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New ASHP system - choices

 
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Air Source Heat Pumps (ASHPs)
Last Post by Derek M 2 years ago
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 Derek M
(@derek-m)
Illustrious Member Member
Joined: 5 years ago
Posts: 4425
26/04/2024 9:55 pm  

Posted by: @harriup
↑

All heat pumps will hit the point where their minimum output exceeds the heat being taken out by the heating circuit, not oversizing too much means that happens at a higher outside temperature. How the pumps handle the lower demand is really hard to pin down. I know my system when using the weather compensation curve worked just at its minimum power but cycled up to 6 times per hour. Switching to the room adaptive mode slowed the cycle to once per hour or so but with a more complex power consumption as it it often puts a burst of energy in before a pause. But overall this a more efficient mode - better COP. Looking at some similar systems on heatpumpmonitor, though, displays such dissimilar working patterns that it is hard to differentiate what elements are created by the way the house heating system is responding or is programmed.

 

 

In an ideal world a heat pump unit would contain two compressors, a nominal 12kW capacity heat pump having one compressor rated at 8kW and one compressor rated at 4kW. The 8kW compressor providing heating during most of the Winter period, with the 8kW plus 4kW meeting demand during particularly cold spells.

As milder weather conditions are encountered the system would changeover from the 8kW compressor to the 4kW one, thereby allowing a much lower thermal energy output without the possibility of cycling, and probably providing an increase in overall efficiency.

Unfortunately we don't yet live in an ideal world.

I believe that most, if not all, heat pumps control the stopping and starting of the compressor based upon the temperature of the LWT and RWT, such that when either of these temperatures reaches a defined limit, the compressor is started or stopped. This of course is more likely to happen during milder weather conditions.

The 'on' period and the 'off' period is therefore dependent upon the limits set within the controller and the actual temperature measurements produced by the LWT and RWT temperature sensors. One of the major determining factors affecting the frequency of cycling could be the location of the LWT and RWT sensors within the system. If the sensors are located within the outdoor unit they are likely to cool at a more rapid rate than if they were located within the home's thermal envelope, and therefore increase the cycling frequency.

 



   
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