@terry1812 The MMI and Homely are connected but the data comes from the heat pump cabinet via a D.comm unit attached to P1 and P2 connections in the MMI. The D.comm is the communications channel like modbus. Regards, Toodles.
Toodles, he heats his home with cold draughts and cooks his food with magnets.
@terry1812 I think that on the Daikin MMI, setting the emitter type to radiators allows for a DT of 5 C; the emitter type set to UFH allows for a DT of 10 C. Regards, Toodles.
Toodles, he heats his home with cold draughts and cooks his food with magnets.
@jamespa interesting. Yes I am, and I have been since 22nd October only using weather comp. The 16th Jan was when BG made the changes to remove the hive and extra pump and convert the buffer to volumiser, plus add the madoka, which replaced the Hive. So I am using full weather comp. I have only 1 zone and all TVR’s are set to max. If you are correct about the LWT and the system ignoring RWT , then the system would be happy as soon as it got the right leaving water temp, and what does it do then?
Maintain the LWT by adjusting its heat output (if the required power is within its modulation capabilities)
Yesterday, I wanted to check a couple of things, I wanted to make sure the pump was capable of higher flow rates , I wanted to see how long it took to reach the LWT and how long it took to get to a 7L/m flow rate. I turned the system off, that is to say I changed tge setting for heat from on the off at 11am. At 9pm I turned it on again. It took 24 minutes to get to its target LWT and at 29 minutes it got its flow rate to 7L/m. Now it’s happy and now it’s only variable ( unless outside temp changes) is RWT, within the range of 10 or less. Only if the RWT goes outside of 10 does the flow rate increase from 7L/m.
so we come to the crux of the issue. If outside temp is -2 and say the house is at zero. The heat pump will give max power, no question, but it will only do this until such time as the RWT is above 10. By my calculations this happens at around indoor temp of 13c, given outdoor temp remains at -2. It happens because the returning water temp has now risen sufficiently to deliver a heating delta t of 10, at which point the flow rate drops to 7/l m and , if the formula is correct the heat output at that time drops to 4.88kw.
This is surely OK. Its adjusting its heat output according to the load, higher output while its heating up then lower output while its cruising along. My Vaillant does exactly the same with its heat output, it just doesn't bother changing the water pump speed.
If the temp in the house then drops below 13 and the Delta t rises then the flow rate will increase again until it hits delta 10.
im not sure what you are saying here. Is it the temperature in the house or the LWT that it actually responds to. Its should be the LWT, and the heat pump should be maintaining it at or around its target value according to the WC curve. If its not maintaining the target LWT value then something is wrong, or you have a thermostat somewhere (possibly in the Daikin controller) that is overriding the WC. Can you clarify what exactly happens please?
I understand that there are 3 settings possible with the Daikin, Radiator, fancoil and UFH . The delta t for radiators is 10, whilst that for fancoil and I think UFH is 5. I understand from various forums that some people with radiators use the fancoi setting. I haven’t modelled what that would do, but of course it would keep max output for longer. I’ll do that modelling and see what that does. Clearly once you get there though, the same rule applies and the max output drops more.
I would not run with a detlaT of 10, it will add about 8% to your heating bills (relative to DT5) because it reduces the average radiator temp for any given flow temp, so you will need to increase the flow temp to get the required output from your radiators. If you can change this parameter, I would recommend you do so.
So going back to yesterday, I can see , or think I can, that the Daikin has a priority 1 to get LWT to target but priority 2 is to get delta T to max if 10 ( in my case, because I’m on the radiator setting). If the Daikin doesn’t care about delta t, then once it was at LWT why wouldn’t it just keep pumping away until delta t was zero?
Because it cant, for deltaT to be zero (or more accurately near zero) the water pump would have to be working extremely quickly in order that the heat emitted by the radiator (which is determined by the difference between the average radiator temperature and the room temperature) cause only a small drop in water temperature. You would soon reach the point where the water pump hits its limits.
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.
@toodles Thanks. That shows pretty much the same picture, with the heat pump going off and on, but maintaining a pretty steady temperature. What is your heat loss figure and associated temp range?
@terry1812 According to the survey Octopus Energy carried out, 6.5 kWh. The range of LWT is 30 - 50 C, It was set to 35 - 45 C until recently but I found the extreme cold left us with a rather cool living room for a few hours in the morning; I had the lower temperature adjusted down to allow the heat pump to ‘find its’ own level’ so to speak during mild weather. I felt that Homely should be able to make the decisions more intelligently with less restrictions. Certainly in the cold weather, the comfort level was being maintained 24/7; I have yet to see if the lower end benefits from the wider range as the change was only made recently. Regards, Toodles.
Also, if Octopus installed yours, and assuming you are an octopus energy customer, you can get the very affordable maintenance package they are offering, is that correct?
If the temp in the house then drops below 13 and the Delta t rises then the flow rate will increase again until it hits delta 10.
im not sure what you are saying here. Is it the temperature in the house or the LWT that it actually responds to. Its should be the LWT, and the heat pump should be maintaining it at or around its target value according to the WC curve. If its not maintaining the target LWT value then something is wrong, or you have a thermostat somewhere (possibly in the Daikin controller) that is overriding the WC. Can you clarify what exactly happens please?
@jamespa , I’m having to think about this very carefully, so I need to tackle it in chunks. So this one first. I’m assuming there is a steady state until something happens , if the temperature dips then, other things being equal, the RWT will start to decrease and so the Delta T will increase . If as I assumed the delta T was already at 10, to give the max output, then an increase in the Delta T to say 11, would cause the flow rate to increase to bring it down again. It has no other option at that point, I think, because it is already maxed out on flow temp. So the system is simply responding to the change in the Delta T, which has moved over target. I was sure I had also seen somewhere but can’t find it at present that the UFH was set to a delta T of 5. Also the only meaning of the Fixed in the radiator row, is that 10 is a system target. Because it seems to happily moved around below 10 but not over. Other than of course in a system start up scenario.
If the temp in the house then drops below 13 and the Delta t rises then the flow rate will increase again until it hits delta 10.
im not sure what you are saying here. Is it the temperature in the house or the LWT that it actually responds to. Its should be the LWT, and the heat pump should be maintaining it at or around its target value according to the WC curve. If its not maintaining the target LWT value then something is wrong, or you have a thermostat somewhere (possibly in the Daikin controller) that is overriding the WC. Can you clarify what exactly happens please?
@jamespa , I’m having to think about this very carefully, so I need to tackle it in chunks. So this one first. I’m assuming there is a steady state until something happens , if the temperature dips then, other things being equal, the RWT will start to decrease and so the Delta T will increase . If as I assumed the delta T was already at 10, to give the max output, then an increase in the Delta T to say 11, would cause the flow rate to increase to bring it down again. It has no other option at that point, I think, because it is already maxed out on flow temp. So the system is simply responding to the change in the Delta T, which has moved over target. I was sure I had also seen somewhere but can’t find it at present that the UFH was set to a delta T of 5. Also the only meaning of the Fixed in the radiator row, is that 10 is a system target. Because it seems to happily moved around below 10 but not over. Other than of course in a system start up scenario.
Sorry I'm still not sure what you are actually observing (what do you see as opposed to what do you think might be the explanation)
The heat pump can change two things namely
1 the amount of energy it puts in. This will affect principally the flow temperature and the energy emitted by the radiators. It will have only a small effect in deltaT
2. the water pump speed. This will affect delta T. It will have only a small effect on the flow temperature and energy emitted by the radiators.
Neither control loop will be perfect, so there will be deviations from the targets. But what you should observe is that it tries to reach the targets and most of the time does so.
Is this what you observe and if not how and when does it deviate?
The first is the important one, the second very definitely secondary (as I have mentioned many, possibly most, heat pumps don't bother with the second control loop, they just leave the pump operating at the flow rate needed when output is at max. Unless pipe noise or erosion is a problem, the second control loop adds little or no value and in fact has some disadvantages.)
Anyway, assuming your water pump can cope I would operate at dt5 not dt10, which should mean you can drop the wc curve 2-3C saving 6-8% in energy consumption.
This post was modified 2 weeks ago 4 times by JamesPa
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.
@jamespa I think I have explained what I have observed. The heat pump hits the target LWT first, then it hits the RWT and finally it hits the flow rate. It does modulate. I can see that the refrigerant temp changes quite a lot, the leaving water temperature also fluctuates, the most fixed parameter is the flow rate, which averages 7, moving as it does between 6.9 and 7.1.
on the issue of delta t , I can’t see any benefit of changing to the 5. Firstly it tends to sit between 6 and 4 anyway, and I’m assuming therefore that I already have that efficiency gain but 5 gives a lower energy output which would seem to offset the gain anyway?
@jamespa I think I have explained what I have observed. The heat pump hits the target LWT first, then it hits the RWT and finally it hits the flow rate. It does modulate. I can see that the refrigerant temp changes quite a lot, the leaving water temperature also fluctuates, the most fixed parameter is the flow rate, which averages 7, moving as it does between 6.9 and 7.1.
OK that now makes sense, basically both control loops are operating as expected.
on the issue of delta t , I can’t see any benefit of changing to the 5. Firstly it tends to sit between 6 and 4 anyway, and I’m assuming therefore that I already have that efficiency gain but 5 gives a lower energy output which would seem to offset the gain anyway?
If you changed the target deltaT to 5 I presume that the heat pump would compensate by increasing the speed of the water pump thus maintaining the energy output (ie the second control loop would do its job). Obviously it might not do that but the manual and your observations suggest it would.
The efficiency improvement comes about because, for any given LWT, the average radiator temperature is lower if the deltaT is higher (eg LWT40, DT 5, average rad temp 37.5; LWT40, DT10, average rad temp 35.). So a higher DT means you have to set the LWT higher to get the required output from the radiators. For each degree the LWT is higher, you lose 2-3% in efficiency. Pretty much the whole reason we run ASHPs at a low DT is to improve efficiency. The 'cost' of this design decision is we need wider pipes and stronger water pumps, which is why pipework upgrades are sometimes required when moving from a boiler to an ASHP. Obviously its your choice what you do.
This post was modified 2 weeks ago 2 times by JamesPa
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.
Thinking about installing a heat pump but unsure where to start? Already have one but it’s not performing as expected? Or are you locked in a frustrating dispute with an installer or manufacturer? We’re here to help.
