We are just over 1 year into having a Vaillant Arotherm+ 7kW heat pump. Overall it is working well but I am slightly disappointed with my space heating SCoP - currently sitting at ~3.6. I had hoped it would be 4.0 or more. We have also been using the heat pump for cooling - which achieves a high COP in summer - and raises our headline average to ~3.7.
I suspect that the main problem is that we had underfloor heating installed throughout the ground floor, as part of a major renovation but it was done before I knew much about heat pumps. It is very well insulated under the screed but was done with 200mm pipe centres - requiring a higher flow temperature.
Would anyone be willing to take a look and see if there’s anything else obviously off in my data?
Two contributors in my opinion: First, a design temperature of 45°C at -1.7°C is quite high for UFH. From your data, it appears the actual flow temperature at design temp is lower though. Secondly, and probably more impactful are the defrosts. Look for example at Dec 5th, you are constantly defrosting and it appears the heatpump struggles to keep the house warm. This eats huge amounts of energy. Even at 5°C outside you’re sometimes getting almost half-hourly defrost cycles (e.g. Dec 20).
With all this in mind your SCOP appears quite good actually!
Yes, I have tried to drop the value on the Vaillant weather compensation curve - I currently have it set to “0.50”. So the flow temperature doesn’t often reach 45°C.
What can I do to try and reduce the number of defrosts? The heat pump is in front of a south-facing wall but in winter it is in the shadow of our neighbour’s house all day.
And probably not much that can be done to reduce humidity?
I fear there’s not much you can do. Temperature and humidity are the main culprits and if you’re in a wet climate, you’ll have these issues if temperatures hover around zero. This is in fact so bad that a heat pump’s power output is up to 20% below quoted nominal output by the manufacturer, at least for Vaillant units. Have a look here for example: Vaillant maximum output capacity testing
You mention UFH on the ground floor only and a mix of both new and old radiators upstairs. I think your radiators are setting your heat pump flow temperature. The radiator water temperature is then typically blended down for UFH operation. I assume you have a blending bypass valve arrangement to reduce the hot water temperature to the UFH. A 45C MWT with a 200mm pitch would generate a heat output of 121 W/m2. That’s a lot of heat into a room. If it was operating with a MWT of 35C, then it generates 84 W/m2. That’s still a decent amount of heat into a room. The 200mm pitch may not be the issue.
Your upstairs radiators will most likely have set the design choice for the MWT =45C. This should be part of your heat loss calculations for each room. As you know, the radiator heat output is dramatically affected by lowering your MWT. At 45C you have 40% of the radiator output against the typical rating, but at 35C it is 21%.
If you could lower your MWT of 45C at your -1.7C design case, then your COP should improve. You can get a fair idea from the valiant heat curves, but you are already operating near the 0.5 curve and a water flow temp of ~39C. The challenge is whether you can still lower the heat curve value, but still warm the upstairs and downstairs rooms. If this is possible, then your COP would increase.
My question is when you attempt to drop the heat curve further, is it the upstairs or downstairs rooms struggling to get to the room temperature, or both?
I hope the plot here helps. It shows the heat delivered by the UFH with a pitch of 200mm against a K2 type radiator. The radiator size is chosen to deliver 48 W/m2 at your original design case for an notional room size with a flow temp of 45C. The 48 W/m2 was based on your 6.5kW ASHP in a 136m2 house. (6500/136=48 W/m2). It suggests the UFH with 200mm pitch may not be the constraint. The UFH (200mm pitch) should typically offer higher performance than a radiator system. Unless it has a larger than normal room floor area unheated, or a carpet, or wood laminate covering with a TOG>1.0. If you have an UFH mixing valve, then that could adversely affect the UFH flow temperature. It was difficult for me to understand the heavily insulated piping around the UFH manifold.
