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How quickly would you want/expect heat recovery to take from setback?

AndrewJ · 2026-10-17T09:44:16Z

Now into year 2 with my Cosy 9. I currently have it set to keep the house at 22c (runs off a thermostat if you recall) with a setback to 21c 23:00 to 03:30. Hot water runs 02:30 to 03:30. I have the house set to warm back to 22c at 03:30 because I’m on an overnight tariff of 6.6p until 05:30 so it make sense to do “heavy lifting” before the peak period starts and it pulls on my battery. Last night at 23:00, the house temperature was 22.6c and the heating hadn’t been on all day because it was warm enough to heat the house from just the sun. OAT dropped to 6.2c (reported by the Cosy unit) at 03:30 with the house temperature at 21.2c. At 07:30, the house was at 21.7c and it finally reached 22c at around 09:30 when the OAT had reached 9.5c. It took 6 hours to raise the temperature by 1c. My flow temperatures are 50c@-10c OAT and 33c@12c OAT. This is a drop from 36c@12c OAT I ran last year. As far as I know the WC curve is linear. At that flow temperature setting, the house can reach temp and I have to say it is more comfortable for having the heating on for that time period because the internal air actually feels warm. Is there any consensus on how long you would want to take to warm up by 1c? Is there anything in the above that says I may have dropped the flow temperature too far given it’s likely to get colder? Perhaps there is scope to drop the flow temperature further? Really it’s more a discussion point than an actual query: if it meets temperature, we are comfortable and it isn’t costing the earth then all is good right!Remember, Octopus control their Cosy units via a thermostat with the hysteresis set to -0.1c to +0.2c. So with a setting of 22c, heating comes on at 21.9c and off at 22.2c. These are reported temperatures of course, I doubt the sensors are that accurate! On a semi-related subject, I’m inclined to think it isn’t worth doing a setback for just 3.5hours. I suppose then it becomes more a question of whether the flow temperature is sufficient to recover from the 1hr water heat cycle when it is colder.

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AndrewJ

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Topic starter 15/11/2026 3:51 pm

Posted by: @cathoderay

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I still think other weather factors apart from the OAT may be the cause of what you have observed. Standard weather compensation only sets the Set Leaving Water Temperature, and that indirectly sets the energy use. But on one day you might have favourable weather conditions eg solar gain, with the indoor air temperature (IAT) responding well, while on another day with unfavourable weather conditions eg rain and wind chill, the same amount of energy used struggles to make much difference because your heat loss has increased.

....

By the way, have you posted your design conditions heat loss? I've had a quick skim through the thread and couldn't see it anywhere.

Actually, good point. Yesterday was sunny for a good chunk of the day and could well have had some impact - it's been rainy and cloudy here all day and very little sun. If I was take a basic assumption of two bodies plus TV in the evening then that will also keep the temperature up (given our insulation.) I think the bigger indicator here then becomes what happens overnight when we are in bed and the sun has disappeared - and yes, it's colder as well. That can be seen on the temperature graph and it clearly wasn't able to maintain or recover the heat lost back to the target temperature. That itself might well confirm what you say and indicate that the flow temperature is now too low.

I haven't posted design heat loss in this thread, but it is in a larger Cosy 9 thread I initiated. Octopus calculated this at 8.5kWh at -3.4c. A subsequent heat loss survey by them after I was struggling to keep temperatures down determined it to be 7.5kWh. However, they also came back at that time with a revised radiator schedule that put 1kWh MORE heat into the house at the design temp. I sort of lost some faith with them at that point. It's also worth mentioning in the year before having the Cosy installed I used 9000kWh of gas for heating and hot water.

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