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Why Discharge Line Temperature is a Useful Reading
Since I started in the trade, we would take discharge line temperature in the winter on a heat pump system. You can easily check the discharge line in the winter, while suction superheat and even subcool can be more difficult to access.
The old-timers who trained me would say that the discharge temp will be “about 100 degrees over outdoor ambient” when a properly functioning heat pump system is running in heat mode. That rule of thumb is actually pretty close, but it's isn't exact—and what happens if you get a different reading?
First off, if your discharge temperature (as measured with a thermometer at the compressor) is over 225°, you have an issue. At that temperature on the discharge line, you will have an internal compressor temperature of over 300°, and the oil will begin breaking down, so even if you check for no other reason, check to make sure you are under 225°.
For what it's worth, when I check A/C systems, I commonly see discharge line temperatures between 160 to 180 degrees on properly functioning systems at typical summer conditions in Florida.
A high head or low suction pressure can cause a higher discharge line temp. Think of it this way: if the compressor is working against a higher pressure in the discharge line, it will need to do more work (higher compression ratio) to get it to that pressure.
This increases the electrical consumption of the motor in the compressor, which adds heat energy. There's also the simple fact that higher pressure = higher temperature on the basic physics side.
If the suction pressure is low, the density of the refrigerant is also lower (again, basic physics), which means that the compression ratio will be higher to get it up to the head pressure, AND lower-density refrigerant won't be able to cool the compressor as well because there just aren't as many refrigerant molecules passing through the compressor crankcase (refrigerant mass flow rate decreases with low suction).
If your suction pressure is low, but the superheat is low (low evaporator airflow or heat load), it can cause LESS of a discharge temp increase than if the suction is low due to low charge, restriction, or evaporator underfeeding. That is simply because the suction temperature is lower, but a low suction temperature is still less important to compressor cooling than a proper mass flow rate. In other words, a correct compression ratio and proper suction pressure are more important to compressor cooling and discharge temperature than suction temperature alone.
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 a high head will also cause a high discharge line temperature: overcharge, low condenser airflow 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 airflow 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, low suction pressure, low subcool)
- Severe overcharge
- Low condenser airflow
- Restricted metering devices
- Other restrictions (liquid line drier, suction line drier, kinked lines, clogged screens)
- Low discharge line temp can be caused by
- Overcharge (slight to moderate)
- Low load/airflow (in some cases)
- Compressor not pumping (low compression)
Keep in mind that keeping the discharge line and compressor temperatures in check will greatly increase compressor longevity with refrigerant-cooled compressors. It is worth noting that severely high compression ratios (low suction, high head) are more common in refrigeration applications as a cause of compressor overheating.
In A/C applications, it is more commonly caused by the high suction temperature/low mass flow rate associated with low charge or restrictions. However, instances of overheating due to dirty coils and poorly insulated suction lines are also quite common.