Daikin 8kw flow T and power are hunting and will not settle down

Hi
My Daikin 8kw heatpump seems to spend a lot of time trying to find the correct flowT and power to run at and ends up cycling between 2 points for a long time .See attached.

Pump also only runs at a very low flow rate of 6.9 to 7.1 lm which it also cycles between. With emitters set to radiator or fancoil

Is this due to radiator size and how they are balanced?

Is this correct as I cannot see how to adjust to stop this happing

Many thanks

Mark


They all do this.

7lpm is the minimum flow rate.

The flow rate is set by the dT you have requested.

If the radiators cannot deliver all the heat being produced then the heat pump slows the water down effectively reducing the heat output. Once the pump is at the slowest it can go that is where it sits.

Daikins do not work like most other heat pumps.

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Thanks for info, it’s good to know.
Re the DT , the pump runs my system at between 3 to 5 DT on both fancoil set with a DT of 5 or radiators set with a DT of 10,

I assume this is also a quirk of Daikin

Regards

Mark

No, that is because of the size of your emitters.

At the flow temperature you are running, at the lowest flow rate, that is all the heat they can deliver to your property.

If your emitters were bigger and your heat loss was higher, the dT would go to what you have set, that is 5c on fan coils in your case?

Once the dT is at 5c (if that is what you have set) if it starts increasing the flow rate will increase delivering more heat.

Daikin focus on return temperature.

If you set a flow temperature of say 32c with a dT of 5c, the heat pump will try and keep the return temperature at 27c.

It will lower the flow rate until that is achieved.

In your case, the flow rate won’t go low enough to get to the requested dT all of the time.

If you want a higher flow rate then you need to set a lower dT.

But then you will find that it is not very good at holding a steady flow rate.

My experience is that the flow rate is only steady if you set the dT to an extreme which either forces it to run at minimum or maximum flow rate.

Anything in between that and it can’t do it.

Then you end up with a hunting flow rate on top of the hunting flow temperature and electrical input.

All this is inefficient and I can observe my efficiency improving as it becomes more stable.

The hunting flow temperature and electrical inpt knocks a fair bit off the COP.

My experience is that the Madoka and modulation just make it worse.

So, I don’t use them, I am not suggesting you don’t, but with all that going on nothing is fixed and looking at the data it’s just all over the place.

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With my 9kW, I don’t have detailed monitoring, but from watching mmi info and measuring pipe temperatures, return tended to like being 4 degrees below flow. It has felt a lot more stable after configuring dT to 4. It now tends to settle with a flow rate slightly above the minimum - perhaps that helps.

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Thanks for the detailed explanation, that is very helpful

Regards

Mark

I used to run my 9kW on the radiator setting with a dT set of at least 8c

It was always the most efficient way of running it. It never used to get to the dT of 8c but it kept the flow rate steady at the minimum 10lpm. That made it most efficient because it takes out one variable.

I do the same with my 8kW.

I couldn’t do it at first beacuse I couldn’t set an overshoot of more than 1c in the radiator setting. A firmware update fixed that.

Now I am on radiators at a dT of 10c.

I have tried every possible setting, ufh and fan coils at dT’s between 2c and 10c.

Low dT’s cause other problems.

So my best efficiency is radiators at a dT of 10c (that I rarely see) with the flow rate on the floor.

Just so you know, I do have detailed monitoring with heat and electricity meters.

Hi Mark,

I think there are two behaviours - a macro and a micro - a wow and a flutter - and Matt’s explanation may be more relevant to the macro level. It is the macro level behaviour that I observed in my earlier thread

I think in today’s thread you are observing the micro fluttering level, which is another fingerprint of this heat pump. Here’s my 8kW unit the last 24 hrs showing a few episodes like yours

I haven’t figured out the circumstances that trigger this fluttering behaviour, but I have an Octopus engineer coming next week (to discuss the “Daikin dip”) and I’ll add this fluttering issue to the list of things I’ll ask them about.

Hi yes I have both

It would be good to see what octopus have to day
Please keep us updated .

I am sure some of my issues is down to having to throttle back some of the rads that are oversized.

I also have 1 big rad (1.8h x 0.5 k2) that only gets worm at the top and the bottom is room temp.

At installation octopus said this was airlocked and would work its way out , which is not the case so will be calling them today as I am sure it’s a pipe issue causing a very slow flow to that rad

The Daikin dip is easy to explain.

It is caused by the aggressive nature of the start of a heating cycle.

At the start the flow rate goes to maximum and the set flow temperature is reached pretty quickly.

This gives a lot of heat and the return temperature quickly rises and eventually exceeds what is set by the requested dT.

After about 20 minutes the flow rate starts to decrease to bring the return temperature down.

Then the flow rate sits at minimum as it tries to bring the return temperature down to where it should be.

At this point the dT is small and the flow rate is at minimum giving a lower heat output than what you are going to eventually end up with.

This shows as a dip, a large amount of heat that decrease to minimum and then slowly recovers as the return temperature falls.

It’s a characteristic of the Daikin controls.

I minimise it by restricting the maximum flow rate. This limits the initial amount of heat and electricity consumption and stops the return rising more than it has to in that first 20 minutes or so.

We were all talking about radiators being too big in another topic.

With my system I have more than 300l of water so even after the first 20 minutes or so it hasn’t all been through the heat pump.

This is where monitoring comes in, just watching what happens and fine tuning to remove the `problems’

If you have radiators with trvs that are opening and closing this just adds another variable into the equation.

Everything that is constantly changing upsets these heat pumps.

My experience is the more things that are stable the better.

However, it all works and it’s pretty good at heating a house and holding a room temperature.

The only issue is that you can’t have maximum comfort and maximum efficiency. That is my experience.

So it’s all a compromise, like most things in life.

All of my radiators are warmer at the top than the bottom, if think you’ll find that is normal and more noticeable because of the lower flow temperatures.

If the compressor isn’t stopping, just varying its speed (power consumption), I don’t think there is so much to worry about.
However, the very short time-period of the oscilations shown in your screenshot suggests to me that you may have a “short circuit” somewhere. For example a radiator (maybe towel rad) close to the heat pump, which is not properly balanced and is allowing warm flow water back into the return circuit rather too easily.

Every one of these heat pumps behaves like this.

It is just a `characteristic’ of them.

Thanks all rads are balanced apart from towel rad which I will check ,

Thinking about it, could it be the pressure regulating valve as I am not convinced this was get set up at install but just adjusted to a random position.

As I have lots of rad that do not have trv’s can this be closed completely ?

There is nothing wrong with your heat pump @markr

It is perfectly normal for these.

I suspect there is a problem with control of refrigerant flow within the heat pump, but on the whole they work OK.

There is nothing you can do about it, me, and many others have tried every possible setting between us and this is just how they are.

You can shut the abv fully as long as you have enough radiators that won’t close.

Mine is doing it, I can’t stop it and I don’t know why it does it. No Madoka, no modulation, fixed lwt only but it does the same. Really annoying!

Hi Adam, if that were the case, wouldn’t the monitoring show the oscillations on the return temp rather than the flow temp?
Cheers, David.

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Just had octopus engineer round as 1 of the rads is not working ,only way to get it hot is to turn off other radiators.

He look at the abv and it’s letting 29lm of water through when fully closed on the test run.

Which must be effecting the system so will be back next week to replace

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Can you get 29lpm?

How do you know it is going through the abv?

System was put into calibration mode for abv , pump set to high and the isolation valves closed for central heating, I assume the 3way valve was set for heating .
At this stage abv should be adjusted to allow a fixed throughput as specified by Daikin.

However closing/opening the abv had no effect on flow rate , it stayed at 29lm, engineer then opened the heating valves to allow flow through rads and again flow rate did not change

He said this was quite a common issue and would be back next week to replace the valve.

At the same time it was agreed he would fit the Spirotech SpiroVent which I had just purchased which was nice of him, will have to keep him top up with coffee and biscuits

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