Thanks @jamespa for showing the variability. Unfortunately for me taking a gas meter reading involves a trip outside the house! I did gather data over 14 days and found that I used 382.96 kWh to heat my house to a comfortable temperature over 109.5 degree days. So that's 3.50 kWh per degree day.
I'm not clear though on how to express that, or extrapolate, this heat power usage into the typical worst-case energy needs that a heat loss survey would use. Can you or anyone help me? I think in my area they use about -2 degrees C as the low temp range.
OK so assuming a base temperature of 15.5, -2 will be 17.5 degree days. So your heat loss at -2 is roughly 3.5*17.5/24=2.6kW which is consistent with what @old-scientist concluded a few posts back. Based on 22hrs heating you need ~3kW
If you can find a ~4 kW heat pump that is genuinely a 4kW heat pump, not a software downrated 5-6kW one, that could be a good choice otherwise you are probably looking at a 5kW heat pump and a bit of batch heating in the shoulder season. The extra capacity will help with DHW but you need to be sure you have enough system capacity (ie water in the system) adding a 2 port volumiser (not a 3-4 port buffer) if necessary. Do check that is not a downrated pump of higher capacity. The 5kW Vaillant for example is 5kW but the 3kW Vaillant is the same hardware software limited. I think that all the Daikin R32 models 8kW or less are the same, not sure about their R290s. Dont know about other makes off hand. Physical dimensions, compressor type (if specified) and refrigerant change are the give-aways. If they are all the same for 'larger' and 'smaller' models then they are the same machine.
Is 3kW sensible given construction and floor area (if you provide some details Im sure various will comment). If so then its probably as good a value as any.
Don't allow anyone who wants to fit a 4 port buffer, low loss header or plate heat exchanger between pump and emitters anywhere near.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Wow, thanks for that @jamespa! I actually did 15.0 degrees for the degree day output, so can adjust slightly.
I have a feeling that a calculated heat loss (with the various estimated parts of it like air exchanges) will produce a higher number, but I think in no case should I accept a unit over 5 kW. I have some interest in Ebac heat pumps and got them to give me some info on how low it will scale. It was more info than I expected, and I have nothing to compare it to... but maybe you can give your thoughts. Page 6 of the attached file!
... the property was built in aprox 2020, about 170 sq M, ground floor plus two above, the top floor being essentially a pre-converted loft room, but the ceiling is still reasonably high. Doors and windows are UPVC. Walls are exterior brick and not quite sure what is in between inner and outer walls. It does hold heat in pretty well. Even though it was new, we retrofitted UFH on the ground floor. Radiators on 1st and 2nd floor are type 22 (I think). I have been able to run the gas boiler at about 35 degrees flow temp and it is warm enough.
This post was modified 4 weeks ago 3 times by sussex-greenish
Wow, thanks for that @jamespa! I actually did 15.0 degrees for the degree day output, so can adjust slightly.
I have a feeling that a calculated heat loss (with the various estimated parts of it like air exchanges) will produce a higher number, but I think in no case should I accept a unit over 5 kW. I have some interest in Ebac heat pumps and got them to give me some info on how low it will scale. It was more info than I expected, and I have nothing to compare it to... but maybe you can give your thoughts. Page 6 of the attached file!
The heading on the table is presumably meant to be kW not kWh!
- the 9kW one will be cycling almost continuously with a min output of 2.7kW at 30C Personally I would avoid.
The 5kW (6kW if you operate at 35C, if the numbers take into account defrost) one could still cycle at 7C (o/p1.96kW at 30C), but definitely better and possibly you wont do much better than this - here is the Mitsubishi R290 6kW data (the 5kW is same at bottom end) - remarkably similar!
You probably need to make sure you have a reasonable system volume to ensure slow rather than rapid cycling, may be worth adding a volumizer depending on the system volume calculations. Earlier today I reviewed a post with a 14kW heat pump in a 7kW house. The striking thing is that on times are so short even close to the onset of defrost cycling that it never really gets going. Im thinking that the system volume is too low in this one as well as the pump being oversized, probably something to be avoided.
@jamespa Thanks again for the great comparison. Yes, I certainly was not considering the 9kW one! And I did notice they had the units wrong on there (kWh instead of kW). I also meant to say I have type 21 radiators but I suppose the info without knowing the sizes or the rooms they are in doesn't mean much anyway.
One thing I don't quite understand ... running open-loop (no buffer tank) how does one know if I have sufficient water volume (system capacity)? Pure estimation? Drain the system and see how much water comes out?
One thing I don't quite understand ... running open-loop (no buffer tank) how does one know if I have sufficient water volume (system capacity)? Pure estimation? Drain the system and see how much water comes out?
Radiator manufacturers publish the volume of water in their radiators. For any given height and type its proportional to length so they often just give a figure per m. If you are talking standard rads it wont vary much from manufacturer to manufacturer so the figures from Stelrad or Myson for example are good. You can estimate pipe lengths from your house size (you dont need to know the exact figure or routing and from that come up with a system volume. Obviously draining the system is another approach!
Given your numbers I would say you want to accommodate at least 1kW of over capacity in the mid range, ideally 2kW. Assuming a flow temperature hysteresis of 5C (which is a reasonable value), and say you want a minimum of 20mins run time, that means you need ~ (20*60*2000)/(4200*5) l of water in the system = 115l. if you have the space you could just put in a 100l volumiser!
If you need extra system volume use a 2 port volumiser not a 4 port buffer tank. 4 port buffer tanks, low loss headers and plate heat exchangers, all of which separate the heat pump from the emitters, are bad news. Lots of installers put them in sadly, dont allow it. 4 ports bad, 2 ports OK.
Here are the figures for the Vaillant 5kW. Possibly very slightly worse than EBAC or Mitsi. Its not always easy to find figures for minimum output sadly.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
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