Get the survey done correctly!

FlorayG

A little story…

...I decided to get a heat pump installed by Octopus. The installation went smoothly last July and I had all new radiators as well. Very happy.

Come winter and the heat pump was working constantly and the house just never was warm enough. Several engineers ( I think 6) sent out to investigate and endless phone calls with them. Meanwhile my fuel bill was higher than ever.

Finally an engineer said " I don't think your survey was done right" and a different surveyor arrived; spent 3 hours measuring and calculating and finally said " Nope, this Cosy 6 installed isn't big enough for thisproper6you need a Cosy 9 and two of the radiators are too small"

So by the end of January I had a new, bigger heat pump and suddenly the house was warm and my electricity bill minimal. Plus, Octopus gave me £400 compensation

So just make sure your surveyor is competent!

QrizB

I think @matt_drummer had a similar experience with Octopus, only the other way around. The original surveyor calculated the heat loss to be much higher than it really was so they fitted a heat pump that was far too large and wouldn't modulate low enough, which meant it kept cycling and the efficiency was low (a poor COP).

FreeBear

Part of the reason I fitted a heat meter to my boiler was so that I could monitor how much energy was being used to keep the place warm. Logging the data alongside temperatures inside and out also provides a picture of heat loss. On the back of the graphs, I'm of the opinion that I need around 2.5-3kW to maintain the temperature, and 5-6kW to heat the place up (with a peak of around 7kW). So a 7-8kW heat pump that has a 4:1 modulation range should be ideal.

NedS

Glad to hear you got it sorted, sounds like a good outcome.

Most installers seek to over specify by around 30% relative to the heat loss to give sufficient capacity to recover from defrost cycles on very cold days, and to allow for recovery from time out for hot water heating which could easily be an hour plus per day for smaller heat pumps / homes with low heat loss.

So if your heat loss is 2.5-3kW, theoretically a 4kW unit should be sufficient and any of the 5kW units should be a good choice.

michaels

https://forums.moneysavingexpert.com/discussion/comment/82052267#Comment_82052267

So design is for a nominal coldest day and should really only cover keeping temp constant, heating required hot water and running defrost cycle at that temp. They are not generally designed to let the house get cold and then rewarm it.

So in your example if 3.5kw keeps the temp steady at 21c on that -3c day then you will perhaps only need 1.5kw on a plus 7 day and your suggested 8kw unit will end up cycling on and off to limit output to this level. There are lots more plus 7 days than -3 days.

Reed_Richards

I'm of the opinion that I need around 2.5-3kW to maintain the temperature, and 5-6kW to heat the place up (with a peak of around 7kW)

To heat your house up as fast as possible, you need to first heat the water in your system up to the maximum permitted output temperature of your heat source (i.e. boiler or heat pump). The time required to do that will depend on the maximum output power of your heat source and the volume of water in your heating system. But once that maximum operating temperature is reached then the rate of warming depends entirely on the output capacity of your radiators. So if you want a heating system that can warm up your house quickly, you need to over-size the radiators, with a larger output capacity than a correct survey will tell you that you need. Bigger radiators will be more expensive and may take up more wall space, but you could feel that it's worth it.

FreeBear

OK. Some data - 8th of January of this year. Dropped to just under 0°C during the night, hitting -0.4°C around 09:00. By noon, it had warmed up outside to 2°C where it stayed for most of the afternoon. When the heating kicked in at 08:30, internal temperature was 18.5°C, rising to 19.5°C by 10:00. From 11:00 through to 16:00, I was experimenting with a fixed flow temperature without any thermostatic control - Wanted to see how the system worked at 40°C. At 16:00, internal temperature had crept up to 20.4°C, although how much of that was due to solar gain, I wouldn't like to say, but Wundergorund is saying it was cloudy for most of the day (perhaps I need to start logging sunshine levels).

Total heat output was 23kWh for 24.43kWh of gas.

I don't aim for heating up as fast as possible - To do that with the existing radiators would mean running with a 70-80°C flow temperature which would kill efficiency. Quite happy with a 1°C per hour with a 50-60°C flow with the boiler range limited to ~7.5kW.

Hasn't really been much below freezing at all this winter, so don't really know how well the system responds at -3°C or even -5°C. If we get a big freeze next winter, I'll run the fixed flow temperature experiment again.