Reuben Saltzman

Gravity Furnaces

If you’re buying a home with a gravity furnace, you should have the furnace replaced.  Gravity furnaces are those huge ‘octopus’ furnaces that can just about fill up a whole room with ductwork.  They are called gravity furnaces because it’s gravity that distributes warm air – the warm air weighs less than cold air, so it rises.  These furnaces don’t have blower fans, and there is little that can go wrong with them.  While I rarely find any safety issues or problems with gravity furnaces, the main reasons to replace them are money, efficiency, and comfort.


The biggest concern for most people is the money it takes to heat a home with a gravity furnace.  Gravity furnaces typically cost about twice as much to operate as a modern forced air furnace, because they are terribly inefficient.  Gravity furnaces just have a huge flame that warms up the air in the ductwork, and all of the exhaust gas that leaves your home through the chimney is wasted heat.  On a gravity furnace, about half the heat generated goes up the chimney, making it about 50% efficient.  Newer furnaces can be as high as 98% efficient.


While money and efficiency go hand-in-hand, I’m listing them separately because replacing your old gravity furnace is also good for the environment; the more efficient your furnace is, the less greenhouse gases get released in to the atmosphere.  Replacing old gravity furnaces is a ‘green’, environmentally responsible thing to do.


Your home will be much more comfortable with a forced air furnace.  Old gravity furnaces operate by allowing the heated air to rise up the middle of the home and the cool air falls back down along the outside walls, making the middle of the house warm and the outside walls cold (see diagrams below).  Additionally, with a forced air furnace you’ll now have the option of adding central air conditioning, which is not possible with a gravity furnace because there is no blower fan to distribute the air.

Gravity Furnace Diagram Poorly Located Supply and Return Registers

With all the benefits of replacing a gravity furnace, why don’t more people do it?

Cost.  A gravity furnace and the ductwork for a gravity furnace will almost always contain asbestos.  An asbestos abatement contractor will need to remove the old furnace, which obviously drives up the cost of the replacement.  The ductwork will also need to be modified, because the new furnace should have smaller supply ducts going to the outside walls, and larger return ducts on the inside walls.  This is the opposite of how gravity furnaces are designed to work.

If nothing else…

Besides all of the logical explanations for replacing or not replacing a gravity furnace, you should also consider the emotional aspects; most home buyers that I work with are very nervous about buying homes with gravity furnaces.   I always wonder how many potential buyers already passed on a home just because they were worried about the gravity furnace.

Reuben Saltzman, Structure Tech Home Inspections – Email – Home Inspector Minnesota


No responses to “Gravity Furnaces”

  1. kelly
    October 30, 2011, 10:42 am

    thanks! helped with my homework!

  2. John Green
    November 20, 2011, 3:11 pm

    Question on gravity furnace electrical system. Is their a need for and outside electrical supply to operate the unit. My daughter just moved into house with this type unit and the main burner will not light. Pilot light is on but not a very big flame.

  3. Karen Collins
    October 4, 2012, 10:47 am

    Would you please be able to give me an idea of approximately how much it would cost to replace my gravity fed furnace?


  4. Reuben Saltzman
    October 4, 2012, 12:08 pm

    Hi Karen,

    You would need to contact an HVAC contractor in your area. The prices for this work vary quite a bit.


  5. Steven
    December 6, 2012, 8:31 pm

    I personally prefer gravity furnaces over the HVAC one’s for the simple reason that they’re simple, pretty much maintainence free and they keep the living space warm and comfortable. Unlike the HVAC furnaces that have all the electronics and blower, gravity furnaces only have two components to them; a thermocouple and a gas valve.

    One of the other reasons I like gravity furnaces is because once the desired temp is reached (via the thermostat setting), you will still feel heat from the furnace. Remember, a gravity furnace is a pretty large box and when the innards are heated up they will stay heated for a while. With the newer forced air furnaces or HVAC, once the temp is reached the burners and blower shuts off. If your place cools down rapidly the furnace will constantly turn off and on.

    To answer John’s question on whether an outside electrical supply is needed for a gravity furnace, the answer is NO. The voltage required to open the gas valve is in milivolts and that is supplied by the thermocouple. The thermocouple is activated by the lit pilot..

    If you have a pilot but the furnace does not come on it could either be one of 3 components; the thermostat itself, the thermocouple or gas valve. To check if the thermostat is working all you can bypass it at the gas valve. The valve will have 4 screws, 2 for the thermostat wires and 2 for the thermocouple wires. To check all you need to do is jumper the two leads on the gas valve that reads “thermostat”. If you hear a click that’s an indication that the gas valve is opening. A few seconds later the burners should fire up. If you don’t hear a click and the burners don’t fire up then you’ll need to check the thermocouple.

    To check if the thermocouple is working all you need to do is run a volt meter across the two leads and see if it registers the required millivolts with the pilot lit. If there’s an insufficient reading you’ve got a bad thermocouple. It’s about a $35 part. If the millivolt reading is within range then you’ve most likely got a bad gas valve.

    It’s a process of elimination and only takes a few minutes to run through them.

    My gravity furnace heats up the living space in a matter of 3 minutes and when the burners shut off I can still feel heat pouring though my vents. I’ts very comfortable and noise free.

    The new HVAC furnaces are indeed energy efficient and will heat up a house quickly due to the blowers, but they’ll run about $2500-$3500+. Where the bigger cost comes from is if your old gravity furnace has asbestos around it and it’s ducts. There’s a 99% chance it will, so you’ll need to have an abatement company come out and remove them which could run another +/- $3,000-5,000.

  6. Liz carter
    February 16, 2013, 8:35 pm

    I’m purchasing a townhouse that is listed with gravity heating and forced 1 A/C, how is that possible?

  7. Reuben Saltzman
    February 18, 2013, 4:57 am

    Liz – they could have installed a high-velocity AC system separate from the heating system, but I’ve never seen this done. Highly unlikely.

  8. Joe Nero
    May 18, 2013, 6:39 pm

    Dear Reuban,
    With regard to Steven’s positive opinion (Dec.06, 2012) on Gravity furnaces and the fact that it does NOT require an electrical feed, I dare say you could be a bit better informed and your opinion less biased.

  9. Lonna Haney
    May 29, 2013, 3:04 pm

    Each winter when we start our heater, which in cavity fed (from 1936) it smells something horrible then i guess it burns off. Can you tell me why it smells? Also our intake is just a wood grate in our hallway with no filter on it, and I don’t see where you can put a filter. Is that normal?

  10. Reuben Saltzman
    June 4, 2013, 3:56 am

    Lonna – yes, those conditions are both normal.

  11. Patricia Smith
    October 21, 2013, 6:47 pm

    I have gravity furnace. Does chimney need to be cleaned & if so, how often?

  12. Reuben Saltzman
    October 22, 2013, 3:46 am

    @Patricia – no, gas appliance chimneys do not need to be cleaned.

  13. Lonna Haney
    October 22, 2013, 2:27 pm

    Karen, My contractor estimated $15,000 to replace my gravity furnace.Concerning Joe’s comment, Yes, 110V elect is turned into miltivolt at the heater. Telling someone how to test the thermostat over the internet is so wrong!!!!! Hope the heater doesn’t blow up in their face.I would get a repair professional to do that. If you don’t know what your doing, don’t do it!!!!

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