Teachable Moments – Home Edition

by: Mike Ranieri, PE

I’ve lived in my current home for two years. For two years now, we have dreaded the coming of winter. Winter means colder weather, and colder weather means “Heating Mode” for our heat pump. During “heating mode”, a heat pump reverses its refrigeration cycle. This makes the outside coil function as the evaporator and the inside coil function as the condenser. It is at this time that our outdoor unit would become heavy machinery. There would be a very loud buzzing, as the compressor would rattle to life, echoing through our neighborhood.

graphic courtesy of US Dept of Energy

It was until last night that I believed this was a normal effect of the cycle reversing. Three nights prior to my teachable moment our heat pump stopped working. It was the weekend and we did not want to pay weekend rates for a service call. Though I have worked in the electrical and mechanical industry for 30 years, I had absolutely no experience working on air conditioning systems, and had no idea where to start looking.

graphic courtesy of US Dept of Energy

I did notice, however, that there was a soft clicking noise coming from our heat pump. So, I turned off the circuit breaker and removed the small service panel at the corner of the unit to see if I could find the source of the clicking. There was a small black relay and a round, silver capacitor nestled in the corner of the unit.

This was my teachable moment. I remembered from somewhere in my past that capacitors are used to give motors the “oomph” they need to start. Outdoor heat pump compressors need lots of this “oomph” to get their cycle going. Without the “oomph”, the motor draws more current than it needs to run. When electrical parts draw more current than they need to run, they complain. Their complaint is vocalized in a 60Hz hum. The more overloaded they become, the louder the 60Hz hum. At that moment, I realized that the capacitor in the outdoor unit had been in a failed state for at least two years! After two years of complaining, the relay had also failed, hence the clicking sound. I ordered a new relay and a new capacitor online and had them “overnighted” to our home. It probably took me two or three times as long to make the switch than a service man would have taken, but I was able to install the new parts and turn the circuit breaker back on.

I walked back inside the house and realized: In the time it took me to walk to the back door, the heat pump had silently started! Hmmm… it’s not normal for the outside unit to make lots of noise.

Lesson Learned: When your A/C screams at you, listen!

Published on January 04, 2018
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