We’ve just had an 11kW Altherma installed by Octopus and after setting up monitoring, a Shelly ProEM in the distribution box by the heat pump and a P1P2MQTT module, I was surprised to see the unit running with a power draw as low as 720W. At other times it does pull the more normal 850W - 920W.
We do have the pump limited to 60% power, maybe it is related to that. We are trying to tame the unit as I think we have won the prize for the most oversized unit yet…
Octopus heat loss calcs were 8.4kW @ -3C
A steady state 12 hour overnight run @ 0.6C (DeltaT 21C) retained a steady state using an average 2.9kW of heat generated.
I have the same unit and its absolute max output over the last two years when at zero c outdoors has been 8.4kw… lower than in the Daikin tech specs due to rapid defrosts. So yours would be perfectly sized IF the heat loss calcs were correct. Therein lies the problem. Check the ventilation loss calcs first as they are not really calculations but straight guesses of air change rates. Most houses have much lower air change rates and bathrooms and kitchens especially do not have the extra ventilation from extract fans on 24hrs a day.
Incidentally my units min power draw when heating is closer to 900w.
I think the unit is great, solid, well put together and barely audible when it is dialled fully back, as is the case for 90% of the time. It won’t be a disaster if Octopus stand by their figures but will just mean we will be looking at an average COP below 3 which would be a shame.
The house is a 25 year old barn conversion, thick stone walls and poor wooden double glazing, but with a good layer of insulation under the floor, on the internal walls and in the loft. Gas usage for 2024 was 11,000kWh for heating, HW and gas hob which would also indicate a much lower than 8.5kW heat loss but in retrospect I think the Octopus heat calcs just applied the standard losses for what appears to be an old building with solid walls and leaky windows.
As for the lower than normal power draw, I’m scratching my head on that one, the Shelly is right next to the heat pump with a CT on the incoming live to the distribution box and the second CT on the live feed to the heat pump. Both report the same value within a couple of watts.
If Octopus heatlose is overstated a little, then the flow temperature at -3 can be lower then 50c, hence giving higher COP then their calculations claims.
Your input power draw matches what I see on my unit and I’m running my circ pump at 70%. It alone accounts for 150w at that speed, rising to 200w or so at 100%.
I’m another with an 11kW beast that’s oversized for the property (~5kW max). It’s an older generation so the lowest modulation input is around 1100w. But behaviour is similar.
Data here.
I found I had to raise the warm end of the weather dependant settings to 32C at 10C outside to get a decent run time otherwise the heat pump would cycle on/off too frequently as it couldn’t loose heat quick enough. I now end up with longer off periods as the house has warmed up/over heated.
I have a 9kW Daikin Altherma installed and it normally sits at 900 - 1000w when stable. This seems to prove that Daikin HP’s are all the same but adjusted to the kW stated by the company.
I don’t know about a secret, but lots of trial and error.
Trying to get it to heat as gently as possible within the range it can operate at is the trick. It really only settles down for me when the outside air temp (OAT) is less then 5C. When it’s warmer the house over heats.
Things I’ve done.
Bigger radiators, & balancing the radiators.
Set heat pump target deltaT to 8C, the only way to control flow on Daikin’s. I’ve tried both ends of the dT spectrum, on this unit being well oversized for me, I have found a wider dT to be better, as have others.
Not set the warmer limit on the weather curve to low. When I had the curve at 25C at 15C outside, the heat pump couldn’t emit enough heat at its lowest limit to keep running for any decent length of operation so cop takes a bit of a hit, but more annoyingly the pump ramps up to full blast (how all Daikins start a heating cycle), overshoots target, then shuts down. I have had to settle with more a boiler type operation to manage the heating.
As you pointed out, on the plus side it can heat 300l of DHW nice a quickly.
To get a decent COP from the 9-16kW Daikins you need to be able to dissipate ~4kW at the chosen flow temperature. Assuming radiators, you need a lot of area to dissipate the heat below ~35C. I found that 34-35C is the practical lower limit to get a COP above 3.
I’ve never understood the “won’t do below 30c” thing I’ve read from various members. My EDLA11 seems quite happy to run at its lowest output. Feb 26th is a good example of a cloudy but mild day (mild is 6c upwards):
I get about half an hour compressor on, half a hour compressor off. There is no full blast initial heating, it just tops up the water temp. Yes, it overshoots with ease, but I think that’s more to do with my heat pump being about 2x oversized.
If I tried (and I did) to run with a higher flow temp at the upper end of the curve, we’d either overheat the house or the heat pump would be turned off completely. I’m not sure which strategy is best for effiency; cycling on flow temp or cycling on room temp.
I wonder why I don’t seem to have the same issues with sub 30c flow temps as others do.
This is exactly what people are saying when they say these heat pumps “won’t do below 30C”.
The grey line on your graph is the target flow temperature, the flow temp is consistently 3 or 4C above the target. Your system isn’t running with a flow temperature below 30C. Instead the flow and return temperatures increase until the flow exceeds the target temperature + overshoot value, at which point the heat pump shuts off.
I’d agree that a single on/off count per hour is fine, your system isn’t short cycling…but it is cycling and it definitely isn’t running in a steady state with flow at or below 30C. If you look at the most efficient running Daikin Althermas on heatpumpmonitor.org, none of them are attempting to run at flow temps below 30C, so the empirical evidence suggests that these systems run more efficiencly at the lowest temperature that the system can maintain without cycling (the “low and slow” approach always mentioned for heat pumps).
But running most efficiently (highest COP) shouldn’t really be the goal, surely it is staying comfortably warm in your home / paying as little as possible / minimizing energy impact on the environment (delete and prioritise as appropriate for yourself). So if the way your system is running meets these criteria for your home, then that’s absolutely fine.
Yes, I see what you mean now. I assume, at least in my installation, the lower capacity 8kw unit with its lower minimum output, would still overshoot in the same conditions but not as frequently?
I am toying with the idea of installing a thermostat with a narrower hysteresis. Ideally, I want to remove the dependency on Home Assistant and the Cloud and have a normal, self sufficient, locally managed heating system. I’d still use weather compensation but for days like we’ve had recently with lots of solar gain, I’d rather it just turned off instead of letting the house overheat like the Madoka does.
Negotiations with Octopus are still ongoing. I think getting blood out of a stone would be easier.
Other heatpumps will measure how many degree/minutes they are over target flow temperature, then remain off until same amount under target. Hence their average heat input is the same as if they had kept to target flow temperature.
