Replace 66y old Gravity Furnace with New Furnace and AC or not

11 Replies

Should I replace my gravity furnace from 1952 for my rental property located in Minnesota? It's 66 years old so I am worried it may just die in the middle of next winter. I'm not sure how long they are supposed to last. It's spring right now and I have already gotten bids for installing a new furnace and central air. The bids range from $6500 to $11,000. 

I'm torn right now, do I wait until the furnace dies or do I replace and install AC? I told my friends who rent from me that I was planning to replace it with a high efficiency furnace and central air but I did not anticipate it costing this much. I'm currently house hacking and live on the other side of the duplex from my friends. I'm also planning to install AC on the side I live on which will run me about $3,000 since I do not need a new furnace and ductwork because the side I live on is forced air. I can not raise the rent when they renew since their incomes are lower. They plan to move out August of 2019 which is when I plan to raise the rent and move in someone new. Basically I feel as though I will be giving subsidized housing at the rent they are paying and adding a new furnace and ac.

My other option is to buy energy star window units for them and let them use those. They pay their own utilities. Also, I had the center for energy and environment come test their furnace which tested out at 75% efficiency, my furnace is 80% so not much better. They also updated the house for them to make it more energy efficient. I realize I got myself into a sticky situation.. this is my first rental property. If i do not do this I'll be over promising and under delivering but I also didn't know it would be this expensive since this is my first time doing this. There is nothing in the lease that states I need to replace the furnace or install central air.

Please help! Or give me some insight!


I chose not to and just sold my duplex with the gravity furnace in it.

It’s not a great investment to replace them if they’re still working. I’d have center point come do an inspection and tune up and call it a day.

Get a window unit for ac. You’ll only need it for two months!

Those gravity furnaces have no moving parts and seem to last forever. I had one in a 100-yr old house in Ohio and when it went out I had to spend like $35 for a new thermocouple.

Like @Jordan Moorhead said, a window a/c or two will be much cheaper now and get your friends through the dog days of summer.

Unless the current tenants are willing to pay a higher rent to justify the cost, tell them to suck it up. They rented it with a gravity furnace and can continue renting it with a gravity furnace. It's not as nice as forced air heat and A/C? Then they should have rented something else. 

You have to be careful about tenants moving in and then asking for you to fix things, particularly things that aren't broken.

I agree with everyone - gravity furnaces essentially have no moving parts so there's nothing to break really.  There's a gas valve and you need to light the pilot if it burns out but that's pretty much it.  I actually think they provide nice heat too - nothing blowing the dust bunnies around.  I replaced one with a high eff furnace and A/C and it cost about $13K but that was in my own house and I added a second story to it. I wouldn't do it for a rental.  If you want a nicer option for A/C, consider a mini- split, they're great and very efficient.  I'm planning to install them in a duplex with radiators.

I’ll agree with the others. No need to replace it.

Even a high efficiency unit might save a few dollars each month. Not enough to really make a difference unless you’re calculating savings over 5-10-20 years.

I am with @Jordan Moorhead and @Scott S. , leave it and get a couple AC units.  The older the furnace the more reliable they seem to be from an operating standpoint.  The old furnaces don't have the safety switches and everything else which are common in new furnaces.  The switches are for safety but these switches are often the reasons a furnace won't start up in the fall.  If you keep an old furnace just have someone check the CO2 level periodically and make sure it is operating fine.

Thanks for everyone’s reply! Based on the input, I decided I’m gonna buy some energy star rated window ac units.

A gravity furnace is not a forced air furnace (no mechanical fans pushing air) so it heats up air in one central location which then rises through the ductwork to heat the house and then when the air cools and it becomes more dense then hot air and thus falls down through the return ducts back to the furnace to be heated again creating a slow cycle of convection.

This is such a refreshing thread, every other forum immediately suggests replacing gravity furnaces because of efficiency, but most calculations that suggest 50% efficiency ignore the fact that they use almost no electricity and some of the heat "lost" is in fact lost to the basement which I'm many cases still contributes to heating the living space thru the floors,. I purchased a small home with a gravity furnace roughly 20 years ago and the realtor at the time suggested requesting replacement and I'm still happy I didn't as the heat was silent, comfortable, and required very little maintenance, with a replacement furnace only expected to last 15 years at a $3-4k cost it's hard to imagine why replacing a furnace that's been trouble free for 80 years and uses no filters or electricity to save maybe $30-50 in gas 3 months out of the year is the commonly suggested advice

@Josh Cook

"I decided I’m gonna buy some energy star rated window ac units."

Why ????

If your tenants want AC have them buy their own window units. Make sure they are installed professionally or you could have serious water and mold issues.

I have tested the efficiency on one gravity air furnace and a super knowledgeable energy efficiency guru I know has tested four of them. The results are a FAR CRY from the 50% number bandied about. I am not aware of ONE test that indicated merely 50% efficiency on a gravity air furnace. My furnace was designed to be used with natural gas. It is 86-87% efficient. I plan to take it apart this summer and clean the burners (hack saw blade to the slots) and seal up any gaps in the cast iron puzzle pieces that comprise the combustion chamber (but first I will blow theatrical smoke inside the chamber in order to suss out where any potential leaks might be). So mine is 86% efficient and might end up slightly higher after cleaning burners likely left in place for the last 80+ years.   My friend tested gravity air furnaces designed to be fueled with natural gas and found that they were 85% efficient. He also tested two that were designed for coal and then converted to natural gas. I don't know if it was the original design or the conversion that caused the inefficiency but they were about 65% efficient. I have no other details about these but it would be interesting to see data on tested efficiencies from original designers of these cast iron heat sinks. BTW, I kind of think of them as gas fired "masonry furnaces."  Masonry furnaces are fireplaces centrally located in a house that have massive amounts of mass around a long winding flue pipe. (Masonry furnaces often have a warmed bench, elaborate designs, bread oven, boot warmer, and are architectural centerpieces of the interior of the home.) The idea is to burn wood hot 1-2 times a day, heat up all that mass and then allow the mass to heat the home throughout the day.  My gravity air furnace has a LOT of cast iron mass. Its quiet. Simple. Efficient enough (I put my money into "air sealing" and wall and attic insulation).