Facts: Gravity Air Furnace

fleuchaus

After enduring too many years of reading claims about gravity air furnaces that were based solely on "conventional wisdom" without referencing ANY TESTING or verifiable data I decided to share what I have learned and tested.
Terms: Gravity furnace, gravity air furnace, hot air furnace, hot air heater, octopus furnace
Fuels: coal, gas, oil
Conversions: add fan, change fuel source, add mass
Dangers: asbestos on ducts, heat exchanger leaks, asbestos under the base of the unit (exposed when removed), asbestos sometimes on the furnace itself.
Address Dangers: asbestos on ducts or furnace: encapsulate with rewettable lagging cloth; heat exchanger leaks: seal with appropriate sealant and monitor CO
TESTED Efficiencies: 83% NG, working as designed; 83% NG working as designed; 65% NG converted from coal. Tests conducted with three different combustion analyzers.
Benefits: quiet, nearly unbreakable, little service required per decade or quarter century.
Periodic Safety Inspections: gas leaks, friable asbestos, CO, chimney
My Opinion regarding cost/benefit of replacement: if existing gravity furnace is 80+% efficient then keep the furnace and spend what it would cost to replace it on air sealing and insulating the house, encapsulating any asbestos (EPA recommends encapsulation over removal), and if AC is desired then adding a few mini-splits for AC.
I am interested in facts not preferences or opinions. Aside from basement ducts being in the way for homes with lower ceilings and when there is a desire to use the basement more fully, what are your thoughts on keeping a system like this that requires 1/50th the maintenance (in my experience) of a modern forced air furnace?

Jamie Hall

The only real disadvantage that I know of to gravity hot air furnaces -- and the place I care for has one, although it's not in service at this time -- is that they can take a long time to get heat to more distant places, particularly where the elevation difference is small. Unlike fan forced scorched air, which is almost immediate. Once they do start to circulate, though, they work fine.

Other than that... my usual comment: if it ain't broke, don't fix it.

John Mills_5

Combustion efficiency tells how well the gas is burning when everything is up to temp. AFUE, how modern furnaces are rated, look at all aspects of the operation. An octopus or gravity furnace will pour lots of heat up the big chimney until the heat exchanger gets warm enough to put some heat in the house. Then when the stat is satisfied, heat is still pouring up the chimney for 10 minutes or more while the HX cools down. Also, you are using a lot of house air in the combustion process. There is air for combustion and air for dilution up the draft diverter and that goes on 24 hours a day. So while your CA may say 83%, AFUE is more like 50%.

Around 1990 I lived in a 1937 house with the original forced air furnace converted to gas. I kept daily therm usage vs outdoor temp. Finally convinced the landlord to boot it - shoulda seen the huge cracks in the HX. He put in a 65% GE natural draft furnace with spark. Gas usage dropped like a rock. Would have been really nice had he bought the 80% I quoted him through a friend I was working for.

Tim McElwain

As a 76 year old service tech who has worked on hundreds of those and back in the day converted many to a atmospheric gas Conversion burner from either oil or coal. I have also added a fan system to some of them. I am in agreement with John Mills posting. The overall efficiency is lousy. When fuel was cheap who cared, not the case today.

ratio

The squatter who lived at the little place my brother bought had the solution to the efficiencys inherent with a gravity furnace. He 'fixed' the gas meter so it didn't register any more. The gas company was like "ummmm, looks like you need a new meter" when we had service restored.