CH flow and return pipe sizes

[jacobball]

Hi

May sound a silly question but should CH flow and return pipes (normal 3 bed semi with fully pumped system) be in 22mm and tee'd off in 15mm only up to the rads to get enough flow rhough to them?

 

[gas4you]

Usually yes.

15mm pipe work will efficiently carry up to around 14,000 btu of heat, so 22mm is used off the boiler and for the main carcass as you describe, unless it is a much larger system where 28mm or greater may have to be used.

 

[jacobball]

thanks gas4you - i take it this also applies with a combi (sealed) system that is under higher pressure? or is 15mm sufficient with sealed htg systems?


one other question - when a boiler is installed, ie 18 KW, what excatly is this 18KW? is it the power used by the boiler say over an hour? or whenever the boiler fires?

 

[gas4you]

Pipe sizing is applicable to what ever type of boiler/system you have.

Even if your heat needs were only about 4Kw (15mm size) it is still best to have the pipes off the boiler in 22mm to allow the heat to leave the boiler quickly.

18Kw is the maximum heat ouput the boiler can deliver. It is in effect 18Kw/hr.

If a modern boiler, then it will only use this maximum at start up in the morning when all rads are cold, usually.

Once the boiler detects the return water is warming up it will modulate the burner down, probably to a minimum of about 7-9 Kw, so it is only burning the minimum amount of gas and producing enough heat for the systems requirements at any specific time.

 

[D_Hailsham]

18Kw is the maximum heat output the boiler can deliver. It is in effect 18Kw/hr.

18kW is another way of saying the boiler produces 18 kilojoules of energy every second.

If an 18kW boiler is run continuously at full power for one hour it will have provided 18 kilowatt-hours of energy.