Altherma 3 Modulation

I’ve got a 6kW Daikin Altherma 3, and like many installations, I believe this is significantly oversized. A worst case estimate is a heat loss of around 3.5kW per hour at -2, and of course at average temperatures this drops to about 1-1.5kW. I do not have monitoring although a deployment of ESPAltherma should give me some more granular data. My question is does anyone have any actual proven knowledge about how much these can modulate down to and the effect on cycling below that? For example if it could modulate down to 2kW does this mean that if the emitters can give off 2kW and the house heat loss is 2kW that it won’t cycle. I have searched far an wide for answers and so far have got numbers ranging from 25% to 60% of maximum capacity at most!

Welcome, Chris, to the OEM forum.

Do you really mean this? kW is already a rate at which energy is moved around. If you mean kWh/h, I take it to mean the average over an hour, this reduces to kW anyway - but in this context it can make sense, more often as kWh/day.

I wasn’t entirely sure which unit to use actually, I was trying to get at the fact that it would require let’s say a 3.5kW heater running constantly to match the heat loss of the house at -2. I know that over a day if you multiplied that by 24 it would be 84kWh of heat required as you say. So if I understand you correctly then yes I just meant 3.5kW!

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I have this problem with my Daikin 9kW Altherma 3

In theory it produces 4.6kw of heat at 35c flow, 7c outside at a COP of 5

But my house does not need 4.6kW of heat and my radiators can’t deliver it at that flow rate in any case (at the moment)

My heat loss is around 5kW at 21c/-2c

So my heat pump produces less heat than it could when it is 7c outside, I have to run at a higher flow temperature and the COP suffers as a result.

The heat pump consumes the same amount of electricity as once it is at its minimum there’s nowhere else to go.

I am part way through radiator upgrades, my COP is better and the flow temperature can come down a bit but now the house just gets hotter quicker.

I find I have to let the house get hotter than we want or the heat pump runs in shorter cycles less efficiently.

Somebody else may be able to explain what happens in more detail and at a more technical level than me.

When my radiator upgrades are complete I will have to find a better way to operate but for now I just want see what I can get out of it in terms of COP and work my way down.

I may end up with a smaller heat pump.

My radiators are big enough for my house as they heat it to 22c at a flow temperature of 30c, it’s just the COP is poor doing this as my radiators aren’t big enough for the minimum heat output of the heat pump at lower flow temperatures.

My heat pump is massively too big in my opinion due to serious errors in the heat loss calculation and the tendancy for installers to over size, a double whammy.

I was told by a Daikin engineer that it was a good idea to have installed a larger heat pump than I needed as I might want an extension to my house in the future!!!

Yes, precisely what I’m finding albeit with under floor heating. It’s all very well saying to run it low and slow (which I do completely agree with given a perfect install), but when oversized it results in completely unacceptable sizing! Is your system on OEM that is viewable? Whilst mine isn’t, it would be interesting to see the cycle pattern of someone’s who’s is and compare. It seems to me that Daikin units can only modulate down to an absolute minimum of 45% of their rated output so in the case of both our systems, there already running at the minimum at -2/-3 let alone when it’s 7 degrees outside

Hi Chris,

Mine is the Octopus install in Ipswich.

My solution at the moment is to let the house heat up a lot!

I want to find out what I can get out of the heat pump and longer runs are best but that results in too much heat.

We end the day at 25c, not really ideal.

We have nowhere for the heat to go and shorter cycles are not very efficient, I don’t think we use any more or less electricity whatever we do.

And that is the problem, a heat pump that puts out 4.6kW at the right COP when you don’t need it and only 7.5kW when the defrosts really kick in.

I have asked before, what sort of home has a heating requirement that ranges between 4.6kW and 7.5kW?

Can you cycle the whole heating with the room thermostat? I mean run the heating until the house is warm enough, then keep it off until it’s cooled down a bit. So cycling to maintain house temperature, instead of cycling to maintain flow temperature. This should, I think, result in better COP, as well as reduced consumption.


Hi Tim,

Yes I can keep the house temperature lower with the Madoka room thermostat and I intend to do that in due course.

The last month or so has been an experiment, I want to coax the best COP out of it I can just to prove it can do it.

Then I will turn it down a bit.

I’m not worried about the extra electricity consumption right now, its just part of the capital/setting up cost.

I don’t think the COP will improve, the COP improves the longer it runs but maybe a different approach will work better.

I will wait until I have completed my radiator upgrades and the possible addition of my office to the system and then see where I can get to.

I have spent quite a lot of time on this over the last few weeks and see little value in messing with it much more until I have the `final’ solution.

I think there is a decent compromise in there somewhere with my massive radiators, it will never be the best but a middling ok is achievable.

A heat pump that is massively too big is always going to be too big at best.

My understanding is that the 9kW, 11kw, 14kW and 16kW are the same heat pump with the same compressor and circulation pump and the only real difference is the software and fan speed.

It is also my understanding that they all run at the same minimum level of electrical input so the 16kW version has a really good range of operation if you have a home that can use it and that gets worse as you go `smaller’

Somebody will correct me if I am wrong but that is what I have understood.

Those of us with more modest abodes have the runt of the litter, they are all the same but just turned down to varying degrees at the top end.

I guess the same applies to the 4/6/8kW models?

When I first started thinking about getting a heat pump I read so much about running low and slow and not being able to chuck in heat like a gas boiler on demand. My heat pump feels no different to my gas boiler, it comes on and the heat builds really quickly and I really don’t feel the difference. I think this must be a big clue to the over sizing. It really hasn’t been anything like I was expecting.

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Ahh that’s really interesting, and crazy that if you have the bottom of the models in the range you are at such a disadvantage. The Altherma 3 is for an annexe and we have a Kensa 13kW for the main house which is absolutely atrocious at cycling as it has no modulation at all, the main house heat loss is around 8kW but when the ground is warm it’s pumping out 20kW whether you like it or not! But can’t do anything about that one so was more focused on the ASHP. Just had a look at your graphs and noticed that it seems to be running in 2 big blocks each day with minimal cycling but with quite high flow temperatures… is this a happy medium you’ve tried to find to stop of from going on some endless cycling loop all day?

Hi Chris,

With the help of some posters here we have found a place where the heat pump operates well, a sort of equilibrium.

My radiators can’t cope with the heat output of my heat pump at lower flow temperatures so this is where it will run for the longest and most efficient, the COP improves as the run progresses…

I can run at 30c flow but the COP is worse and it cycles more because my radiators cannot deliver the heat produced.

I am in the process of changing all my radiators, I did upstairs on Saturday and it has helped.

The problem I will run into when the radiators are all changed is that even at lower flow temperatures that my radiators can now handle the heat produced will be more than my house needs so I either have to let the internal temperature rise or get the heat pump to cycle more often.

I will have to either find a balance between internal temperature, COP and electricity consumption that is acceptable and best or get my installer to put the correct size heat pump in.

The big blocks are a clue, my heat loss is quite low so the temperature builds in the morning, goes off on the overshoot and then comes back on in the afternoon and evening at a higher room temperature and overshoots again, goes through the night with nothing and then starts again.

At the moment I am experimenting, I just want the heat pump to run for as long as possible so I can see what it can do. When all my radiators are changed I will focus on making it as efficient as I can in terms of comfort and electricity use, the COP will be out of my hands with this heat pump.

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Interesting discussion. I went through on the same process in the last (my first) heating season. I have started an 11kW “noname” heat pump, and spent two months to try to set up all the PID values on the compressor and the circulating pump, to reduce the cycling frequency. In my case, I have a couple of fan coils, which made the process even more difficult. Finally I replaced the HP for an Altherma 3, 9kW unit. I have to admit the 7kW would be more adequate, so the HP is still oversized. Here is what I found during setting it up:
-flow rate needs to be reduced, to do not exceed 25l/min. (pump speed + valve)
-I’m using UF heating as emitter, to keep DT as low as possible.
-I set the overshoot to low(ish) 2 degrees
-took me quite long time to get the weather dependence right.
-my room thermostat is only work as a “timer” and a manual on/off switch. I cannot run the HP all day on our climate.