What happens to the suction pressure and discharge pressure when the load on a heat pump in cooling mode decreases?

When the load on a heat pump in cooling mode decreases, the suction pressure typically goes up while the discharge pressure goes down. This phenomenon occurs due to a reduction in the heat load that the heat pump has to manage.

As the cooling load decreases, the heat pump's compressor has to work less hard to remove heat from the indoor space. With less heat being absorbed from the indoor coil, the refrigerant returning to the compressor is cooler and at a lower volume. This leads to a decrease in the heat extraction process, consequently resulting in a higher suction pressure because the refrigerant is not evaporating as quickly.

On the other hand, as the compressor's workload decreases, the discharge pressure, which is a function of how much work the compressor is doing to send the refrigerant to the outdoor unit, tends to decrease as well. There is less compressive force exerted on the refrigerant due to the lower load, hence leading to a drop in discharge pressure.

By understanding the relationship between load and pressure in the context of heat pumps, it becomes clearer how a decrease in cooling load leads to the described changes in suction and discharge pressures.