Hi
link below, I hope it helps
Please note I had issues with air in system on DHW so numbers for water will be incorrect for first couple of months
@markr . When my heat pump was running in the Dec - Mar period I was getting a cop of around 4, but it wasn’t delivering max heat, so at peak I was consuming about 20kwh per day and getting 80kwh of heat. However that is just less than 4kw heat output per hr, whereas I would expect between 7.5 kw per hour in the cold weather. This is where the limitation comes in. On DHW, I was getting around a cop of 2. Though for reasons I can’t explain after they came to fit an external sensor that dropped to around 1.5. No apparent reason for it , but the immediacy of the change is suggestive like.
Reduce your dT to 5c and your flow rate will increase when it gets cold enough.
The flow rate will always be at the minimum of around 7lpm when the set dT has not been achieved.
You can’t achieve a dT of 10c at the flow temperatures you are using and therefore the flow rate never increases above minimum.
This limits your heat output.
I don’t know, but I suspect your radiators are not big enough to emit the maximum heat output of the heat pump at the flow temperatures you are using.
What is your total radiator capacity at dT50c?
If you are running our system with homely Could the issue be down the homely app. Changing the weather cure /offset or any of the “eco”modes automatically in the background to improve scop and not managing the heat output on cold snaps ?
@markr . Hi Mark, that’s helpful. So what I see is that you were peaking at around 2.7kw of heat in Jan and Feb. But you had good room temps. What was your heat loss measured at?
@markr , I’m not using the homely app.
The maximum heat output of the unit is limited by the amount of heat your radiators can transfer. Evidently, your radiators are capable of transferring approximately 4.68kW with an average temperature somewhere in the region of 42.5c-37.5, depending on what your DT is actually settling on.
If you want more heat to be emitted, you have two choices:
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Raise your flow temperature, which will result in a higher DT but keep the flow at the 7L/min minimum it’s already stuck on. It’ll also reduce your efficiency.
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Install larger radiators, such that your rooms are still evenly heated but your DT is increased as more of the heat is shed. Your flow will continue to sit at the 7L/min minimum. This option involves upfront expense, but allows you to choose lower flow temperatures for efficiency.
This ‘problem’ is nothing to do with the Daikin unit, it’s to do with physics.
Hi Ben,
The flow rate varies to maintain the set dT.
If the dT set is so big that you can’t get it, then the flow rate stays locked at minimum.
Once the set dT is achieved and starts increasing the flow rate increases to bring the dT back down to the set level.
But, in this case, if the limit of the radiators has been reached nothing will give any more heat other than raising the flow temperature.
@matt-drummer , hi Matt. That’s an interesting proposition. In theory that would halve the output, which at some point might be compensated for by increased flow rate. Do you know that will work? Yes we discussed the radiators before, they are definitely big enough and I recently had Daikin and supplier looking at that and they confirmed they were ok.
I know, I’m not disagreeing with any of that. A higher flow temperature means means a wider gap between leaving and return (because the pump probably won’t speed up - see below) which means greater heat transfer.
Assuming that the DT setting on the unit is 10, and the actual DT @ 45c leaving in practice is nowhere near 10, I would expect that either solution (larger radiators or higher flow temps) would still leave the DT sub 10 and therefore keep the flow rate on the floor.
Hi Ben,
If I tried to run at 45c I would exceed the maximum output of the heat pump, ie, I couldnt do it.
My radiators will get to a dT of 10c at 40c flow temperature or less.
At that point my flow rate increases.
Hi Terry,
Once the requested dT is achieved and then exceeded the flow rate increases to bring it back into line.
The reason you never get to a dT of 10c is that your radiators are not big enough to do that with your heat loss and the flow temperatures you are using.
Where is your data so we can see it?
Sure, in your case that would happen, but Terry definitely isn’t in this scenario - as you’ve said in your next post!
I think we’re operating in total agreement, even if we’re saying it in different ways 
Yes Ben,
But Terry needs to understand how it works.
The flow rate is only at minimum up to the point the requested dT is not achieved, once it is, the flow rate increases.
This is how these Daikin heat pumps work.
The dT controls the flow rate, set it bigger than you can achieve and the heat pump sits at minimum flow rate, set it too small and the heat pump sits at maximum flow rate, anything in between, the flow rate varies to maintain the set dT.
The flow temperature controls the heat output.
The bigger the emitters, the lower the flow temperature you need to hit the maximum heat output of the heat pump.
If the emitters are not big enough to output 8kW at the maximum flow temperature achievable then that is your limit.
If, as in my case, the radiators are `too’ big, then you limit the maximum flow temperature you can run.
If Terry tells us what his total radiator capacity is then we will know.
If Terry shows us his data then that will help.
Octopus calculate heat loss at just under 6kw,
British Gas was 5.4kw
I calculated it to be 5kw using heatpunk.
If I look at Heat demand explorer in heatpumpmonit it shows around 3.6kw
Hi Ben,
Yes, this is Terrys issue.
I am sure that he thinks that if he could increase the flow rate he would get more heat.
As you know, he won’t!
All that will happen is the dT between flow and return temperatures will reduce.
The heat output will remain the same.
This is because the radiators are at their limit at the chosen flow temperature.
As we have both said.
@matt-drummer , Matt what do you need beside the radiator sizing. I’ll pull it together and post it tomorrow.
Hi Terry,
Just total radiator output will do, but if you are not sure just put up the Octopus radiator schedule.
Do you have data to look at?
The total at 50c is 25.7kw
