# Battery backup for furnace



## jbrooks19 (May 22, 2014)

I am working on a design for a battery backup for my furnace in the event we lose power, i currently have 8 Lead Acid (Rechargeable) Batteries rated at 12v and 12Ah each.
Wired in parallel ill achieve 12v and can run 12A for up to 8 hours, which should run my furnace for around 7-8 hours. The means of charging will be electric with solar backup. I have most of the components except the solar. So ill start wiring it in a few weeks and maybe next year add the solar part of it. Not to mention, the batteries are new and i got them for free from work because the piece of equipment they came out of was "Defective" and needed disposed. (It was a battery powered 2-stage snow blower, the blower motor was bad but the charger and batteries were all still good)

I will post pictures as the project progresses and keep everyone up to date.

EDIT: I will be using a power inverter to convert the 12v DC to a usable AC current. (Still need to buy one of these)


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## paraquack (Mar 1, 2013)

Sounds interesting, can't wait for pix.


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## Smitty901 (Nov 16, 2012)

The fans on a furnace draw a bit of power and the modern ones also have blower fans It would take some major power to run one for a long length of time . At least it seems that way. I have a generator and a wood/coal back up


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## budgetprepp-n (Apr 7, 2013)

That is an interesting idea,,,, That might work.
Let us know when you try it out. Boy just think if you had one of the old style gas 
furnaces that didn't have blower motor. 
(I think that was called gravity heat)


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## Will2 (Mar 20, 2013)

how many batteries is it?

Are you taking into consideration that your batteries will drop voltage as they drain?


How many watts does your furnace surge at. 

There are lots of inverter options based on your wattage needs. If you are trying to keep your price down you can stick to about your surge or peak wattage draw for the inverter max.


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## MrsInor (Apr 15, 2013)

Furnace backup = fireplace.


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## bigdogbuc (Mar 23, 2012)

Definitely let us know how it works! I often think how I would run my furnace if I lost power? Which, winter is about the only time that sort of thing happens. Just when you need it most.

I don't have a generator or anywhere to store one and keep it from getting stolen. I rent a house, but it's older, in a medium density neighborhood, and have no shed or garage. And since I rent, I'm not building or buying either one. 

Interested in your results.


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## StarPD45 (Nov 13, 2012)

budgetprepp-n said:


> That is an interesting idea,,,, That might work.
> Let us know when you try it out. Boy just think if you had one of the old style gas
> furnaces that didn't have blower motor.
> (I think that was called gravity heat)


I heat my whole house with a propane floor furnace. No electricity required. I added an aftermarket set-back thermostat which runs on 2 AA batteries. 
When we moved in here 30 years ago, we last power almost weekly. That's the main reason we went this way.
They are still available. Empire and Williams are two current brands.


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## Chipper (Dec 22, 2012)

Wood stove fan?? Low tech is best.


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## Will2 (Mar 20, 2013)

I forgot to add that you should check with your furnace manufacturer if the control board - furnace can operate with PWM 
(pulse modulated wave, or if it needs a pure sine wave.

AC devices, especially sensitive electronics which are more common as things get newer and smaller can have more stringent requirements.

While things like power tools are often fine with PMW - medical, audio equipment etc.. can be thrown off heavily by PMW. 

This is true when using backup generators also.


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## jbrooks19 (May 22, 2014)

Thanks for the input guys. My furnace is gas and only has a blower motor and thermostat to run from the batteries. My degree is in Industrial Electricity.. it's kinda my "Cup of Tea"  not to toot my own horn.. This is a sturdy older furnace so PMW and minor power fluctuations won't bother it. I am going to run it thru an inverter to clean up the sine wave before it hits the furnace anyways.


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## PaulS (Mar 11, 2013)

jbrooks19 said:


> Thanks for the input guys. My furnace is gas and only has a blower motor and thermostat to run from the batteries. My degree is in Industrial Electricity.. it's kinda my "Cup of Tea"  not to toot my own horn.. This is a sturdy older furnace so PMW and minor power fluctuations won't bother it. I am going to run it thru an inverter to clean up the sine wave before it hits the furnace anyways.


12v x 96Ah = 1152 Watt hours. The AC motor on your furnace uses 120v at 10amps running 120v x 10 = 1200 watts. To start that motor is going to take close to 20 amps. It looks to me like your battery backup (with a 100% conversion - impossible) will run your furnace for less than one hour.

I think you need to rethink this and use some larger batteries - say 8 each 60 Ah batteries?


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## Smitty901 (Nov 16, 2012)

Living in Rural Wisconsin we have had a few power outages at the worst time. Power companies share repair people across state lines and large areas so major cities get help ASAP but those of us the are out side town don't count.
That is ok I guess it makes us stronger. Some years ago I installed a vent less LP heater 33,000 btu as a back up It requires no power at all.
Not prefect but does work in a pinch. Also a wood/coal added on furnace that thing can keep the house livable in the worst of weather.
We do have furnace wire to quickly hook it to one of the small generators if need be.
Options are they key not be tied to one way to work around power outages.


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## jbrooks19 (May 22, 2014)

I stand corrected. Its been awhile since iv'e done the math on power consumption..I believe you may be right. I'll do more math and probably still go ahead and build this and test to see how long it lasts.


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## jbrooks19 (May 22, 2014)

Then add more batteries as the budget allows. This is mainly a test run anyways, i eventually want to be able to run my furnace for 24hrs on battery backup.


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## Smitty901 (Nov 16, 2012)

jbrooks19 said:


> Then add more batteries as the budget allows. This is mainly a test run anyways, i eventually want to be able to run my furnace for 24hrs on battery backup.


 Because if the power goes out here we do not know weather it will be an hour or 2 days the first thing we do is lower the thermostat to so it does not run as often .
24 hours on battery may take some work but it can be done. Question is it worth the cost long and short term. A small low cost generator can be had for less than the barratries may end up costing. And we all know no battery last forever.


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## Will2 (Mar 20, 2013)

I thought I posted this already but be sure to check for whether you need a pure sine wave inverter or if your furnace can handle PMW (pulse modulated wave)

Some electronics are sensitive (including some heating systems, but most medical and audio equipment). More and more stuff is sensitive these days due to smaller electronics components that are more sensitive to dirty electrical signals. 


Also unless you are getting cheap batteries one 100 AH battery might run 200 dollars. However not sure if you know electronics

You have a 70 percent conversion loss there abouts from DC to AC (about 30% less energy) plus the efficiency of your inverter may reduce that a bit. Note that 110 volts or 120 volts is 10x the amount of energy (in this case potential difference)

formula for ohms law is I=V/R or watts law P= VI If it takes 9.17 times the energy flat out it means you will get 1/9.17 amperage at 110 volts as you would at 12 volts. Now add the loss and it goes down to about 1/12.97 so 8AH at 12 volts is more like 0.07 amps or 70 miliamps at 110volts. So an 8 AH battery turns into a 0.615 AH and 8 of those batteries turns into just under 5 AH total at 110 volts.

I could be wrong but this is my basic understanding of your situation.

If your furnace needs sine wave and you plan on going for a better setup down the road I suggest getting a good sinewave inverter that allows back feeding into the grid, it will keep more value.

Also take into consideration you may be able to directly feed 12v into your furnace which will remove conversion loss and reduce the need for an inverter, as some blower motors may run at 12v, check with your furnace manufacturer.

This is the formula you will need to determine Amps

P / V = I

watts divided by volts equals amps

so say your furnace is 600 watts

600 / 120 = I

but you have 12 volts batteries so

600/12 = I

note that for the 120 supply you have to use 5 amps for the 12 volt it is 50 amps.

Mow multiply that by 1.3 and you have 65 amps (or amp hours) 

So you need about 103 12 volt batteries to get 8 120 volts 8AH battery (my math could be off on that.


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## jbrooks19 (May 22, 2014)

Attached


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## paraquack (Mar 1, 2013)

Don't forget to keep your temp down during battery use.


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## PaulS (Mar 11, 2013)

jbrooks19 said:


> View attachment 5588
> 
> 
> Attached


The math is better there. You should also rate your inverter for the starting amperage of the electric motor which can be much higher than its running amps. The motor should have a tag that shows start-up current draw.

If you pay attention to the details then you can make this work.

One more note: Some electric motors need pure sine wave or they will chatter and work poorly. I would suggest that you use a pure sine wave inverter. They are much more expensive but are worth the extra price in the life of the equipment run off it.


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## Smitty901 (Nov 16, 2012)

small cheap generator looking better and better with each new post


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## PaulS (Mar 11, 2013)

I have a rather large generator but with a generator you need fuel, oil, spare parts and so on. If you have lots of space you might get a years worth of fuel and several years in spare parts but what do you do after that? Do you have the manpower to protect that generator 24/7? Do you have a way to wire it in? Do you realize that being the only one in a large area with lights is going to make you a target?

My generator is for short term emergencies - to be shared with neighbors who can use it to keep frozen food frozen or to run a furnace and refrigerator for a few hours. If the SHTF it will not be used. It will be packed away for some future emergency. I will go dark so I blend in with those folks around me - I am more likely to survive that way and my neighbors are less likely to turn on me because I have something they don't. (although the few neighbors I have have their own generators.) That is one of the nice things about rural living. People tend to be able to get along without much help.


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