How to reduce frequency of defrosts on Daikin Altherma?

Hardware Heatpump
21

I think I understand what you are talking about now Dave.

Yes, it’s annoying with Daikin, they just keep going even though they have reached the set RWT, they always want to exceed it, and then you get what I call, the Daikin dip, whilst waiting for the RWT to reduce to what it should be.

But this is all part of the way they are controlled, they cannot control the flow rate enough for me, which is why I do what I do. It’s just all so unstable.

See post number 10, you can see the difference in mine.

22

@chrisg

All my settings

Fixed lwt - no wdc, no modulation, no room temperature control.
Pump limited to 60% (this only affects space heating, DHW is unaffected)
Quiet mode (quietest)
Radiators with dT of 10c (the lowest you can set, you can set more I think!)

That’s it I think.

23

Thanks :slight_smile:

How are you finding quiet mode - is it having an effect on defrosts?

24

I don’t know Chris, it may well do.

I only turned it on to help with DHW.

I tend not to change anything too often now.

I run off batteries and use IOG but I am limited in how much I can charge and discharge.

Not that it matters much, but keeping the electricity down helps when it is cold with very little solar generation.

25

Hi Matt,

This is the part of your argument that I don’t understand.

Can you be more verbose about which dT and which heat loss you are referring to please.

If the dT you are referring to increasing is between heatpump flow and return then wouldn’t that imply that your room is getting colder? And if that is the case how do you correct that?

26

Hi David,

My `argument’ only applies to my house.

I am sure I have always made that pretty clear.

It’s a long and boring story that I have been through many times before and it all goes back to the initial survey.

It’s quite boring now, so I will keep it brief.

We have a conservatory that shouldn’t have been in my heat loss calculation, but due to a muck up by the first surveyor, it was.

It’s unheatable, but Octopus now insist that it is as I use it as a habitable room.

Although it is only 13% of the floor area, it is probably 40% of the heat loss.

And this is the root cause of all my problems.

My heat pump at 8kW is way too big for a house with maybe only a 2kW heat loss, or just a bit over.

I haven’t analysed this in any detail and haven’t really thought too much about what goes on, but something like this.

We have big doors between the house and the conservatory, so it can be open plan, or closed, or somewhere inbetween.

If it is sunny it gets warmer, if it is cold it gets colder in the conservatory. It acts as some kind of big heatsink and has a disproprtianate affect on how much heat the house needs and how warm I want to let it get.

Now it is shut, so it gets colder but the rest of the house is Ok.

If it gets a bit warmer outside, I open the doors and then turn the heat pump up a bit.

With care, I manage to keep the house warm, but the conservatory is variable!

I’m sure you get the idea.

So, all this is for my house and my particular issues.

None of it is very satisfactory, but it’s what the `experts’ gave me.

What I do about it, I don’t know yet.

We are not that sensitive to the actual temperature of the house within a bit. We accept that if it’s cold outside the house will be a little cooler and vice versa.

As long as it is warm enough we don’t care very much what the actual temperature is, all that we don’t like is it varying too much, as long as it is sable, it’s fine with us.

I think the bottom line for me is that I never really need much more than the minimum efficient output of this heat pump, so I end up in a very narrow window of operation.

27

To try to clarify my thinking… We have control over two parameters: the LWT and DT. (And hence indirectly the RWT.) The HP has control over a couple of variables: the power P and the flow rate F.

At equilibrium (when both LWT and RWT are correct), P and F are not independent: If it doubles both P and F, then (within the heat pump) nothing changes: twice the energy is delivered to twice the volume of water, and the temperature rise is unchanged. The RWT will change, because the change in flow rate alters how much heat is given out to the house in the radiators, but that will be a very gradual change over quite a long timescale. Esp. if you have a volumiser which is already at the desired RWT. (My system is plumbed such that it flows from HP through the radiators, then the volumiser before returning to the HP. In fact, I also wonder whether there are any stratification effects within the volumiser, which is a vertically-mounted cylinder - the water could be slightly cooler at the bottom than the top.)

So what I don’t understand is what the HP can use to choose appropriate values for P and F. If it chooses too low, there’s not enough heat going into the system and RWT will fall. If it chooses too high, there’s too much going in and RWT will very slowly rise.

My system seems to struggle to switch from a high-power state to a low-powered state after a defrost cycle.

Presumably your system is permanently in a state of “RWT too high”, and so the HP is incentivised to choose the lowest P and F which are consistent with raising the incoming water up to the desired LWT ?

28

I don’t think the Daikin controls are particularly sophisticated.

Other manufacturers use degree minutes and a fixed flow rate.

Daikin control RWT.

If I understand @johncantor correctly, this is a hang over from the days when heat pumps didn’t have inverter control for the compressor.

At the start of a heating cycle, flow rate goes to maximum for about 20 minutes. This is to get the water in the system up to the requested flow temperature.

It seems to ignore the RWT at this point.

If the RWT rises too much, it will use any overshoot to keep a reasonable dT. If the overshoot set isnt enough and RWT is too high, the heat pump stops and then has another go in a few minutes.

But it starts again completely from scratch, so another full 20 minutes at full flow rate, which will fail again for obvious reasons.

After about 30 minutes the flow rate drops to minimum as it tries to lower the RWT that is now higher than what it is aiming for.

So you get a great big slug of heat followed by not very much heat (relatively) as the flow rate is reducing and the dT is small.

Then, over a period of time, the RWT reduces to where it needs to be.

After that, the heat pump tries to keep the RWT in line by varying the flow rate. My experience is that it can’t do this very well as the flow rate swings massively from high to low with big peaks and troughs in the heat output.

All of this is bad enough at a fixed lwy but once you introduce a wdc and modulation into the mix, then everything is constantly moving and it is never stable.

My RWT is nearly always too high compared to the set dT so the flow rate is always at the minimum after the initial 30 minutes of a heating cycle. I don’t think the heat pump is bothered about this, there is nothing else it can do. I suppose it could decide to raise the flow temperature, but it doesn’t, it is fixed, more or less, at what I set it at. As long as the RWT isn’t close to the LWT it doesn’t care.

I would assume the power required is just that based on how much water is flowing, the dT, the flow temperature requested and the outside temperature.

This is a Daikin trying to run at too low a flow temperature relative to the heat loss of the house and emitter capacity. It’s just too aggressive with the flow rate, too much heat.

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This is mine with a lower dT, the flow rate variations are mad in my opinion with lots of spikes in heat output. It is also moving the flow temperature with the flow rate, not one thing is steady.

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And another. the flow rate is at both extremes just to keep the set dT of 4c, it can’t do it.

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And here, a dT of 3c is great when it was cold, max flow rate and stable, just like it is when I run it at the lowest flow rate. The theme here is that a constant flow rate is stable and efficient, a wild flow rate makes a mess.

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I think perhaps the biggest issue is that I can see it.

Without monitoring I would be oblivious to it. Maybe that is the best way and possibly why Daikin don’t make this information available as standard?

1 Like
29

cheers for everyone’s input
i have now turned off modulation (from 4 )
i have changed my heat curve slightly
i have put my dT to the maximum of 12 ( my thinking that if the heat pump is working at its maximum out put anyway i might as well put this heat in to the house ( this might end up being a mistake but im giving it a go )

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1 Like
30

this change to the dT seems to have a affect on the the flow rates changed made just after 8 am

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31

It isn’t behaving like mine.

My flow rate is always on the floor.

You are still in fan coils I assume?

32

Yes still
Fan could because rads dT is not adjustable of 8

33

Radiators on mine is set at minimum 10c and you can increase, it will not go to 8c.

I would have thought a dT of 8c would be enough?

1 Like
34

I will
Leave it on fan coil until tomorrow morning then change it to rads to see if it makes a difference

35

wonder if something like this would help ?

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couldent wait i am now on radiator and a dT of 12

36

I think i am going to move to pure weather compensation without modoka influence ,
i noticed since changing my latest setting that the system is reacting to room temp , ie its trying to meet room target temp before anything else , i suspect im going to end up with settings like you eventually with LWT control , but i do want to go through the journey to get there

will alter to pure weather comp tomoz

37

I think you will need a wdc, more than I do anyway.

What I think will work is limiting your pump speed with the bigger dT.

Then you won’t get so much power when it starts up and hopefully the ice will hold off for longer.

1 Like
38

my dT setting is 12 and dont think i can get any higher and with my current flow temps its unlikely to get a higher actual dT than it currently is

39

I am not talking about dT.

I am talking about limiting the pump speed to 60%, you won’t need even that, but it is as low as you can go.

Your flow rate will be 7lpm, but you want it to get there asap.

You are currently getting 23lpm at the start.

I think this is the cause of the problem.

1 Like
40

@matt-drummer do you remember where the setting is for limiting the pump speed ??