RUNNING YOUR GAS FURNACE FROM A GENERATOR OR INVERTER

As I get older it seems that cold bothers me more than heat. I live in an area where winter temperatures dip below freezing from October to March and can go below zero from November to February. My preps include several ways to stay warm. They include a portable propane heater and propane, wood for the fireplaces, EWC sleeping bags, and a low wattage electric blanket that can be run off a battery and an inverter. I put these preps away for an emergency where no services are available.

Homes in my area are heated with natural gas. The great thing about natural gas is that it runs on a self-contained system. It is a self-pressurized system that doesn’t need the power grid to deliver gas to your home. Natural gas will still be flowing into your home even after the power grid is down for 30 or even 60 days.

The big exception is an earthquake. The natural gas system is shut down after an earthquake to avoid fires from leaking gas due to broken or cracked pipes. Anything else, including a hurricane the size of Katrina, will not affect the delivery of natural gas to your home.

The only thing you need to keep your furnace putting out warm air to your home is a way to run your furnace blower motor. Furnace blower motors range in size from ¼ horsepower to ¾ horsepower. A 1/3 horsepower motor and a ½ horsepower motor are the most common. I have 2 furnaces in my house. This allows me to heat each half of the house separately and to use smaller more energy efficient furnaces.

So, the $64 question is how do you get power to your furnace blower motor legally and safely?

DANGER!!!  There is a lot of bad information on the web. A lot of information that can kill you, kill an unsuspecting repairman, or burn down your house. I am deadly seriously! Pun intended. Don’t – Don’t – Don’t use a suicide cord (homemade cord with 2 male ends) and back-feed power through a house outlet as many people suggest.

The safe and legal ways are a Furnace Transfer Switch or a Furnace Interlock Device. Both make sure your temporary power from a generator or inverter cannot mix with your grid power. A transfer switch can run $100 or more plus installation costs. An interlock device can be $50 and can be hard to find for just your furnace.

My preference is to wire your furnace in such a way that it meets code, prevents mixing temporary and grid power and costs less than $10 per furnace in parts.

First let me link you to a YouTube video. http://www.youtube.com/watch?v=DZ16PKs46g0
The video is too long. He takes a lot of time (40 minutes) getting through the installation. It could have been edited down to probably 20 minutes. That said, he shows a simple, safe and inexpensive way to set your gas furnace up to run on a generator or inverter during an emergency.

I did the modifications on both of my furnaces in less than an hour and for a total cost for both furnaces of less than $15. The biggest savings was not buying a replacement cord (pigtail). They can cost $12 each at Home Depot. Instead I bought 2 power strips at Home Depot. They had 14 gauge, 3 wire, (14/3) 4’ long cords. I cut the power strips off at about 2 feet long. This left a 2 foot long plug and cord which was plenty long for my setup. The cost was $4.50 for each power strip. My furnaces already had metal boxes on them. I just had to buy 2 – outlets, 2 – face plates and 2 – ½” punch-out cord clamps.

How much Electricity do I need to run a Furnace Motor?

Depending on if you have an old or a new furnace will make a big difference on how much electricity your furnace blower motor will use. Old furnaces will use more energy. New furnaces are more energy efficient.

An old 1/3 horsepower motor will use up to 1400 watts on start-up and a continuous 800 watts while running. An old ½ horsepower motor will use up to 2100 watts on start-up and a continuous 1200 watts while running.

A new 1/3 horsepower motor will use up to 900 watts on start-up and a continuous 500 watts while running. A new ½ horsepower motor will use up to 1500 watts on start-up and a continuous 800 watts while running.

The bedroom area of my house is serviced by a furnace with a new 1/3 horsepower motor. Using a “Kill-a-Watt” meter, I found that it actually draws 850 watts at start-up and 440 watts continuous.

For me, a 1000 watt generator or inverter could be used to heat one-half of my house. I have a 2000 watt generator and a 2000 watt inverter connected to a battery backup system. I’ve tried running my furnaces with both systems and they work great.  How long will my generator run my furnace? The answer is as long as I have fuel for my generator. You don’t run your generator and furnace constantly, just intermittently as you need heat.

How long will my battery backup system run my furnace? A little quick math. I have 3 -125 amp hour batteries in my battery backup system or 375 amp hours. The furnace uses 400 watts or 3.33 amp hours (400 watts / 120 volts = 3.33 amps). You don’t want to use more than 50% of your battery capacity, so I have 187 amp hours to use. At 3.33 amps, I could run my furnace for 56 straight hours. If it was moderately cold and I ran the furnace 15 minutes every hour, the battery system would provide heat for almost 10 days without any recharging. If it was really cold and I ran the furnace 30 minutes every hour, the battery system would provide heat for almost 5 days without any recharging.

Another possibility is to connect an inverter to your car battery and run your car in a ventilated area during the time the furnace is on and your car battery is providing power.

One last note, use a good extension cord between your generator or inverter and the furnace. Use a heavy duty 12 gauge grounded cord. Harbor freight sells a 25 ft 12/3 cord for $19.99 and a 50 ft 12/3 cord for $29.99.

If you are not comfortable doing the modifications yourself then get someone knowledgeable to do the work.

That’s it. Keep warm.

 

Ed Rogers
Copyright “Keep It Simple” 2012. All Rights Reserved.

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