Again, thank you for another thorough post, @derek-m - will revert once I've digested / understood and prepared a cohesive, relevant response!

Posted by: @derek-m

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I believe that the FTC controller can only vary the water pump speed if the actual water pump is capable of having its speed varied. An easy way to check is change the speed setting from 1 to 5 and listen for a change in pump speed.

If the flow rate is say 10 lpm and the DT is 5C, then a certain quantity of thermal energy will be transported. If the flow rate is now increased to 20 lpm then to transport the same quantity of thermal energy the DT must reduce to 2.5C.

If you can vary the water pump speed between 1 and 5 by means of the FTC, then it may be possible to identify the setting that produces a DT of approximately 5C when the heating demand is at a maximum.

With regard to the required flow rate through UFH as against radiators, I suspect it will depend upon the heating capacity of the UFH and radiators. If you pass 3kW of thermal energy into both radiators and UFH, and both are capable of emitting 3kW of thermal energy, then the required flow rate and subsequent DT should be approximately the same.

@rhh2348

Yes, reducing the flow rate should cause the DT to increase. What actually happens in practice is that say the overall flow rate is 10 lpm, and is supplying 10 radiators of equal size and each have a balanced flow of 1 lpm. If the volume of water in each radiator is 7 litres, then it will take approximately 7 minutes for the content of each radiator to be replaced with a fresh supply.

During its transition through each radiator thermal energy will be absorbed from the water by the radiator body, which in turn will emit this thermal energy to heat the air in each room. If the water is entering the radiators at 40C, during the 7 minutes transition it will start to cool as it loses thermal energy, so may exit the radiator at 35C.

If the flow rate is increased, the water passes through each radiator in a shorter period of time, so has less time to cool. Reducing the flow rate gives the water longer to cool, so the return water will be cooler, producing a larger DT.

The same quantity of thermal energy will be being transferred from the heat pump to the heat emitters, just the quantity of water being used is varying.

Coming late into this. I like the flow explanation re heat emitters. Presumably it's similar at the ashp heat exchanger ie. at lower flow rates the water in the ashp heat exchanger will have more time to be heated and reach a higher temperature. My experience is that lower flow rates cause more cycling - as higher temperatures are reached? Also do higher temperatures reduce the cop? 

Posted by: @sliderule

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Coming late into this. I like the flow explanation re heat emitters. Presumably it's similar at the ashp heat exchanger ie. at lower flow rates the water in the ashp heat exchanger will have more time to be heated and reach a higher temperature. My experience is that lower flow rates cause more cycling - as higher temperatures are reached? Also do higher temperatures reduce the cop? 

The process is slightly different at the heat pump end.

The LWT is set by the controller, either a fixed value or one using WC. If the heat pump is heating the water to a set LWT of 40C and the return water from the heat emitters is 35C, the system should be balanced with a DT of 5C.

If the quantity of thermal energy being emitted by the heat emitters now reduces, the RWT will start to increase. This in turn will cause the LWT to start to increase, since the heat pump was operating to increase the water temperature by 5C.

The heat pump controller will sense this increase in LWT and will slow the compressor to try to balance thermal energy supply to thermal energy demand. This should work fine until the compressor slows to its minimum operating speed, at which point both the LWT and RWT will start to increase and the compressor will eventually be stopped on the high water temperature limit. The heat pump will now cycle.

If the system is operating in a balanced condition as described above, but the flow rate is now doubled, the heat emitters will still be emitting approximately the same quantity of thermal energy, but the RWT will start to increase. The heat pump compressor should remain operating at approximately the same speed, since it still needs to supply the same quantity of thermal energy, but because it is heating double the quantity of water, it only needs to raise the temperature by 2.5C. So the LWT should remain at 40C, but the RWT should increase to 37.5C.

Some heat pumps systems have the ability to vary the water pump speed in an attempt to maintain the DT at 5C.

To try to control the quantity of thermal energy being emitted by the heat emitters, the heat pump controller, operating in WC mode, could lower the LWT, and as described above would also cause the compressor to slow, thereby improving the overall efficiency.

As you can appreciate, there are actually several control systems all working in co-ordination to try to keep the heat pump operating in a reliable and efficient manner. This is why the system needs to be correctly installed and optimised.

Posted by: @derek-m

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Some heat pumps systems have the ability to vary the water pump speed in an attempt to maintain the DT at 5C.

Mine does this. The ASHP is basically never switched off it's either heating or cooling.

Circulation pump is set to run at all times, the heat pump controller changes it's speed dependant on what going on.

When compressor is off the pump runs at min speed around 0.6m³/h. On compressor start the circulation pump increases speed to about 0.92m³/h. Once target temp is reached for discharge flow and dT starts to decrease flow rate is increased to maintain to help maintain dT, gets to 1.03m³/h. Basically dT will continue decreasing until max flow temp is reached and compressor switches off.

Hi everyone. Just got my Ecodan fitted and attempting to find out how to run it to best effect. Slowly picking stuff up on here. It would appear my installer set the system up as you would a gas CH system, ie quick blast in the morning and again in the evening.  I’ve tried to level things off a bit and am playing around with WC curve, but it’s a bit tricky at the moment as it’s so mild. One thing I noticed was on my MEL cloud app it would appear my return temp is higher than my flow. I assume this is impossible and the installer has simply mixed up the thermistors. Will this affect the system performance or is it just for display purposes? Thanks

It will affect system performance as those are the main temperatures the controller will use to switch the outside unit on and off – definitely a case of getting the installer out to rectify the issue.

Posted by: @morrisnotoff

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One thing I noticed was on my MEL cloud app it would appear my return temp is higher than my flow. I assume this is impossible and the installer has simply mixed up the thermistors.

As @harriup says this needs to be fixed. 

Its worth noting that, later in the season, when defrost occurs, there will be short periods when return is higher than flow (because the heat pump is essentially operating in reverse).  But you shouldn't be defrosting at present!

Thanks for the replies. I’ve got the installer out to remedy on Wednesday. Cheers

Hi everyone, 

Thought I would give an update here, and given the recent cold snap in the UK it would seem a good time to do so. We live in Woking, outdoor temp over the past 24hrs has been between -1c and -5c. 

As a reminder, the key issue I was trying to resolve was excessive cycling. The only major change I have made since my last post was shifting the unit into quiet mode (one notch down from normal) as I had the feeling the unit was oversized for our heat loss. My logic was that quiet mode would throttle the compressor's ability to increase the flow temp too quickly. 

See below three different one hour reports that show a nice constant delivery of heat (other than for the defrost cycles). 

The above patterns of flow and return temp are fairly similar (with a nice delta T of 5) when the temperature outside is below 13c. (Anything above 13c, and there is a bit of cycling but maybe 2x an hour, which is better than before where it was cycling 5x/6x an hour.)

Now of course - the only reason I wanted less cycling was to try improve the CoP - my CoP for Jan so far has been 2.8x (which is still poor) but is better than what I was getting last year (2.2x). 

So for those who think their unit may be oversized - considering quiet mode may be an option to explore. 

Happy 2025 everyone! 

Yes, Jake T, I do that and even use the quietest mode in warmer weather. But having downsized from a 14 kW to an 11.2 kW heat pump, I can't use Quiet Mode in below zero C temperatures, especially as I have it timed OFF from 1600 to 1900pm for the Octopus Cosy tariff. The old 14 kW HP was 13 years old and had 2 refrigerant losses and finally a compressor failure which, rightly or wrongly, I attribute to frequent cycling especially in the early years when I didn't know it was happening. So I calculated the heat loss for the house and downsized the new HP. I did find on the old HP that cycling reduced when I increased the flow rate to maximum 30 l/minute, but this might not work in a different house. I make sure that all short pipe runs are throttled down to minimum flow to avoid a water "short circuit". Our year average CoP is 3.5 but CoP has dropped to 2.7 in this January so far. I notice that your flow temperatures of around 40C are much lower than ours which are 45 - 50C in this cold weather. Our high flow temperatures may be partly due to intermittent running of the HP because of the substantial solar heat that we get from almost the entire house having South facing patio doors, and concrete floors which have to reheat when the sun goes down, as well as the Octopus Cosy 3 hour pause. Also, I can't get more than around 20 l/min flow with the new HP, and I plan to try a different flow water pump in the Summer. We also changed to Advanced Auto Adaptation after a few months, for various reasons, and my impression is that cycling (which only happened in the warmer weather) was virtually eliminated and the AAA probably optimises comfort before CoP, which after all is what a heating system is for. Maybe one consequence of downsizing the HP is more frequent defrosting which in this cold weather can be every 45 - 60 minutes, but I can't compare it to the old HP which I couldn't monitor. I share all this as there are always interesting comments on this forum.