Fan Speed v Efficiency?

Air Source Heat Pumps (ASHPs)

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Toodles

(@toodles)

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6432 kWhs

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Topic starter 06/08/2023 2:16 pm

The fan on our ASHP has 4 speeds that may be selected; Daikin’s notes speak of lower speeds for quieter operation but make no mention of performance or efficiency degradation. My thought is that the fastest speed gives maximum airflow across the condenser and thus (probably) produces the greatest rate of heating of the fins. Noise is not really a problem as we unfortunately live in an area with high ambient noise pollution and the vast majority of the time, the fan cannot be even detected unless within approx 0.5 metres of the grill. Running at maximum speed presumably uses more energy than the lower speeds might but is this extra energy consumption providing higher efficiency than a lower speed might? Thoughts please? Regards, Toodles.

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!

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Derek M

(@derek-m)

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06/08/2023 4:12 pm

Posted by: @toodles

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The fan on our ASHP has 4 speeds that may be selected; Daikin’s notes speak of lower speeds for quieter operation but make no mention of performance or efficiency degradation. My thought is that the fastest speed gives maximum airflow across the condenser and thus (probably) produces the greatest rate of heating of the fins. Noise is not really a problem as we unfortunately live in an area with high ambient noise pollution and the vast majority of the time, the fan cannot be even detected unless within approx 0.5 metres of the grill. Running at maximum speed presumably uses more energy than the lower speeds might but is this extra energy consumption providing higher efficiency than a lower speed might? Thoughts please? Regards, Toodles.

That is a good question, though I suspect to provide a definitive answer would require quite detailed testing to be performed under varying operating conditions.

If one considers what is happening within the internals of the heat pump, as the refrigerant flows through the evaporator it changes from a liquid to a gas, and in so doing absorbs thermal energy from its surroundings. The metal of the evaporator therefore falls to a temperature below 0C, and in doing so absorbs thermal energy from the surrounding air. Even without the fan or any air movement due to wind, this cooling effect will cause some movement of the air around the evaporator. The amount of thermal energy that can be absorbed will therefore be dependent upon the temperature of the evaporator, the outside air temperature (OAT), and the flow of air around the evaporator.

For improved efficiency the refrigerant gas needs to be warmed as much as possible before being returned to the compressor, but of course it cannot be any warmer than the OAT. For this to be achieved, sufficient thermal energy must be absorbed by the refrigerant gas from the OAT, which will require a certain volume of air to flow past the evaporator. Once this minimum volume of air flow has been achieved, I suspect that increasing the air flow will have little effect.

As the heat pump works harder, the rate of the refrigerant gas flowing around the system increases, which in turn requires more thermal energy to be absorbed, and hence a greater flow of air around the evaporator. This is exacerbated as the OAT reduces.

I do believe that some heat pumps have the capability of varying the fan speed as required by the operating conditions, but not all of them have this facility.

To try to provide some form of answer to your questions, consider that the amount of power required to operate the fan will probably be in the order of less than 200 Watts, whilst the compressor could require several kW's. Having a little more air flow than the optimal amount would appear to be more beneficial than insufficient air flow.

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