There seems to be a lot of mystery and misinformation about air-air systems. These are basically reversible air-conditioners, and I have personal experience of some very successful installs. However, they don’t suit everyone, and in the depths of winter, they might struggle as they defrost and the COP drops. I occasionally hear wildly optimistic winter COP claims. So, I will be monitoring this unit and try to estimate the COP (heat output / power input)
See my blog from 2021 Is a reversible air-air heat pump a proper heating system?
Monitored Graphs (OpenEnergyMonitor) https://emoncms.org/thehub
The values above are examples, and will vary considerably as the outside air temperature changes.
Why do this experiment?
When talking to installers of air-air split systems in the past, I could find nobody willing to grapple properly with the sizing issue. All wanted to go play-safe and simply go on manufacturers guidelines. like choosing a 3-litre car engine… it will cope with any eventuality. Installers seem to want their installs to be ‘toasty warm’, and who can blame them.
However, I have in the past been surprised how a very small system can (if you let it) produce good-enough comfort. So, at The Hub I have fitted a 3.5kW 12,000 BTU output (nominally) air-to-air heat pump/air-conditioner in a 80sqm poorly insulated community hub. It’s a ElectriQ R290 model. Like many mass-produced products, they are made to a price, and not built to last. Its longevity is a concern.
Work-in-progress… do come back later
Initial findings are positive. All users of the hub are reporting ‘its lovely and warm’. That said the building was often ‘freezing’ before, so compared to that, it is certainly warm. It is an old building with vast shop-front single glazed windows. The control seems annoyingly poor in that it will creep up way past the set-point, then when it does switch off, it takes far too long before it switches on again. There is no control of the differential to my knowledge. On the other hand, the Tuya app to control it is quite good if you are into geeky controls. ‘Scenes’ can be used to change the temperature setting and/or fan speed depending on time and outside temperature.
Mid winter performance will be interesting. I am not expecting it to cope that well with very cold damp condition….. let us wait and see!
How can we estimate the COP?
It is very easy to measure heat delivered to water by measuring the flowrate and the temperature rise, however, it is very difficult to measure heat delivered to air. The alternative is to look at the refrigeration circuit and estimate it from there. Sadi Carnot was a very clever chap and worked out the theoretical efficiency of a heat pump back in 1824. The formula he worked out only used the refrigerant temperatures.. One of it evaporating in the ‘cold side’, the other of it condensing in the hot side.
We must use absolute temperatures (starting at 0 from absolute zero)
We know from experiment that the actual real-life COP for this type and size of compressor is likely to be between 45 and 50%. From this, we can plot an estimated COP on our graphs. At outside temperatures below about 7°C, 45°F, defrosts can occur. We will need to estimate the duration and cooling during this process to allow for it.
Watch this space.