Quote:
Originally Posted by NiHaoMike
Yes, it reduces the delta T the heat pump has to work with.
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I think you mean the larger surface area requires a lower delta T to match the load at constant fan operation. So you want a smaller compressor to provide a lower delta T at an efficient operating cost. (Perhaps you could use a larger compressor with a different amount of refrigerant, but it wouldn't be efficient.. not sure)
Changing fan speed will change the rate of heat exchange, which will reduce efficiency of compressor-to-heat-exchanger-area combination. So, ideally when changing fan speed, you also change compressor rate or surface area (to maximize efficiency of meeting the load.)
However, there are other factors to system design. Humidity control is important to comfort and health. The indoor coil can provide dehumidification to keep levels below a maximum. Rate of dehumidification depends on coil area, temperature, and air flow.
Air speed at ducts affects temperature stratification and air distribution noise levels. Typically, your ductwork is sized to provide a certain exit velocity for a specific flow rate. Changing the fan speed throws off duct design, unless your ductwork contains active components to maintain exit velocities (very unlikely.)
So ideally the fan speed is constant, and compressor rate and surface area are variable. I'm not familiar with variable surface area systems, but I would guess a few rows of coils with electronic valves could work. The compressor rate and surface area change to meet the load at maximum efficiency, and to dehumidify at a desired rate. I suppose you could say, to meet the sensible and latent loads appropriately.. but I find it intuitive to clearly distinguish between latent load and total load.