Sunday, January 23, 2005 - Page updated at 12:00 AM

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Ask the Expert / Darrell Hay

Got gas? How to power furnace in a blackout

Q: I understand that our natural-gas furnace will not operate during power blackouts because there is no power to the electric fan. Power to the furnace is via a direct line; there is no plug. Given budget and desired convenience, I am not interested in a generator solution. However, is it feasible to convert the furnace to a plug-in and then put it on an uninterruptable power supply (UPS) battery unit and have the furnace run when the power is out? How much run time could I expect from a small UPS unit?

A: The answer to your question is dependent on your particular furnace and the capacity of the UPS unit you have. Does the furnace have powered exhaust, or a draft inducer (a "high efficiency" furnace or any model made after 1992)? Does it have spark ignition, a hot surface igniter or an old-school continuous pilot light? What size blower fan does it have? Blower fans typically need 800 watts to run continuously, double that on startup.

Add to that the power for the inducer and electronic ignition (if equipped), and you require a good amount of power to start and run your gas furnace. Most smaller UPS units would be hard-pressed to run modern furnaces more than a very short time.

You might be better off spending $200 on a high-capacity deep-cycle marine battery and an inverter. Or a gas fireplace.

In any case, you need to get the specs and crunch the numbers for your equipment. Keep in mind that technically your furnace needs to be hard-wired — not something the cat could trip over and unplug.

Q: What is the ideal humidity level in a home? My furnace seems to make it too dry in the house.

A: Perfect humidity is 35 to 45 percent. Higher than 50 percent and we start to get problems with dust mites, condensation and mold. Lower than 30 percent and our skin starts to dry and building materials shrink and crack. Be wary about adding a humidifier in our climate. Low humidity is rare. In almost all cases, humidifiers cause more problems than they cure, and in many cases shorten the life of the furnace when (not if) they leak.

Q: You mentioned testing the relief valve on a water heater in your "resolutions" column Jan. 2. My tank is completely wrapped with insulation. Where can I find the valve?

A: Relief valves are located in one of several places: the top of the tank, the side of the tank, somewhere in the top two-thirds, or on the inlet or outlet pipe near the ceiling, within a few feet of the tank.

Darrell Hay is a local home inspector and manages several rental properties. He answers reader questions. Call 206-464-8514 to record your question, or e-mail Sorry, no personal replies.

Copyright © 2005 The Seattle Times Company


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