Auto-Switching Inverter/Charger for Wood Furnace, Pellet Stove, Outside Boiler, Etc.

[TooBizzy]

I recently finished a project that I have wanted to do for a very long time. I heated my house with a Hot Blast Forced Air Furnace for about 20 years. Replaced it 2 years ago with a Firechief. The Firechief is a much better furnace, but that would be another thread! I have always worried about power outages when my wife and I were at work or away from the house, which could allow the furnace to over-heat. So I set out to research and build an auto-switching Inverter backup for the stove.

I checked the draw (Watts) of the 1/3hp furnace blower with a Kill a Watt Meter and it draws 360w. So a 1000w inverter is more than enough to handle this load. I found the 12v 1000w Aims Inverter/Charger that didn’t break the bank. In sizing up the battery bank, I wanted enough capacity to run the furnace at least 7 hours with 50% capacity left in the batteries to keep from over discharging. In a power outage, 7 hours would be more than enough time for the wood to burn down in the stove and for it to be less of an issue.

I settled on (4) 6v deep cycle 242ah batteries wired 2s2p to give me 484ah at 12v. Since my blower pulls 360w @ 120v, my batteries will have to supply 36ah to power the blower. Total capacity is about 13.44 hours run time. 50% of that is a little less than 7 hours.

I’ve attached a few pictures of the finished project. If you have any questions, feel free to ask. BTW, I ran the furnace on backup today for about 4 hours. It took about 15 minutes for the batteries to settle in on the load @ 12.36v. The next 3 hrs 45min the voltage only dropped .1v and the batteries never even got warm to the touch ….So I can see this easily running the furnace for the calculated amount of time. I am more than happy with the outcome! Sorry for such a long post!

 

[dave_dj1]

Toobizzy, very nice project. Now would you lay it out in laymans terms for me? I need this exact set up for my DIY outside wood furnace as I too have had many a worried moment when away or when the wife is home alone. I have a whole house gen set but it's not automatic, I have to start it.

How did you go about making it full auto? How do the batteries re charge?
Any info you could lend would be great.
thank you in advance
dave

 

[TooBizzy]

Hmmm, Laymans Terms! This may take some time! Ha, just kidding! Seems as though, After some lengthy research, they have come out with auto-switching inverter/chargers. The Aims Pure Sine Wave Inverter is just one of them. It does everything for you. If the power goes out, it goes to battery backup within 10 milliseconds. The furnace fan doesn't know any difference. It Works Kinda like a UPS you would put on a computer. But these are heavier built to run bigger loads. When the power comes back on, the inverter switches back to line power and automatically charges the battery with a 3 stage algorithm. Its sweet!

I'm with you, I've been wanting to do this for a long time for the same reasons as you. I have a small generator that I can get out and have the furnace running in minutes (If I'm Home). Then bigger ones I can run the house on. But this will work Automatically if I'm not at home. What a load off my mind!

There are other things you shoudl know about inverters. So I will try not to overload you! There are Pure Sine Wave Inverters that conditions the 120v output to be just like it comes from the power company. This is a good thing! Then there are Modified Sine Wave Inverters that are much cheaper and give you, So called, Dirty power or square sine wave. These are good for resistive loads like heaters, coffee pots, toasters, etc. If you are trying to power motors or sensitive electronics, Pure Sine Wave Inverters is the way to go and won't damage your equipment..

This is the inverter I purchased: http://www.theinverterstore.com/1000-watt-pure-sine-inverter-charger.html

Do you have a Kill a watt meter? http://www.amazon.com/P3-P4400-Elec...F8&qid=1452480093&sr=8-1&keywords=kill+a+watt

This is a very handy tool to have in the shop! You can use it to see how many watts @ 120v a given load is. In my case the furnace blower pulls 360w...So I needed to get an inverter that would power this load. Then you just build your battery bank as big as you need to power your load for as long as you want to run it.

Is your outside furnace forced air or a boiler? If its a boiler, all you would have to do is add up the watts of the water circulator, the stoker fan, and the inside furnace blower to size the inverter and battery bank. This may be a more complex system to set up, but doable. A forced air system would be just like mine. A lot easier to build.

Let me know if you have any more questions. This thing is sweet and I'd like to share what I've learned!

 

[zogger]

Excellent project! One question, observation. The battery bank. I know from my solar work that has to be pressure vented to the outside, is your case designed for that? And also, the bottom lined with a substance like sintra, with some like baking soda loose to absorb and neutralize any spills scattered at the bottom?

Besides that a nice clean project, something all furnaces should have, wood/propane/natgas.

 

[dave_dj1]

TooBizzy thanks for taking the time to lay it out for me and offering your help.
My outdoor unit is forced hot air so only a blower like yours. I don't have forced air induction for the fire.
I have mine hard wired but it would be easy enough to convert it to a plug.
I actually just ordered two power off alarms from Amazon so in the event of a power failure (and I'm home and sleeping) I could get up and start the generator. I would hate to have the stove overheat and get ruined. I think it will be worth the investment just for the peace of mind.

Zogger, good point about the batteries, I know in my camper they are vented to the outside.

 

[TooBizzy]

Zogger, I don't have the vent in place yet, but I do have it and its ready to be installed. I lined the box with plexiglass and it has a plexiglass cover. I have never heard of sintra. Will have to look that one up. Good point on the baking soda. Will add that too.

Dave, Interesting you just purchased the Power Off Alarms from Amazon! Mine came in 2 weeks ago! Power went off about 3 weeks ago and it didn't wake me. My wife woke up and got me up to get the gen started. The power had only been off just a little while, but the part that bothered me is it didn't wake me. My wife has to sleep with noise, like a fan. And it still didn't wake us when it turned off. So now we have the alarms just like you! Next project is to install something to text or call me when I'm at work if the power fails.

 

[TooBizzy]

Zogger, BTW, since you have experience with battery banks, is it ok to vent the box into the attic? Or does it have to go directly outside? My attic is vented?

 

[zogger]

Zogger, BTW, since you have experience with battery banks, is it ok to vent the box into the attic? Or does it have to go directly outside? My attic is vented?

I have only heard to the outside. The system I managed had dual banks and vented right out side. When charging heavy, those liquid lead-acid based batts produce hydrogen and some sulphides. explosive and corrosive. Usually a DC fan is put so it pushes, not pulls, through the box and the gases out the vent. Don't want any possibility of sparks getting to it. Here is a good resource

http://www.homepower.com/

 

[TooBizzy]

Thanks for the link. Lots of good information there! The challenge will be to find circuitry that turns the fan on when charging/discharging. I don't think the inverter has provisions for this and I really don't want to tie into the inverter fan. It is 2 speed and runs accordingly when charging or discharging depending on load and heat.

 

[jwilly]

They make a fan for just for venting the battery box and it comes with a controller that can be adjusted to turn the fan on when the voltage reaches the point where the batteries begin to off gas. I have one on a 24v system that turns on at 25.6 volts. I can't remember where I got it.

 

[TooBizzy]

Thanks jwilley! I was able to find a place that makes the power vent. Here is the link:

http://zephyrvent.com/specifications-and-prices/

 

[ChoppyChoppy]

My brother has a Hot Blast, it doesn't need the blower for it to heat. Your new stove is diferent?

 

[TooBizzy]

I had a Hot Blast that served me well! It lasted about 20 years before the shaker grate gave way! It had several cracks down the front and every once in a while, we would get a wiff of smoke in the house. So I was thinking the internal jacket was cracked as well. This Firechief is a much better stove with a lifetime warranty on the shaker grate. It is built so much heavier and so far I like it much better. Yes It will run without electric, but it can get very hot when the furnace blower isn't moving air through the stove. I had that happen with my hot blast and it wasn't good on it. I was always able to get a gen on it pretty quickly, but it got very hot a time or two! Here lately we've been having more outages that helped nudge this project up the list! Now I don't have to worry about damaging the stove or having it overheat...

 

[dave_dj1]

I got my Kill a watt today, let the games begin.

 

[jwilly]

Thanks jwilley! I was able to find a place that makes the power vent. Here is the link:

http://zephyrvent.com/specifications-and-prices/

That is similar to the one I used, put and inlet on the battery box and exhaust outside. The fan runs almost all the time on a sunny day.

 

[brenndatomu]

I had the same concerns with my Yukon furnace...if the power went out right after I loaded...holy cow, that's a ton of heat to "gravity" to the house. Anyways, I looked at doing what you did and decided not to, because, well, I'm cheap.
I ended up getting a Maverick ET-732 remote BBQ thermometer. http://r.search.yahoo.com/_ylt=A0LE...rmometer/RK=0/RS=MFMUI_MwpsDg1zcm0Ph4TyjXO58-
It has 2 probes, I put one in the flue, and the other in the supply plenum. It is battery operated and has programmable high and low alarms for both probes. I wasn't sure about the battery power but it has not been a problem, they last about a year running 24/7. The range on it is ~300'. It helped me sleep better but I was still concerned about power fail if nobody was around...so I put a heat dump door in the plenum. It has a fusible link that melts at 212* which lets the door drop open. Foolproof, zero maintenance http://r.search.yahoo.com/_ylt=A0LEVu.c_JZW.nsAuFknnIlQ;_ylu=X3oDMTByOHZyb21tBGNvbG8DYmYxBHBvcwMxBHZ0aWQDBHNlYwNzcg--/RV=2/RE=1452764445/RO=10/RU=http://www.yukon-eagle.com/FURNACEACCESSORIES/SAFETYHEATDUMP/tabid/200/Default.aspx/RK=0/RS=cXx5qzLZyerCjkHztZoNsqW2znE-

 

[TooBizzy]

Dave, you're going to like that Kill a watt meter. I've used mine a lot recently. Been testing LED lights to see how much "Cha Ching" I'm going to save on my electric bill!

Jwilly, I think I only want my fan running when its charging. Sounds like yours is solar powered? I don't want to pull any more heat out of my garage than necessary. So I've found a Voltage Control Switch (relay) that will power the fan at a preset voltage during the charge process and turn off when at float charge voltage. Convection should be enough to vent the box during the float stage of charging.

Brenndatomu, I thought about making a heat dump for my furnace, but didn't want to cut into it! I was going to make one myself and use some sort of powered solenoid to latch it shut. If the electric went out, the door would open. But decided to do it this way instead. Thanks for the links though. I like the temp probes! Awesome!

 

[jwilly]

Dave, you're going to like that Kill a watt meter. I've used mine a lot recently. Been testing LED lights to see how much "Cha Ching" I'm going to save on my electric bill!

Jwilly, I think I only want my fan running when its charging. Sounds like yours is solar powered? I don't want to pull any more heat out of my garage than necessary. So I've found a Voltage Control Switch (relay) that will power the fan at a preset voltage during the charge process and turn off when at float charge voltage. Convection should be enough to vent the box during the float stage of charging.

Brenndatomu, I thought about making a heat dump for my furnace, but didn't want to cut into it! I was going to make one myself and use some sort of powered solenoid to latch it shut. If the electric went out, the door would open. But decided to do it this way instead. Thanks for the links though. I like the temp probes! Awesome!

It only runs when it is charging and voltage is above the preset.

 

[brenndatomu]

I thought about making a heat dump for my furnace, but didn't want to cut into it!

It goes in the supply plenum, which it looks like you already cut into...

 

[hupte]

Toobizzy, very nice project. Now would you lay it out in laymans terms for me? I need this exact set up for my DIY outside wood furnace as I too have had many a worried moment when away or when the wife is home alone. I have a whole house gen set but it's not automatic, I have to start it.

How did you go about making it full auto? How do the batteries re charge?
Any info you could lend would be great.
thank you in advance
dave

just an idea I heard you can get an automatic transfer switch that turns on your generator in the event of a power outage.