Quote:
Originally Posted by charlesfl
I understand that less temperature differential means less work.
Less work means less electricity used. How is the electricity used less, less amperage use by the compressor or run for a shorter time.
Sorry, I am not being very clear.
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With respect to total energy consumed, it can go either way. All the compressor really cares about is compression ratio, as far as the energy it consumes. With a constant-displacement, constant-speed, residential HVAC compressor, the current consumed is closely related to the difference between (absolute) suction and discharge pressures. These two pressures determine the compression ratio the motor must work against. When the CR is low, so is the current consumed. As the CR rises, so does the load current.
Run time is more dependent on mass flow of refrigerant. The mass flow versus temperature gradient determines the system capacity and the amount of heat transferred by the phase change cycle.
In cooling mode, the compressor will see moderate suction pressure and high discharge pressure. The condenser pressure follows outdoor temperature, so when it is "only warm" outside, it sheds heat well, keeping CR low. Lots of mass flows, and lots of heat is transferred at high efficiency. The hotter it gets outside, the higher the discharge pressure rises, the more current the compressor draws, and the less mass that flows through it. Run time increases accordingly.
In heating mode, the compressor will see low suction pressure and moderate discharge pressure. The evaporator pressure follows outdoor temperature, so when it is "only cool" outside, it gathers heat well, keeping CR low. Lots of mass flows, and lots of heat transfers at high efficiency. As outdoor temperatures drop, evaporator pressure drops, load current drops, and mass flow through the compressor drops due to reduced suction pressure. Runtime increases accordingly.
If the outdoor temperature drops enough, the compressor will be starved of refrigerant due to very low suction pressure. There will not be enough refrigerant flowing through the compressor to cool the motor windings, causing CR to skyrocket along with discharge temperature. This condition can burn motor windings, valves, and/or oil, causing compressor burnout and system failure.
If the evaporator coil freezes up, the same "starvation" condition is forced on the system due to the drastic reduction in heat flow through the frozen heat exchanger. If a heat pump is not designed for cryogenic conditions, it is equipped with some sort of defrost control to protect the compressor. Some units merely stop the compressor until the evaporator warms enough to support operation again. Others have an active "reverse cycle, hot gas defrost" function. Others have some sort of electric defrost heater built in. Most units will not resume cooling mode of operation until the evaporator is well above freezing temperature.
The ground-source (aka geothermal) heat pump does the best when there is a supermassive outdoor heat source that remains mainly constant in temperature, regardless of outdoor (weather) conditions. Due to the relatively constant temperatures of both outdoor and indoor heat exchangers, the performance of the unit is always very predictable, so heating and cooling circuits and controls can be optimized for maximum energy savings. In extreme weather conditions, the energy savings compared to a standard air-source unit add up very quickly.