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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 11:26 am

I think increasing target temp wouldn’t help unless the flow temp also increased and I don’t have that level of control. Heating the water earlier would give more time of course.

The lack of consistency doesn’t help. Today it has failed to get to target temp at all. Around 8am it had reached 21.7c but it has since fallen back to 21.5c at 11:15 and it’s not had any problem reaching 22c at OAT of 7c before and it started from a much colder OAT yesterday. It looks like it’s using around 0.5kWh currently. I suspect I’m right on the limit of flow temp for the living room heat loss.

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