When there was around 0C I’ve had around COP 2. I was in an impression that when it gets warmer I will get much better cop because more heat is extracted from air. To my surprise today was 15C outside and I started DHW heading from 42C to 50C. I was watching and COP was barely 2.5. Pretty poor! Chart is attached. But what bugs me more is that I went out to have a look on a pump and the fan was barely spinning! Power consumption was increasing with flow temperature raising but the fan was still spinning at almost minimum RPM. No change over a course of heating. I can’t get why simply the FAN is not spinning much faster to get more heat from air at the lower compressor speed. Maybe I’m missing something but does this sounds legit to you ? Or some issue with the pump… thx
That’s a really low hysteresis which will negatively impact COP. If you recharge from 30°C, things will look really different. 2.5 for a recharge from 42 to 50 doesn’t sound too far off.
Hmm, yes but 30C water could be too cold… 30C is in the middle of tank then at top there is how much ? 35C or 37C ?
Nevertheless, it’s normal that the fan running at such low speed ? I thought that it will blow like when there is 0C outside but no, it was spinning very slowly. I’ve heard compressor more than the fan…
No idea about the fan, honestly - I haven’t monitored my fan speeds vs. outdoor temperature yet.
Regarding the temperature of the tank - this really depends a lot on the sensor location, type of tank etc. Here is an example graph from my 300l tank. I typically charge to 47°C and reheat once a day at the warmest time. As you can see on the graph, the temperature at the top is typically quite warmer than at the sensor location in the center. I suggest to play with the recharging hysteresis to remain within your comfort zone while still trying to increase it a bit. Work your way up gradually, see whether the water is ever too cold for you and if so change it back to your last comfortable setting.
Thanks. But somehow I don’t understand that graph. Why on 22 Dec the top is around 47C and bottom at 15C ? I Also have 300L tan, well 280L net volume and I have a probe in the center. I heat to 50C so I guess the temperature at top is probably 55C.
Nevertheless, would be good if you can check the outside FAN RMP in warmer weather, I was surprised by the fact it was barely moving. Pump consumption was above 3KW. Flow rate around 19l/min. dT was around 5 - 6 C so I guess the pump tried to maintain this but really don’t know why it didn’t increase the FAN speed. Usually when there is above 5C outside i have my flow temp set to 40C but and here the FAN moves really slowly but also the pump is running at 800W.
So that why I don’t understand why when heating water and flow temp raising to 60C slowly the FAN is barely spinning. Compressor consumption is increasing of course. Unfortunately I don’t have sensor for outside FAN speed (I don’t know the modbus register for that) so I can only go out and check.
I quicky used up a lot of hot water (visiting family over Christmas). The center sensor only saw new cold water while at the top I had plenty of hot water left. On other days with lower (slower) water usage, top and center are closer together. That’s why I said it’s highly individual and really dependent on your circumstances - there is no optimum that works for everyone. Find your own minimum recharge temperature comfort zone and stick with that.
Reheat from empty cylinder / lowest temperatures (high hysteresis) – no quick top-ups
Choose lowest target hot water temperature you can get away with
Use Eco mode (or some other low compressor / quiet mode)
Schedule reheats when highest outside temperature
Lots more details in my article here:
Looking at your graphs you’re only starting to heat (flow temp) from 40C through to almost 60C.
So the efficiency will always be poor doing that.
All the good COP in a hot water run comes early when the flow temp is at its coldest.
So the emptier the starting point, the colder the water, the colder the starting flow temp, the higher the efficiency.
Our outdoor unit sits below our bedroom window and I’ve noticed that the fan speed seems to operate directly in proportion to how frosted up the evaporator is. Whether its space heating or DHW, it doesn’t matter. It seems to operate as quietly as it can by moving just the air it needs.
I too have noticed the differences in DHW tank temperatures, and wonder about ways of breaking up the thermoclime (layers) - anyone got any ideas? I do not know relative height of the internal thermometer, the cold water infeed, or the hot water take-off points, but when the system thinks the tank has water at 35 and the hot water at tap is roasty toasty hot, I am puzzled.
Anyone any bright ideas?
There was some discussion of this on Daikon Altherma 3, sub 2 DHW COP.
In post #9 I put forward the (pragmatic) view that heating DHW in two or more short bursts may help do this (if you can predict when you want hot water, and are able to programme DHW heating accordingly).
If you don’t want to get bogged down in the theory, you could always give it a try…
Fit a “Domestic hot water (DHW) circulation pump” between the cold water inlet and hot water outlet of the tank. But remember it will reduce COP as the heatpump will never have a cold layer in bottom of tank.
I can’t work out if the speed is directly linked to the outdoor temperature or if how hard the compressor is working or something else is used as a factor in deciding the speed.
My Ecodan appears to get noisy at a very set temperature and below but I don’t know if this is just because the compressor would be working hardest below this.
It might be one of these things that vary between manufacturers.
I don’t know but defo related to outside temp. If there is cold fan is blowing but if there is above 5 or 10 fan barely moves no matter if heatspacing is active or DHW. Compressor running 3kW but it still barely moving. Can’t get it.