ASHP sizing - value of Heat Transfer Coefficient

jamesw · 2024-05-13T16:24:57Z

I am in the early stages of planning an ASHP installation, and am arming myself with knowledge and numbers around appropriate sizing of the heat pump. The point I'd like help with is whether there is any value to me in working out my house's heat transfer coefficient, in my particular circumstances that allow me actually to measure my peak heating load. I have read the excellent article at thiswebsite on the various ways of sizing a heat pump. My heating system has been operating on a weather-compensated basis very successfully for many years using a Vaillant gas boiler and Vaillant controls. It is already well-insulated, at least for a Victorian house. What this effectively means is that I am operating the heating system exactly as it will be operated when I have a heat pump, which will probably be a Vaillant Arotherm Plus since my controls are already compatible with that. For instance, in colder weather my heat source is on continuously (with gratifyingly low flow temperatures) during the day - no cycling at all. Consequently I am able to see, from my hourly gas usage data, what the energy input to my boiler is (in the form of the energy content of the gas it is consuming). With a small adjustment for the efficiency of my boiler, I can therefore deduce the energy output, and hence my heating load. I did this over winter on the coldest days, so I have a very good idea what my steady-state heat load is in cold conditions. I think this may be the most relevant possible information for sizing my heat pump, since it should be sized to deliver this load. There is no point in having a bigger pump (unless I want to make allowance for even colder days or a warmer house), and a smaller pump will be inadequate. Is there something I am missing, or does it sound sensible that my measurements are the best information I can obtain for sizing? Would there be anything to gain from calculating the heat transfer coefficient? I can't see that there would (although of course it might be of general interest, and could serve as a double check - although I think a superfluous one).

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Air Source Heat Pumps (ASHPs)

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Toodles

(@toodles)

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16/05/2024 9:11 am

@jancold The PCM is heated up by a 3 kW element in the version I have (there are others where a high temperature heat pump can do the same thing instead), the unit itself is very well insulated and loses no more than 0.75 kWh of energy in 24 hours. When hot water is required, it passes through a length of pipework inside the PCM material and is heated to approx. 50- 55 degrees C - then to your tap / shower or whatever. (Rather like a Combi Gas Boiler might supply the hot water) I think if my memory serves that the PCM is heated up to 65 - 70 degrees C. and this is why an HT heat pump is required as an LT can’t raise the temperature to that required. We have a Sunamp supplied regulator on the output so that the water is supplied at 45 degrees C. for safety and comfort.

The energy can be stored in the PCM for days and the unit we have takes approx. 11 kWh of storage; sometimes, we don’t recharge for several days if it is dull (little solar PV) and the Agile price is too high for my Scrooge nature to allow it to run. The control wiring for the Sunamp is connected to a Myenergi Eddi and that indicates ‘Max Temp Reached’ when fully heated and then the control system monitors the temperature of the unit. The system may be a little ‘granular’ though as the LED status indicator shows ‘Full’ then a sort of ‘More than Half full’ , then ‘Half Full’ and then ‘Less than Half’ and finally ‘Empty’ there is also a ‘Mains Connected’ LED that indicates charging or at a standby status. This granularity worries some people a little I gather but I am happy with this as we rarely need more than 25% of the stored energy in a day - and the unit is only a quarter the volume of a conventional DHW tank and does not feature all that pipework that tanks seem to bristle with!

The system is also very unlikely to suffer from Legionella problems as water is not stored in any quantities at dangerous temperatures. As you might have detected, I rather like the system! Having very little space for a conventional tank went a very long way towards our decision to have a box in the airing cupboard and still have some room for the linen! Regards, Toodles.

Toodles, he heats his home with cold draughts and cooks his food with magnets.

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bontwoody

(@bontwoody)

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16/05/2024 10:55 am

@jancold To add to what @toodles has said, you can imagine the phase change material as something like candle wax, which will melt before the maximum temperature of the store is reached. A property called the latent heat of fusion means that a lot more energy is needed to raise the temperature between solid and liquid and so this is stored in the liquid material.

I would imagine that the material closest to the coil would solidify initially as it heats the water but might melt again as heat is transferred from the outer volume.

Thermino's are a good idea and small but also a bit pricy and as toodles says need to be heated higher than a cylinder would.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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Toodles

(@toodles)

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16/05/2024 11:05 am

@bontwoody I might add that the control wiring indicates to Eddi that the temperature required by the PCM closest to the element has reached temperature - at which point, the system stops calling for power. The Eddi is set to (I think) 20 minutes of rest and then it will try again to see if the ‘local to the element’ material has dissipated the energy into the mass of the unit and cooled down sufficient to request more energy to heat the element further. I have observed this happening and noticed the Eddi apply more power after this ‘dissipation’ cycle. Regards, Toodles.

Toodles, he heats his home with cold draughts and cooks his food with magnets.

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