# Gas pipe sizing

#### omph

I am currently studying gas pipe sizing calculations and am somewhat stumped by the flow rate.

Since the flow rate is based on m3/per hour, how is this established ? Is it based on the max output of the appliance per hour ?
Lets say I have a 30kW boiler running at max, this would convert to 2.84m3 of gas used per hour - Is this the correct flow rate for the appliance ?

Also, what if the boiler was range rated - lets say a 30kW heat only boiler was reduced to 10kW, would the pipe sizing still need to be for 30kW ?

Better looking at it, than for it.

(I dunno but i would not want the boiler limited by gas flow).

Lets say I have a 30kW boiler running at max, this would convert to 2.84m3 of gas used per hour - Is this the correct flow rate for the appliance ?
I don't see why not. But when calculating pressure drop, also need to include gas used by other appliances off the same pipe, cooker, fire etc.
Also, what if the boiler was range rated - lets say a 30kW heat only boiler was reduced to 10kW, would the pipe sizing still need to be for 30kW ?
I doubt the regs would allow that. And why install a 30 kW boiler if you only need 10 kW? Unless you have a very large house, 30 kW is likely to be from a combi in HW mode.

pipe must be able to supply boiler at its highest setting

pipe must be able to supply boiler at its highest setting

Boiler Setting or installed rating or highest rating?

Highest rating.

Boiler Setting or installed rating or highest rating?
highest rating

I have one more question, is it acceptable to mix and match sizings ? Lets say I have a 12m run on a 22mm pipe that is powering 2 boilers - both boilers have a collective flow rate of 5.6m3.

22mm @ 12m has a max acceptable flow rate of 3.9m3. What if for the first 6m I ran a 28mm, and use 22mm for the rest - is this be acceptable ? 28mm @ 6m = 12m3 which is more than enough, and the 6m @ 22mm = 5.8m3 which is just about enough for the two boilers.

I have one more question, is it acceptable to mix and match sizings ? Lets say I have a 12m run on a 22mm pipe that is powering 2 boilers - both boilers have a collective flow rate of 5.6m3.

22mm @ 12m has a max acceptable flow rate of 3.9m3. What if for the first 6m I ran a 28mm, and use 22mm for the rest - is this be acceptable ? 28mm @ 6m = 12m3 which is more than enough, and the 6m @ 22mm = 5.8m3 which is just about enough for the two boilers.
its basic standard practice

@omph - are you doing the work or a gas safe registered engineer? If the latter, as per @gas112 it’s basics and they should be doing it.

I have one more question, is it acceptable to mix and match sizings ? Lets say I have a 12m run on a 22mm pipe that is powering 2 boilers - both boilers have a collective flow rate of 5.6m3.

22mm @ 12m has a max acceptable flow rate of 3.9m3. What if for the first 6m I ran a 28mm, and use 22mm for the rest - is this be acceptable ? 28mm @ 6m = 12m3 which is more than enough, and the 6m @ 22mm = 5.8m3 which is just about enough for the two boilers.
You are along the right lines but you would need more 28 and less 22 or the pressure drop would be too great. Use your training material as this is basic gas. if your not qualified then leave it to those who are

@omph - are you doing the work or a gas safe registered engineer? If the latter, as per @gas112 it’s basics and they should be doing it.
I'm studying towards it, still pretty fresh and loads to learn but moving along quickly.

CBW
You are along the right lines but you would need more 28 and less 22 or the pressure drop would be too great. Use your training material as this is basic gas. if your not qualified then leave it to those who are
I guess you can say I am a little too eager - doing loads of reading - and am way ahead of the current material (which is a bit boring tbh).
When you say there needs to be more 28mm than 22mm, is there a ratio or method to this ? I am going off the flow rates on the sizing sheets and everything seems good. I guess I should calculate the pressure drop for the entire run but need to get the math around my head.

There's this online calculator if you haven't already got one. I think it has some safety, my calcs give somewhat lower pressure drop

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