The only way to be certain would be to physically trace the pipework from your heat pump to your buffer tank, and then from your buffer tank to heat emitters. Also ensure that any water pumps that may be installed are pumping in the correct direction.
Unfortunately where the pipes run under the floor would make it a very big job to trace the full length. The only pump in the system is in the UFH circuit. Doubting myself I went back with a different thermometer today - a physical one rather than ip-enabled sensors - so no chance of getting mixed up. This is what I found (temps in red)
@squonk It must be the case that the direction of flow on your two connections to/from the ASHP, on the left side of the buffer, is the opposite of what you show your diagram. The temperature is unarguable (you've measured it twice with two different thermometers) so it must be the direction. For you diagram to be 100% correct for temperatures and directions the buffer has to be inputting energy. Which (unless it has an immersion or similar - but it doesn't look like it?) it isn't. so the direction must be wrong.
so you must actually have this (the box is the buffer). which is going to issues with too much mixing in the buffer .all the flow inside the buffer will be vertical rather than mostly horizontal as per proper buffer use. this is nothing to do with the losses due to the way buffers work even when well implemented, this is worse.
BTW there must be another circulating pump on the heat pump side of the circuit. probably in the heat pump outdoor unit if you cant' obviously see it. given that it'll be in the outdoor unit, factory fit, it's extremely unlikely to be pumping the wrong way w.r.t the outdoor unit itself - we can reasonably assume that that is right. you 've said can't trace the pipework route under the floor and I'm not sure there's much point anyway. But it all points to flow direction into the buffer bring wrong. the obvious thing to do next is to look at reversing flow direction on the ASHP side into the buffer, as simply as you can. you want to get to this - note the flow inside the buffer is horizontal.
moving pipes on the input to the buffer will be hard work as its fixed pipework and in a tight space. I'd suggest that a larger project to turn the buffer into a volumiser and make it a single circulation loop, probably over the summer when you don't need heating, might be a time to look at this.
However, having just fitted my own outdoor unit , the thought occurs to me is that reversing the outdoor unit connections would be a) an easy mistake to have made b) easy to revert back. So there is a possible quick fix.
can you do this?
Do you have isolating valves somewhere in the pipework just before the flexible hoses? are you confident enough with a spanner to tackle this? if not then this is easy for any competent plumber, doesn't have to be an ASHP specialist.
1. close isolators
2 disconnect the hoses - at the house end or the outdoor unit end, whichever is easier. The water in the hoses and in the outdoor unit will leak out at this point and will be under enough pressure to spray for the first second or two. the isolating valves stop the whole houses worth leaking.
3 reverse hoses and reconnect
4. open isolators and check for leaks in the connections you just re-made
5. repressurise system . you need to replace the water that was lost in step 2. there'll be some air in it now as well.
6. run pumps if the ashp has a self-test mode to get the air out
The temperatures that you mention are all quite low, which is great for efficiency (but not for a warm house when it's cold out). What really matters for efficiency, is the flow temperature out of the ASHP itself - does it report this? The low flow multiplied by the low deltaT indicates that the heat out of the heatpump is low, I'm sure associated with a cold house. I don't think this is directly related to COP - COP is most affected by the flow temperature, which appears ok, which is a puzzle.
I had a quick look at the arotherm plus spec sheet, and it claims a sCOP of 3.6 even with a flow of 55C. It doesn't say, but I expect that the datasheet assumes weather compensation is enabled for that figure. I can't understand why weather compensation wouldn't be suggested for your setup.
How do you measure the COP? ASHP COP measurements are not MID certified so of unknown accuracy, and it's cheaper to design using less accurate parts. I recommend at least getting an MID certified electrical meter for the heatpump (£50 item). Better to get an MID certified heat meter too (£300 item).
Did you run the original storage heater solution before the ASHP - so do you know the heating requirement? You could look at your EPC (downloadable, just google)- it estimates heating loss/m^2.
I'm not familiar with the arotherm interface, but can you be absolutely sure that the internal immersion is off ?
The only way to be certain would be to physically trace the pipework from your heat pump to your buffer tank, and then from your buffer tank to heat emitters. Also ensure that any water pumps that may be installed are pumping in the correct direction.
Unfortunately where the pipes run under the floor would make it a very big job to trace the full length. The only pump in the system is in the UFH circuit. Doubting myself I went back with a different thermometer today - a physical one rather than ip-enabled sensors - so no chance of getting mixed up. This is what I found (temps in red)
Ian has already come to the same conclusion, that the flow through the buffer tank is incorrect. To prove that the problem is not within the heat pump, which I also doubt, identify the water in and water out pipework at the heat pump and measure the temperature. The water out should obviously be warmer than the water in.
Personally I would be inclined to get your installer back to rectify the problem at their expense, since it may require new insulation and antifreeze top up. It may also educate them to confirm that equipment has been correctly installed before they dash rapidly off into the sunset.
@derek-m the OP already said, the installer was out of his depth and has since retired so the "back to the installer" option doesnt work in this case. Hence my suggestion of hopefully a relatively do-able pipe swap. If the OP isn't comfortable doing it then any plumber can. good point though , if its got glycol in, more careful procedure needed than what I described.
also the OP said its a 45L vaillant buffer. their pics match what you get if you google that.
@derek-m the OP already said, the installer was out of his depth and has since retired so the "back to the installer" option doesnt work in this case. Hence my suggestion of hopefully a relatively do-able pipe swap. If the OP isn't comfortable doing it then any plumber can. good point though , if its got glycol in, more careful procedure needed than what I described.
also the OP said its a 45L vaillant buffer. their pics match what you get if you google that.
Thanks for the clarification. I'm old, and easily confused. 🙄
@derek-m no worries! have to say though this one is a perfect demonstration case for "don't assume; check the basics; measure it to be sure". Its only the OP's measurement of temperatures at your request 👍 that has pointed to what must be happening.
@squonk further to my previous note regarding pipe swap: If the system has glycol in its not as simple as I described. Plenty of info googleable for further reading. I am not able to advise on how to this. May be best to get a plumber used to handling glycol in.
If that is the case , thus adding complexity to the pipe swap and making it not a zero cost option, I would be tempted to hang on for a month or two until its warm enough to take the system down, then think about a rebuild with the buffer turned into a volumizer, single circulation loop.
Thanks for the various answers. The tank is a 45l buffer, no immersion or anything else. I’m not sure it’s as simple as swapping the pipes at the pump end (which would be easy) as I also have a 300l hot water tank which feeds off the pump via a diverted valve.
Getting the installer back is unlikely, so I will need to find someone capable to resolve it.
@squonk run the system in DHW mode and with your thermometers, see if the water going through the diverter valve, on what should be the Hot input to the DHW tank, is actually the hot ?
that tells you more about where the crossed pipe is.
@iancalderbank I assume it does have glycol … but I also assumed the guy knew what he was doing, and look where that got me! There are numerous other problems with the install, so I have asked a proper company to quote for a complete re-install, I think that’s the only way I’ll know it’s done right.
