DEAR TIM: We are looking to buy an older house that has a very unusual furnace. The home inspector calls it a gravity furnace, but I think it looks more like an octopus with these large pipes that protrude out like tentacles from the top of the large, round furnace. There is also some white coarse paper wrapped around the pipes, and a 1-inch thick piece of white cardboard on top of the gravity furnace. Is this a good furnace, or should we negotiate for a new one? Stephanie T., Pittsburgh, PA

DEAR STEPHANIE: Gravity furnaces are a nearly extinct species. They date back to the late 1800's and early 1900's. The large, round furnaces were almost always designed to burn coal, however many came from the factory equipped to burn wood, oil or natural gas. In fact, it was very common to see these furnaces converted from one fuel source to another. The first house my wife and I owned had a coal gravity furnace that had been switched over to natural gas at some point. As best as I could figure, this furnace was originally installed in the early 1920's. It was still working perfectly in 1975 when I turned it on.

The furnaces worked by gravity. The most basic ones only had a few moving parts, and those were usually the doors you opened to insert fuel or remove ashes or cinders. As the furnaces got hot, they heated the air around the core of the furnace's combustion chamber. This air floated up through the large pipes into the house. Gravity pulled down the heavier cold air into the base of the furnace. A slow-motion conveyor belt of air constantly flowed through the furnace as long as fuel was burning inside and residual heat remained in the metal parts after the fuel was exhausted.

The heating ducts where the warm air floated up into the rooms were often installed in the wrong locations. My guess is you might find register covers on inside rooms near the central core of the house. Modern forced-air heating/cooling technology has supply ducts and registers located on exterior walls of homes. This placement allows the conditioned air to wash across the wall surface that is either cold in the winter or hot in the summer.

Modern return air ducts should be located in each room on the wall opposite the exterior wall. This allows the conditioned air to be pulled across the room providing even heating or cooling.

The cold air usually entered the furnace through a giant duct often found in the floor or at the bas of a flight of steps that led to the second floor. The gravity furnaces themselves were often located in the basement of a home which helped supply them with cooler air. The basement location also aided the loading of wood or coal through chutes that were at grade level outdoors. Gravity would make it easier to get the fuel down into the storage area near the furnace.

These gravity furnaces were horribly inefficient. I do not have access to actual numbers, but my guess would be that 50 percent or more of the heat generated by the burning fuel went up the chimney. Compare that to today's super-efficient furnaces that often extract 95 percent of the heat from fuel, and you can see why it might be a great idea for you to replace the large gravity beast, even if it still works.

The white wrapping around the pipes is almost certainly asbestos. It was commonly used as an insulator and fireproofing material on gravity furnaces. It is a very dangerous and toxic material. The fibers in the asbestos are known carcinogens. The thick, white cardboard product is probably pure asbestos. It is acting as a shield that keeps the intense heat of the burning fuel inside the furnace. Do not touch or disturb this cardboard or the wrapping around the pipes.

If you decide to purchase this home, I would recommend that you negotiate the removal of the furnace, the proper installation of new ductwork and expert removal of the asbestos by a certified asbestos remediation company. Do not underestimate the seriousness of the asbestos issue. Be sure your realtor includes language in your written offer that ensures a certified asbestos removal company will get rid of the toxic material.

Keep in mind that just removing the old gravity furnace is not enough. The ductwork within the walls is almost certainly oversized for each room, and it is in the wrong location. If you want your home to be comfortable, you will need to have new ductwork extended to each room and similar return-air ducts installed to get air from each room back to the furnace. The giant return air duct on the first floor needs to be abandoned, and the floor patched if that is where it is located.

Asbestos facts and removal information can be found at the Environmental Protection Agency's Web site: http://www.epa.gov/asbestos/ . I urge you to read all of the information there and become highly educated about this topic.

There are ways to encapsulate the asbestos, and some companies may advise you about this. However since the furnace is so inefficient, the asbestos has to go out with the furnace. You need to make sure little or no asbestos is released into the air or on other surfaces in your home.

Talk to your realtor about making an offer to buy contingent upon getting several estimates for a new furnace, ductwork and the removal of the asbestos. Be sure the language indicates the purchase price will be reduced by an amount that allows you to hire professionals to do all of the work.

I want to give a huge thanks to Tony White for taking these photos. Tony is an appraiser in Cincinnati, OH and owns White Appraisal, Inc.

Author's Notes:

John Podolinsky, who works for the State of Montana in their DEQ/WUTMB/Asbestos Control Program, was kind enough to contact me after this column ran in the Great Falls Tribune:

"I enjoyed your article in the Great Falls Tribune newspaper on Saturday, September 15, 2007, on gravity furnaces and asbestos!!

It was a pleasure to read such an informative article on both subjects. Fortunately, I do not heat my house with a gravity furnace, but I do deal with asbestos on a daily basis.

In my line of business (asbestos regulatory), I witness much asbestos non-compliance based on ignorance. Your article will hopefully educate!

If you plan to provide additional asbestos information to your audience, especially those who live in Montana, feel free to refer to our website below or me.

Thanks again and keep up the great work,"

John Podolinsky
State of Montana
DEQ/WUTMB/Asbestos Control Program
1520 East 6th Avenue
P.O. Box 200901
Helena, MT 59620-0901
www.deq.mt.gov/Asbestos/index.asp

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Comments: Welcome! I, Tim Carter, don't answer questions here. If you post a question here in the Comments Area, perhaps another visitor will help you. You need to go to the Ask Tim page if you want a question answered. Once there, look closely at how many weeks behind we are. Please be patient as you use this free service. If you have an emergency and need to talk to me, there is an option there for you. Heather 24 Jan 2008, 13:32 Dear Tim, I manage a House that has the original coal burning monster gravity furnace. At some point it was converted to burn natural gas which it feasts on with gusto. The heater is simply ineffectual when the burners are adjusted too low. What I want to know is, is it still necessary to have an exhaust duct to draw impurities (and a lot of heat)away from the air going into the House, or is the gas we burn today cleaner, making the exhaust duct obsolete/unnecessary? AsktheBuilder 24 Jan 2008, 16:22 Heather, If you disconnect the exhaust duct, you will be dead in about an hour or less from a gusto-sized dose of carbon monoxide. Your mission, should you wish to accept it, is to replace that beast with a high-efficiency unit. You will be amazed at how you will nearly cut your gas bill by half. Nick 08 May 2008, 16:12 Not all Asbestos is cancerogenic. Short strands cheap asbestos which is used in North America is causing cancer, long strands Russian and Asian asbestos is totally safe and is proven not to cause any cancer. mark nicolussi 06 Nov 2008, 15:01 Good article. I think, perhaps, you underestimate the difficulty of moving ducts - especially to the 2nd floor. Mary McIntosh 07 Jan 2009, 11:51 I have a gravity heat furnace in my house. Only problem I have with it is its dry heat. Is there any way to add moisture in the air inexpensively. Gene 07 Jan 2009, 14:48 You can add moisture with a humidifier..........either a whole house unit built into a main duct or buy a free-standing unit for whichever room needs it. albert landrum 10 Apr 2009, 01:35 My dad has a gravity furnance that does not put out enough heat. The burner is burning but very little heat is coming thru the ducts. Are the flames to low and if so, how can they be adjusted? Dave in Chicago 11 Dec 2009, 07:27 I have a gas fueled gravity furnace originally designed for gas. We tested it's efficiency with a flue gas analyzer. Drum roll please....82% efficient. --Obviously it seemed too high based on the claim made on the Internet. Every claim I have read does not include a reference or measurement. Some admit it's a guess or conjecture. -- My furnace is designed like a very efficient system called a "masonry furnace". I asked Chicago's local energy efficiency guru, John Porterfield, what he thought about my gravity furnace efficiency reading. He said that he had only measured two: a coal fired unit converted to gas was 62% and a unit designed for natural gas, like mine, was 82%. Please measure and verify before making claims. Please don't trust claims without verified research. aka: KJB 03 Jan 2010, 06:56 I've got a gravity furnace that was installed in this house sometime between 1906 - 1912 (that's the best Lennox could tell me based on the model number stamped into the boiler door). It was a coal burner that was converted to gas at some point. The inside of these were covered in a special masonry brick that retains heat and slowly discharges it. These furnaces can be very efficient and use far less electricity than forced air units. During an huge ice storm here, the entire area was without power for weeks. Most houses had no heat because of the power requirements for the fans while we had the same heat we usually had because the only part that needed electricity was the thermostat, which ran on a D battery! So while the gravity furnaces may not be the norm, they can be very effective if used and maintained properly. Arthur 08 Jan 2010, 14:00 I've an old gravity furnace that I cannot afford to replace. But I would like to increase the efficiency of it if possible. First, I would like to ask you if it is possible to vent the combustion chamber. It is a mystery as to how it is getting its oxygen, but I assume that it is pulling it out of the basement. Secondly, I would like to buy some heat retaining bricks that I might place around the combustion chamber to keep some of the heat that is now riding up the chimney. It is an old furnace (75 years) that was once for coal and has been converted to gas and has the valve components where a stoker was probably attached. Any guidance would be greatly appreciated. Thanks!      View all comments *Name: Email: Notify me about new comments on this page Hide my email *Text: Security Image: Security Image Text:   Remember, Tim Carter doesn't answer questions here in the comments. You need to go to the Ask Tim page.