Thermostat could be cause of short-cycling

Q: Eleven years ago, we finished the basement of our home and replaced our 40-year-old oil-burning furnace with an energy-efficient gas furnace. Since the remodel, the new furnace seems to cycle almost constantly, running no more than five to six minutes before shutting off, then starting again within five to six minutes. Recently we had the furnace serviced and were told (by two separate companies) that the furnace shows evidence of unusual wear and will need to be replaced within several years.

The duct work for the old furnace used the space between the joists of the basement ceiling for the biggest cold air return, located in the central hallway of the basement. We have just discovered to our amazement that someone cut a panel about 8-by-12 inches in size from one side of the forced-air heat duct, permitting the furnace outflow to communicate directly with the cold air return. This is within 6 feet of the filters and furnace intake. Perhaps they thought the cold air return was part of the heat-distribution duct?

Does this situation explain the furnace cycling and account for the furnace's premature aging? Either way, does it need to be rectified when we get a new furnace? Access is a problem, as the forced-air duct is otherwise completely enclosed in sheetrock within the basement ceiling. But we will fix it, if necessary.

A: The hot-air duct being routed into the return air side of the furnace seems suspicious at first glance, but it shouldn't directly account for any unusual wear on the furnace. It may have a small part to play in the short-cycling issue, which may in turn have an affect on furnace life. You didn't say what component the heating contractors said was showing unusual wear, but I am assuming this means the heat exchanger.

The cold-air side of the furnace doesn't really care operationally where it gets its air from. In newer homes, air sometimes is drawn directly from outside to pressurize the building and drive stale air outside. Simply drawing some of its own air from the hot side won't affect how it performs other than making the finished product warmer.

If — and this is a really big if — the blended hot and return air were hot enough to allow the furnace to overheat, it would shut down and resume operation in five or six minutes, as you describe. But overheating is going to have another underlying operational symptom than warmer-than-normal return air (such as excessive temperature rise, flame propagation or gas-pressure issues).

The short-cycling you describe can be irritating. It can cause premature igniter burnout and unusual wear on the heat exchanger because of condensation and rust.

But I doubt that the heat-duct arrangement is the reason for the short-cycling. This can be proved or disproved simply by measuring the hot air coming through the filter. Does it rise and continue to rise at a great rate during the five-minute run time? Does the temperature in the home get hot enough to satisfy the thermostat, or is the furnace shutting down for another reason?

More likely it is the anticipator in your thermostat that is causing it to cycle on and off every half degree or so, (rather than have a wider temperature spread of one or two degrees) or that the thermostat is located in an area that is near a hot-air duct.

Was the thermostat moved during the remodel? Consider the location of the thermostat: Hallways are traditionally the best because they have no heat ducts and little airflow or windows.

I presume the heating contractors have checked out the furnace operationally and checked to see that the furnace is sized correctly for the home. A furnace that has too much capacity will short-cycle. Close to half of all homes have too much furnace capacity.

Without performing a proper heat-loss study, a quick rule of thumb is to take your home's heated square footage and multiply by 25, giving you an approximate furnace BTU capacity (obviously this varies greatly with age of the house, window size, ceiling height, insulation levels and so forth). The BTU value for your furnace is printed on the data plate just inside the furnace cabinet.

Some electronic thermostats have an adjustable anticipator setting. Look in the owner's manual and adjust it if possible. And correct the heat duct feeding into the cold air return if for no reason other than it is just wrong.

Follow-up: This note about last week's column is from someone who used to be in the window-replacement business:

I was a bit surprised that one of your readers had made the observation that fixed (windows) tend to fail more frequently than vented (operable). He is absolutely correct.

The most significant difference can be summed up in one word: Screens. Most seal failures occur in areas of higher thermal stress. In other words, in places where the sun heats the windows, then goes away, comes back, etc., providing cyclical stress on the unit.

Because an insulated unit is sort of like a glass balloon, when the air inside is heated, it expands and the glass deflects outward. When it's cold, it contracts and the glass deflects inward, each time moving closer to fatigue failure of the sealant.

The slight shading effect of the screen cloth is enough to provide years of extra life to the protected units, even though they suffer more stress from the vent being opened and closed.

The difference is something I noticed many years ago, but I had a lot more opportunities than most observers. To have caught something like this with relatively little exposure and no particular reason to give it a second thought illustrates the reader's exceptional awareness and observational powers.

Reply: Thank you. I have definitely noticed many more failures of windows on south- and west-facing sides, so this all adds up.

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 dhay@seattletimes.com. Sorry, no personal replies. More columns at www.seattletimes.com/columnists.