I've now had my Daikin Altherma 11kw monobloc installed for nearly two years, about a year ago this forum, especially Derek, helped to correct some of the performance issues we had. With a year of decent running the SCOP for 2022 was a slightly disappointing 2.78, so what's going on?
I also took note of the comments from Graham Hendra in the article What are the risks of oversizing a heat pump? where he says heat pumps work best in the 25%-75% range of their full output. For my 11kw heat pump this would be 2.75kw to 8.25kw output range.
With the meters fitted I have taken daily readings (I don't have access to the high resolution data) and over the last month or so I've been able to accurately record the heat produced by the ASHP to keep the house warm. (And it has stayed warm thanks to the help from @derek-m and others last year). With this data I've been able to calculate the heat demand to see what the actual requirement is to keep my house warm using the HTC method described here to calculate my houses heat loss to compare against the installers 7.9kw. The results were surprising.
A maximum heat demand on the coldest day in December when the average daily temperature here was -2.5C was just 4.5kw, with the average for December being 2.4kw.
I'm running a relatively low weather compensation curve. 40C@-5C outside, 25C@20C outside, and the house has been a steady 20.5C all the time.
If the heat pump was achieving the manufacturers SCOP of 3.98, I worked out for the annual heat used by my property last year (9300kwh) I would have saved around £350 (using the standard rate 0.34p/kwh). That would be a 30% saving on running costs.
So, is my oversized ASHP the route cause of my less than great SCOP? Or is it something else like the internal primary flow pipework to the radiators, much of which is 22m plastic (reused from the previous system)?
I've now had my Daikin Altherma 11kw monobloc installed for nearly two years, about a year ago this forum, especially Derek, helped to correct some of the performance issues we had. With a year of decent running the SCOP for 2022 was a slightly disappointing 2.78, so what's going on?
I also took note of the comments from Graham Hendra in the article What are the risks of oversizing a heat pump? where he says heat pumps work best in the 25%-75% range of their full output. For my 11kw heat pump this would be 2.75kw to 8.25kw output range.
With the meters fitted I have taken daily readings (I don't have access to the high resolution data) and over the last month or so I've been able to accurately record the heat produced by the ASHP to keep the house warm. (And it has stayed warm thanks to the help from @derek-m and others last year). With this data I've been able to calculate the heat demand to see what the actual requirement is to keep my house warm using the HTC method described here to calculate my houses heat loss to compare against the installers 7.9kw. The results were surprising.
A maximum heat demand on the coldest day in December when the average daily temperature here was -2.5C was just 4.5kw, with the average for December being 2.4kw.
I'm running a relatively low weather compensation curve. 40C@-5C outside, 25C@20C outside, and the house has been a steady 20.5C all the time.
If the heat pump was achieving the manufacturers SCOP of 3.98, I worked out for the annual heat used by my property last year (9300kwh) I would have saved around £350 (using the standard rate 0.34p/kwh). That would be a 30% saving on running costs.
So, is my oversized ASHP the route cause of my less than great SCOP? Or is it something else like the internal primary flow pipework to the radiators, much of which is 22m plastic (reused from the previous system)?
It is not an easy question to answer. Dependent upon manufacturers detailed data, I would normally suggest that the maximum output rating of a ASHP should be between 1.25 and 1.5 times greater than the calculated heat loss for the property. This of course is very much dependent upon an accurate heat loss calculation in the first place. The major problem with ASHP's is that they don't have the excess heating capacity often available with gas or oil boilers.
What is the range of COP that you achieved under varying weather conditions? Are you experiencing much, if any, cycling on your heat pump and what is the ambient temperature range over which it may occur?
COP varied from 2.54 to 3.5 over the period assessed, average daily temp -2.8C to 11.5C, with compressor starts per hour of running 0.1 - 5.6.
I managed to take daily readings most days. Here's the table of data I collected:
Looking at the results of your data gathering, I also suspect that your heat pump is oversized, (or could it be that your heat loss is undersized 🙄 ).
Over the past couple of years, as more and more actual real World data has been made available, I have been refining my ideas of how best to operate ASHP's, particularly during milder weather conditions.
If you are maintaining a constant 20.5C throughout varying outside air temperatures, then it would appear that your weather compensation curve is fairly well optimised to the heat loss of your home. Whilst this would normally be good news, in your case it would appear to be causing more frequent cycling than desired, due in part to the fact that your heat pump may be oversized.
To help reduce the cycling and hopefully improve the overall efficiency, I suggest that you try the following:-
1) Set any room thermostats to a setpoint of 21C.
2) Increase the LWT at the warm end of the weather compensation curve from 25C to 30C.
The above should hopefully have the following effect. As the outside air temperature increases, the indoor temperature will also increase slightly, until it reaches 21C, where the thermostat should stop the heat pump. Because the heat pump has been stopped by the thermostat, it will not restart until the thermostat resets at probably 20C, which should hopefully reduce the cycling rate and improve the overall efficiency.
Please give this method a try and report back the results.
@derek-m I don’t have any thermostats other than the Daikin unit that allows modulating leaving water temperature, this can vary the LWT by a couple of degrees if required. I’ll push the warm end of the weather compensation curve up to 30C and report back. I wonder if @heacol has any other thoughts?
If your heat loss really is really 5 kW @-2.5 then I think your 11kW ASHP will have to cycle at 5-6 degrees ambient. Have a look at the attached.
Having said that, based on my experience cycling isn't as bad as some say it is for efficiency, although 4-5 starts per hour is a bit high (btw I wasn't sure how the ASHP could run for >24 hrs in a period?). Your COP figures don't look bad at all. Do you have the official Daikin figures at different LWTs and ambients? I don't think they will be that far off.
BTW I reduced cycling by doing what @Derek-m suggests you do.
@derek-m I don’t have any thermostats other than the Daikin unit that allows modulating leaving water temperature, this can vary the LWT by a couple of degrees if required. I’ll push the warm end of the weather compensation curve up to 30C and report back. I wonder if @heacol has any other thoughts?
Since you don't have any thermostats, have a look at the Daikin manual and see if you can widen the On - Off temperature range of the LWT. If the calculated LWT is say 35C, the controller may be set to stop the compressor if the actual LWT reaches say 38C and switch the compressor back on when the LWT falls to say 32C. This deadband is often adjustable.
2) Increase the LWT at the warm end of the weather compensation curve from 25C to 30C.
The above should hopefully have the following effect. As the outside air temperature increases, the indoor temperature will also increase slightly, until it reaches 21C, where the thermostat should stop the heat pump. Because the heat pump has been stopped by the thermostat, it will not restart until the thermostat resets at probably 20C, which should hopefully reduce the cycling rate and improve the overall efficiency.
I've done similar with my Grant Aerona 10kW. Room t/stat set to 24degC. Warm end of WC is 32degC at 20degC.
My main thermostat is a Hive, which I've discovered is useless as an ASHP master thermostat as it has a ridiculously small fixed hysteresis of 0.1degC. It now calls for heat all the time as it was cycling the compressor and circulating pump multiple times an hour. The Grant will modulate down to about 25% power consumption, 0.8kW or so, the maximum power when charging HW is around 3.2kW. 25% modulation equates to a heat output about 2.5kW.
My installer wanted to fit a 13kW Grant, based on their 10kW estimated heating load at -3.4degC using 'Heat Engineer' software. That assumed all rooms heated to 21-22degC with no diversity. I prepared my own heat loss calcs, which totalled 7.7kW based on more realistic requirements of 18-19degC and about 80% of the house volume heated at any one time. I used a high infiltration rate, as we have a 5kW log burner which is lit most evenings in winter, and draws combustion air through the fabric. I probably over-estimated that a bit. I find 3-5kW seems to keep the house cosy at 18-19degC. At 7degC ambient my calculated heat loss is 4.3kW.
Because the Grant 13kW is not connect and notify, we hit a snag. The DNO wanted £800 to upgrade the house fuse from 60A to 80A. They were told to 'do one'. In hindsight, I'm glad I opted for the 10kW unit, using my calculations not the installers. It had adequate capacity during the two week December cold snap, but the frequency of defrosting was an issue on a couple of days with freezing fog. That would be so for any unit.
The turn down on the 13kW Aerona would be similar at 25% or 3.25kW, but it would cycle more often due to being a larger capacity unit. 'Right sizing' an ASHP seems important.
If excessive stop/start cycling due to the ASHP being oversized is an issue, could installing a large capacity well insulated buffer tank could be an option, space permitting, to increase system volume? It would slow the system response pre-heat time down considerably however.
The system is a Daikin Altherma. Heating schedule is set in the main controller, I have 20.5C through 6am-10pm then set back to 19C over night. Heating usually only kicks in over night on the really cold evenings. The system has a Daikin wired remote thermostat that reports back the temperature to the main contoller so that the LWT is modulated. You can’t control anything through this remote thermostat, it just displays the local house temperature.
The radiators all have TRV’s except the bathroom. They are all set to max, except the one in our bedroom which is permanently shut off.
I don’t have access to the high resolution data from the meters unfortunately, that’s reserved for the project and they won’t share it. But I can read instantaneous readings. With the lower LWT I had deltaT would hover around 2-3C. At higher temperatures (above ~35C) it’ll be closer to 5C. I’ve changed the curve this afternoon based on the tips above and the house is too warm. It’s currently running with a LWT of 35C at 10C outside, but I’ll give it a day to settle down after the changes.
The deltaT often closes up with warmer ambient temperatures, flow rate does vary from 22l/m down to 10l/m
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