Puzzling Heat Pump Efficiency Data (Daikin Altherma 3 HHT)

[dMb (davidMbrooke)]

Hi Chris,

@ChrisG-S

As you can see, the electricity input value based on the Daikin sensor is sometimes lower than the value based on the dedicated consumer unit readings. But it is also sometimes higher. I thought this would be impossible! Everything is fed from the dedicated consumer unit.

The implication is that one (or both) of the electricity metering devices is inaccurate - because as you say, the heat pump “Sensor” value should never be smaller than the “Dedicated Consumer unit” value (which is measuring the supply to the heat pump plus some other devices).

To be confident of the readings they need to be coming from a meter which is certified to an accuracy standard - typically the Measuring Instruments Directive regulations.

What sort of metering do you have on the dedicated consumer unit?

@ChrisG-S

After a lot of discussion and looking at my own data, it may be that it comes down to minimum heat-pump output and control behaviour, not thermodynamics.

There are always limitations with a heat pump’s variable compressor speed control. It’s therefore almost universal to have some degree of ‘cycling’ when only a modest amount of heating is required. (It’s not that long since heat pumps only had fixed-speed compressors and had no option but to cycle, and most models cope with cycling fairly well.)

For the EPRA18DAV3, Daikin’s technical details summary shows:

• Minimum heating output: 4.4 kW
• Nominal heating output: 9.0 kW
• Maximum heating output: 12.2 kW

@ChrisG-S

The problem gets worse as LWT reduces because it gets to its minimum power sooner.

Is that right? Thinking out loud: At a lower LWT, the radiators will lose less heat to the rooms (at a given room temperature) so ΔT = LWT - RWT will indeed be smaller and less heat will be transferred into the house, which increases the tendency to hit the lower limit of heat output. Running at a higher LWT will result in more heat transfer to the house (because LWT - Room Temperature is greater), so it will be easier to stay above the minimum heat output and avoid cycling.

It all depends how much heat the house actually needs though - and how good the radiators are at transferring that heat at a lower LWT. If you’re dumping 4.4 kW to the house (at LWT = 55) but it only wants 3 kW it will just ‘cycle’ on the room thermostat instead.

A lower LWT is certainly always better for (instantaneous) CoP.

So perhaps your Daikin unit is cycling more at the lower LWT (and cycling is reducing efficiency) - but why is that reduced efficiency not showing in the heat pump’s own CoP readings for LWT = 50 - and does that mean you can’t trust the Daikin’s reporting of electricity consumption?

David