The Monster in Your Basement – the Gravity Furnace

The gravity furnace is also commonly called the Octopus furnace because it has long ducts coming out of the central unit. It can be quite a sight to behold and even scares some buyers the first time they see it. These types of units were installed in homes built in the late 1800’s and early 1900’s. There are still many homes in Minneapolis and Saint Paul which have this furnace as their heat source. Slowly, home owners are replacing them with more efficient HVAC units, as well as replacing the duct-work throughout the home.

The concept of the Octopus Furnace is that heat rises and cold air falls. Heated air rises through the heat ducts and then the cold air sinks, entering the return air ducts, where it is reheated again. The original fuel source for early models was coal, but many since then have been converted to natural gas or oil. The above diagram is a great example of how it works.

Reasons you might want to replace your Gravity Furnace:
  • energy efficiency – gravity fed furnaces are 50% less efficient than a conventional heating system
  • most gravity furnaces contain asbestos. Asbestos is not harmful if left untouched, but if you do plan on replacing your gravity furnace, a licensed abatement contractor will most likely seal off the basement and safely remove any hazardous material.
  • they take up an enormous amount of space in the basement
  • you cannot install a whole house air conditioning system with this type of furnace

I have shown homes to buyers with Gravity Furnaces and many are shocked to see one for the first time. Some even fear purchasing the home, but they are easy to get along with. If you plan on purchasing a home with a Gravity furnace, DO think about replacing one in the near future. It will save you money in the long run.

22 Responses to “The Monster in Your Basement – the Gravity Furnace”

  1. Kassie says:

    We have one and until we get the $15,000+ to get it replaced, it is staying.

    Two more reasons to get rid of it:

    1) Asbestos (or however you spell it)
    2) Scares small children.

    • Sara says:

      Can you tell me what the vents or registers look like? And the stove itself? Was it converted to run on gas or oil? I’m writing a novel and putting one in the basement. What a fabulous metaphor–quiet, no fans, and the heat just seeps upward.

      The asbestos is only a problem if it’s disturbed, I’m told. I don’t have one, but my furniture near our forced air heating gets cracks from the dryness. Boo hoo.

      Did you ever get rid of it?

  2. Kassie – I had not thought about it scaring young children, but I can see how it would. Thanks for letting us know what the cost is to replace one! Wow, I really didn’t know it got that high!

  3. DethWench says:

    Scares young children?? I am from Minneapolis, and one of my scariest memories from college was going into my friend’s grandma’s basement with a candle and discovering an octopus furnace!

  4. stewart says:

    Do not replace that furnace. Great antique value,re-wrap pipes,no fans to repair and no power to operate and quite, not to mention other positives. More heat/cooling, just insulate home. Attic, windows and such. I have the same furnace (PG&E) friends say they are awesome and they do not break at all.. 1918 is my model, as is still going strong. Including my wedgewood gas stove(both have not been replaced). total for a 2300 SQ ft home is less than 60.00$ per month.. If it does not break, do not replace. Those home improvement guys do not always give you the truth.. Stu

    • Heather says:

      I have one in this old house I recently bought. This is my first winter in the house and I love it. I think I will keep it for as long as it works. Its quiet, heats well, and I think it looks cool it matches everything else original to the house. I think my house is about 2300 sq feet as well and my first full gas bill was $100 not to bad. I might get the asbestos removed I havent made up my mind on that one yet.

    • Sara says:

      Can you tell me what the vents or registers look like? And the stove itself? Was it converted to run on gas or oil? I’m writing a novel and putting one in the basement. What a fabulous metaphor–quiet, no fans, and the heat just seeps upward.

  5. Joan says:

    I owned an 1896 home in Kenwood for 26 years. It had a gravity-system furnace converted from coal to gas in the 1920s. It cost me 59 cents in repairs in all that time—a switch wore out. It was the most comfortable heat I’ve ever experienced, the heat flowing gently upward from the cast-iron floor vents, the air circulating via cold air returns made of beautiful hardwood grids in the floor. A few years ago I had an energy-efficient Bryant furnace and central air conditioner installed in the house I now own, and I hate it. No matter where I sit, the forced air streaks across the floor and hits me. I actually have to put boxes a few feet in front of the wall vents to deflect the air current—a low-tech answer to a high-tech problem. I can’t remember what the gravity furnace cost in heat, but I’d pay it happily to have that comfortable system now.

    My children were not afraid of the octopus. They spent many hours under its tentacles, building ships that did not float, Star Trek models that did not fly and tall Lego towers that crashed to whoops of joy. But of course, they, like the children of Lake Wobegon, were above average. 🙂

  6. Trisha says:

    Hi we had one in our basement it always scared me when I went down to the basement, but what I don’t understand was how did the heat get in to my third floor bedroom. There were no registers on the floor but there was this scary looking square hole in the wall with a cement patch in the middle, I always put things in there as it was big enough to store books! Is that where the heat came from? I have asked a million people and no one seems to know what I am talking about

  7. Scharlene Torres says:

    I’m house hunting in Connecticut and today i saw a house built 1813…GORGEOUS.. And in the cellar lived the Octopus furnace.. my partner was the one to find this monster and came up with the WHAT IS THAT FACE. I instantly fell in love with this house as I see nothing but character and beauty in the homes off old…

  8. john says:

    To all who think it is scary, they work well, are the quietest and a lot more efficient than the hvac industry would have you believe. As for the fear factor? DEAL with it, or are you afraid of the dark too? Stop watching Home Alone!

  9. Katelyn says:

    I have a octopus furnace unhooked in my basement and looking to see what they are worth.

    • Jon B says:

      Katelyn, the system is not worth much . the most expensive stuff that might get you some money is the iron and steel from the inside of it and the door itself. Everything else is either scrap metal and most yards wont take cause of the asbestos on it. Contact your local scrap yard and see what they are paying out for it. Hope this helps..

  10. Dee says:

    Owned home over 30 years with an Octopus. 1400 sq ft home in Michigan. No ducts to the upstairs, so the upstairs felt 5-10 degrees colder. I didn’t think heating bills were expensive at all, but 1400 sq ft isn’t a huge home. Never serviced once in over 30 years – can’t beat that!

  11. Renee says:

    What is mine worth?

  12. xuji says:

    I hear all this talk about gravity furnaces are 50% efficient – very low, compared to modern gas furnaces – 90%. OK, so WHY are they less efficient? is it merely the absence of a fan to move the hot air? or what are other MAJOR factors creating the difference in efficiency? Don’t tell me “well, uh, they are just designed better,” oh yeah, well WHAT are the specific characteristics that create the improved efficiency?

    • Mike says:

      I stumbled upon this thread, I will offer these tidbits , I am a general contractor , mechanical contractor, and owner of a nice old gravity coal furnace, running it for 35 years now, bituminous (soft coal), now the unit works regardless of the fuel used wood or coal, but much better on coal, next, the efficiency rating on any heating system is measured on the input and output , meaning the production of heat gained and utilized, and the residual that is exhausted, the older units do expell more up the chimney than does a contemporary unit, an example an electric heater is a 100 percent efficient unit as all the energy produced is used within and none is explelled. The efficiency is lower on older units , but the fuels used then we’re typically very reasonable in the region, so the priority was on production of heat , you can strip additional heat from any device with additional equipment, however the solid fuels systems require some draft, suction if you will up the chimney, this basically is accomplished by heat rising, this flow is essential in the operational sequence of these units, so some loss is acceptable.. modern units are now so efficient that they require a draft booster (inducer) to pump the residual gasses thru plastic exhaust tubing (pipe), low temps realized ..hope this helps

  13. Eric M Haller says:

    We just bought a BEAUTIFUL 2300sqft1918 craftsman home In Washington state (I know it’s not Minnesota) with a Round Oak Moist air gravity (octopus) furnace. it was already converted to natural gas. It has a blower box with changeable filters that help circulate the air through the house. We had a furnace tech come to Inspect it & he was not surprised it was still working perfectly after 100 years.
    He said it was 40% efficient at best. I’ve been working on tightening up the house with caulk, paint, insulation in the attic & such. We will need to have the asbestos abated
    From the furnace in the near future. We have considered upgrading but until then, we will keep running the “beast”.

  14. G.B. says:


    If you still have your old octopus, you may find the burner & the gas conversion parts have value because some of them are becoming hard to find.

  15. Morris Knox says:

    My dad had an old home near Duluth that had an iron octopus that had been converter to fuel oil. He had no complaints about heating the house with it, but one night there was a loud BOOM!, the whole house shook, and thick black smoke belched from the hot air ducts and cold air returns alike. Dad and my stepmother ran to the neighbor’s house to call the fire department. Fortunately, the ‘explosion’ and aftershock wasn’t an explosion at all. It was the rusted out cast iron firebox collapsing in on itself. The outer shell of the heat exchanger remained intact, shielding the surrounding basement from the open flames, even as the smoke and years of accumulated soot, dust, and ash poured into the main floor of the house. Every inch of the house had to be professionally cleaned by a fire recovery service.

  16. ERIC ANDONIAN says:

    We had 3-4 of these monster units in our basement (Southern California 1925 Spanish Colonial house, 3700 sf). They had electric switches with 3 lights on them. We would push and hold the button for each setting (low, med, high). You could hear the buzzing while holding button down (magnetic relay?). It worked really well (1968-1980). Sometime in the late 80’s the new owners put in forced air.

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