# central heating pipe sizing

#### abh

Could anyone help me with information regarding the amount of heat pipes can carry? Searching through previous posts and the internet, there seems to be a bit of variation.

The situation is a 4 storey house. Boiler in basement. Completely new central heating pipes. 2 zones. Basement and ground floor (zone 1) in 22mm with 15mm to radiators. Second and third floors in 22mm with 15mm to radiators. Cylinder, pump and motorised valves in basement with boiler. Heat load for zone 1 is 11kw, zone 2 is 9 kW, cylinder 2 kW.

I was thinking that pipework to and from boiler before it splits at motorised valves would be OK in 28mm, but some information suggests that 28mm won't deliver enough heat. Should I use 35mm? Some posts suggest that 28mm is OK, though.

Any advice appreciated.

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anyone?

15mm = 4Kw (approx 14000btu)

22mm = 18Kw (approx 60000btu)

28mm = 30Kw (approx 120000 btu)

Thanks for that Dave.

I don't mean to question your knowledge, but I have seen other threads that suggest

15mm - 6kW
22mm - 13kW
28mm - 22kW

and it's obviously important I get it right. Do you know where I can get information about this to read up on?

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Nominal Pipe size Internal Area (m2) Water flow rate (litres/sec) Maximum theoretical heat flow rate. (Ignoring resistance)

(Kwatts )

8mm 0.0000363168 .054475217 2.506kw
10mm 0.0000608212 .091231851 4.197 kw
15mm .000149571 .0224356839 10.320 kw
22mm .000339795 .509691992 23.446 kw
28mm .000564104 .846156565 38.923kw

1 kw = 3402btu

Hope this gives you the values you were looking for.

To be on the safe side and to allow for the resistence of fittings etc i would reccommend that you went for around 75% of these values as a maximum

few boilers actually have 28mm connections,

therefore the only benefit you have from having such large bore pipe is reduced frictional losses.

This business about saying a pipe of bore x can transmit Y kW of heat is a bit of a misnomer because its largely dependent on the flow speed. I would also point out that as your rooms warm up the heat load reduces.

Personally I think you would be fine as you are. Upgrading to 28mm will cost you a fair wack and also you have to decide if you think its worth the ball ache!

Oh dear!!

Even the learned sanji has forgotten to mention that at a given flow rate the heat transferred depends on the temperature differential.

Most condensing boilers will operate with about 20°C diff whereas old standard efficiency was a nominal 11°C.

This means that at the same flow rate you can get twice the power output with a condensing boiler.

In practice its not quite so simple. The flow temperature was a nominal 80°C on the old boilers whereas now its designed around 70°C.

Tony

The pipe sizes would depend on the design, how the boilers are connected, make and model of the boilers, flow rates, through the boilers and system. And the pumps used

A small 50mm header connected to the boilers, in the same tapping size would be plenty, be it 22mm or 28mm, although you may need an shunt pump to prevent nuisance tripping

Agile wrote

This means that at the same flow rate you can get twice the power output with a condensing boiler.

So the greater the difference between F&R then the smaller the mass flow rate of water required and so the smaller the pipe work diameter needed. Yes ?.

Specific heat of water= 4.186 x 20C = 83.72

If so then why do the following graphs show the system with the increased TD between flow and return indicating a lower fractional heat output for a corresponding flowrate ?.

Could anyone explain what 'nuisance tripping' is, please?

So does that imply that my planned layout would not be sufficient? ie. 28mm flow and return, splitting into two zones, each zone distributing heat in 22mm? Should I use 35mm, or a header?

thanks for your help everyone, so far!

Could anyone explain what 'nuisance tripping' is, please?

So does that imply that my planned layout would not be sufficient? ie. 28mm flow and return, splitting into two zones, each zone distributing heat in 22mm? Should I use 35mm, or a header?

thanks for your help everyone, so far!

When the heat doesn't clear the boiler quick enough

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