Gas Pipe 15mm vs 22mm

[Truman]

Thanks to this site I've learned quite a bit about plumbing and heating (from a totally hands-off perspective) and so when I had my open vented system converted to a sealed combi system I was a bit alarmed to come back and find the plumber connecting up my new 30kW combi to the existing 15mm gas pipe via a reducer. The boiler itself has a 22mm inlet for gas, plus the length of the 15mm gas pipe was approx 3.5 metres back to the gas meter tee.

I questioned him about this and he said he'd checked and the boiler would get enough gas and it would be OK, but on my insistence he changed the run to 22mm. Now then, I've been through the MI's and it doesn't state that 22mm must be used, nor does it mention if there's a limited run that can be made in 15mm. So, my question is, was I correct to insist on this and was he trying to pull a fast one? Surely no 30kW combi on full power for DHW could get by on 3.5 metres of 15mm gas pipe?

I know this is all academic now, and the boiler has a full 22mm run now - I'm just curious. And yes, the guy is on gas safe.

 

[Billy Bob Bob]

Thanks to this site I've learned quite a bit about plumbing and heating (from a totally hands-off perspective) and so when I had my open vented system converted to a sealed combi system I was a bit alarmed to come back and find the plumber connecting up my new 30kW combi to the existing 15mm gas pipe via a reducer. The boiler itself has a 22mm inlet for gas, plus the length of the 15mm gas pipe was approx 3.5 metres back to the gas meter tee.

I questioned him about this and he said he'd checked and the boiler would get enough gas and it would be OK, but on my insistence he changed the run to 22mm. Now then, I've been through the MI's and it doesn't state that 22mm must be used, nor does it mention if there's a limited run that can be made in 15mm. So, my question is, was I correct to insist on this and was he trying to pull a fast one? Surely no 30kW combi on full power for DHW could get by on 3.5 metres of 15mm gas pipe?

I know this is all academic now, and the boiler has a full 22mm run now - I'm just curious. And yes, the guy is on gas safe.

no 3.5 m of 15 mm would not cope,depending on various factors,bends,tees,length of run,other appliances etc 28 mm may also be needed

 

[Truman]

Thanks. I didn't think it was, and I'm certainly no RGI. There were at least 3 x 90 degree elbows on the 15mm. For reference I also have a gas hob and a connection for gas fire (though I don't have a gas fire at present), all piped off the same run back to the meter.

 

[tamz]

A 3m (straight) length of 15mm will discharge 2.9m3/hr of gas with an allowable pressure drop of 1mb over its length. Add 1/2m to the length for every bend or tee for calculation.
A 30 kw boiler uses approx 2.8m3/hr (it depends on the calorific value of the gas which varies region to region for an exact figure).
Without getting too technical you were right. but it wasn't necessarily dangerous.

 

[kirkgas]

Also some boiler instruction say pipe to boiler must be equal to or larger than connection at boiler, or it will say must be piped as per BS 6891, which requires the calculation Tamz did, to prove an adequate supply

 

[Truman]

So WTF was he thinking then?

 

[tamz]

He wasn't

 

[Truman]

Hrm well I've tried the 4 rings on the hob all on, then opened a hot tap to full blast and the hob rings don't even flicker which is good. I doubt with the 15mm that they'd have stayed lit.

 

[tamz]

They would have of that i am 100% sure

 

[Truman]

OK, so why the need for 22mm then? I thought it was to ensure all gas appliances worked ok if on full bore at once. I'm learning all the time here! Still glad I insisted on 22mm though

 

[AWheating]

your appliance is designed to work with a certain gas supply. Its your engineers job to ensure the appliance has this so the boiler works as required. Undersizing could have no other effect than the appliance does not work as well as designed, it can also have the effect of a dangerous situation.

 

[Rickster123]

Let's have a pic of the install, I'm intrigued.

 

[tamz]

OK, so why the need for 22mm then? I thought it was to ensure all gas appliances worked ok if on full bore at once. I'm learning all the time here! Still glad I insisted on 22mm though

I'm not going to give you a lesson on gas as sometimes a little knowledge is a dangerous thing.

You did right to pull the guy using your little bit knowledge. He should have known better.

You were never placed under any danger from what he did although it wasn't right.

I can guarantee you he will do the same again on the next job similar to yours because he is a thick :nono: and there will be no recall.
Such is the nature of our trade.
The same as every other field, there are good guys and bad guys, conscientious guys and those who couldn't care less. Those who know, those who should know better and those who couldn't care less.

 

[Truman]

Absolutely spot on there tamz. What annoys me is I thought he was one of the good guys, not the corner cutters. Oh well you live and learn. I know if I'd been in the wrong about the size he'd have told me to f off rather than begrudgingly do the job he knew he should have done in the first place. You may well be right that it wouldn't have been dangerous but that the appliance wouldn't be operating at its best. Like buying a fast car then sticking a brick under the accelerator.

 

[tamz]

Kind off but your boiler has a sort of turbo that means it will get what it needs in this situation. (think about it)

Usually it all comes down to time and money or laziness. Times are hard and money is being squeezed so corners are being cut.
It's not right but it is the way many operate and it will never change.

I'm being brutally honest here but there are a LOT of good guys out there whop do things as they should be done. Unfortunately it's knowing where to find them as most good tradesmen are bad salesmen.

 

[happyflyer]

Truman,


No one is coming out saying its was a hanging offence, Tamz indicates it was marginal and the PD from calculation would be marginal, maybe that's what the GSR man thought too, bit silly for such a short bit of pipe
not to change it to 22mm, probably a more pointed question would be this:

When the GSR clocked the gas meter rate with the boiler on full fire what would the acceptable % reduction would he walk away from, if your man did in fact do a gas meter rate test
maybe he got the full rate, what would we do then.

You see you can calculated all you want and worked to the recommended and allowable PD meter to appliance, but you still need to do the rate test, it may have been OK.

BTW are you sure the boiler gas final connection was indeed 22mm, what make was the 30Kw Boiler

 

[happyflyer]

Absolutely spot on there tamz. What annoys me is I thought he was one of the good guys, not the corner cutters. Oh well you live and learn. I know if I'd been in the wrong about the size he'd have told me to f off rather than begrudgingly do the job he knew he should have done in the first place. You may well be right that it wouldn't have been dangerous but that the appliance wouldn't be operating at its best. Like buying a fast car then sticking a brick under the accelerator.

Most drivers on the road need a brick under their accelerator, let me get to work faster.

Baldy Old Git