Why Discharge Line Temperature is a Useful Reading


Ever since I started in the trade we would take discharge line temperature in the winter on a heat pump system. The reason for this is that in the Winter the discharge line is easily checked while suction superheat and even subcool can be more difficult to access. The old timers that trained me would say that in a properly functioning system the discharge temp will be “about 100 degrees over outdoor ambient” when a heat pump system is running in heat mode. That rule of thumb is actually still pretty close, but it’s isn’t exact…. and what happens if you are getting a different reading?

First off, of your discharge temperature (as measured with a thermometer at the compressor) is over 225, you have an issue. At that temp, the oil will begin breaking down, so if you check for no other reason, check to make sure you are under 225.

High head and/or low suction cause a higher discharge line temp. If your suction pressure is low but the superheat is low (low evaporator airflow or heat load) it will cause LESS of a discharge temp increase than if the suction is low due to low charge, restriction or evaporator underfeeding.

You can also see an increased discharge line temp if you have a high suction superheat at the condenser due to an uninsulated or improperly insulated suction line.

On the condenser side, anything that causes high head will also cause high discharge line temperature. Overcharge, low condenser air flow due to improper motor or blade or dirty condenser coils. In the case of heat pump units running in heat mode, the most common causes are dirty air filters or other indoor air flow restrictions (because the condenser is now inside during heat mode)

In short… high discharge temp can commonly be caused by

– Low charge (high suction superheat)

– Severe Overcharge

– Low condenser air flow

– Damaged condenser fins

– Restricted metering device

– Other restrictions (Liquid line drier, suction line drier, kinked lines, clogged screens)

Low discharge line temp can be caused by

– Low compression ratio (failing compressor)

– Low condensing temperature (Outdoor temperature)

– Metering device too far open / flooding evaporator coil / Low Suction Superheat

— Bryan

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