Gravity floor furnace on an inspection report

What is a gravity or floor furnace, and what should you do about one?

A gravity furnace is an old furnace with no blower fan, and a floor furnace is a single heater hung below one room's floor. Before your deadline, have a licensed heating, ventilation, and air conditioning (HVAC) contractor test either one for carbon monoxide and bid its replacement. Then ask the seller for that bid amount.

Key takeaways

• The International Association of Certified Home Inspectors (InterNACHI) says replacement is usually recommended, even for a unit that still works.
• Across the cited ranges, converting a gravity furnace to gas forced air adds up to about $5,860 to $15,330 with new ductwork and haul-away, before any asbestos removal.
• Thick white wrap on the furnace may be asbestos. It needs its own bid from a qualified asbestos professional.

How do gravity and floor furnaces work?

With no fan to push it, warm air rises from a gravity furnace through ducts in the walls and floors to the rooms above, which is why some people call it an "octopus furnace." InterNACHI says they first burned only coal, and many were later converted to oil, gas or propane.

A floor furnace is a single heater under one room. A Texas residential code posted on UpCodes defines it as "a self-contained furnace suspended from the floor of the space being heated, taking air for combustion from outside such space." Your local building department or a licensed HVAC contractor can tell you which code applies at your address.

Is a floor or gravity furnace safe?

Treat it as unknown until a licensed HVAC contractor tests it. InterNACHI lists one hazard to check on gravity units: "carbon monoxide in the supply air, which can result from openings in the seams of the burner chamber." The Environmental Protection Agency (EPA) warns that you "can't see or smell carbon monoxide, but at high levels it can kill a person in minutes."

Ask the contractor to open the unit, inspect the burner chamber and test for carbon monoxide, before any money talk. If anyone feels sick near the unit, EPA says to open doors and windows, turn off combustion appliances and leave the house, then go to an emergency room. From outside, call 911 or your gas utility, then the HVAC contractor. Gravity furnaces have the same combustion-air and venting requirements as forced-air furnaces, says InterNACHI.

What are the disadvantages of a gravity furnace?

InterNACHI calls its few advantages slight next to its drawbacks.

• Wasted heat. By one estimate InterNACHI cites, only half the heat warms the house and the rest goes up the chimney. Many modern forced-air furnaces are 95% efficient.
• Slow heat. It can take a long time to heat a room after it's turned on.
• No central air. With no blower, a gravity furnace can't be paired with central air conditioning.
• Oversized ducts. The old ducts are the wrong size and in the wrong place for a modern furnace. They get modified or abandoned when the furnace goes.
• Asbestos. Asbestos was used to insulate gravity furnaces as they went in. InterNACHI says it "should not be handled or disturbed," and that buyers "should negotiate the price of the house to include the cost of removing the asbestos insulation."

InterNACHI also says a working gravity furnace doesn't have to be replaced. Replacement is usually recommended anyway, because units still running are past their life expectancy and lack the safety of modern forced-air systems.

How much does it cost to replace a floor or gravity furnace?

It depends on what goes in. A gas furnace in place of a gravity furnace is priced here with new ductwork, since the old ducts get modified or abandoned. A floor furnace can be swapped for a new floor furnace, or for a ductless mini-split that needs no ducts.

Here's a gravity furnace converted to gas forced air, using the low and high ends of the cited ranges. The low end is $3,800 for the furnace, $2,000 for ductwork and $60 for removal, or $5,860. The high end is $10,000, $5,000 and $330, or $15,330. Asbestos removal comes on top.

Kept prices every line of an inspection report for $79, usually under 4 hours, and marks the ones that need a professional quote.

Who can price it before your deadline?

A licensed HVAC contractor. Ask for one visit that covers the carbon monoxide test, the unit's condition, and a written bid for the replacement you want. HVAC inspection before buying covers what that visit includes.

If the inspector noted white wrap around the furnace or its parts, get an asbestos professional's bid too.

What should you ask the seller for?

Ask for a credit or price cut equal to the written bid for the replacement you'd actually choose, plus the asbestos bid if there is one. Name the scope and the trade on each line: new furnace, new ductwork, removal of the old unit. What to ask for after a home inspection shows how this fits next to the rest of your list. The buyer repair request letter template gives you the wording.

If the seller says no, decide whether you'd still buy at this price with the replacement on you. When to walk away after a home inspection walks through that call. Your contract sets the deadline and terms, so confirm them with your agent.

Are floor furnaces still made?

Yes. Williams still lists floor furnaces. A new one doesn't tell you anything about the one in your report. The contractor's nameplate check does.

Do people still use gravity furnaces?

Some do. InterNACHI says they went in from the late 1800s to the mid-20th century, and they "still perform acceptably where they exist in some older buildings." If your report also flags a newer forced-air furnace near the end of its life, see a 20 year old furnace on an inspection report.

How do I know what the rest of my report is worth?

Price the heating items together, then the rest of the list.

If the furnace is one line of a long report, upload your inspection report to Kept so your days go to the HVAC contractor's test and bid.

Sources

• InterNACHI, Gravity Furnace Inspection
• EPA, Protect Your Family and Yourself from Carbon Monoxide Poisoning
• UpCodes, Floor Furnace definition
• Williams, Floor Furnaces
• HomeAdvisor, Furnace Replacement Cost
• Bob Vila, New Furnace Cost
• Bob Vila, Mini-Split Installation Costs