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Can I use my electric furnace fan to move heat from my wood stove?

I have a 2000 SF home (approx 1000 foot each level, lower is "daylight" meaning one side is underground on the lower level and one side of the lower level is entirely open). I have two Blaze King wood stoves (catalytic converters) on each level but my thermostat for my furnance is on the main level. If I use my wood stove upstairs I don't heat my downstairs (where all my water piipes are) and if I use my downstairs wood stove it won't heat the upper part of the house.

My question is, can I use my downstairs wood stove and run the fan only on my electric furnance (which is also downstairs and fairly near the wood stove) to heat my entire house? Pros and cons?

Asked by ann greene
Posted Sat, 12/15/2012 - 01:28

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2 Answers

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Ann,
Furnace fans are notorious energy hogs; some draw 800 watts. That's a lot of electricity. Of course, in winter, the waste heat from the electric motor heats your home. But you're still using 800 watts of electricity, which costs more than firewood.

The traditional solution is to cut a hole in your floor (above the downstairs wood stove), and install a steel grate in your floor.

A better solution is to find out why your home is so leaky that a single wood stove can't keep you warm. It's time to get an energy audit. I bet that a blower-door test will reveal lots of air leaks in your home. If you plug them, you may find that you are perfectly comfortable.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 12/15/2012 - 07:57

2.
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It takes a LOT of cross sectional area and well placed grates, or a very high temperature difference (such as directly above the wood stove) to move a lot of heat between floors by convection. At a high delta-T, the conduction through the ceiling/floor would still exceed the convective transfer, but with a grate directly above the woodstove the delta-Ts are high which takes much lower air movement to transfer the heat. It still takes a substantial cubic feet of air per minute rate to move much heat, whether by convection through grates, or with an air handler.

The specific heat of air by volume is about 0.018 BTU per cubic foot per degree F, or about 1BTU per cubic foot at a 5F delta. If you're moving a full 1000cfm from the warm space to the cooler space with an 800 watt air handler with a 5F temperature difference it's about 60,000BTU/hr, (which may be more than of the total output of your wood stove) and even at a 2F delta it's 24,000 BTU/hr- which sounds pretty good, right? But...

That would assume 100% of the return air entering the air handler is coming from the space with the wood stove, and 100% of the supply air is being delivered to the cooler space. But in fact it's drawing and supplying air from/to both spaces, and leaking along the way. In practice it's unlikely that you'd be getting better than 6000 BTU/hr of heat transfer out of it, with maybe another ~1300BTU/hr of direct heating out of the windings of the blower motor (assuming something like half the motor heat ends up in the upstairs.). Putting the same 800 watts into a pretty-good mini-split heat pump would deliver more than 7000BTU/hr (except when it's below +15F outside), directly into the room, and would not be increasing firewood use.

Looking at the specs on the Blaze King catalytic wood stoves ( http://www.blazeking.com/EN/wood-stoves.html ), any one of them would be be able to heat 100% of a current code-min 2000' house in most of the US (assuming you have glass in your windows, and something better than plastic sheeting for exterior doors.) Even the Sirocco & Chinook would likely cook you out if the specified 30,407 BTU/hr average output over 8 hours is put into play for the over night.

And if it's NOT roasting you overnight with a full-load setting it to ''High" you have some serious weatherization issues to attend to, which as Martin aptly points out, would go long way toward fixing the upstairs comfort issues with only the downstairs stove burning, and you'd burn a lot less wood too.

Many homes with daylight basements only have insulation (if any) on the daylighted side, and have huge heat losses out of foundation, and air sealing & insulating the foundation, foundation sill & band joist would automatically freeze-protect the plumbing without directly heating the space, so long as the upstairs was fully conditioned & heated.

Answered by Dana Dorsett
Posted Tue, 12/18/2012 - 18:20

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