@terry1812, welcome to the forums and it sounds like you’ve been on quite a journey with your Daikin system. It’s frustrating to hear about the lack of support from both the installer and manufacturer, especially when you’ve invested so much time and effort into getting things right.

It’s good to hear that the physical installation by BG seemed solid, with experienced installers and a tidy job. However, the handover and ongoing support clearly fell short, which is a common theme with many heat pump installations. The issues with the Hive controller, buffer tank and conflicting pumps are all too familiar, and it’s great that you managed to diagnose and demonstrate the problem yourself. It’s a shame it took escalating to the CEO to get any meaningful action, but at least the system is now running better after the changes.

The ongoing issues with the Daikin ONECTA app are disappointing, especially given how crucial remote monitoring and control are for heat pump systems. It’s worth persisting with Daikin support to get this resolved, as it’s a key part of managing your system efficiently. You’re not the first homeowner that’s had these issues.

Regarding maintenance contracts, it’s concerning that BG have backtracked on their initial promise. For your warranty to remain valid, you’ll need to find a qualified heat pump engineer to carry out regular servicing. Many independent heat pump specialists offer maintenance packages, so it’s worth reaching out to a few local companies to compare options.

On the topic of weather compensation and the 4.88 kW max output, it’s possible that the system is limiting itself to maintain efficiency and avoid short-cycling. Weather compensation works best when the system can run for longer periods at lower outputs, but if your heat loss calculations or system design don’t align with this, it can lead to the issues you’ve described. It’s worth asking Daikin or a heat pump specialist to review your system settings and ensure everything is optimised for your home’s needs.

It’s clear that you’ve put a lot of thought and effort into understanding your system, and while it’s been a steep learning curve, you’re now in a much better position to manage it. Hopefully, with the right maintenance and a bit more fine-tuning, you’ll be able to get the most out of your heat pump.

OK I found the original post about 4.88kW and I also agree with @editor above.

Firstly It wont help to operate with the thermostat as opposed to on WC, if the flow rate is not sufficient its not sufficient!

Your system should be able to deliver the full output of the heat pump and 7l/min is too low a flow to do this.  The relevant deltaT in this case is between flow and return, normally designed to be 5C at the lowest OAT.  So 7l/min is 7*4200/60*5 = 2.5kW.  However some heat pumps modulate down the flow rate to keep deltaT constant.  Yours may well be doing this and if it is then 7l/min might be the value at a higher OAT.  Others operate all the time at a fixed rate sufficient to deliver the maximum power, which means that deltaT reduces as the OAT rises (and hence flow temperature and heat demand drops).  I dont know which category Dailkin is in. 

Have you actually witnessed the flow rate when its cold outside and the heat pump is operating at the maximum flow temperature on the WC curve?  If so what is it and what is the deltaT (flow to return) in this case.

It is possible that the water pump needs adjusting.  This should have been checked when the buffer was removed, because it would have changed the conditions under which the water pump is operating.

If you could post some more data including

• Flow rate
• Flow temp
• Return Temp
• OAT

(ideally when its cold or when you are actually experiencing a problem, as opposed to anticipating one)

it should be possible to work out whats going on and point you in the right direction.

@jamespa , Hi , thanks for your response and Mars’ too. So I got into this just really trying to ensure what was what in trying to understand what amount of heat energy was being delivered at various temperatures etc and how this worked with the radiators. So for heat energy the formula is the one you used just now. So in weather dependent mode the system, as I understand it, is only really concerned with 2 things, the Target LWT , the Delta T . In the Daikin system the upper tolerance is set at 10c delta T , for radiators. So if the delta T is too wide it’s going to up the flow rate to bring delta t down to 10 or less . When delta t is at 10:or less and it’s usually less, the flow rate always drops to ~7L/m. If I watch it it actually flickers between 6.9 and 7.1.

mostly it has been fairly accurate in hitting the target LWT, based off the WDC. Although not so much this evening. So having done those two things the system is content, it’s not interested in how much heat it’s putting out nor is it concerned with temperature. Now I call that steady state, ie it’s hitting its target LWT and it has delta t at or below 10. But in steady state the flow rate is always the 7L/m, in weather dependent mode. Now the 4.88kw was me being generous using the max of delta 10. Mostly the delta t seems to fluctuate between 6 and 4, so quite a lot less output, as the only variable in weather dependent mode seems to be the delta t. Is OAT, outside temperature? 

answering your specific questions, yes in weather dependent mode the flow rate is always 7. I run the system 24/7, except today for a little experiment because the weather was good for it.  So running constantly the system is always in steady state. The delta t, as I say does fluctuate quite a lot but usually between as I said above 6-4. I think the water pump is working fine. Flow temp obviously varies with the outside temp, being on the wdc. Usually it tracks it pretty closely, as in what target I’d expect to see from the WDC is usually what the target is. And clearly the delta t is a function of RWT so it varies a little relative to the LWT, within that 6-4 range. 

so the system does seem to be designed this way, or maybe not. I know because he said, that @toddles uses homely and that apparently dynamically changes his flow rate. Which would enable him to skirt the issue. But others may not. As I said, it seems to me that when we choose weather dependent we are setting it to try and deliver a temp we are comfortable with, but the system doesn’t know that, it only knows that we want , for the sake of argument a 45c LWT, and it knows it has to deliver a delta t of 10 or less. So given that once delta t is at that target level the flow rate will have dropped to its minimum of 7L/m. It’s in the algorithm, I assume, after all why would it go higher if it’s meeting all its targets. 

So there is an anomaly or not depending on your point of view because , because the weather dependent mode isn’t looking at temp, and the maths show that the max at 45c flow temp , 7L/m flow rate and a delta t of 10 is 4.88kw and as you point out and I know is correct, it’s usually lower. So at -2c, I would not be getting the system design output of 7.3kw ( my heat loss at -2 targeting 21c). Unless there is something else going on I Don’t understand. Mostly the outside temperature has been not much below 5, when it has been lower, I have noticed the dip in the temperature in the house. I am usually looking for around 18c downstairs, which is my comfort zone  and at 5c and above I usually get it, but the upstairs bedrooms are quite a lot lower. 

Does that help at all?

Posted by: @jamespa

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Firstly It wont help to operate with the thermostat as opposed to on WC, if the flow rate is not sufficient its not sufficient!

I forget to address this, because I think this is critical. The Daikin can increase its flow rate, but it needs to be called on to do so to meet something it isn’t meeting. Clearly in weather dependent mode it’s meeting everything. I was thinking that perhaps in thermostat mode it’s going to be calling for more heat , well heat anyway, so presumably the Daikin knows it needs to put more heat out and I assume it would aim for its maximum output. And it can only deliver that if it ups its flow rate, and I’m guessing that as it doesn’t seem logical that it will do it in weather dependent mode it ought to be doing it in thermostat mode otherwise it will never deliver its supposed max output. 

I don’t have any reporting add ons, but it would be interesting to see, if those that do report this sort of info and whether or not if you can find one on WDC and one on thermostat and see what they show for flow rate at low outside temps, if all that data is available. Good thought, I’ll have to see what I can find on that.

Posted by: @terry1812

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Posted by: @jamespa

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Firstly It wont help to operate with the thermostat as opposed to on WC, if the flow rate is not sufficient its not sufficient!

I forget to address this, because I think this is critical. The Daikin can increase its flow rate, but it needs to be called on to do so to meet something it isn’t meeting. Clearly in weather dependent mode it’s meeting everything. I was thinking that perhaps in thermostat mode it’s going to be calling for more heat , well heat anyway, so presumably the Daikin knows it needs to put more heat out and I assume it would aim for its maximum output. And it can only deliver that if it ups its flow rate, and I’m guessing that as it doesn’t seem logical that it will do it in weather dependent mode it ought to be doing it in thermostat mode otherwise it will never deliver its supposed max output. 

Im sorry but I think you are getting confused.  The statement above is either not correct or there is something wrong with your system that needs to be fixed.  The Daikin will certainly be capable of operating at full output in weather compensation mode without any external stimulus.  However it will only do so if it needs to do so, and even at a low OAT it may well be that it doesn't need to do so because the energy being supplied to the house is enough to meet the demand.  You dont need Homeley or any other external controller to make it do this, but you may need some tweaks to the system if it hasn't been set up correctly

Posted by: @terry1812

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So in weather dependent mode the system, as I understand it, is only really concerned with 2 things, the Target LWT , the Delta T . In the Daikin system the upper tolerance is set at 10c delta T , for radiators.

In principle/primarily its only concerned with one thing, the target LWT (which is dependent on the OAT). The basic feedback loop which governs the system is that the heat pump will try to meet the target LWT by by adjusting the amount of energy it puts into the leaving water.  If it does this successfully then the LWT will cruise along nicely at the target value, and the RWT will be whatever it is determined by your radiators.  To first order, your heat pump doesn't care about this. 

The heat pump can fail to do this for two principal reasons:

• The demand from the house (ie heat lost by the radiators) is lower than its minimum steady state output.  In this case the LWT will overshoot and the system will shut down for a while.  It will then start up again.  This process will repeat and you have cycling.
• The demand from the house (ie heat lost by the radiators) is higher than its maximum steady state output.  In this case the LWT will never reach the target.  The system will keep trying and will continue to work continuously with the LWRT below target

There are various other scenarios but these are the major ones.  In your context a third may come into play:

• The demand from the house (ie heat lost by the radiators) is higher than the minimum steady state output of the heat pump but restrictions in the water system mean that the water does not flow fast enough to conduct the heat from the heat pump to the radiators.  In this case the LWT will overshoot, the system will shut down for a while.  It will then start up again.  This process will repeat and you have cycling.  The difference between this case and the one above is that in this case the house will not reach temperature because insufficient energy gets to the radiators.

The above is a description of how the fundamental control loop works.  Your heat pump may in addition measure the LWT and turn the water pump up or down to maintain a particular delta T (if its maintianing a deltaT of 10 thats too high for efficient operation).   However this is very much secondary and also unnecessary, many heat pumps dont bother.

You fundamentally need to be aiming to operate on weather compensation, anything else will cost you dearly. Im still not clear whether you are doing this and whether you are getting warm.  So can you clarify

with all TRVs up to max, any thermostats ditto does the house reach the desired temperature?  If it doesnt, or it overshoots, in what circumstances?  What is the difference between LWT and RWT according to your heat pump

@terry1812 Before I had Homely smart control, I had been tweaking the Daikin WC curve but was battling against wide temperature (OAT that is) variations; nevertheless, the extremities of the range were the only difficulty I experienced with the ideal settings. For most of the curve range, I found that WC managed to achieve our required comfort level within a degree C or less. This was a temporary situation as we were just waiting on the delivery of the Homely controller in the late Autumn.

So in summary, the Daikin MMI settings for WC were very consistent apart from the very cold or mild settings probably due to my own incompetence rather than the inability of the equipment to do the right thing at any given OAT.

With Homely control, I find that the greatest variation from required comfort level is ~0.6 degrees C upward. I am attaching a plot from Homely Dashboard for the last 24 hours showing IAT, OAT, flow temperature, flow rate and estimated kW output. You will note the wide variation in flow rate throughout the period. Regards, Toodles.

@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? 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. 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. 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. 

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?

@toodles , Hi, that’s a very interesting chart. So your ( very high) temperature is consistent within a relatively narrow band. Your flow rate is all over the place and your pump is forever stopping. You are using temperature control presumably, as it looks as if it’s switching the heating off as soon as a certain temperature is achieved and then switching it back on when its drops below something. Mine just runs steadily all the time, which I think is what they are designed to do. My variability comes when delta t drops and output drops, and vice versa. What sort of cop are you getting with that arrangement?

@terry1812 The Homely Dashboard reports 4.57 for that period. (Pinches of salt are supplied as required 😉) Regards, Toodles.

Further to the above, yes, we like a comfort level well above that of many! 22.4 is where our baseline is (We actually like 22.5 but allow a little for Homely’s habit of always being a little above to ensure one is never cold I think!). The starting and stopping seems to have happened since the very mild weather started recently We are all radiator throughout and when Homely was set up, the requirement is that WC is turned off to allow Homely to take over fully. Regards, Toodles.

@toodles well that’s a very respectable cop. Does it get its info from the MMI.

@jamespa . James the spreadsheet may explain what I think is happening. I have this as an outstanding query with Daikin, but I haven’t had any response, I will try them again. In the Daikin installation manual, it says the min flow rate for DHW is 25l/m, so when the water comes on in half an hour I’ll have a look. It also says min flow is 6, but I’ve never seen it below the 6.9-7.1 on mine. In the manual it also says “ Emitter type influences the maximum modulation of the desired LWT. It is important to set Emitter type correctly and in accordance with your system layout. The target delta T for the main zone depends on it.” 

The manual doesn’t say what this modulation range is. 

In the spreadsheet I made assumptions about how long it might take to get to delta t. Based on my test yesterday it may be that the delta T would be achieved more quickly and therefore the max available heat would drop more quickly. Thoughts?