Boiler Flow Temperature for Different Outside Temperatures

Reed_Richards

Astria said:

Lower return water temperature is known to:

• Slower corrosion rates
• Less O2 released to attack metals
• Less Thermal Shock to the system and components
• Better on the expansion vessel as the membrane is kept cooler
• Reduces cavitation at pump and fittings
• Less noise/ creaking in the system
• Increased comfort through reduced heat gradient in the room
• Increased comfort through steady emitter output
• Safer
• Cleaner Air within the house
• Less loss through pipes in unheated areas
• higher comfort at lower room temperatures
• More efficient combustion/ heat transfer
• Condensate can clean heat exchanger and ensure better heat transfer
• More extracted latent heat from added condensing
• Longer burn times, less cycling of the boiler and boiler wear and tear from turning on and off the ignition sequence.

The list goes on...

I have highlighted the one that @Londoner_1 didn't seem to believe in.

Qyburn

nxdmsandkaskdjaqd said:

Qyburn said:

I did this for commercial builds, using mixing valve rather than varying boiler temperature but the final effect is the same. We used a ratio of 2:1, a 1 degree change in outside temperature made a 2 degree change in flow temp. With a design upper and lower limit. 

Is it possible for you to say the following:
-5C = x flow temp
0C = x flow temp
5C = x flow temp

I'm afraid I don't remember, but it would be different in each case. To take your example it would be something like 80 deg at -5C, 70 at 0 and 60 at +5. Or 70, 60, 50. The actual figures would depend on the building and the plant. These were all retrofit so we had the benefit of seeing how system worked under the old controls. I should also say that the 2:1 was a limit of our system at the time, but seemed to work OK.  When I made a dedicated controller for mixing valves we could use any relationship.

Reed_Richards

The two factors that control your Weather Compensation settings are the intended operating temperature of your radiators and the outside temperature when the output from your radiators is just enough to keep your house warm.

Suppose your radiators were sized for a Delta T of 50 C.  That means that to maintain your house at 20 C the average radiator temperature is 20+50= 70 C.  that might be achieved by setting a flow temperature of 80 C and balancing the radiators so the return temperature is 60 C.  Or 75 C Flow and 65 C return would give the same average radiator temperature.

Now suppose the radiators are sized so that your house will maintain 20 C inside when it is -10 C outside.  When it's 5 C outside you house will lose heat at half the rate it would when it's -10 C outside.  So you need only half the heat output from the radiators.  Although radiator output is not quite linear with Delta T, it's not far off.  So at +5 C outside you would only need Delta T to be about 25 C, which could be 55 C flow and 35 C return (or 50 C flow, 40 C return).

Generally when a heating system is installed the heating engineer will design around a particular Delta T for the radiators but using only rough calculations of heat output requirements for each room.  When there is a cold spell when the outside temperature does not change too much you could experiment to find how low you can make the boiler temperature to just keep the house warm enough and you might then have enough data to work out what weather compensation settings you need.