When you throw your hand over a running condensing unit, you’re feeling the release of heat absorbed indoors, in the suction line, and from the heat of compression. All of that heat has to be rejected in the condenser via desuperheating, condensing, and subcooling.

Putting your hand over the condenser can allow you to feel your way through the system and determine if it’s rejecting an appropriate amount of heat. If you don’t feel a lot of discharge heat (or feel too much), then you need to investigate why that is. However, you can also use temperature probes to get a quantitative measure of the temperature (although that is variable). In some cases, you can expect that temperature to be half of the CTOA; radiant gains and probe placement can also significantly impact your temperature readings.

You might even get different measurements on identical systems (especially in light commercial applications with several units in a single area). You can measure several similar systems and find an outlier.

To measure the condenser discharge air temperature, you have one probe measuring the ambient temperature (away from the sun) and another probe measuring the discharge air temperature. A typical modern system may have a differential of 8-20 degrees (F). Older systems, systems with high loads, and refrigeration units are likely to have higher differentials.

If the discharge air temperature is too low, you might consider checking the fan blade (size and rotation direction). High discharge air temperatures could be due to a high load or a dirty condenser coil.