Thanks for the replies and opinions.....
If a room is at 15C the rediction in output from a radiator at 45C compared to 65C is only 40% and of course with less output the stored heat lasts longer as well.
Agreed and I understand your point completely with regard to temperature and time from the store. However, if I need 12kW to keep the place sensibly warm in the morning then I need 12kW don’t I? Once the rads start dropping they won’t be transferring 12kW as the temperature difference is just not there to keep the heat transfer rate high enough. I’ll try turning the combi CH right down and see if I can get some cooler flow temperatures to compare to at the moment.
I suppose this is why people talk of having oversized radiators to deal with lower circulating temperatures.
Just because the radiator design temperature is 65C doesn't mean that you have to run them at that temperature, you'll still get useful heat into the house at 35C.
This is the advantage of weather compensation, it adjusts the flow temperature so that the heat output matches the heat requirement. In normal coolish UK weather that will give a flow temperature of 30-45C. The temperature will only get up to 50 or more in very cold temperatures, which aren't that common, in the southern half anyway. The other advantage with a store based heating system is that the mixing valve on the return reduces the flow back to the store and creates and maintains stratification, so the high temperatures for DHW are kept for as long as possible.
Your figures look about right to me. I have a 2500l store and only need to light the boiler every 4-5 days in cool weather; it's been light 6 times in the last month with an average temperature of about 8C. I do let the temp drop to 45C though but it generally only gets heated to 75-80C.
I agree with your first point completely. I guess my point is that if I need the amount of heat transferred from the rads that I think I do, I simply won’t get it if the radiator temperatures are lower than planned. That said, I think that the “whole house method” and even normal radiator sizing is based on quite bad weather – something like minus 5 outside? On the majority of days I could probably get away with less heat and therefore lower radiator temperatures and consequently more time from the whole system.
The sticking point here is that theoretically I actually need 17kW for the whole house. I already dropped that to 12kW for the sake of my comparison based on my estimation of what we radiators we do run on “normal” winter days.
At 17kW output I’m actually down to 84minutes of CH with 65’C flow temperature. If I go for 17kW output but drop the useable temp to 40'C then I get approx 3hrs of CH – which would be fine. But I’d need larger radiators wouldn’t I?
I will do some reading on weather compensators, sounds like an excellent idea to alter flow temp based on actual temperatures and therefore actual heat requirements. I can see how that would yield efficiency improvements on any days that are warmer than the situation the CH was designed for.
Do people fit weather compensators to the CH outputs from thermal stores?
