Reuben's Home Inspection Blog


Gravity Furnace? Get A New One.

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 the air in the ductwork, and all of the exhaust gas that leaves the 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 96% efficient.

High Efficiency Furnace Diagram

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', socially 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.  At least five digits.  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.

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 - Minneapolis Home Inspections

Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 15 commentsReuben Saltzman • March 07 2009 06:49AM


WOW that is very informative. I am glad I live in Fort Myers Florida and don't have a furnace.

Posted by Deanna Casalino, Fort Myers Florida Homes, (Remax Realty Group) over 11 years ago

Reuben one more benefit to replacing a gravity furnace. You gain storage area in the lower level! I have seen furnaces the size of a small car replaced by a HVAC system with a much smaller footprint.

Posted by James Gordon, REALTOR, PBD SFR SRS (Sibcy Cline Realtors®) over 11 years ago

Speaking of furnaces in our WWII victory ship the SS American Victory we have 2 50 ton boilers that operate on 450psi. We use 44 gallons per mile


I will post a blog if you want to learn more about it. Charles Wheatley Fort Myers Florida


Posted by Charles Wheatley, Remax Realty Group (Remax Realty Group) over 11 years ago

Hi Reuben - Great article about gravity furnaces! I decided to replace my own not long ago. I used only 2/3 of the actual cubic ft. of natural gas as the entire year before (September to September - taking into account an entire heating season). Two additional benefits to homeowners that make the switch: 1) Ridding the home of asbestos wrapping on the old duct work that is a health hazard 2) Having a more comfortable home due to the ease of controlling the humidity level in a proper range.

Downside? The gravity furnace operated flawlessly only requiring a seasonal filter change. The new  gas furnace (Bryant) control board went out after only 17 months. Though part was covered by warranty, the service call, troubleshooting time, and warranty fee was expensive.

Posted by Jim Dvorovy, REALTOR - Canton Ohio Real Estate (Cutler Real Estate) over 11 years ago

Reuben, great post as usual.

Posted by Tad Petersen / Home Inspector, Mpls (Safeguard Home Inspections, Inc.) over 11 years ago

Deanna and Tad - Thanks!

James and Jim - Excellent points, especially about the extra space gained - duh!  Why didn't I list that?

- Reuben

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 11 years ago

Back when I was a home inspector (2001-2005), I had the pleasure of being one of three team members inspecting a beautiful old house out in Julian. It had an "octopus" gravity furnace, with all the ducts nicely insulated, so to speak. It was an oil burner, and the service tech was there the same time we were.

House in Julian

1937 gravity furnace

Posted by Jim Frimmer, Realtor & CDPE, Mission Valley specialist (HomeSmart Realty West) over 11 years ago

Well, I see Jim's been here, too, and has stolen my two best pictures. I guess he can do that since he helped inspect that home.

Posted by Russel Ray, San Diego Business & Marketing Consultant & Photographer (Russel Ray) over 11 years ago


hi, i'm Twanda,  great blogs, very helpful,. However, i'm a new home owner, i bought an old house with a gravity fed heater, what did i know?, NOTHING, 5 mths later my christmas tree was brown on one side, dead and flaking fast, needless to say, the heater broke. carbonminoxide killed the tree. the insurance co. sent this guy out to fix it, he did, with a gas/electric efficiency furnace GMP125-4 80 afue, i thought it was to big when he turned on the blower and it riped the carpet back. but then again. it's an old house, and old carpet, okay. it also took up more floor space than the gravity fed. but it worked it was warm and i was happy, for the moment until i went to the basement to look at his work, i had questions about everything. why there was so much foil tape? those wires, are they just supose to hang there? why is there a huge hole in the side of the furnace? His answers were, the Ins. Co. doesn't pay him to do the cosmetics. I could pay him on the side to come back, and he would after his shift for a far price.

ANYWAY, Have you ever seen (inspected) or heard of such thing happening?

 i made due with what i could til now, i'm searching to correct the things i have done or could improve,  like the air return, the ductwork, is the filter in the best place.

i didn't pay the guy to come back, but a gave him a great tip.."dont' let the door hit you on your way out", My assortment of raw emotions got the better of me. i couldn't even call the ins. co. to complain i was a wreck i was no good to talk to anyone about it.

i'm sorry for venting and being so winded. btw. are you married? [just joshen ya] THANK YOU,


Posted by Twanda Wright almost 11 years ago

Hi Twanda - tons of foil tape, hanging wires, and a huge hole in the side of the furnace?  Those aren't cosmetic items.  

I don't know how the codes are where you live, but here in Minneapolis, replacement of a furnace requires a permit.  The permit fee pays for a city inspector to come out to inspect the work, to make sure it's code compliant.  It sounds like this never happened at your house.  

I've see furnace installers do sloppy work before, but I've never heard of an insurance company replacing an old furnace that went ka-put.  Was this a warranty company, or your homeowners insurance?  If it was your homeowners insurance, you're luck to have a brand new furnace for free!  If this was part of a home warranty, why wasn't a permit pulled?   

If you want to post a photo of your furnace installation, I'd be happy to look for any obvious defects.

Thanks for reading!

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 11 years ago

For whoever posted that last turd on my blog, does rel= "nofollow" mean anything to you?  

It should.  

You don't know what you're doing, you're wasting your time, and you look like a fool.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 11 years ago

I am borrowing the comment in #13 to use...  I get turds on my blog so often.


Posted by Maureen McCabe, Columbus Ohio Real Estate (HER Realtors) almost 10 years ago

lol - please do!

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 10 years ago

I just bought a 1960's log cabin that has this beast in the basement.  The house doesn't have any duct work and uses room air conditioners to cool the house.  What I am looking at right now is a ductless system.  Would you have any other suggestions to get it cool in the summer and warm in the winter?  The bummer is that the seller had to put in baseboard heating units in the upstairs area so it would pass for financing.  But I am not a hot weather person and need to get a different option for cooling.

Posted by Denise Oppenhagen about 9 years ago

this is for everyone out there,,,,,,,i am an hvac tech with 25 yrs of experience,in all types of heating and air conditioning,refrigeration, i also do solar,geo thermal,,,,,,,please take my advice,,,when u purchase a house,seek out professionals,in the areas that concern you,,,if u are concerned with heating and air conditioning,,,call an hvac company,,,,if u are concerned about your plumbing ,toilets, bathtub ect,,,call a plumber,,,,and so on,,,do not truly rely on house inspectors alone they are a basic inspection.i know u will spend a little more money [couple hundred] but it wil give a better peace of mind or bargining egde or both.i have seen this too many times,,,example;my customer bought a house had an home inspector look it over,had a building inspector look it over,,,passed with flying colors ,,then he called me,i took a look at it,and found a cracked heat exchanger in the furnace[3yrs old]and a septic leak in a finished basement.both health issues,,enough of that.......gravity furnace works great in small hunting cabins and work shops ,very dependable,u dont need electric,and they were made up to the late 1980s,,,,,,,ocassional use only,,,,they came in all different shapes

Posted by blitzkrieg over 7 years ago