Thermostatic Expansion Valve
Is it necessary?
A thermostatic expansion valve in a swimming pool heat pump is designed to calibrate the amount of refrigerant the heat pump needs to operate efficiently at varying outdoor temperatures. The alternative to a thermostatic valve is to use a calibrated flow rater.
The flow rater delivers an equal amount of refrigerant to the evaporator. This system is calibrated in a temperature controlled laboratory where the conditions are simulated to be at their coldest.
Most pool heat pump manufacturer's use a thermostatic expansion valve because it is so much easier to install and needs no calibration. Maybe they do not have access to a lab to calibrate a flow rater.
But, there can only be advantages to having a flow rater instead of a thermostatic valve. The flow rater is more efficient at low temperatures when heating will be in demand. That is also when a heat pump is the least efficient. So, a flow rater that is calibrated at these temperatures will be more efficient than the thermostatic valve. Also, the flow rater has no moving parts. This means that the thermostatic valve has a higher risk of failure than the flow rater. The repair cost of thermostatic valves will also be higher.
The Elite, Hybrid, and Sunriser models all have calibrated flow raters. They are designed to operate at low temperatures for highest efficiency and reliability.
The colder the outside air gets, the less efficient a heat pump will be. A heat pump is energy efficient because it doesn't produce heat. It transfers heat from the air and sends it to the pool after compressing this heat.
A Elite 100 pool heat pump will have a COP of 6.8 at 80F but will be reduced to 4.9 at 50F. The BTU output will also be less. The warmer the swimming pool is, the faster it loses heat. Below 40F, the heat pump cannot efficiently restore the evaporating heat back to the pool. The electronic control is set to stop the heat pump below 40F to prevent useless consumption of electricity.
What is COP?
COP is short for Coefficient of Performance. The COP number basically means how much heating value you get from every dollar of electricity you spend.
A simple way of understanding is to compare with other heating equipment.
- Electric resistance heat has a COP of 1
- Highest efficiency gas has a COP of 0.95
- Heat pumps have a COP of between 3 to 7