The Flickering Light Bulb In Your Furnace

Every winter, at least one of the properties I manage experiences issues with the heat. Usually the first sign of trouble is when it fails to turn on in that shoulder season in October. You know, when it dips into the 50s and below at night. Tenants call me to say their radiators are cold, and I call the plumber. Then suddenly, the heat comes on! I cancel the plumber. All is well... until we get a 10 degree temperature drop.  Inexplicably, the heat goes out again. Sometimes it kicks on again before I can get the plumber over there, so we leave it again. But the writing is on the wall: I know that it's only a matter of time before the problem is back, and usually in a matter of days.

Signal Relay - this little guy might be to blame if your boiler
doesn't come on when it should, but then comes on by itself later on.

This week I finally put a name to the most likely culprit: signal relay. A signal relay is a switch that turns on the circulating pump (which draws hot water up through the radiator and back to the furnace to be re-heated). They are electrical circuits, and they fail. It's hard to say how often they fail - and there might be more than one if the furnace has zoned heat.  But, if your heat fails to go on when it should (and may or may not come on later without a plumber's help), or goes off at an inappropriate time (either to come back on or not), you can put money on a signal relay failing or having failed.

If you're wondering why shouldn't we have the plumber in even when the heat starts working by itself, the answer unfortunately is because you can't tell a signal relay is broken unless it's actually not working. Think of a light bulb in a chandelier. Sometimes, the filament in one bulb disconnects from one of the two contact wires and the bulb goes out. You think to yourself "I need to replace the light bulb", but before you can, the filament reattaches and the bulb starts working again.  Now, you can't remember which bulb it was (unless you're really good like that), and you don't want to change out the good light bulbs because that would be a waste... so you have to wait until it goes out again before you can identify it.

Same thing with a boiler. If the signal relay is causing the problem but it's still working intermittently, then the heat is on while it's working, and if tested the relay will not seem broken. So, unfortunately usually the heat has to be off for the plumber to find the problem.

The good news is it's usually an easy fix if your plumber has a compatible part with him. Make sure you can tell him the make and model of your furnace when you call. Otherwise he may not have a compatible part and may have to come back the next day - meaning more hourly charges and possibly a very cold overnight.