Wednesday, March 9, 2011

Thermal fuses: Often the symptoms, rarely the problems

* I would tell you not to try this at home, but that's pretty much the only place you can try it. So I will tell you to turn the power off at your circuit breaker panel if you decide to try this at home.

** Turn the power off at your circuit break panel if you decide to do this at home.  If you don't, then you might as well consider this to be a manual on how to possibly kill yourself.

*** I really didn't set out to create that kind of manual, so turn the power off at your circuit breaker panel if you decide to do this.

Late last fall, I noticed that the air coming out of our heat registers was not overly heated when the auxiliary heat strip—the heat element located inside the furnace—was called for by the thermostat. When the day came that it took the furnace more than three hours to bring the house up to 70 degrees from 62, I decided it was time to figure out what was going on.

Being an inspector, my first action was to measure the air temperature as it came out of the heat registers, and to compare that reading to the temperature at the cold air return register. With the house temperature at 70, the cold air return measured right around 70.  That makes sense.  Unfortunately, the air gently coming out of the heat registers was only a few degrees warmer. When the heat strip was working properly, that air had previously been around 120 degrees. Obviously something was amiss.

So I went up to the attic with a few screwdrivers and wrenches. The first step was to remove the service panel cover on the furnace/air handler unit.

Six screws later, it was out of the way.

After checking a couple of connections and making sure that there was electricity where it should be, I turned off the circuit breakers in the furnace itself and in the circuit breaker panel—the breakers on the furnace are basically glorified light switches under most circumstances, and I didn't want power coming to the furnace at all. So after turning the circuit breakers off at the breaker panel, I removed the breakers in the furnace and then removed the heat strip unit.

Initially, I looked at the heater elements themselves in order to make sure they weren't broken or grounding out on anything. Somewhat disappointed to see that they were fine—as it was my theory that the heater elements might have been the problem—I then looked at the thermal fuses that are mounted at the base of each heater element. Bingo—two of the three had melted, leaving only one of the three heater elements functional. 

You can see in this picture that the bottom one is still connected, whereas the top two have gaps.

Thermal fuses, like their electrical counterparts, shouldn't be replaced without figuring out the cause of their failure. They fail for a reason, and “they got tired” is almost never that reason. So while replacing the links would have given me my heat strip back, it would have been only a matter of time before I would be repeating their replacement. So it was time to investigate further: what caused the thermal links to fail?

To understand the investigation, the function of thermal fuses needs to be understood. And in order to understand that, one must understand the relationship between an electric heater element and the air it heats. The heater element heats the air as the air passes over it. What does the air do to the heater element? It cools the heater element by absorbing its heat. So what would happen if there weren't enough air passing over the heater element? Trouble—that's what would happen. The coils would keep getting hotter until they either melt and break, melt the structure that supports them, or possibly—and worse—start a fire.

Thermal fuses are a safeguard against such circumstances. Thermal fuses melt when they are subject to a temperature above a certain point—and by melting, they sever the connection between the heater coils and their supply of electricity. So if it should ever be that the amount of air moving across the heater coils is not enough to keep the from overheating, the thermal fuses melt in order to keep the heater coils from achieving potentially destructive temperatures.

So, back to me sitting in the attic, staring that the furnace. Why did the thermal fuses melt? I'm thinking it's the result of an insufficient supply of air, but what would cause that? I check the filter, which is about two months into its six-month life. The filter is pretty clean, so it goes from suspect to potential witness in my mind.

Next comes the motor in the furnace. For some reason, I glance at the sticker on the service panel cover. What a mess of numbers. Hmmm...why did the manufacturer make that chart?

I look closer. I see a column titled “HEATER KIT MODEL USED.” This looks promising. Then I notice another column, which is titled “M.S.S.” This is initially cryptic and worthless. So I dig out the installation manual for the furnace and look for anything that could shed light on the sticker's meaning. In 65 pages of text, I find one sentence which vaguely mentions different heat strip kits requiring different fan motor speed settings. Suddenly the “M.S.S.” column comes to mind—motor speed setting? Sure enough, the column under these letters contain either the letters “L” or “H.” So I look back to the manual to find out if the motor has multiple speeds.

Sure enough, it does: low, medium, and high. I find my heater kit (HKR-15C) on the chart on the service panel cover and see that it requires that the blower motor be set to “H.” Unfortunately, the furnace comes from the factory with the blower motor set to its lowest speed. Well, that's handy to know. So I find the three wires that determine the motor's speed, read in the manual that the black wire needs to supply the motor when the highest output is desired, and plug it in where the blue wire (lowest speed) once was.

Months later, the results speak for themselves. No more melted thermal fuses, the air coming from the heat registers is around 120 degrees, and all is well. So if you've got an electric furnace or a heat pump with a heat strip, and if you notice that the air isn't as warm as it once was when the heat strip is engaged, this may or may not be your problem—but if you're feeling up to the task (turning the power off at the breaker panel makes you infinitely more “up to the task”) (seriously, turn the power off at the breaker panel), it's a great place to start your investigation. After you check your filter, of course!