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Why I hate Heat Pumps


Thermal relays and blast heater connections

Well, maybe that’s a bit of an overstatement but when something fails by design when you really need it, it tends to foment hate and discontent.

This morning I woke up and something didn’t seem quite right.  I looked at my projection weather station image on the ceiling and noted that it was 7 degrees outside.  Nice.  I then noticed that the room was 43 degrees.  “Ahh”, my half-asleep self said, “The power must be off”.  Then my little-more-awake self realized that I’d not be able to see the projected image if that were the case.  As I woke a bit more I realized two things.  The heat pump was running and I was COLD.  My wood stove fire had burned out and the heat pump had kicked in at 55 deg.

Now I keep my room about 50 degrees when I’m sleeping so that temperature wasn’t all that unusual but it was different.  I got up, stumbled into the living room and looked at the thermostat.  The blue light indicating that the “blast coils” or resistance heat was on.  To be expected in this kind of cold.  I also noticed that the heat pump compressor was running.  It should not be.  Too cold for heat pumping.

The real pisser was when I felt the air coming from the duct.  Cold.  The blast coils were not on.  I slid on some coveralls and went outside and took the covers off the heat pump control compartment.  A quick check with a DVM showed me the problem.  The thermal relays (see photo above) were receiving power but were not picking up and energizing the heat coils (white terminals in the photo above).  I quickly realized the problem.  They were too cold!


Thermal Relay Close-up

Here is a close up photo a thermal relay (not the same ones as above.  I had a better picture of this one)  This thing is called a relay but it really isn’t.  The “thermal” part means that it uses a heater instead of a magnetic coil.  All this thing consists of is a Klixon-type thermal switch attached to a heating element.  Apply power to the element and in a few moments the Klixon disc closes the contacts.  Remove power from the element (“coil equiv”) and in a few moments the Klixon cools off and the contacts open.  It has a built-in time delay which is handy for example, for controlling the fan and allowing it to run awhile after the heating or cooling turns off to recover that last little bit of heat or coolth.  No time delay relay is necessary.

All this assumes, of course, that the heater can heat the Klixon disk enough to cause it to click.  In single digit weather (and probably low teens), it does not.  That’s the core of the problem.  The device that is supposed to turn on the blast coils when it is too cold for the heat pump to work fails when it’s too cold for the heat pump to work. Unbelievable.

My short term solution was to drag a heat gun out there and warm them til they clicked.  That got me immediate heat.  The intermediate solution was to build up the fire :-)  The long term solution will come when the weather moderates.  Then I’ll install a pair of conventional contactors in place of those thermal abominations.  In the meantime, I tucked some fiberglass insulation around the relays.  That’ll conserve heat enough that they should operate.

I’m making this post for two reasons.  One, to blow off steam.  Two, to alert you that if you suddenly don’t have any heat and your heat  pump is a package unit with everything outside and especially if it is an economy model like my Rheem, be aware that nothing is broken.  Don’t call a service man or if you do, don’t let him change out those thermal relays.  They’re not broken.  Get ’em working with a hair dryer or heat gun and then tuck some insulation around them.  Or have the service guy do it if you’re nervous around high voltage.  Print off this article and show it to him if you like.