A heat pump reverses the refrigeration cycle, making the indoor unit the condenser and the outdoor unit the evaporator. A reversing valve shifts the flow and turns the suction line into the discharge line and vice versa (though there are some runs of common suction and discharge lines). The liquid line stays the same in heat mode, but the flow direction changes. Trane systems have somewhat unique configurations, though.
The discharge line, suction line, liquid line, and saturation suction temperatures can give you a pretty good idea of how a heat pump is running. In general, the discharge line temperature should be 100-110 degrees above the outdoor temperature. The suction line should be 5-15 degrees colder than the outdoor temperature, and the liquid line should be 5-15 degrees warmer than the indoor temperature. The suction saturation temperature will typically be 20-25 degrees below the outdoor temperature.
Common issues with heat pumps come from the auxiliary heat. Not all thermostats are compatible with multi-stage heating, including the heat strips. Other times, the auxiliary heat wasn’t set up properly in the first place despite being wired into a compatible thermostat. The thermostat can send the call on the heat strips immediately if you activate emergency heat, but it can also call for emergency heat if the setpoint is high enough. To find out if the heat strips are actually calling, take the amp draw on your high voltage wires (240v into the air handler).
It’s usually easy to prove heat kit failures (such as by looking for open safeties). While it’s usually acceptable to quote the whole heat kit for replacement, we need to make sure we know which problem we’re solving. We have to address underlying problems, like airflow problems.
Airflow has a huge effect on the heat pump when it’s in heat mode. The blower needs to be set up to handle multi-stage heating; if the blower can’t hit the high stage, the system might go out on high pressure due to the buildup of heat and pressure. You also need to be checking the filter and whether your coil is clean outside.
When setting the charge, it’s best to recover the refrigerant, weigh it, and add it back in with the required additions or subtractions. You can also use the HVAC School calculator to see if you need to add a different amount of refrigerant than the factory charge due to line lengths. On a heat pump, we charge the system through the common suction port when the system is running.
Bert also answers questions from the class and finishes by instructing the class to start with what they know first: the main rules of thumb, checking if the air filter is dirty, etc.
Please check out this class for more information on heat mode: https://www.youtube.com/watch?v=IoBiyEpaZAw&t=390s&ab_channel=HVACSchool
We recommend reading Craig Migliaccio’s book as an additional resource: https://www.acservicetech.com/ac-book
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