Replace our earlier Maxa 6kW heat pump for a 4kW Haier in January.
Our heating in house is all UFH within 100mm concrete screed at 300mm centres. Also connected to the system is a summer house with a fan coil. The total heat requirement is around 3.5kW at -9.
Original plan was to run pure weather compensation, but running practical flow temps at a typical 7 Deg day was really too low to get a sensible dT and a return temp that would allow compressor restart if it stopped. Also got quite a bit compressor hunting when heat pump had run out of room to put heat into the floor. Kept trimming back flow temp and it got better, but still not great. So reverted to running against a house thermostat, which help but did not cure the issue.
Working with AI to diagnose we found during hunting the heat pump control was doing things with the flow temp and due to the floor being a huge buffer not getting the feedback it expected (flat return temp) control went into chasing itself mode.
April came along and heating season finished. But we run cooling so gave some place to experiment. We got similar symptoms in cooling but not so extreme. So I decided hydraulically seperation with a CCT close coupled tee), running ASHP pump in fixed speed and secondary pump as close as I could get so little or not distortion occured.
The CCT seems to give a little more return temp feedback and the heat pump runs very pretty well.
Will wait to see what the heat season brings.
Current setup, CCT, fixed pump speed, no buffer or volumiser, single zone (house and summer house), UFH over a 192m² and a single fan coil, plus have just added a cool to MVHR outlet) but with multiple room sensors, each can call for heat or cool, but whole system goes or off all at once. WC curve is flow 26 to 28 degs @ 10 to -5 degs outside temp.
A years worth of data Haier installed Jan 2026 and CoP has got better as system tuned. Current 3 monthly CoP is 6.1. I don’t do DHW with heat pump. Would rather let the heat pump full use of cheap periods heating or cooling and let the immersion run at night in cheap period or day time when solar will provide the energy or again in a cheap period.
With your very low heating flow temperatures, it possible a 4 port buffer would work well. My thinking is having the heatpump 5c above the UFH flow could be less costly than the heatpump compressor frequency bring very unstable.
But if your heating energy requirements are as low as I expect they are, is a heatpump justified compard to a simple emersion heater in a buffer tank?
One of my main drivers for a heat pump was cooling. But running costs soon stack up, with an immersion, let’s say the average heat loss is 1.5kW, so multiply by 24 that is 36kWh. I could do that all at 10p (due to having a battery) so £3.60 per day heat cost or £108 per month or around £600 per year. If I average a CoP of 5, those numbers drop to 72p, £21.6 and £120. So around £480 savings per year.
But on the cold days the immersion would run all day/night and then take in peak rates as well, making the pay back for the heat pump shorter.
I self installed, ASHP cost £2100, so in under 5 years the heat pump has paid for itself and it gives me cooling in the summer and shoulder months.
Can you calculate from the weather forecast how many hours of heatpump use is need to put into the large floor thermal mass to keep property warm for next day? So rather than controlling heat input based on flow temperatures control it using a TPI type setup with the flow temperature allowed to increase.
Or use a “degree minute” type system so the heatpump runs for 20 minutes with no limit on flow temperature than closes down until the flow temperature have reduced by the amount of overshoot of heat input.
(Hopefully you can also lock the heatpump into low modulation other than when full output is needed.)
I could and did play with that in the past. Generally did that with gas boiler and have done it with ASHP in the early days of having one, mostly due to being on E7 with the heat pump. Found with a heat pumps too busy playing catch up, so letting the room sensors do there thing on a small hysterisis is a more gentle process.
But now try to keep all things heating and PV without internet inputs. Use home assistant mostly for monitoring (or at least anything that can leave me with cold house or PV not doing as expected). So use the external room sensors and they seem to work well. All are hard wired to an ufh controller (it does heat and cool) and its output feeds in to the heat pump via a wireless switch. The UFH controller has an external air temp sensor that kick off any demand for heat if outside temp is 10 degs or above for 12 hrs.
Do have a predicted room temp algorithm in home assistant it predicts room temps in 1 and 4 hours, also played with that, so heating started early and made a flat room temp. But wasn’t worth having heating tied into home assistant, so gave that up.
Close up showing return temp not changing to big changes in flow temp being input by the heat pump controller (green and red) during that part of the day CoP drops to 4.0.
Below is a cooling run after installation of the CCTV now have good tracking of flow and return temps
Notice the power spikes - one at 1700, seems to do this every few hours in heating and cooling, not sure why assume to move away from a fixed running position to exercise things.
