Running a Furnace off of an inverter

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Old 05-27-16, 09:49 AM
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Running a Furnace off of an inverter

Hi...This is my first time posting here and I'm a DIY guy but not an electrician. I understand the basics.

I've recently installed a furnace in our mountain cabin which runs on Solar electric power. I have 5 deep cycle batteries that have always done the job well. Until recently I ran a 1500 watt pure sine wave inverter which handled everything I needed. When I started the new furnace up (1/3 horse power fan motor) it tripped the reset on the inverter. I assumed that it simply wasn't powerful enough for the surge from the blower motor which I estimated at 1750. So naturally I spent the money for a 5000 watt inverter. I figured better safe than sorry! I hooked the whole thing up and the blower kicked the inverter out again. I'm feeling a bit defeated and wondering if I needed to go to the larger inverter at all.

The system is wired as follows: Solar panels into a solar controller...controller into battery bank...battery bank to inverter...inverter to breaker box...to a 20 amp breaker for the furnace...breaker to a double box...into an outlet...out of the outlet to a single pole switch which turns the furnace on and off.

I think that's all the info you might need to diagnose this...I appreciate any help anyone can give me! Thanks!

Steve
 

Last edited by Steve Duprey; 05-27-16 at 10:07 AM.
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Old 05-29-16, 04:44 PM
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What is the wire size from the batteries to the inverter? What is the distance?

Have you tried to measure the voltage drop using a voltmeter as the furnace starts up, both at the inverter input and at the furnace?

I am estimating 300 amps from the batteries for a few to several seconds as the furnace starts up.
 
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Old 05-29-16, 04:49 PM
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What type furnace did you install? Are you measuring just for the motor, or are there any type of heat strip elements in the play?
 
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Old 05-30-16, 06:09 AM
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Not to familular with solar,but increasing the inverter size without increasing the battery capacity seems odd,is the breaker that trips protecting the battery string?
 
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Old 05-31-16, 09:21 AM
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I'm using 12 gauge wire to connect the batteries to each other and a standard jumper cable (probably 10 gauge) to connect the battery bank to the inverter. After talking with a couple folks here the consensus is that I need to increase the gauge on all these to allow for more power to get to the inverter when the surge happens.

I'm not exactly sure how to perform the measurements that you're asking for.
 
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Old 05-31-16, 09:24 AM
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It's a goodman 60,000 BTU LP Furnace., There shouldn't be any other draw besides the blower motor.
 
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Old 05-31-16, 09:49 AM
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I'm not exactly sure how to perform the measurements that you're asking for.
Using a multimeter set to DC voltage measure the voltage where the batteries connect to the inverter before you start the furnace. Have someone turn the furnace on at the thermostat and as it tries ti start observe the voltage at the connection of the batteries to the inverter.

With the multimeter set to AC and furnace off measure the voltage where the furnace connects to the inverter. Have someone turn the furnace on at the thermostat and as the furnace tries to start observe the voltage where the inverter connects to the furnace.

A set of alligator clips that slip on the multimeter probes would be useful for doing this. Clip the probes to the connections and set back and watch.
 
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Old 05-31-16, 11:50 AM
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Thanks Ray...I'll give a try tomorrow and let you know what I get!
 
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Old 05-31-16, 12:01 PM
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Does this furnace have (a) a constantly burning pilot light, (b) a spark ignited pilot light or (c) a hot surface ignitor? Does it have a combustion air blower?

What is the required input voltage for the inverter? If it is 12 volts then it will draw a maximum in excess of 400 amperes to output 40 amperes (5000 watts) at 120 volts. The #12 conductors connecting the batteries (I assume in parallel) are woefully small as would be the conductors (#10?) between the battery bank and the inverter.

If you were using the 1,500 watt inverter with a 12 volt input the minimum size of ALL the battery conductors would be at least #4 and #1 would be far better. I don't know that I have ever seen a 5,000 watt inverter that would connect to 12 volts.

So, please answer the questions about the furnace and also what size batteries you are using, the required input voltage to the inverter and the actual conductor sizes on the battery side.
 
 

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