It’s been a few weeks since the heat pump was commissioned. We have had an up-and-down sort of September, and following a few days of summer returning, we’ve had a cold snap. In previous years we always tried to hold off turning the heating on as long as we could, waiting at least until the clock went back at the end of October, but this time we kept the heat pump switched on the whole time as we were told by the installation engineer to do, and played around with the controls to adjust settings instead. Here are some initial thoughts.
The way a heat pump heats a house is quite unintuitive for us who are used to gas central heating. An air source, air-to-water heat pump is a low-temperature heating system. Last winter I had the gas boiler’s flow temperature* as low as possible, which was 43-45°C (*flow temperature is the temperature of the water that the boiler heats to before it is circulated through the pipework to radiators). But 45-45°C is still quite hot if you think about it – if it was bath water, that would be considered scalding hot, and you would probably need to add a lot of cold water to be able to soak yourself in it. With the heat pump, too, 45°C is near the top end of what you want the flow temperature to be. Unless it’s icy cold outside, it is said that the optimal heat pump flow temperature is a lukewarm 35°C. That’s barely your own body temperature. So when you touch your radiators, it’s like touching a human skin, not cold for sure but certainly not warm the way you expect them to be. It’s hard to believe that radiators that are only human-body warm can heat the entire room, but so far they seem to do that just fine.
The control panel and the app that came with the heat pump system has different operating modes: TIME-CONTR., MANUAL and HEATING OFF. Confusingly, MANUAL setting is basically a fully automatic mode, where you simply set a desired temperature, and the heat pump operates to maintain that temperature. The installer told me that I should leave the control in this setting as it is the most efficient way of running the heat pump. It’s basically just like you would use your fridge – you generally leave the dial inside the fridge alone and let the appliance to do whatever it needs to do to keep the content chilled/frozen. You hear the pump waking up into action, put cold air inside and stop when it’s done, all without you turning it on and off.
However, you might (as I do) want to turn the heating down during the night when nobody is up, and if you have a variable tariff (as we do – Octopus Flux tariff in our case), want to try to make the most of the discounted ‘flux’ hours (2-5 am) and not to let the heat pump to come on during the peak hours (4-7 pm), you are tempted to switch to the TIME-CONTR. mode, where you can set different desired temperatures at different times of the day. I’ve been experimenting with this setting to see how effective this approach is. I’m also considering switching to a tariff designed for heat pump users called Cosy Octopus, which gives you multiple discounted periods but with an even higher peak rate than Flux. I will report back how we fare.