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Can I make an uninterruptible power supply for my furnace? Answered

So, last winter, my area got TONS of snow, very cold temperatures ( I spent 20 minutes dropping droplets of water on an iron railing and literally watching it freeze), and then we lost power. Our furnace runs on natural gas, so it would've worked, except that the fan inside it and also the ignition system (I'm pretty sure) wouldn't work without power. I'm thinking of getting maybe 10 or so surplus marine/car 12 volt batteries and a high-power inverter, and running the furnace off normal 120 AC, until power goes out, then a relay ( or some solid-state circuitry) would trigger the batteries ( in parallel), which would charge either off of a solar panel outside, or wall power once the power was back on. Also, the furnace has what looks to be a pretty basic motherboard inside of it, would I need an inverter that makes a really sine-y sine wave for that, or could I get away with a cheap square wave one? Also, it's wired directly into its own circuit, though I'm pretty sure it's still 120 V. Comments, advice, etc. is welcome. Thanks!



7 years ago

Most of the common gas fired furnaces around here use 24VAC as the control voltage on the circuit board. The Transformer is 120VAC Primary and 24VAC sec. I have only seen some of the high efficiency units (like a Mitsubishi ductless system) that uses DC current.

You can do the job with one twelve-volt battery with deep cycle car batteries being the best and ordinary car batteries being the cheapest to maintain. You only need one battery for this application, as well as an inverter and an off-the-shelf battery charger. You probably don't need a huge inverter. The fan will be your biggest draw, so look at that and see how many watts it says on the label, then add at least %50. If it says 200w, get a 300w inverter. If it says 500w, get a 750w inverter. Etc. Either way, an average car battery should run your furnace for 20 hours easily. You can also buy a pre-made battery pack to do the same job. They're sometimes mistakenly sold as "electric generators," by the way so keep your eyes peeled for that phrase.

We have a new Lennox furnace that won't run with our Honda generator because of "harmonic distortion," or, per the above, "a cheap square wave." So do inverters throw bad sign waves off a battery, or is this not a concern? Is my only concern the watt rating on the inverter I buy, or do I need to worry about quality of the power as asked in the original post?

Using this approach, I could charge the battery from my generator if the outage is a very long one.

Thanks for the question and answer–I've been going crazy trying to find a solution to my problem: the irony of having a wonderful new high-efficiency Lennox furnace that won't run when we need it most (during an extended power outage in the middle of winter).

Yeah, if it gives you trouble on the generator, you should probably get a better inverter with a reasonably clean output.  The package of a good one will probably have a picture of the kind of wave it produces.  Energizer and Mastercraft are two brands that have good models with lots of info on the box.

All the same, Honda's aren't known for clean power, just good engines.  It might be an isolated problem, and you might try any other power source and have it work.

I did have another thought:  Break the furnace down into key components. 

The blower/fan is probably AC, and wants AC power.  It won't be too picky and will run off your generator or inverter with little or no trouble.
The printed circuit board wants DC power and gets it from a transformer connected to the mains.  If you're using a battery and an inverter, you're building a circuit that goes "battery-inverter-transformer-circuit board."   You could also just find out what kind of DC power the board wants and give it that directly from a DC source, going around the transformer.  If you have a buddy who is familiar with electronics, fill him in and get his help with this one.

Ah, that is an excellent thought! The furnace circuit diagram suggests exactly what you say and this seems less messy than other approaches.

I would still need to do something with switching between normal power and the generator power above and beyond our GenTrans, but that switch would just be for the DC circuit ("going around the transformer") at that point.

(I did try a newer Sears generator, but the furnace coded the same way.)