I have a super insulated, envelope home. I have R-88 in the ceiling and R-44 in the walls. Living in Northern WI I do have heating requirements, but nowhere near the output of the 90,000 BTU furnace that was originally installed. After living in the home a few years I decreased the burner orifices to give a 50,600 BTU output. This was better and I have used it for the last 20 years. Now my furnace is 33 years old and I am looking for the "Right" furnace to install.

I have connected my furnace to my computer and have recorded every burn cycle for the last year. I have read my gas meter to calculate the amount of gas used per minute. I then calculated that my furnace requirement is about a 25000 BTU furnace. The smallest furnace I can find on the market is a 40,000 BTU furnace. I calculate the 40,000 BTU furnace about twice as much heat as I need.

I do not want to decrease the output of my current furnace (by reducing the number of gas nozzles), as I am concerned with effecting the efficiency of the furnace.

What are my options for such a small heating requirement?

  • VTC as a shopping question which are not the type normally entertained on this site. Reason being that they become out dates and are very specific to the OP's situation and of less interest to the site visitors.
    – Michael Karas
    Dec 20, 2016 at 14:40
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    I feel like a lot of questions on this site get flagged with the "that's not really our kind of question" issue, which is frustrating. I see two useful nuggets within this question that you might want to draw out, thus avoiding what @MichaelKaras is bringing up. First, where can you find a NG furnace with <40k BTUs? Second, are there any tips and suggestions for sizing and/or finding an appropriate furnace for a super-tight, super-efficient home? If you rephrase your question, you may be able to avoid the wrath of the admins ;)
    – pbarranis
    Dec 20, 2016 at 15:20
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    Have you considered a modulating or multi-stage furnace? Dec 20, 2016 at 15:36
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    Are you set on natural gas, or open to other energy sources? Are you set on a forced-air furnace?
    – mmathis
    Dec 20, 2016 at 16:11
  • @pbarranis Where to find a furnace < 40k BTUs is still a shopping question, but your second point is the essence of the question. I've suggested an edit to remove the shopping aspect and highlight that.
    – mmathis
    Dec 20, 2016 at 16:12

2 Answers 2


Low ambient mini-split heat pump is a typical approach - might want a couple-three (the 9-12K BTU/hr ones tend to be more efficient than the bigger ones, and the "1-compressor - 1 inside unit" also tend to be more efficient than the "1 compressor - multiple inside unit" versions, though the initial cost will be more.

An alternate approach for just heat, and using your ducts as is, would be to replace your furnace with a "fan coil unit" (water to air heat exchanger with a blower fan) and connect that to a highly efficient gas water heater. A circulating pump would also be required, and perhaps a heat exchanger to isolate the systems (or find a coil unit that is safe for potable water contact and use a stainless steel pump.)

On the third hand, a natural gas parlor heater (fake wood stove) in that size range could provide the heat, (or a pellet stove if you wanted to change fuel sources, but at the current time gas is probably the most economical heating fuel if you have it) and then you might still want some sort of fan on the ducts to help even out the temperatures around the house. These are available with controls that do not require outside electric power to operate, and can be quite an attractive focal point.

Ground source heat pumps are a lovely idea that suffer from often being not economically viable - A minimal GSHP setup is typically 5-10X the cost of low-ambient air-air units, though they are far more resilient in deep cold since they operate from deep groundwater temperatures that are usually stable at 52F or so.

  • Forgive a possibly stupid question, but my heat pump stops being efficient around +40 degrees outside. I live about 6-9 hrs drive south of northern WI (where OP says he lives). Can a heat pump be even remotely efficient when it's -10F outside??
    – pbarranis
    Dec 20, 2016 at 18:30
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    Yours can't - there are heatpumps that ARE made to operate down to -15F and still be useful - you have to start with buying one of those, not one of the others. See the first two words in the answer.
    – Ecnerwal
    Dec 20, 2016 at 18:32
  • Sure, the principle of refrigeration works with any gas, and hypothetically with the right type of freon, it could be optimized for -15F. It's a design choice by the manufacturer. One thing enabling better operating range is electronic motor controls (e.g. flux vector drive) that let them spin the freon compressor at an ideal speed for conditions, rather than the only choice being 3450 RPM. Dec 20, 2016 at 20:56
  • Point being that it has not been hypothetical for at least 2-3 years (possibly longer) now. These have been a game-changer in cold climate heating options.
    – Ecnerwal
    Dec 20, 2016 at 23:29

Yes, you are right that there's a difference in how the snowbelt vs. sunbelt heat their homes. Part of your solution could be there.

Heating in the snowbelt is almost always a large fuel-fired furnace with forced pumping of working fluid (i.e. forced air). This requires both fuel and electricity, if either one fails, the home becomes uninhabitable -- and that's just accepted somehow. /boggle/

An all-but-lost style is the "gravity furnace" where the heated air is meant to rise by convection, up a carefully designed ducting system. This obviously requires a relatively vertical house and one designed for this. This could work without electricity, but since a forced-air furnace is a drop-in replacement, I'd imagine most have been converted.

Heating in the sunbelt is a simpler affair. You often see all-electric heat, and those are easily seprately zoned, so you only heat the spaces you are using. You often see all-gas heat, by which I mean a gas furnace that does not require electricity. For instance, my home is cozy warm in a power outage. You also see forced-air systems, but the motivating factor is A/C not heat.

These two ways of thinking are so different that frankly, installers in the snowbelt are not conversant in the sunbelt style. Stores in the region don't even stock the gas furnaces. I'm not even sure city inspectors would allow a sunbelt style.

The achilles' heel of the sunbelt systems is carrying the heat around the house. (sunbelters don't need heat in every room, nook and cranny since pipe freeze is not a big issue). Electric heat can be carried around the house on 12 AWG wire, with cheap $50 Cadet baseboard heaters at point of use (separately zoned too; nice.) But gas-only furnaces rely on convection to carry heat around the house, and that only works with certain topologies.

Another option which is all-electric is a heat pump. Recently, they have been getting quite good. Ground-sourced heat pumps are even better, but rather costly to install. Since they only move heat, not create heat, they are much more efficient than a heating element or burner. You need a unit designed to work in "rather cold" conditions, otherwise it will need a thermal-heat backup, either electric heating elements or gas. In that case I would look at that gas-only furnace.

They come in several types; including floor furnaces (set in the floor with a grating, and I've seen an $800,000 house with one of those!) Or a "baseboard" style embedded into a wall with outlets on both sides of the wall. Or a vertical, up-the-wall "wall heater" designed to reside embedded into an interior wall, with gratings on one or both sides.

The gas wall furnace I linked is typically jetted for either 25,000 or 35,000 BTU if single-sided, or 35-50,000 BTU if double-sided. Like any passive gas heater, it needs a vent more-or-less straight up, and entirely properly built on the outside. You see a lot of negative Amazon reviews from people who have no idea what they're doing and blame the product.

The upshot is, you have a really exceptional home, that deserves a really exceptional furnace. With such efficiency, you are not far away from power-outage resiliency with just a little application of "solar off grid" type technologies to protect essential loads like refrigeration, auiliary lighting, router and tablet charging so you can watch Netflix in your cozy home. However, a balky blower to push air around the house is an impediment to such an achievement.

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