Thanks for sharing your setup and the details of your situation, and it sounds like you’ve been on quite a learning curve with your heat pump system. It’s great to hear that the forum has been helpful so far, so thanks for joining.

The short cycling issue you’re experiencing is a common problem when the system isn’t properly balanced or sized for the load. The fact that the compressor is running for less than 10 minutes at a time suggests that the system is either oversized for your heating needs or there’s insufficient water volume in the circuit to absorb the heat output effectively.

The Midea technician’s suggestion of adding a buffer tank could help, as it would increase the water volume and reduce the frequency of cycling. A volumiser is probably, in my opinion, the far better route to get more volume.

Also, since you’re already using weather compensation, try lowering the flow temperature further, especially during milder weather. This will reduce the heat pump’s output and allow it to run for longer periods.

It’s also worth noting that Madrid’s mild climate means your heat pump should be able to run very efficiently, so getting the system balanced correctly will make a big difference to both comfort and energy use.

I don't have first hand experience of fan coils but get the impression that they have much lower water volumes than panel radiators. I think that the technicial is correct that adding more water volume to the system will reduce the short cycling. The more water in there is in the system then the longer it will take to heat it up or for it to cool down when the heat pump stops.

@johnr  , they are definitely small in size, so the water volume cant be that much, unfortunately the user manual doesnt list the water volume so im helpless short of disassembling one and measuring the water volume.

@editor  , i did ask them about volumiser but he didnt understand or it was lost in translation ( spanish is not my native langauge) , after extensive search for such product in spain , i simply couldnt find any, only listing for what translates to buffer tank ( i could tell since they come with 4 connection ports) , i just read on another post here in the forum, that a 4 port buffer tank can be connected as a volumiser, by using only 2 connections. whats your take on that? and if so how should it be connected and would the secondary pump also need to be placed differently 

unfortunately the same midea technician quoted 3000 euros just to add the buffer tank and redo the pipe work, which i find to be crazy, so im trying to understand the full process and see which part i might be able to do myself. since i can find such tanks from plenty of suppliers for under 400 euros.

I agree with the above  Fancoiks have a very small water volume and you have a fairly high capacity heat pump in a mild climate.  Short cycling is to be expected.

I would have a volumiser fitted not a buffer tank.  A volumiser has only two ports, buffer tanks 3 or 4 ports.  Both are just tanks.  Unfortunately the terminology is used a bit loosely and may anyway be different 8n Spain.  2 ports good, 3 or 4 ports bad.  I would say make it as big as you can, if you have room for 200-300l that would be good, if nit fit what you can.

It will still cycle, you can't change that, but the cycles should be longer

That's a great photo to show of the pipework.  It appears to show a low loss header (black vertical cylinder), which has been bypassed.  There are two quarter turn valves which are set to direct the return flow back to the indoor unit.  In effect the low loss header has been replaced with close coupled tees.

Fan coil units need quite a large temperature difference between the hot water and the cold air, othewise they dont transfer much energy.  It could be that the return water temperature from the fan coils is quite warm and being routed directly back into the indoor unit.  Lining up the valves to use the low loss header might reduce the hot water cycle back to the indoor unit and reduce cycling.

Please also consider the heat pump will cycle if it's output energy can't get out of the fan coils.  If you increase the flow temperature you will get more energy out of the fan coils (bigger temperature difference between water and air) and the system will cycle less.

Regards

Bob

@bobtskutter thanks for the reply and the explanation on the pipes, i did turn the valves as you advised and now the return flows through the low loss header, i took a note of the exact time i changed it, and will monitor for 24 hours and check back on the number of cycles and length.

in terms of flow temp, i'm running on a curve that for now is keeping the house at the desired temp so increasing the flow temp ( via a more steep curve) will just make the house too warm .

and as per the other advices above, i'm looking around for a volumiser tank and what are my options to get it installed in order to increase the overall system water volume.

@jamespa thanks for the reply, and clarifiying, i indeed now learnt that the goal is a 2 port tank, but if i cant find that (in the spanish market) will a 4 port tank work if only 2 ports are connected ( i saw a couple of videos showing that, but cant tell if its the correct approach)

Posted by: @rmy

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will a 4 port tank work if only 2 ports are connected ( i saw a couple of videos showing that, but cant tell if its the correct approach)

Yes, just blank off the unused ports.

As you have fancoils I would put the volumiser in the flow, not the return, it might just soften the experience during defrost (there is some good natured disagreement over this).  I would plumb the incoming from the HP at the bottom, and the outgoing to the emitters at the top. 

Some volumisers have an immersion heater in them, this is not essential to their function but can provide backup if the heat pump fails but the water pump is still working, a fairly unlikely circumstance to the extent I cant really see the point.  It can also provide additional oomph at very low temps if the heat pump is a bit undersized.  Im neither recommending an immersion or not, but presumably its cheaper without!

@jamespa thanks for confirming, as i can find plenty of options now . one more question, would the secondary pump need to be moved from the current location ( in other words. the tank should be before or after the pump on the flow line) ?

Posted by: @rmy

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@jamespa thanks for confirming, as i can find plenty of options now . one more question, would the secondary pump need to be moved from the current location ( in other words. the tank should be before or after the pump on the flow line) ?

I forgot you had a LLH and secondary pump.  Is the service technician who visited proposing to remove it or leave it and add a buffer tank/volumiser in addition?

Personally I would ideally try to lose the LLH and secondary pump at the same time - but am I right is saying you have bypassed the LLH anyway?  Is it all working with 2 pumps and a bypassed LLH.  If so anywhere convenient in the flow will be fine.

@jamespa he didnt give any details, and the proposal sent is just stating tank install without any further details

and yes the LLH was bypassed. and the secondary pump is right after it, so what i conclude is that I can just connect the tank simply after the pump on the flow, so the total water volume in the system is increased.

that sounds straight forward and hopefully translates to simpler and cheaper installation cost.

thank you for the great help