How come the temperature is sometimes not maintained?

Question

First let me come right out and say that I am a software guy and this seems like a hardware problem, so I already feel out of my league. That said, here's the deal:

Background: I have two-zone "hot air" heating in my home driven by air handlers in my garage which generate heat from propane stored in an underground tank near my driveway. There are two thermostats, both of the new touch-screen type. (I'm pretty sure the thermostats are installed correctly. One was installed professionally, and both respond properly to the fan controls.)

The Problem: Sometimes, like about 40% to 50% of the time, the temperature in one or both of the zones drops way below the target temperature. This is worse overnight, since I'm not awake to notice it and make a temporary fix. The temporary fix? Simply switching the thermostat off and then on again. When I do that I can hear a "click" and soon after that warm air is blowing from my floor and wall vents.

I guess there is an intermittent failure to communicate between my thermostats and the air handlers. What I don't understand is why, when the temperature is below the target temperature, the thermostats don't resend the "call for heat" signal. The thermostat clearly knows the ambient temperature, as it displays it. Why does it take a "reboot" to resend the signal? And the larger question, of course: how do I fix it?

Answer 1

If this is indeed two independent systems in one house (as it sounds like it is), the symptom you describe actually sounds like a common furnace problem, and you happen to have both doing it at the same time.

• Flame sensor problem (I've had this happen myself)

  The flame sensor is a usually L-shaped piece of metal that literally sits in the path of the flames, and detects their presence. It is a safety shut-off feature so if there are no flames, it turns the gas off. It gets dirty over time, and at the beginning will happen only occasionally, and will gradually happen more frequently as it gets dirtier. Cleaning it with some steel wool once a year will usually solve the problem. It is usually connected to the control board using a white high-temperature wire.

• Igniter problem

  Could be a bad igniter, which will cause the gas not to ignite at all. This could happen intermittently.

• Exhaust blocked/backflow

  There are sensors to detect when exhaust gas isn't being vented properly, so if your chimney or vent is slow or clogged, it may be causing this sensor to trip and shut off the furnace. This is less likely to be an intermittent problem, but is worth checking out.

In all cases, when there is a problem, the furnace will not try to start again until you cycle the control signal on the thermostat or cycle the power to the furnace itself.