In this short podcast, Bryan answers a listener-submitted question and explains why heat pumps get a bit weird: when it comes to superheat in heat mode. He also explains how we can move heat from outdoors to indoors, even in temperatures below freezing (cold temperatures just have less heat, not zero heat).

In order to move heat in very cold conditions, we need very low suction pressures and cold coils, which gives us a high compression ratio. Superheat will be affected by these conditions. Remember: superheat that is too low can cause floodback, and superheat that is too high can cause the compressor to overheat.

Superheat is easy to check in cooling mode, but it's harder in heating mode, especially since the suction line is at the outdoor unit. In heating mode, we can only measure superheat between the coil outlet and the reversing valve inlet, which is a very short run of tubing.

The superheat will also be less stable in heat mode in cold weather, and it will have a wider range of "normal" values than cooling mode, depending on the conditions. Superheat could even drop to zero with some fixed-orifice metering devices (which would cause floodback). That's why many of these heat pumps have accumulators, which collect liquid refrigerant to protect the compressor. Even though TXVs attempt to maintain superheat, you may still see some variation in heat mode during cold weather. EEVs are common in ductless systems and are highly controllable but maintain relatively low superheats by design; fast metering control, intelligent logic, accumulators, and low refrigerant charges allow them to avoid floodback in low temperatures.

In any case, spikes and drops in temperature can cause the superheat to jump or collapse because the load changes (as the outdoor coil is the evaporator). Frost buildup on the coil also inhibits airflow and heat transfer, causing the superheat to change as the evaporator pressure and temperature drop. Defrost also introduces chaos to the equation. All of these should influence your judgment when checking superheat to diagnose or commission a system in heat mode.

 

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