Gas pipe sizing

FreeBear

Having a new boiler fitted, and have some concerns about the pipe being specified. So crunched some numbers, and hope someone can double check my figures.

Boiler is rated at 30KW

Length of pipe required between meter & boiler - Approximately 8m.

Number of 90° elbows to be used - 10.

Using the Copper Pipe Sizing Charts from https://www.gb-gas.co.uk/download.html, the pressure drop for 22mm pipe is 0.0684 milliBar per metre (0.4069 mb for 15mm).

Effective length of 90° elbows - 15mm, 0.4m. 22mm, 0.6m.

Engineer is proposing to use 15mm for the last metre (and 4 elbows) with the remainder in 22mm (6 elbows)

So that equates to an effective length of pipes as [15mm] 1m + (4 x 0.4m) = 2.6m. [22mm] 7m + (6 x 0.6m) = 10.6m

At a gas flow rate at 30KW, this gives a total pressure drop of (2.6m x 0.4069mb) + (10.6m x 0.0684mb) = 1.78298 mb

If he were to use 22mm from meter to boiler, the effective length would be 8m + (10 x 0.6m) = 14m with a pressure drop of (14m x 0.0684mb) = 0.9576mb.

Assuming the maximum permissible pressure drop between meter & boiler is 1mb*, using even a short length of 15mm pipe means this is not achievable. With 22mm pipe, it is going to be close.... Do I do one of the following -

1. Let him fit 15mm pipe and then report him to Gas Safe when the pressure check fails (and probably have to pay someone else to correct the work).
2. Tell him to stop work and demand a different engineer completes the installation (adds more delays).
3. Crunch the numbers in front of him and insist he uses 22mm (or even 28mm) and no 15mm.
   

*) https://registeredgasengineer.co.uk/technical/low-pressure-and-under-sized-pipework/

BS 6891(1) states that, for natural gas installations, the design pressure loss between the outlet of the primary meter installation and the point to be connected to any appliance inlet shall not exceed 1mbar at the design installation maximum flow rate

Zandoni

I've just had a new boiler fitted and it's perfectly normal for the last stretch of pipe to be 15mm, many boilers including Vaillant have a 15mm connection. When he commisissions the boiler he will run many checks to ensure you have the correct pressure and there will be no need to report him.

ThisIsWeird

Hi FB.

Yes, it's weird that a combi boiler that will in almost all cases require at least a 22mm supply pipe ends up having a 15mm gas inlet. That really confused me when I fitted my GlowWorm 15-odd years ago.

10 elbows is a lot! Are they all standard solder elbows, or 'swept', or formed in the pipe?

Like you, I did the calcs for my own job, and it was 'ok', and a pressure reading once done confirmed this. I still see a low-burning gas ring 'flicker' whenever the combi fires up...

What to do? I dunno. But since you have done the calcs, and therefore have concerns - whether legit in practice or not - then it surely bears a chat? From what I've seen, most plumbers do not do the calcs, but rely on 'experience' - not sure whether that is a good thing. Eg, before I fitted my own boiler, I had a few plumbers out to quote. They all said 'the pipe needs replacing with 22mm copper (the old one was lead!), but not one did a calculation.

So, a chat for reassurance, and then check the benchmark - I understand it should have the boiler's running gas pressure as one of the results? You will then most likely have the boiler serviced after a year or so to maintain the warranty? You could always use a different GS and ask them what pressure figure they get. If there's any concerns, you kick & sue the bottom of the first guy, and report him.

ComicGeek

FreeBear said:

Having a new boiler fitted, and have some concerns about the pipe being specified. So crunched some numbers, and hope someone can double check my figures.

Boiler is rated at 30KW

Length of pipe required between meter & boiler - Approximately 8m.

Number of 90° elbows to be used - 10.

Using the Copper Pipe Sizing Charts from https://www.gb-gas.co.uk/download.html, the pressure drop for 22mm pipe is 0.0684 milliBar per metre (0.4069 mb for 15mm).

Effective length of 90° elbows - 15mm, 0.4m. 22mm, 0.6m.

Engineer is proposing to use 15mm for the last metre (and 4 elbows) with the remainder in 22mm (6 elbows)

So that equates to an effective length of pipes as [15mm] 1m + (4 x 0.4m) = 2.6m. [22mm] 7m + (6 x 0.6m) = 10.6m

At a gas flow rate at 30KW, this gives a total pressure drop of (2.6m x 0.4069mb) + (10.6m x 0.0684mb) = 1.78298 mb

If he were to use 22mm from meter to boiler, the effective length would be 8m + (10 x 0.6m) = 14m with a pressure drop of (14m x 0.0684mb) = 0.9576mb.

Assuming the maximum permissible pressure drop between meter & boiler is 1mb*, using even a short length of 15mm pipe means this is not achievable. With 22mm pipe, it is going to be close.... Do I do one of the following -

1. Let him fit 15mm pipe and then report him to Gas Safe when the pressure check fails (and probably have to pay someone else to correct the work).
2. Tell him to stop work and demand a different engineer completes the installation (adds more delays).
3. Crunch the numbers in front of him and insist he uses 22mm (or even 28mm) and no 15mm.
   

*) https://registeredgasengineer.co.uk/technical/low-pressure-and-under-sized-pipework/

BS 6891(1) states that, for natural gas installations, the design pressure loss between the outlet of the primary meter installation and the point to be connected to any appliance inlet shall not exceed 1mbar at the design installation maximum flow rate

A 30 kW combi is likely to have a gas input of more than 30 kW though, probably more like 3.2 m3/hr rather than 2.8. So the pressure drops per m length are going to be higher than you've used above, and well above 1 mbar for 100% 22mm - what the installer is suggesting would give a pressure drop over 2 mbar.

They would have to use 28mm for at least half the installation length to comply.

Zandoni

It seems to me that everyone is going crazy about gas pipe sizes, have a look at this. https://londonboilercompany.com/gas-pipe-size/

ComicGeek

Zandoni said:

It seems to me that everyone is going crazy about gas pipe sizes, have a look at this. https://londonboilercompany.com/gas-pipe-size/

Not really - it really is a very simple calculation to undertake, that's why there are detailed gas sizing charts out there. Putting a finger up in the air and guessing isn't needed, it's simple to check. That video link has absolutely nothing to do with the gas pressure losses due to pipework, it's to do with the operating pressure of old vs modern gas valves. I completely agree with the link that you don't rely on the gas pressure reading on the old boiler - but if they had bothered to spend 5 mins calculating the system pressure loss then they would have known the approximate result before they started work.

If one installer hasn't included for upgrading the gas pipework then that is an unexpected cost for the consumer - the consumer could have gone with someone who would have done it correctly, but was slightly more expensive due to rightly allowing for gas pipework replacement. 

Boilers may work at a lower gas pressure than 20 mbar, but just because it does doesn't make it compliant with the regs. Most boilers would work down to 16/17 mbar but doesn't make it compliant.

Zandoni

The video doesn’t just just discuss the difference between old and new gas valves, it’s point is that you can’t tell by testing the old boiler. I have just had three quotes for a new boiler and all of them said they will test at the end to see if my gas pipe would need upgrading. 

In my case it’s certainly not easy to do any calculations because my gas supply is in barrel which is buried in the floor. Luckily for me mine was OK and that saved a lot of upheaval.

FreeBear

ThisIsWeird said: 10 elbows is a lot! Are they all standard solder elbows, or 'swept', or formed in the pipe?

It is a long run, going round two corners of the building, and through two walls. Using 10 elbows would be worst case - It may well be that two or three could be swapped for bends, and thus reduce the effective length by 0.4-0.6m (not enough to make that much difference to the calculations).

ComicGeek said: A 30 kW combi is likely to have a gas input of more than 30 kW though, probably more like 3.2 m3/hr rather than 2.8. So the pressure drops per m length are going to be higher than you've used above, and well above 1 mbar for 100% 22mm - what the installer is suggesting would give a pressure drop over 2 mbar.

They would have to use 28mm for at least half the installation length to comply.

Had used 3.7m³/hr when I first did the calculations, but dropped it down just to be gentle (that and wasn't sure which column of figures to use in the installation manual).

Did query pipe sizing with the installer yesterday - He acknowledges that 1mb is the maximum pressure drop, pointed out the gas connection on the boiler is only 15mm, and his proposed run of pipe will be fine. So I'll let him carry on and make sure he does all the paperwork.

Should (when ?) the gas supply fail the pressure drop test, and they try to charge me extra for a 28mm upgrade, I'm ready for a fight.

FreeBear

Zandoni said:

The video doesn’t just just discuss the difference between old and new gas valves, it’s point is that you can’t tell by testing the old boiler. I have just had three quotes for a new boiler and all of them said they will test at the end to see if my gas pipe would need upgrading.

For me, the old boiler is out of commission, so testing the old boiler is not an option. In addition, the new one is being installed in a different location, so a new supply pipe is required. The installer doing *this job is winging it, and I'm pretty sure he hasn't done any calculations to ensure the pipe is going to be big enough.

Zandoni

FreeBear said:

Zandoni said:

The video doesn’t just just discuss the difference between old and new gas valves, it’s point is that you can’t tell by testing the old boiler. I have just had three quotes for a new boiler and all of them said they will test at the end to see if my gas pipe would need upgrading.

For me, the old boiler is out of commission, so testing the old boiler is not an option. In addition, the new one is being installed in a different location, so a new supply pipe is required. The installer doing *this job is winging it, and I'm pretty sure he hasn't done any calculations to ensure the pipe is going to be big enough.

I’m sure it’s not his first rodeo, but if your not happy just tell him the size of pipe you want and be prepared to pay a bit more.

ComicGeek

Zandoni said:

The video doesn’t just just discuss the difference between old and new gas valves, it’s point is that you can’t tell by testing the old boiler. I have just had three quotes for a new boiler and all of them said they will test at the end to see if my gas pipe would need upgrading. 

In my case it’s certainly not easy to do any calculations because my gas supply is in barrel which is buried in the floor. Luckily for me mine was OK and that saved a lot of upheaval.

But testing the old boiler tells you nothing about the pressure losses in the pipework between the meter and boiler. The company fails to state that the more onerous requirement is to achieve a maximum pressure drop of 1 mbar from meter to boiler - it doesn't matter if the lowest working pressure of the boiler is 16/17 mbar, it still needs to achieve the maximum 1 mbar drop.

But what if the better answer is to install a slightly smaller boiler (with lower gas flow rate) rather than digging up your floor and replacing the pipework? Bit late if the larger boiler is already installed.

I still think it's ridiculous not to do the calc first.