Is my Daikin Alther...
 
Notifications
Clear all

Is my Daikin Altherma 11kW heat pump oversized?

11 Posts
5 Users
1 Reactions
2,063 Views
(@hydros)
Estimable Member Member
326 kWhs
Joined: 2 years ago
Posts: 59
Topic starter  

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?

The setup is pretty simple, an ASHP monobloc, through a backup heater (that isn't switched on), to the diverter value, past a auto bypass value (that is fully closed), round the radiators, through a small 18ltr volumizer and finally back to the heat pump. The system has meters fitted for the electrification of heat demonstration project so accurate performance can be monitored. Its very similar to the Setup 3 in Brendan's article on this site, How to correctly install heat pumps so that they work properly and efficiently - Renewable Heating Hub.

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. 

Heat Demand Screenshot

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)?


   
Quote
(@derek-m)
Illustrious Member Moderator
13722 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4165
 

Posted by: @hydros

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?

The setup is pretty simple, an ASHP monobloc, through a backup heater (that isn't switched on), to the diverter value, past a auto bypass value (that is fully closed), round the radiators, through a small 18ltr volumizer and finally back to the heat pump. The system has meters fitted for the electrification of heat demonstration project so accurate performance can be monitored. Its very similar to the Setup 3 in Brendan's article on this site, How to correctly install heat pumps so that they work properly and efficiently - Renewable Heating Hub.

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. 

Heat Demand Screenshot

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?

 

This post was modified 1 year ago by Derek M

   
ReplyQuote
(@hydros)
Estimable Member Member
326 kWhs
Joined: 2 years ago
Posts: 59
Topic starter  

@derek-m 

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:

Heat Demand Screenshot 2

   
ReplyQuote



(@derek-m)
Illustrious Member Moderator
13722 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4165
 

Posted by: @hydros

@derek-m 

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:

Heat Demand Screenshot 2

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.

 


   
ReplyQuote
(@hydros)
Estimable Member Member
326 kWhs
Joined: 2 years ago
Posts: 59
Topic starter  

@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?


   
ReplyQuote
(@kev-m)
Famed Member Moderator
5561 kWhs
Joined: 3 years ago
Posts: 1299
 

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.


   
HydroS reacted
ReplyQuote
(@derek-m)
Illustrious Member Moderator
13722 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4165
 

Posted by: @hydros

@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.

 


   
ReplyQuote
(@allyfish)
Noble Member Contributor
3119 kWhs
Joined: 2 years ago
Posts: 379
 

Posted by: @derek-m

Posted by: @hydros

@derek-m 

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.

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.

 


   
ReplyQuote
(@heacol)
Prominent Member Contributor
1884 kWhs
Expert
Joined: 3 years ago
Posts: 328
 

@hydros How is the system being controlled? You mentioned a set temperature, where is that being set?

What control do you have on the radiators and in what rooms?

Do you know the difference in flow and return temperatures and the flow rate, you should be able to get them from the heat meter?

Does the delta T remain constant, and does the flow rate vary with the ambient temperature and heat output?

Professional heat pump installer: Technical Director Ultimate Renewables Director at Heacol Ltd


   
ReplyQuote



(@hydros)
Estimable Member Member
326 kWhs
Joined: 2 years ago
Posts: 59
Topic starter  

@heacol 

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


   
ReplyQuote
(@hydros)
Estimable Member Member
326 kWhs
Joined: 2 years ago
Posts: 59
Topic starter  

@kev-m 

Thanks for the form. It does seem to tie in with what I'm experiencing. Cycling starts to increase at 4-5C outside temperature.

The official Daikin figures for the Altherma 11kw Monobloc EDLQ011CV3

Condition 1: heating Ta DB/WB 7°C/6°C - LWC 35°C (DT = 5°C) | SCOP 3.98

Condition 2: heating Ta DB/WB 7°C/6°C - LWC 45°C ( DT = 5°C ) | SCOP 3.09

The last 24hour average temperature has been around 6-7C, LWT around the 35-38C mark. DT of 5C. 24hr COP 3.1. Compressor starts 2.0 per hour

For info. Comparing the last 24 hours since I've raised my weather dependent curve to a comparable day in November:

29/11/22 5.5C ave. outside, 18.2kwh used, 56.5kwh heat produced, COP 3.11. Compressor running 12 hours, 2.0 starts per hour

08/01/11 6C ave. outside, 18.4kwh used, 57.4kwh heat produced, COP 3.12. Compressor running 12 hours, 2.0 starts per hour


   
ReplyQuote



Share:

Join Us!

Latest Posts

Heat Pump Humour

x  Powerful Protection for WordPress, from Shield Security
This Site Is Protected By
Shield Security