Why Is My Furnace Running In The Summer?

Q: The gas furnace in my 4-year-old home comes on seemingly for no reason and runs for several hours most days, yet it stays comfortable the entire time. What is wrong with my furnace?

A: Nothing is wrong with the furnace. All homes built since July 1991 have had state-mandated, interior-mechanical ventilation systems. You have what's called a make-up air system built into your forced-air furnace's return air plenum.

An adjustable timer located on the exterior of the furnace periodically activates the blower fan inside the furnace (the one that pushes air to each individual register). No heat is produced during this cycle unless the thermostat is calling for it.

You may notice a separate pipe leading into the return-air system from the exterior. This pipe has a mechanical damper somewhere along its length that opens when the timer activates.

The damper controller is mounted on the pipe exterior - it's about two packs of cigarettes in size. The 6-inch pipe allows exterior air to be sucked directly into the furnace return-air system and into the house.

The house stays comfortable in summer because the furnace is not producing heat. During the heating season, when the outdoor air would cool down the house, the thermostat responds by signaling the furnace to produce heat and suck in fresh air from the outside simultaneously.

In addition to the make-up air system, your house would have a means of exhausting the stale interior air and reducing the positive pressurization. A whole-house fan, usually mounted in a hallway, is a dedicated fan for this purpose and is connected to the same timer.

Other houses (and those without forced-air or central-heating systems) meet the exhaust requirement with an additional timer collocated with a laundry or bathroom fan. Make-up air can also be accomplished with sliding or clipping window vents. Yes, the irony is tremendous; pay beaucoup bucks for air-tight windows, then pay more to put a hole in them! (I have window vents in my own home . . .)

Bringing cold air into a home during winter through a 6-inch pipe may seem like lunacy in the energy-efficiency department. That may be true, but for about five years prior to 1991 we built homes air tight with no means of mechanical ventilation. Many had interior condensation problems and people got sick because of lack of air exchange. Older homes had air exchange naturally - it leaked in. Newer ones need to do it mechanically.

Q: My son was recently injured playing with our electric garage-door opener. The door momentarily trapped him and was squeezing his leg until I heard his screams, ran into the garage and reversed the door direction. Does this happen often and what can be done to prevent reoccurrence, other than supervising my kids better?

A: I'm not aware of any particular statistics on this phenomenon. I have heard of it happening before, however. I'm sorry about your son and hope he wasn't badly injured.

These doors should have reversing mechanisms that will make the door go up when met with resistance. When adjusted correctly, you can reach over and grab the door on its way down, triggering it to reverse.

Many times they are not adjusted correctly. The adjusting screws are often located on the exterior of the motor casing, at the rear. Too much sensitivity will cause the door to reverse during normal operation downward. It will take some trial and error.

Newer doors have the electric eye sensors as well as a conventional reverse sensor. I know many children love to play with and try to beat the electric eye by jumping over it. An attractive nuisance, I guess. I would have done that as a kid, myself.

Tip from a reader

Last week's column addressed using diesel fuel or form oil to treat forms before pouring concrete to make for easier and cleaner removal. I got an e-mail from Joel Shank, who suggested using vegetable oil.

Never tried it myself, but why not? Definitely higher in saturated fat but more environmentally friendly. But what about that wonderful smell of diesel and all those big stains it puts in your clothes when mixed with dirt? I'd miss that. Not! Can I get canola oil in 5-gallon buckets?

Have a happy July 4!

Ask The Expert answers readers' questions every Saturday. Send questions to Ask The Expert, Seattle Times, P.O. Box 70, Seattle, WA 98111, or call 206-464-8514 to leave your questions on Ask The Expert's recorded line. E-mail address is dhay@seattletimes.com

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