What Do I need To Run A Gas Furnace?

[Arlou52]

I have acquired a solar panel system for free from a family member and have decided I might set it up for my own daily and possibly emergency use. It has plenty of power for my daily needs but what about this. My 110 AC furnace draws 13 amps on start up and 6.5 maps continuous operation. I want to be able to use this 12 volt solar panel system to run this in an emergency. I now have five 100 watt panels, a 1000 watt inverter, and a 250 AH 12 volt battery bank. I doubt this will do it but how much do I need to increase all of these to run this gas furnace if I decide to use this system for more than normal daily use? I have no idea how to calculate this.

[bcroe]

Solar is a lousy backup system, in part because the sun is frequently poor when you
need it. I solved the furnace problem decades ago cheaply, with a 4KW generator set
(battery start, bat maintainer used). I have only used it 4 times since then. Bruce Roe

[bob-n]

Bruce is right. Solar is a lousy backup.

You can do the calculations if you want. When you're done, you'll know why Bruce told you that it was a poor backup system. Let me help you with some of the calculations. It's pretty easy. The hardest part is making accurate assumptions. I going to throw in wild guesses and show you where they lead.

To get started, assume that it is sunny every day, and that your solar panels get 5 hours of strong sun every day. That's a very generous assumption, but let's go with it.

Also assume that you are willing to heat the house for those five hours and let it cool down naturally the rest of the time. That won't be pleasant, but again, let's go with it.

You need 6.5 amps at 110V to run the heat. Amps x Volts = Watts. That means that you need a continuous 715 watts to run the heat. You have 5 100-watt panels, so immediately, you can see that it's not enough. You'll need at least 3 more 100-watt panels. Inverters are not perfect at converting panel output to usable power. Some are 95% efficient, some quite a bit lower. You may need 4 panels or 5 panels. If you want heat when it's a cloudy day, you'll get roughly 1/3 the power out of the panels if it's a little cloudy. Clouds really hurt solar production. So if you want the system to run continuously on a cloudy day, you're going to need 20 more panels.

You say that the furnace requires 13 amps to start. From the same equation, you calcualte that the 1000 watt inverter can only deliver 9 amps. 1000 watts = 9 amps x 110V. You're going to need a larger inverter.

To make the place comfortable when you lose power, you're going to have to reassess the assumptions. You need to know how many hours per night the heat will really be on. Let's assume that it needs to run for 4 hours every night, and get that electricity from the battery. That means that your battery must put out 715 watts for 4 hours, or 2860 watt-hours.

You have a battery rated 250 amp-hours at 12V or 3000 watt-hours. That could just barely do it for the above assumption.

The most efficient heating system is engineered to run continuously on the coldest day. So if you lose power during a bitter-cold storm, you need to have enough battery to last you much longer than 4 hours, at least long enough until the sun comes out and the panels clear of snow and ice. That means at least 4 times the battery capacity you have now.

Once the sun comes out and the panels clear, you're again getting solar energy. Great! But your batteries are dead. Yuck. You need heat, but you sized the solar panels to just barely run the heat. There's no energy left over to recharge the batteries. So you're going to need even more panels and an even larger inverter.

I hope this helps.

[Keylan]

The gas furnace still needs electricity. If your gas furnace doesn’t have electrical power, it won’t run. Definitely, not recommended as a backup.

[Mike90250]

Gas forced air furnace needs
power for brains and ignition (50w)
power for blower motor (500 - 1500w)

There are Gravity Wall heaters, that use a thermopile powered sensor and control valve, and hot air comes out the top.

Gravity wall heater gas.jpg