Radiator Temperatures and Outside Air Temperatures

[Johntheo5]

In view of recent threads re getting boilers to automatically achieve flow/return dTs of up to 20C maybe this will give some idea of how reasonable boiler efficiencies might be obtained by simply adjusting the boiler setpoint temperature with respect to the OAT, (Outside Air Temperature) on boilers without Weather Compensation. Its assumed that the rads are sized for a minimum OAT of -5C and a required room temperature of 20C, I have used two flow/return dTs (at 100% rad output), the 10C figure might be the more usual dT with all rads opened up and on, fairly meaningful condensing occurs from OATs of 5C and higher and boiler flow temp of 55C/60C and lower.
Just changing the rad temperature by 10C for each 5C change in OAT will work quite well, my daughter uses the table below and achieves very reasonable condensing with excellent boiler efficiency with a 20kw Vokera Vision 20S.

Boiler Temp OAT Boiler Output
75C -5C 100%
65C 0C 80%
55C 5C 60%
45C 10C 40%
35C 15C 20%

 

[Johntheo5]

Correction (Rev1) to the 20C data, I hadn't done all the settings at a constant dT of 20C, its a bit academic anyhow as the pump speed probably couldn't get low enough and the boiler wouldn't be happy with those very low flowrates at increasing OATs.

 

[ericmark]

My house has double-glazed windows, and is reasonably well insulated, also the living room is heated with the sun through the windows, often the outside temperature goes down with clear skies, but also clear skies means sun heats the house more, so really can't see how knowing outside temperature really helps, at least not without also knowing a lot more about the weather, sunshine, wind direction etc.

 

[Johntheo5]

Thats true, all my main "living quarters" are south facing so the solar effect has a huge influence on the room temperatures during the day, but my roomstat/TRVs take care of that and in the winter evenings with no solar (sun) effect from say 5 to 130 the next morning when we retire, the OAT/boiler flowtemp relationship works pretty well, if one has weather compensation then the curve can be easily changed to give optimum results.