you have to do or the more duct leakage you might have.


AB: Having a return in every bedroom has some drawbacks. So putting a central return and then having a return air pathway reduces the duct area, and that often helps with airflow too, because one of the biggest problems with getting the right airflow in duct systems is the static pressure is too high. There’s too much resistance in the duct system and a lot of that happens on the return side because they don’t have enough return area. And if they’re running a small duct to each bedroom and putting a small vent in there, you may not have enough return duct area for pulling in enough air. So that’s where a lot of the resistance comes from. Instead of doing that, you make a return air pathway, and there’s different ways you can do that.


AB: The traditional way is the door under-cut, and that can work fine as long as the air flow is low enough. In a house where you’re pumping a lot of air into the bedrooms, if it’s an oversized system, which is common, or if it’s an old house that needs a lot of air pumped into the bedrooms, the undercut might have to go halfway up to the door knob [laughter] to get enough return air pathway. But typically, an inch might do it. And also, it’s not just the air… The return air pathway is not just under the door, because interior doors don’t have weather stripping and there’s leakage around the sides and the top as well, so that adds up. And John Simlhac, contractor and a really smart guy in Virginia, has done measurements of this and said, they can go up to about, I think, 75 CFM of supply air into a bedroom and use the door undercut and leakage around the return air pathway around the sides and top of the door.


RS: Allison, can we back up just a second? 


AB: Sure.


RS: You’re a pioneer in this field, right? 


AB: Oh, I don’t know about a pioneer.


RS: Oh, come on. You’re being…


AB: A lot of people been doing this before me. I am a good publicizer of what other people have done.




RS: Well, okay. So how did you get so interested in building science and HVAC layouts and all this other kind of house science geeky stuff? 


AB: That’s a great question. So where do I start? Well, I’ve been interested in environmental stuff for a long time, and then energy efficiency, and then once you get into energy efficiency, if you start getting into making houses more energy efficient, then you discover building science, and that opens up a whole world to you. Then you can get into indoor air quality and all kinds of other stuff. So I guess the real point where my life changed was when in 2001, I bought some land and built a house, and I didn’t know anything about building a house. I was the general contractor on it, and I had never built anything bigger than a bookcase before that. So it was quite an adventure, may be a full hardy adventure. But I learned a lot. And lived in the house for three years, and we won’t talk about why I left the house because of the divorce and how it affected me but…




RS: Sorry. Don’t make me laugh, but I feel your pain.


AB: Yeah. So that’s how I got into building science, by building the house. I spent two years building it. I didn’t know much about building science. I took a class at Southface in Atlanta, three-day home building class and learned some of the basics there, and then built the house and learned more. I didn’t know how to do Manual J then or anything, but I knew that I needed a Manual J when I built the house. So I hired a contractor and told him I wanna Manual J, and he did a Manual J. He said… It was a 2000 square foot house on top of a 1000 square foot basement inside the building enclosure. Not finished, but still inside the condition space. So about 3000 square foot total. He came up with three tons. That’s pretty good. That’s definitely not rule of thumb sizing for air conditioning in the South. That would be six tons for that house, because they typically use about 500 square feet per ton. Well, later I learned how to do Manual Js myself, and I repeated the Manual J and did it correctly, and I came up in one and a half tones. Even though he was half the size of what a rule of thumb contractor would have put in, he was twice as big as I would have put in if I had known then what I know now.


RS: What exactly is a Manual J? 


AB: Good question. Manual J, it sounds kinda abstract. It’s a protocol and it’s a manual. It is actually a book that describes all the details of the protocol for doing a calculation to find the heating and cooling loads on the house. So how much heating the house needs in winter at the design conditions, how much cooling the house needs in summer at the design conditions, and then you use that, the results of the load calculation to pick your equipment, size your equipment properly to meet the loads.


Tessa Murry: And all HVAC contractors use this Manual J when they’re installing systems in houses, right? 


AB: Right. [laughter] No, they don’t. And there are a lot of contractors that know how to use it, and running an HVAC… I’m not gonna throw all the HVAC industry under the bus here. It’s difficult sometimes because you’re competing against a lot of low-bid guys and… Well, yeah, they don’t all use Manual J. They do go with the rules of thumb. There is actually, for existing homes, there’s a better way to come up with the sizing that you need.


RS: And just talking about how infrequently people actually do calculations, I recently had my furnace and AC replaced, and I had, I think six or seven different contractors come out to give me bids, and one of all of them did any type of measurements of the house. She went around, she measured all the windows, did all the stuff to do a calculation to figure out exactly what I needed. All the rest of them said, “So what size you got now? Does that seem to be working for you? Okay. Well, that’s what we’ll stick with.” That’s as far as anybody else went as far as trying to calculate the right size for me.


AB: Yeah, well, and so you were lucky if they stuck with the same size, because sometimes they will put it in a bigger system. [laughter] That happened. Bryan Orr at the HVAC School, and you probably know who that is. No? 


RS: No.


AB: Oh, you gotta go online, look up HVAC School, Bryan Orr. He’s in Florida, so opposite climate to you guys. But he’s got a whole lot going on and he’s got people from all over, a lot of different climates working with him. So I recently saw an article if his about the case in Florida that had… The house had serious problems, and the when the house was originally built, it had one sized system and then I think it got sized up the next time it got changed and it was sized up again. And that caused all kinds of humidity problems in the house and mold. And so, yeah, sometimes they do go up in size.


AB: What I alluded to a minute ago, though, there’s a better way to size your air conditioning system or heating system for existing homes, and that is, while your system is actually working properly, find the days when you’re close to the design conditions, your design… Outdoor design temperatures for your location. Here in Atlanta, our summer design temperature is 92 degrees. That means only 1% of the time that we’re above 92 degrees. So 92 degrees is the temperature that we usually use for designing air conditioning systems in Atlanta. Our winter design temperature is 23 Fahrenheit, which means 99% of the time, the temperature is above 23. And so we designed heating systems for that temperature. So you find those days when it’s 92 in the summer or 23 in winter here, and then, where you are, you’re -10, I think, or…


RS: I thought it was like -20, but it may have changed.


TM: I just googled this and I don’t know if this is correct, but it says -15 for Minneapolis-Saint Paul.


RS: Yeah, that sounds right.


AB: After I said -10. I just remembered Gary Nelson in Minneapolis, I visited him when I was up there, with you guys, and he told me that the design temperature there is -15. He heats his house with a one and a half ton heat pump and to get down to -28, and the heat pump did fine.


TM: Yeah, that’s impressive. That’s really impressive.


AB: So, yeah. So the best way to size a system for an existing home is to find out how much runtime you get with the system that you have on your design day and in winter, your design day and in summer. And I did this with the condo I used to live in and on design days I was getting like 30 minutes of runtime on my air conditioner, 30 minutes out of an hour. So that means it was probably about twice as big as it needed to be. I had a two ton system, I could have put a one ton system in, except conventional systems don’t come in one ton size. Or I could have got a two stage two ton system which goes down to one ton, but if you look for those days, when you’re close to your outdoor design temperature and time, how much your system runs, that’ll get you ready for the replacement, so you can probably go down in size a little bit.


AB: Now oversizing furnaces is not as big a deal as oversizing air conditioners, especially in human climates. Oversizing air conditioners is not a good thing because of humidity. When you oversize the air conditioner, the air doesn’t pass over the coil enough to dehumidify properly. You end up with too high humidity. And let me back up a minute. I said oversized furnaces are not as big a deal as oversized air conditioners. It doesn’t mean they’re not a big deal because any time, any system that’s oversized is gonna have a short cycle, it’s gonna go on and off more frequently. And that can lead to comfort problems because you get these blasts of hot air, these blasts of cold air, and then it goes off. And depending on how bad the building enclosure is and how hot or cold the day is, you’re gonna cycle between these feelings of comfort and discomfort.


TM: So just to clarify and make sure I’m understanding right, that a furnace that would be properly sized, Allison, if it’s -15 here in Minneapolis, that furnace would run most of the hour to keep up with the thermostat, keep it satisfied, and keep the house warm. If the furnace only ran for 30 minutes out of an hour when it was -15 outside, then that furnace would be oversized. It’s pumping in more heat than the house needs. Is that what you’re saying? 


AB: Yeah. With furnaces, you wanna have some buffer because you are gonna have the -28 days there.


TM: Yeah.


AB: In Minneapolis. And so I wouldn’t size it right on the nose there. Although Manual S, the air conditioning Contractors America protocol for sizing equipment says you can size it at 100% of the load. But the load that you calculate with Manuel J has some oversizing built into it. Well, I’ve been saying 10 to 20%, but John Proctor in California recently told me that they’ve done some stuff and found that it was more like forgetting the number, but it was a lot more than what I have been saying.


TM: Okay, so maybe your furnace would run 40 to 50 minutes out of an hour and that would be…


AB: All of the design there, yeah, yeah. Then you have buffer.


TM: Yeah.


AB: Also, there’s thermal lag built into your building enclosure. So the issue with you guys in a really cold place is when you have like a week or two weeks of really cold temperature where you’re like -20 to -30 for a week or two. And I think you have that kind of weather sometimes.


TM: We do.


AB: I don’t know why you live there, but…




RS: Yeah, we don’t either.


BO: That’s a normal week in International Falls.


AB: Yeah, I’ll take Atlanta.




BO: Yeah, no doubt. A couple of thoughts. One, is that something else we always tell our clients about is just the wear and tear you have on your appliance, because it’s not the running that causes the wear and tear, it’s the warming up and cooling down, the expansion and contraction of that metal constantly, and that’s what leads the failed heat exchanger on your furnace and lots of other problems. However, to your point, how it’s not as big of a deal on a furnace, so many of the furnaces being installed today are either dual stage or triple stage or multi stage, where when it first turns on, it’s not running at 100% capacity. It’ll kind of ramp up, it’ll run at a lower B to you for, I think it’s like 15 minutes typically. And if it hasn’t satisfied the thermostat in those first 15 minutes, then it’ll kick up to the highest gear. And so because of that, I’ve had HVAC contractors tell me it’s not as big of a deal to oversize it as it used to be on a single stage furnace. What do you think about that, Allison? 


AB: Absolutely, I agree with that. I’m a big fan of variable capacity and multi stage equipment. Most of our designs are for variable capacity equipment in many splits, ducted and ductless, and that’s what I’ve got in my house, that’s what we’ve got in our office. They work really well and recently installed Emporia View Electricity Monitor in my house and it’s really interesting to watch the, power ramp up through the day and then come back down. It really works, that the system runs almost continuously. In summer time at night time, it goes down to its lowest setting, which is basically just the blower. The compressors are off most of the night, and then it’ll come back up as it heats up in the next day.


TM: So, Allison, I have a question, quick question for you about heating in really hot climates. You wrote in a blog recently… What is it called? Ten Building Science Lies to Watch Out For. And point number two was that all homes in the south spend more on air conditioning than on heating, which is actually false. And it kind of blows my mind. You think that in a climate like yours down in Atlanta, that you’d be spending a lot more money on cooling your house, electricity, all that, but that’s not actually true. Can you can you kind of expand on that a little bit? 


AB: Yeah. So there’s some nuances to that as well as there is with everything. If you go to the US Energy and Information Administration, USEIA, their website, and you can look at the data from their residential energy consumption survey. RECS, R-E-C-S, Residential Energy Consumption Survey. I recently posted this graph on LinkedIn that their most recent data they’ve published on this from 2015 shows that in the South region, which is the states from Texas to Delaware and then South down to the Atlantic and Gulf of Mexico, we use twice as much energy in our homes for heating as we do for cooling in the south region.


TM: Wow.


AB: For the US as a whole, it’s much higher. And your region, it would be even higher than the average for the US.


RS: Yeah.


TM: Why is that? Is that because these houses don’t have very much insulation in them compared to cold climates and so it takes more to heat them or that they don’t have central heating systems that are efficient, and so you’ve got all these different space heaters and everything trying to heat spaces. What’s the deal? 


AB: There’s several factors to that. So one of the biggest ones is that there’s a lot of heating done here in the South with gas. So when you’re burning a fuel directly inside the house and with new equipment, the efficiency for that starts at 80%. So you’re getting 80% of the energy into the house, but you’re burning that fuel right there on site. And with air conditioning, you’re moving heat, you’re not getting it from an energy exchange like burning gas. You’re moving heat from inside, outside, using electricity to do the work. And you can generally move about three units of heat out of the house for every one unit of energy that you put in.


TM: Okay. Yeah.


RS: Sure.


AB: But the catch there is that the electricity has an efficiency of about 35% when it’s generated at the power plant, because we burn coal and gas for a lot of our electricity and nuclear, that’s some of it, then a little bit of renewables and some biomass. So what the EIA data show is the site energy and electricity. When you look at just the electricity being used, you’re missing the two-thirds of the electricity that was lost at the plant and in the lines being delivered.


TM: Wow. So it’s not necessarily that the houses are a lot more leaky, they don’t have very much insulation, so it just costs more to keep the heat in and keep them warm. It’s more so just the fact that adding heat costs more energy and money than it does to remove heat like you would with air conditioning.


AB: As part of it, yeah.


TM: Part of it.


AB: And I’d say that’s probably the biggest part of it. But also we do more heating than cooling here in Atlanta and a big part of the South. Unless you’re on the Atlantic Coast from, say, Virginia down and then all of Florida and then all across the Gulf Coast. If you’re close to the coast, you’re using more for cooling than heating probably. But once you get up into the Piedmont area where we are in Georgia here, we have about 3000 heating degree days, and well, we used to anyway. [chuckle] Last several years we were like 2400 or so. We do a significant amount of heating and our heating season lasts… We start heating sometime in November, usually or maybe October, and then we still do a little bit of heating in April.


AB: Our cooling season here is maybe May through September, sometimes into October a little bit. And May and September aren’t that much heating often. Well, April and September certainly are much heating. May is picking up, so there’s… I think there’s less overall heating. And also the Delta T for cooling is smaller. Delta T is temperature difference between inside and outside. We cool the house to say 75. The outside temperature is say, 92, 95. So we’ve got a 17 to 20 degree Delta T for cooling, but we’ve got about a 40 degree Delta T for heating, 70 degrees and say 20 degrees or 23 degrees, something like that. So we’ve got a bigger temperature difference for heating.


TM: Right. Yeah.


RS: Makes sense.


BO: Okay. I have to ask this question ’cause I’ve been just chewing on my tongue. What career did you jump out of, to jump into this? And I mean, clearly you were fascinated by it because you win 150% at it. So give me some sense of Allison pre-2001 and then…


AB: Well, okay, I grew up in…


BO: ‘Cause I can read all about him, post 2000.


AB: Sure. I grew up in Cajun country. My mom moved us down there when I was 10 years old and then I lived in Florida for nine years, so I was an alligator hunter.


TM: No way.


AB: Yeah, that naturally led me into building science.




RS: Of course.


TM: Well, that’s where I found a love for building, for houses as well. I was down in actually in the bays of Louisiana. Which part of Louisiana are you from? 


AB: Chauvin, Lukayou…


TM: Chauvin? Wow, okay.


AB: You where that is? You know where Cocodrie is? 


TM: Yeah.


AB: You do? Oh, yeah.


TM: Yeah.


AB: So Chauvin is on Highway 56.


TM: Yeah.


AB: A few miles north of Cocodrie.


TM: Okay, I was…


AB: Cocodrie, for those of you who don’t know is the end of the road in south Louisiana.


TM: Far as you can go.


AB: Yeah, and people know it because it’s a place that a lot of people go to go out and go fishing.


TM: Yeah, I was living in Thibodaux for a year.


AB: Wow.


TM: I don’t hear a Cajun accent at all, Allison.


AB: Well, I’m from Texas and I got teased out of my Texas accent when I was 10 years old, 11 years old, and I don’t think I ever picked up a Cajun accent.


TM: Bummer, it’s a great sounding accent.


AB: Yeah.


BO: Okay, I have a second question.


AB: Well, I should back up. I never was an alligator hunter, that was made up.


RS: Oh…




BO: I like that story. Okay, so how many systems in this country are oversized if you had to place a percentage on them… On unemployment? 


AB: Oh a percentage, not a number. I was gonna say that’s probably a 51,275,000. I think.


RS: That was my guess.




RS: Yeah.




BO: Wow.


AB: What percentage are oversized? Probably 80%, 70%.


BO: Wow. Holy cow.


AB: Yeah. Lots of ’em.


BO: It feels like it’s not laziness, but I don’t know what other basket to put it in, it feels like when companies started scaling and buildings had to be scaled and built quickly, did we lose some of our sense of the details in that process? 


AB: Yeah, there’s a whole lot to it. If you’re not gonna do a real load calculation, it’s safer in the mind of the contractor, it’s definitely safer to put in something that is gonna be too big, than to put in something too small. If you put in something too small, you’re gonna get the call back saying, “Hey, this isn’t cooling the house, this isn’t heating the house”. In a place like Minnesota, that could be a life-threatening situation if it can’t heat the house, so it’s easier to go that route. And I think that’s the main reason for it is they’re not doing load calculation, so they’re over-sizing.


BO: Allison, you were talking about run times and recording run times and figuring out if your equipment is sized properly. What does that look like? Is it a person like me sitting on a stool in front of my furnace with a stop watch waiting for it to turn on and off and spend an hour down there with a book or a podcast, or do you have other fancy tools and equipment that will measure this, and even with these multi-stage units, are you measuring ramp-up time and peak time and all of that, and calculating it, or is it just more straightforward? 


AB: Yes, so you can do it sitting there on a stool with a stop watch, if you live in a place like the condo I used to live in, that’s not hard, you just sit in the living room and every time the system comes on… And I used to have to turn up the TV if I was watching TV anyway, so just start the timer. [chuckle] That’s one way to do it. But a better way is to use data loggers, if you use these data loggers, you can get… This is not the one you would use, but it looks something like this, this is from Onset, it’s the HOBO data logger, and they have one called a motor run timer. Well, like this with my air conditioning system at the condo, they have little magnets on the back and it detects a motor nearby because the motors use magnetic fields, and every time that magnetic field kicks on or it goes off, it causes the data logger to start recording.


AB: And so it just tells you whether the motor is running or not running, and if you have a multi-stage or a variable capacity system, then you’ll need something more sophisticated to see what level it’s operating at, but if you have the standard fixed capacity system, just the motor run time logger, it’s all you need just motor on, motor off and that’s what it tells you.


RS: And that’s assuming that you have your fans set to auto, not on.


AB: Of course. Right. Because if you had the fans set to on, then yeah, it’s just gonna be fan on and run time is 100% in that case. But if you’re doing air conditioning, you could put it on the outside unit, ’cause that’s gonna go on and off with the cooling.


TM: Those data loggers something that you can just buy off of Amazon and how expensive are they? 


AB: They make a lot of different kinds. This one is, I think, a hundred and something. I think the motor run time logger is a hundred something, they have a lot of temperature and relative humidity data loggers, which we have a bunch of ’em used for a lot of things, they’re 120 bucks or something.


TM: Okay, I might just prefer a stop watch.




AB: Yeah.


RS: I like the data logger, I may have one of those on my wish list soon.




TM: I knew it. Yeah.


RS: Of course. [chuckle] Now Allison, I’d like to hear your thoughts on having your fans set to on versus auto, both for heating and cooling, how should people set their thermostat? 


AB: Which is… The most common answer for any builder to answer this question, it depends.


TM: It depends.




RS: That’s Tessa’s answer for everything.


AB: I wrote an article a few years ago that a lot of people like called, “How to talk like a building scientist.” I think that was the first thing I said.




AB: If you put the fan in the on position, that is good for filtration, and a lot of contractors will say, “Oh… ” They’ll sell you UV lamps and special filters and things, and they’ll say keep your fan in the on position that way that air will keep moving through and it’ll keep getting cleaned. And that’s fine sometimes, the problem with keeping the fan in the on position is… Well, number one, depends on how much energy your fan uses, it could be a big energy hog, if you’ve got a really resistive duct system and an old inefficient fan motor, then that could use a lot of energy keeping that in the on position. In the humid climate though, in cooling season, it can be a problem because if you keep the fan on the on position, when the compressor goes off and the air conditioning coil starts warming up, you’ve got all that water sitting on that coil, if the fan is continuing to blow air across it, you’re just evaporating that air back into the air stream and putting it back in the house.


TM: It acts as a humidifier, whole house humidifier. Kind of? 


AB: Yeah. And I did an experiment in my condo with this a few years ago, I turned the fan to the on position, I left it there for, I think a week, and very quickly, like within a day or day and a half, the humidity went from an average of 58% to about 70%.


TM: Wow.


AB: You don’t want your humidity to hang out at 70%, and then when I turned it down at the end of the week it took a little bit longer to come back down. So another reason not to keep your fan in the on position is, if you have ducks outside the enclosure, like in an attic or in an unconditioned basement, if you have unbalanced duct leakage, well, if you have any duct leakage outside of the enclosure, the fan continuing to run is gonna cost you because you’re losing conditioned air, you’re bringing in unconditioned air, and if you have unbalanced duct leakage, that changes the infiltration on the house too, so you may be leaking more air through the house as well as the ducts.


BO: Interesting.


AB: But if you have all your ducts inside the building enclosure like I do here, and if you’re not in a humid climate, or if you’re not at the humid time a year, it’s okay to put the fan on the on position and get more air cleaning going on.


BO: Allison, what’s your definition of humid? 


AB: My definition of humid.


BO: Could you give me an above or a certain percentage? 


AB: Yeah. So let’s start with the design conditions recommended in Manual J. 75 degrees Fahrenheit for cooling and 50% relative humidity. That corresponds to a dew point of 55 degrees Fahrenheit. A dew point is the temperature of condensation, that means if you’ve got air with a dew point of 55 degrees and you’ve got, say, a cold water pipe that’s at 50 degrees, you’re gonna get condensation on that water pipe. We don’t have this problem in the South, but in some places, people get condensation on their toilet tanks. Have you seen? You probably see that there.


RS: Oh yeah, we have that, for sure.


TM: Yeah, yeah.


AB: That does not happen down here. Our water temperature in the toilet tank… I’ve been measuring my water temperature coming into the water heater and our incoming water temperature over seven years is about 74 degrees. If I’m keeping the house with a 55 degree dew point, I am not gonna get condensation on the toilet tank because of that.


RS: I’m sorry, but that’s just gross. I can’t picture turning on my faucet and drinking 74 degree water.


AB: Yeah.


RS: Like, come on.


TM: Wow. Yeah.


BO: ‘Cause you’re not married to an Indian lady who does all that all the time, that’s the way they like the water, warm.


RS: Okay, alright. Good, good.




AB: So the definition of humidity… I like to think in terms of dew point, what is the dew point temperature of the air? 55 degree dew point is our conditioned air for cooling season, and that’s a nice humidity level. When I look at the Weather app to see how humid it is outdoors, I don’t look at the relative humidity, I look at the dew point. Our dew point up here right now is probably in the 70s, 71, 72 degrees outside right now. That’s sticky. If I’m just sitting in the backyard in the shade and there’s a little bit of a breeze, that’s fine, but if I’m cutting the grass or going for a run or even for a little walk, I’m gonna feel that and it’s not gonna be that comfortable. If the outdoor dew point is between 60 and 70 degrees Fahrenheit, that can be moderately comfortable, especially the closer to 60 it is. If the outdoor dew point is below 60 degrees, that’s open the windows weather, because that… And I wrote an article about this last year, I think, What humidity is, is it okay to open the windows? And it’s not relative humidity, its dew point. If the dew point is below about 60 degrees Fahrenheit, then you can open the windows and you’re not gonna make the house too humid.


BO: Taking notes here because, Tessa, I have to talk to you offline. I have a situation we need to deal with at the cabin, so…


TM: Oh-oh.


BO: Yeah. Okay, Reuben, I know you wanna get back to returns in sleeping rooms, so let’s kinda pull it back to where we started because there’re some loose ends.


RS: If you hadn’t turned it over to me, we would have gone off without a hitch and we could have asked Allison more about his background, but I just jumped right in with my questions on it, yeah.


TM: After the chase, yeah.


RS: You were telling us about different ways to do this towards the beginning of the show, Allison. And we kinda left it on how you would set these systems up and why they’re better, and we didn’t quite finish that discussion about all the pros and cons.


AB: About the pros and cons or about the different methods? 


RS: Both.


AB: Okay, ’cause I remember, yeah, I remember we talked about… We started going down the path of return air pathways and how can you do it, and I talked about door undercuts, but I didn’t talk about ways other than door undercuts, and there are better ways than door undercuts. If you’ve got just a little bit of air going to a room and in high performance homes with low loads, you may have only 50 CFM, cubic feet per minute of air going into a room, and then the door undercut is usually fine for that. But a lot of times you’ll need more than that. And so there are things called transfer grilles or jumper ducts. And a transfer grille can go into a door. In my condo, I installed transfer grilles that go in the bottom of the door. I cut the bottom of the door out and I just screwed these things on and it took like 10 minutes to install them and it made a big difference. I talked earlier about the pressure difference between the room and the hall when you close the doors and turn on the air handler.


AB: In my condo, we didn’t have returns in the bedrooms, and though we had one bedroom that was worse than the others, and it was about six or seven Pascal, so the pressure, with the door closed and the air handler running. That’s pretty high. Energy Star New Homes says that you have to measure that pressure difference and it’s gotta be below three Pascals. It’s kind of an arbitrary number. Why do they choose three? I don’t know. And you could have chosen two or any number really. You want it kinda low. Three is a reasonable number. Six or seven Pascal is kinda high. So I installed this thing, and I don’t remember what my final pressure difference was after I installed it, but it was very low, it was definitely below three, so it gave the air a way to get out. It’s just a little grille and it had a cardboard baffle to give some sound dampening effect. You can saw these things through the wall as well. And Tamarack is a great company that sells these things. You can go to their website, or something. They sell them. They go in the wall or through the door.


AB: So that’s transfer grilles. You can also do a transfer grille with just two grilles on the wall. I’ve seen bad ways and good ways of doing the transfer grilles through the wall. A bad way would be like just cut a hole above the door and put a grille on each side and you’ve got light and sound that can go easily through there, that’s not great. You wanna separate those things. So if you find a wall where you can put one high on one side of the wall and one low on the other side, and then you cut the sound and light transmission a lot. And if you’re gonna do that, you wanna duct that completely, don’t just use the wall cavity. The code still say you can use wall cavities, and lined wall cavities for returns, and this will be a return air pathway, but it’s best always to duct anything that’s moving that kinda air, ’cause it could be connected with something else otherwise.


TM: I’ve got a beef with central return systems, Allison. And I know you actually mentioned this too in your blog, talking about just how noisy they are sometimes. I’ve lived in places that have just one central return, and when that furnace kicks on, it’s just this huge suction and a ton of air is moving through it and it’s really loud. So what’s… What’s a way that you can combat that problem? 


AB: Well, my house… So my condo had a central return and it was very loud. I told you I had to turn the TV up because it was right there by the TV and I’d have to turn the TV up every time it came on. In the house I’m living in now, I’ve got two systems. The two air handlers that are encapsulated in the attic. One is for the bedrooms and it’s got one return in the hallway. And the other is for the living room, kitchen, dining room and it’s got two returns. One in the dining room, one in the living room, and I cannot hear when the air conditioner is running and when it’s not. There’s two reasons why they’re noisy. Number one, it’s something we talked about earlier that a lot of systems are oversized. When you put an oversized system in, it makes a lot more noise and to get it not to make a lot more noise, you have to size the next bigger and especially on the return side.


AB: Then that’s the other problem it causes a lot of noise is that return grills are often undersized. They need to be bigger. Then if you can’t get one big enough, if it would take like the whole wall, then split it up. That’s why I have two in my living room. That air handler is one and a half tonnes so it’s about 600 CFM, cubic feet per minute of air flow, and I’ve got two 16 × 25 grills for it. And I’m running Merv 13 filters on them too, so I’m getting good filtration. I’ve got very low pressure drop. Filtration and pressure drop’s a whole other issue, but they’re very quiet and they work well. It’s all a matter of sizing the ducts and sizing the equipment properly.


TM: Okay, so if it’s done correctly and sized properly, you shouldn’t have that really loud noise going on? 


AB: Right.


BO: And then so if you’re gonna go with a central ducting system or an individually ducted system at the room, it really sometimes boils down to economics and how much do you wanna spend on the system and can you run the ducts, is their room in the joist or whatever, or is there any other limiting factors that would cause somebody to go one way or the other? 


AB: If you’re doing in the central return, it needs to be big and you need to have space behind it as well because you don’t want just a shallow box. Well, we like to put filters at the grilles and so with the filter in there, you definitely need space behind it. You want ideally six inches behind the filter, and we specify two-inch thick filters so you need about 8 inches from the grille for the return.


BO: Reuben, do you have any other lingering questions? 


RS: I’ve got about 13 of them but I think we’re kind of out of time here.




BO: Allison, there’s always a part two. I hate to say that but we never get through our list of questions with any guests and we’ve done this for almost… We’re beginning our fourth year and we stumble and fall at the finish line every single time because there’s always so much more to ask.


AB: Yeah. Good, better than the awkward silence.


TM: You know, you changed my mind, Allison, about central return systems.


AB: Oh really? 


TM: After I read your blog, I was like, “Okay, you know what? I think I can get behind these.” Because I too, like Reuben, had always thought that when a house just had a central return or a few central returns, that it was just because the whoever installed it was being cheap but in your article, you brought up so many good points that if you do it properly and you size those returns properly, that you’re saving money overall in the system, less duct work, you also have less energy usage or less energy being lost if you’ve got ductwork going through unconditioned spaces as well, or duct leakage, which all houses have duct leakage. So you’re saving money with less ductwork in general and then you can have these high Merv rated filters that are installed on these central returns that filter all the air coming through the system in different ways, in addition to maybe a filter at the furnace. And so you’ve got potentially better air quality with the central return system. So all those things seem like just pros in my mind that I hadn’t really thought about before.


RS: Yeah. Yeah, changed my mind too.


AB: Alright, good.


BO: That’s how you do it Allison. Just one building geek and another inspector at a time, you can change a lot of minds. Personally, I’m really excited to have been able to have this conversation with you. I’ve never personally met you but I have read your blog a lot. It’s great stuff. Even if you’re not a house nerd, it’s really good for understanding how to set your house up to succeed. So thank you for all the work you’ve done and thank you for your time. It’s been fun. Where should everybody go to get the best information? 


AB: Well, the Energy Vanguard website is at and there’s a lot of other good websites too but you can find those. I’ve got links to those on my blog. And also, I have a book coming out in October, I’m hoping. I just got good news from the publisher this week. She got quotes from two printers. Earlier, she had told me that all the printers were saying 15 weeks to print from the time they get it. One of them now is saying 9 or 10 weeks, so yay! 


RS: Yay! 


TM: What is the book about? Can we ask? 


AB: Oh yes. The book is called, “A House Needs to Breathe… Or Does It?”




RS: You just pulled the rug out from under Tessa’s. [laughter] Or does it? 


TM: Oh. Well, I will be excited to get my hands on that, Allison.


RS: Tessa, I’ve already pre-ordered a box of 20 or whatever to give all the inspectors at Structure Tech, so…


TM: I’m so excited.


BO: I was just looking at the pre-order list yesterday, I think. Reuben, I saw your name up at the top there. Yeah.


RS: It’s on there, absolutely.


BO: Allison, I’m gonna kinda put a close on this one but thank you very much. We really appreciate your time. And as always, if you have questions for the podcast or comments, Reuben, where do we send them? 


RS: Email us at


BO: Thank you very much. Thank you everybody for listening. We appreciate it. You’ve been listening to Structure Talk, a Structure Tech presentation. My name is Bill Oelrich, alongside Tessa Murry and Reuben Saltzman. Thanks for listening. We will catch you next time.




0:44:02.2 BO: Hi everybody. Bill here again with Structure Talk. We really wanna thank you for listening to this podcast. It’s been a ton of fun for us to put this presentation together. And if you could, we would love it if you would go to any of the podcast platforms where you find Structure Talk and leave us a rating and subscribe to the show. You can also subscribe to our blog at and, of course, you can listen to the show on the internet at Thanks again for listening. We appreciate the support. And if you have any suggestions for show topics, please email them to Thanks for listening.