# Boiler gas supply pipe sizing

Discussion in 'Plumbing and Central Heating' started by struc, 5 Apr 2016.

1. ### struc

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Hello,

I have a few questions about gas supply pipe sizing calculations I was hoping for help with.

Q1. I am looking at replacing an existing combi boiler with a Worcester Bosch 38CDI. I've had a quote saying that the pipe may need upgrading to 28mm and that upgrading the first 5m should be enough. However, the existing gas supply pipe is 22mm diameter and I estimate has an effective (i.e. including allowance for bends) length of around 26m. Having looked into the calculations (for example using http://www.installeronline.co.uk/wp-content/uploads/2013/06/Size-matters.pdf ) it seems that for a WB 38CDI which requires a flow rate of 4.2m^3/h around 12m of 22mm diameter pipe would produce 1mbar pressure drop meaning that far more of the pipe would need replacing than quoted. Am I missing something here, are these calculations quite conservative?

Q2. The gas pipe runs from the meter on the ground floor, to the boiler which is also on the ground floor, via the upstairs (i.e. under the floor boards upstairs). If it is required to upgrade more of the pipe then it is going to be difficult to replace the portion of the pipe that runs from the ground floor to upstairs as it is behind kitchen units. One thought I had was to replace the pipe on the ground floor and the pipe that is upstairs and leave the difficult to access pipe that runs between the two floors. The guy didn't think you could do that (and like I say didn't think it was necessary anyway) and seemed to suggest that the upgraded portion would need to all be at the start of the pipe to have an effect. Obviously he does this for a living so I took his word for it, but given it's a pressure loss calculation I wasn't sure why that would be? What I mean is, are you not effectively just summing the frictional losses along the pipe for a given flow rate, so it wouldn't matter where each pipe was in the system.

Many thanks

Last edited: 5 Apr 2016
2. ### Dan Robinson

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Yes, but gas is compressible, so downstream pipe can be effective, but it depends on other appliances.

Last edited: 5 Apr 2016
3. ### Agile

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Yes, you are right its just summing the pressure drops.

He is wrong and that's a common misconception even amongst RGIs who are expected to understand it all.

But the problem is that most have not received basic scientific training and don't automatically think about the problem correctly.

The part near the meter usually has a lot of elbows and renewing that is most beneficial. Sometimes the pipework near the boiler can be usefully increased.

Tony

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4. ### struc

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Thanks for the replies! Does anyone have any thoughts on my first question. i.e. a 22mm diameter pipe that is 12m long with a flow rate of 4.2m^3/h would get a 1mbar pressure drop. Therefore as I have around 26m of pipe I’m going to need to replace alot more than the quoted 5m of pipe.

5. ### Steelmasons

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I'm sure the RGI who's quoting the gas supply upgrade will take care of this for you , failing that then I suggest you book yourself on a gas course.

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6. ### struc

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He says upgrading the first few meters will be enough. I'm just concerned in case this is wrong and I have a boiler installed that it is a nightmare job to adequately supply gas too.

I've had a few quotes, the first guy didn't mention the gas supply would need upgrading. The second guy mentioned that it may do but left it at that. Though he didn't know the routing of the gas pipe so had little to go on. In the intervening time before the third guy came i traced the rough route of the gas pipe and asked him about upgrading the pipe. After receiving the quote i looked into how do the calculations as non of the guys seem to have done any and because it will be difficult to replace where the pipe runs from down stairs to upstairs. Having done the calculation i can't see how replacing the first few meters is going to be enough unless I've misunderstood the tables or they are very conservative.

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7. ### durhamplumber

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To achieve 4.2cubic metres of gas /hour from a pipe 26m effective length,the whole run needs to be 28mm,surprising,but true.This gives a 0.9mb drop,and 1mb drop is specified as the max by WB.As we know there is a huge difference between the real world and ideal world,but any problems and the pipe is too small,WB will walk away.
If the pipe could be a mixture of 22 and 28mm pipe,it makes no difference whether the pipe is at the beginning middle or end,that stuff about compression of the gas in the pipe is nonsense.

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8. ### struc

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Thanks, Unless anybody disagrees with this, then I guess I'm probably better off getting another guy to do the work then.

As you have suggested it's relatively close to 1mbar drop assuming 28mm all the way, and if i was to keep the pipe that runs from downstairs to up stairs at 22mm the numbers show a pressure drop of 0.98mbar, which seems very tight given all the uncertainty (ie the exact pipe length, the type of bends etc). Am wondering if I might be best off reducing the boiler to a 34CDI (which requires lower gas flow rate) then I have a better chance of not having to upgrade the pipe going from downstairs to upstairs. ahhh, decisions... needless to say the wife is busting my balls about why i haven't got the new boiler sorted yet.

9. ### struc

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.... would it be bonkers for them to use 35mm diameter pipe where necessary. For example 22m of 35mm diameter and 4m of 22mm diameter looks like it would be well in. If so i guess i don't need to worry if they can't get to all the pipe to change it.

10. ### durhamplumber

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Why do you need such a big Combi,28kw is more than adequate for most heating systems,and gives a good shower.On the other hand,if you wish to run a shower and bath at the same time,even a 40kw Combi will struggle.

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11. ### struc

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Good question. Basically, we're currently in the process of getting a new bathroom. As part of that we will be changing from our current electric shower which is pretty pathetic, and the wife says cannot wash her hair, to a mixer shower. It will have one of those rainfall showers heads and a handheld shower. I wanted to make sure the boiler didn't struggle when the rainfall shower was used and as it's around £100 to move up a model I was playing it safe and going bigger. Obviously didn't know about gas pipes then though...

12. ### Phil12345

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What's your mains flow and pressure struc?

13. ### struc

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The flow rate is currently only around 16 litres per min. But we currently have a lead supply pipe which I was going to get replaced. Am assuming the flow rate will be much better once that's done.

I don't know the pressure. I believe I need some sort of device to measure that which I don't have.

14. ### BigSnoopy01

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16lpm is not enough for the boiler your looking at as this flow will be shared between the hot and cold. The guy whos quoted the upgrade seems to be looking to do the job correclty as many would break the regs and manufacturers instructions and not even upgrade. phone him up and tell him to price for a full 28mm gas carcus ontop of the quoted given.

whats the new showers max flow rate and the heads flow rate?

15. ### PullerGas

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This is clearly DIY gas work!!

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