Condensing boiler, flow temp vs gas use
Posted: December 14th, 2022, 7:41 pm
Yesterday I put the heating on with the boiler flow temp set to 68C. Return temp is generally 45-50C.After a couple of hours, the chill was off the house despite being about -3 outside. The lounge hit a balmy 17C.
Reading the meter before and after showed gas use was a lot lower than feared, certainly lower than the 30kWh the boiler is rated at, and came in at roughly £1.85/hour. It was so much lower than I expected I checked the boiler and looked up the model no and GC no to check. As we have microbore pipes and the flow is poor, but heat loss from the pipes embedded in the wall is massive, presumably the boiler cycled. The room stat definitely didn't click as it was set well above ambient.
So today we tried it with the flow set to 50C, which the boiler spec sheet says is most efficient of the temps tested. Return temp was (according to Draper's finest IR thermometer and some tape on the pipe) the predicted 30C once the heating had been on a couple of hours. As MrsF was WFH we splashed out and turned the rads on in her office, but turned the bedroom TRVs right down. Checkin at 18:30 the hourly gas use was right down, about £13.20 for the 8.5 hours since it was turned down, plus 2 hours earlier and one last night before bed, all at 65C, and 30 min of hot water. Tonght we';; use the immesrion.
Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct. They also say your home will still be hot but take longer to get there, this is not correct. Because despite closing all the curtains at 4pm, and having had the heat on all day and the TRV open, the lounge is still 15C. And now the gas fire is on. I've just turned the flow temp up to 55C.
Our lounge is 6x4m, with 3 external wall, and because it was designed for the views it has 11m2 of glazing, and solid floor. The radiator is 1400x700 and double fluted, so at 11000 BTU at delta T65 (the difference between rad temp and room temp) is it well over the 8800 BTU the room requires. Except that at Delta T50 that drops to 8000 BTU, and at Delta T30 (which is about what we had today, the rad was 38C in the middle) the output is only 4000 BTU. And I expect doubling the radiators would increase the gas use.
Paul
Reading the meter before and after showed gas use was a lot lower than feared, certainly lower than the 30kWh the boiler is rated at, and came in at roughly £1.85/hour. It was so much lower than I expected I checked the boiler and looked up the model no and GC no to check. As we have microbore pipes and the flow is poor, but heat loss from the pipes embedded in the wall is massive, presumably the boiler cycled. The room stat definitely didn't click as it was set well above ambient.
So today we tried it with the flow set to 50C, which the boiler spec sheet says is most efficient of the temps tested. Return temp was (according to Draper's finest IR thermometer and some tape on the pipe) the predicted 30C once the heating had been on a couple of hours. As MrsF was WFH we splashed out and turned the rads on in her office, but turned the bedroom TRVs right down. Checkin at 18:30 the hourly gas use was right down, about £13.20 for the 8.5 hours since it was turned down, plus 2 hours earlier and one last night before bed, all at 65C, and 30 min of hot water. Tonght we';; use the immesrion.
Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct. They also say your home will still be hot but take longer to get there, this is not correct. Because despite closing all the curtains at 4pm, and having had the heat on all day and the TRV open, the lounge is still 15C. And now the gas fire is on. I've just turned the flow temp up to 55C.
Our lounge is 6x4m, with 3 external wall, and because it was designed for the views it has 11m2 of glazing, and solid floor. The radiator is 1400x700 and double fluted, so at 11000 BTU at delta T65 (the difference between rad temp and room temp) is it well over the 8800 BTU the room requires. Except that at Delta T50 that drops to 8000 BTU, and at Delta T30 (which is about what we had today, the rad was 38C in the middle) the output is only 4000 BTU. And I expect doubling the radiators would increase the gas use.
Paul