Are gravity furnaces still around? Do they need to be replaced?

It has been so long since I saw a gravity furnace in a property that I was shocked when I saw not one, but two in a duplex I showed recently in south Minneapolis. The property was in immaculate condition and updated with exquisite taste, which made the furnaces even more surprising.

There were two of them… one for each side of the duplex, and much more modern than the white ‘octopus’ in the left photos above. The white asbestos wrap makes me think of a mummy ready for Halloween!

Gravity furnaces are often nicknamed ‘octopus’ furnaces because of their multiple ‘arms’. They date back to the late 1800’s and early 1900’s and were originally designed to burn coal but later switched to burn oil or natural gas.

There aren’t too many left but you still sometimes find them in older homes, especially ones where the owners have lived for many decades. Although they are only about 50% energy efficient and take up huge amounts of space older homeowners often don’t replace them because, like the Energizer Bunny, they just keep going and going and going… with few moving parts, there isn’t much that can break down.

As its name implies, air circulation in a gravity furnace relies on gravity… the principle that warm air rises and cool air falls. There typically is no fan on an older gravity furnace… that is why there are so many ‘arms’ and why they are so large. Since no fan is pushing the air, the air pressure is lower as the warm air gently enters the room and there tend to be fewer hot spots. However, there is no way to provide central air conditioning or warm air to the basement with this type of furnace… and there is no filter since that obstructs air movement. Note the modifications in the newer model in the photo on the right… with a fan and smaller, fewer ‘arms’.

A past client asked if he needed to replace his gravity furnace before selling his home… it was working fine! The answer was no, there was no safety or code requirement to replace it. But I still advised him to replace it… because it often scares buyers away or they will ask to have it replaced before closing or ask for a big price reduction. Either way, sellers end up paying for it and it is better having it taken care of up front.

Inspector Reuben gives three main reasons for replacing gravity furnaces…

1. Money

Gravity furnaces typically cost about twice as much to operate as a forced air furnace because they are terribly inefficient. With a gravity furnace about half the generated heat goes up the chimney, making it about 50% efficient vs efficiency up to 95% with most modern furnaces… meaning lower heating costs.

2. Efficiency

The more efficient your furnace is, the better it is for the environment because less greenhouse gasses get released into the atmosphere. A new forced air furnace is also more space efficient because it is smaller, freeing up space for other things!

3. Comfort

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. This makes the middle of the house warm and the outside walls fairly cool. Additionally, with a forced air furnace you have the option of adding central air conditioning and zoning/electronic control devices.

I am adding a 4th reason…

4. Getting your home sold

When I started selling homes in 2000 there were 577 single family houses sold in Minneapolis and St. Paul with gravity furnaces. So far this year there are only 80. This means that many buyers will walk away from even considering a house with a gravity furnace because they have other choices. Why do they walk away?

Cost. Most buyers will want the gravity furnace replaced, but it isn’t as easy as simply buying a new furnace.

Gravity furnaces and ductwork almost always contain asbestos, which means hiring an asbestos abatement contractor to remove the old furnace. The ductwork will also need to be modified, because the new furnace should have smaller air ducts going to the outside walls and larger return ducts on the inside walls (opposite of gravity furnaces).

Most buyers are nervous about buying a house with a gravity furnace. And because of everything involved in replacing a gravity furnace, most buyers simply do not want to deal with it even if they have the money to do so.