APR Control vs. Hot Gas Bypass

How hot gas bypass works

Hot gas bypass is a false-loading capacity-control method. It diverts high-pressure discharge gas and injects it immediately after the TXV (the metering device), mixing it with low-pressure liquid to raise the evaporator temperature and create an artificial load. With that fake load present, the compressor keeps running even when the space does not actually need the capacity, which stops the unit from short cycling. It is a proven approach that has kept compressors online for decades, and on some systems it remains a reasonable fit. But it fools the system with a false load rather than reducing the capacity the compressor produces, so it does not lower compressor power the way real modulation does.

Where hot gas bypass falls short

The trade-offs come from that false load and from the plumbing hot gas bypass requires. Four limitations show up most often in the field.

How APR Control is different

APR Control takes the opposite approach. It is an external compressor unloader: it diverts refrigerant from the compressor outlet back to the suction inlet, reducing the compression ratio so the compressor produces genuinely reduced capacity matched to the real load. Instead of adding a false load, APR Control improves the refrigeration effect during part-load operation. By improving the quality of the refrigerant, the ratio of liquid to gas, in the evaporator as it modulates, it raises evaporator efficiency and compressor performance. So the unit runs at part load with lower power consumption and better latent (humidity) removal, the benefit hot gas bypass gives up. APR Control is a suction pressure-activated mechanical valve, UL Listed, that installs as a retrofit without the third-line plumbing or the added charge that micro-channel condenser coils cannot accept.

It is also not an evaporator pressure regulator

Engineers sometimes group the APR Control with evaporator pressure regulator (EPR) valves, the "other evaporator pressure control" that the energy codes reference alongside hot gas bypass. The two work in opposite directions. An EPR valve restricts the suction line to limit mass flow, which creates lift, a larger pressure difference across the compressor, and raises power draw while it is active. The APR Control does the reverse: it reduces lift by unloading the compressor, so power draw falls at part load. Same category name in the code, opposite effect on the compressor.

Where APR Control wins

• Real part-load operation from reduced compressor capacity, not an artificial load
• Increased latent heat removal for better humidity control, instead of the latent capacity hot gas bypass usually eliminates
• Lower power consumption at part load: 16% lower compressor power draw at part load (manufacturer lab testing)
• Less compressor wear from cycling: 77% fewer compressor starts (Independent Field Study)
• Retrofit install with no third line on splits and no added charge that micro-channel condenser coils cannot handle
• Genuine continuous capacity modulation, consistent with the part-load capacity-control intent of ASHRAE 90.1, where a hot gas bypass false load does not reduce compressor capacity

APR Control is not hot gas bypass. For the full technical comparison, see the whitepaper in the resource library, or talk it through with a Rawal Devices Engineer.