Blue light mystery is easily solved for homeowner

Inspector's Eye

April 22, 2001

The blast of wintry air last week reminded me that it's not uncommon for people to get confused about their heating system.

Questions range from:

Does my house have a heat pump or just a furnace?

How come the only time my heat pump blows nice hot air is when the blue light is on?

Why does our new house seem colder than our old house even though both thermostats were set at 66 degrees?

My answer: Did you ever think that maybe your thermostat is wrong? For a quick fix, try adjusting it to whatever setting keeps you comfortable, then lower it a degree at a time until you can just barely stand it. That's where you should leave it if you want to save fuel.

But if you want the thermostat to keep the temperature at 70 when it's set for 70 - you need to get it adjusted or replaced.

Call a heating contractor and ask to have your thermostat checked. It may just need to be calibrated; sometimes that is as simple as loosening the mounting screws and leveling it. Or it may be time for a new thermostat.

Another sign of a malfunctioning thermostat is if the heat stays on too long and makes the house too warm.

As to the question of whether a home has a furnace or a heat pump heating system, homeowners can generally assume that they have a heat pump if the thermostat has an "emergency heat" setting on the selector switch. The homeowners can use the "emergency heat" if there is a problem with the heat pump or until the heat pump is repaired.

The "emergency heat" setting, which normally activates a red indicator light on the thermostat, will shut off the heat pump and will heat the house using only backup heat, usually electric-resistance heating coils built into the heat pump.

The resistance heat produces air that is very warm and comfortable - warmer than the heat pump alone can achieve - but it is much more expensive to operate than the heat pump. That's why the resistance heat is used only as a backup.

The red light that comes on while the emergency heat setting is selected is there as a warning and a reminder that you've selected an expensive heating mode.

Similarly, if a blue or green light is illuminated on your thermostat while the heat pump is running, it indicates that the backup heat is running along with the heat pump. This happens automatically. Expect to see the blue light on when it's very cold outside (less than 35 degrees).

When it's that cold out, a normal heat pump can't provide enough heat without help from the backup heat, so the backup heat activates automatically. The backup heat is very warm, and relatively costly. So, when the blue or green light is on, the heat will be toasty, and expensive. That's unavoidable in very cold weather.

But, if the blue light is coming on regularly when it's not particularly cold outside - if it's in the 40s or warmer - something is not right, and your next electric bill will be unnecessarily high. It could be happening because you are in the habit of adjusting the temperature setting, which can inadvertently activate the backup heat - that's why it's more economical to "set it and forget it."

Dean Uhler has been a home inspector for more than 12 years and is president of Baltimore-based Boswell Building Surveys, Inc. Uhler is a member of the American Society of Home Inspectors (ASHI) and is the treasurer of the Greater Baltimore Chapter of ASHI.

Questions, with name, address and daytime telephone number, about homes and home inspections can be faxed to 410-783-2517, e-mailed to or mailed to Inspector's Eye, Second Floor, 501 N. Calvert St., Baltimore, Md. 21278-0001.

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