dont forget that any part of the gas run can be routed externally, even if it had to go a bit further it could be upsized accordingly. any rgi should be able to work out the pipe size required to ensure your new boiler works properly.
Condensing combi and size of gas supply pipe
- Thread starter Shiveringinsuffolk
- Start date
Sponsored Links
28/30Kw combi is going to draw approx 3-3.3m3 gas /hr.
OP has stated 14m of pipe, which that alone if straight is on the limit for that volume of gas. Add to that the 6 or 7 bends he mentions and we are now undersized.
Then add to that the other appliances, not forgeting the general rule that if 2 appliances you double the equivalent length of the pipe so each appliance only has 0.5mBar drop to it and you soon see why 28mm will be need for a fair bit of the distance.
Manufacturers assume that we will size the gas pipe correctly, but then reduce down for the last few meters to 22mm.
OP has stated 14m of pipe, which that alone if straight is on the limit for that volume of gas. Add to that the 6 or 7 bends he mentions and we are now undersized.
Then add to that the other appliances, not forgeting the general rule that if 2 appliances you double the equivalent length of the pipe so each appliance only has 0.5mBar drop to it and you soon see why 28mm will be need for a fair bit of the distance.
Manufacturers assume that we will size the gas pipe correctly, but then reduce down for the last few meters to 22mm.
Thanks for all the answers - been very helpful.
Gas4you: "In terms of percentages, I would say it will be 80% certain you will need a gas pipe upgrade" is exactly what I didn't want to hear, lol - I was hoping for 80% the other way!
Changing tack a little, is the a condensing combi out there that would give me a reasonable performance which would almost certainly not require a gas pipe upgrade? With the vast majority of the existing pipe under the floor, and a very difficult to handle route from boiler to gas main, I really do want to avoid upgrading the gas pipe. It's a old listed timber-frame property so has a pretty unusual layout. My old Potterton Puma 100E gave a pretty decent performance so it wouldn't need to be any better than that.
I'm not too worried about getting a slower water flow rate (as long as the boiler still works), but i did read somewhere that an undersized gas pipe can cause your cooker hob flame to go out if too much gas is sucked away to other appliances. I slighly lower performance I can put up with, but I obviously don't want to do anything risky safety wise.
Gas4you: "In terms of percentages, I would say it will be 80% certain you will need a gas pipe upgrade" is exactly what I didn't want to hear, lol - I was hoping for 80% the other way!
Changing tack a little, is the a condensing combi out there that would give me a reasonable performance which would almost certainly not require a gas pipe upgrade? With the vast majority of the existing pipe under the floor, and a very difficult to handle route from boiler to gas main, I really do want to avoid upgrading the gas pipe. It's a old listed timber-frame property so has a pretty unusual layout. My old Potterton Puma 100E gave a pretty decent performance so it wouldn't need to be any better than that.
I'm not too worried about getting a slower water flow rate (as long as the boiler still works), but i did read somewhere that an undersized gas pipe can cause your cooker hob flame to go out if too much gas is sucked away to other appliances. I slighly lower performance I can put up with, but I obviously don't want to do anything risky safety wise.
As Dave has explained above, its obvious to anyone that the gas supply will need upgrading. To become an RGI you will have learnt pipe sizing.
What you "feel" about the regulations is not relevant. We are ALL expected to abide by the regulations. That includes you !
If you dont bother then I am sorry to hear that you dont want to but I would have to define you as a cowboy making substandard installations.
If you think thats unfair then please explain why you think you should undercut the observant installers who ensure their installations are all kosher.
Tony
Sponsored Links
tony is right, just because a boiler has a particular size gas inlet doesnt mean you run it in that size! each job is different and needs working out on site. im surprised there are people who think pipe sizing is not important.
I assess ACS gas candidates.
About a third can't do the calculations properly.
About a third don't realise that the calculations are hopelessly optimistic.
A few think you can go by the size of the inlet to the boiler ( they usually fail)
About half have no idea why one length of straight 22mm pipe will have a much higher pressure drop than another at the same flow rate. (So their installations always have a drop which is too high.)
It would be interesting to run a poll of gas men, on how often the "less than 1mbar drop" requirement is actually achieved on a 24kW+ installation, where the pipe length is say more than 10 metres. I'd estimate about 10% !
About a third can't do the calculations properly.
About a third don't realise that the calculations are hopelessly optimistic.
A few think you can go by the size of the inlet to the boiler ( they usually fail)
About half have no idea why one length of straight 22mm pipe will have a much higher pressure drop than another at the same flow rate. (So their installations always have a drop which is too high.)
It would be interesting to run a poll of gas men, on how often the "less than 1mbar drop" requirement is actually achieved on a 24kW+ installation, where the pipe length is say more than 10 metres. I'd estimate about 10% !
Perhaps I am in the half which have no idea but I suppose you really mean to say than some know what causes a higher than calculated pressure drop.
Typical causes are
size discontinuities at couplers
cutting with a pipe slice but not reaming the cut end afterwards
Internal solder blobs
Roughness from corrosion
Roughness from flux left inside
Actual manufacturing roughness
squashing the pipe during transportation
and probably a few more!
Have I missed much Chris? Or were you looking for something completely different?
Something different:-
Gas loses more pressure drop when going upwards than downwards!
Thats about all I can think of now!
Tony
HI,
I am thinking of replacing my 20yrs old Potterton Kingfisher II & interested in Gas Pipe Size issue.
BG came out & said there may be an issue with pressure, If I accepted the quote they would check the pressure.
No mention of calculating pipe length bends etc.
They are unable to do the calculations because my meter is about 100 meters away at the bottom of my drive.
Can't remember the reason why now now but 20 years ago when we built the house BG laid a new mains some distance up the road and we had two new supplies/meters at our gate and I think they felt our expense to get the rest of way to houses.
From Road with v.large 2" yellow pipe, reduced to go through meter then back to large yellow pipe (Think about 2") 90+ meters to a few meters short of house, I think to 22mm under concrete & tiled floor 4m to boiler.
Do you think the reduction at the meter then opening out to the larger pipe will have a significant effect on pressure. BG thought prob. be OK.
Why are these these calc's needed. If a pressure gauge is put on the pipe just where it would go into the boiler and measured with any other gas applicances flat out, surely taking pipe size into accout this would show flow rate and any fluctuations if held on long enough. There must be hundreds of situations domestic or industrial where there is no idea where the pipe run is.
Why do these new boilers require so much larger supply , my boiler use to run gas oven (Not any more), hob, fire & 100,000 btu inefficient boiler without any problems. If these new HE boilers are meant to save 30% gas why do they need a bigger supply, do they burn super hot/fast?
Off the subject a little, my Boilers on internal wall, existing flue straight up through roof, looks a lot bigger than new ones, do you think the new flue could be run up inside the existing. (might physcially not be enough room, not sure how think double wall was) woud save reflashing and messing with tiles.
Is above small conseravatory, BG wanted £600+ for scaffolding and to re-flash or a good few hundred £'s to route the flue out of the way of the conserv. and through the tiles aagin, meaing leaving the old, disused flue.
Regards
Obviously I have no training in Gas.
I am thinking of replacing my 20yrs old Potterton Kingfisher II & interested in Gas Pipe Size issue.
BG came out & said there may be an issue with pressure, If I accepted the quote they would check the pressure.
No mention of calculating pipe length bends etc.
They are unable to do the calculations because my meter is about 100 meters away at the bottom of my drive.
Can't remember the reason why now now but 20 years ago when we built the house BG laid a new mains some distance up the road and we had two new supplies/meters at our gate and I think they felt our expense to get the rest of way to houses.
From Road with v.large 2" yellow pipe, reduced to go through meter then back to large yellow pipe (Think about 2") 90+ meters to a few meters short of house, I think to 22mm under concrete & tiled floor 4m to boiler.
Do you think the reduction at the meter then opening out to the larger pipe will have a significant effect on pressure. BG thought prob. be OK.
Why are these these calc's needed. If a pressure gauge is put on the pipe just where it would go into the boiler and measured with any other gas applicances flat out, surely taking pipe size into accout this would show flow rate and any fluctuations if held on long enough. There must be hundreds of situations domestic or industrial where there is no idea where the pipe run is.
Why do these new boilers require so much larger supply , my boiler use to run gas oven (Not any more), hob, fire & 100,000 btu inefficient boiler without any problems. If these new HE boilers are meant to save 30% gas why do they need a bigger supply, do they burn super hot/fast?
Off the subject a little, my Boilers on internal wall, existing flue straight up through roof, looks a lot bigger than new ones, do you think the new flue could be run up inside the existing. (might physcially not be enough room, not sure how think double wall was) woud save reflashing and messing with tiles.
Is above small conseravatory, BG wanted £600+ for scaffolding and to re-flash or a good few hundred £'s to route the flue out of the way of the conserv. and through the tiles aagin, meaing leaving the old, disused flue.
Regards
Obviously I have no training in Gas.
Big Nose, you have got a pretty good idea of whats involved but you should start your own thread.
If the actual pipe sizes/lengths are not known then as you suggest its possible to measure. Thats needs a gas engineer.
We dont know what power boiler they are suggesting so I cannot comment on pipe sizes. It depends on a calculation of the house heat losses.
Generally current flues need to go to an outside wall to eject horizontally. Other arrangements are sometimes possible but NOT putting anything up you old flue.
Tony
If the actual pipe sizes/lengths are not known then as you suggest its possible to measure. Thats needs a gas engineer.
We dont know what power boiler they are suggesting so I cannot comment on pipe sizes. It depends on a calculation of the house heat losses.
Generally current flues need to go to an outside wall to eject horizontally. Other arrangements are sometimes possible but NOT putting anything up you old flue.
Tony
Hi,
Thanks for your reply,
Quite large 5 bed house, 16 rads.
Current Potterton Kingfisher II 100,000btu (I think = 30kw) easily copes, usually setting 3-3½ (of 5) perhaps 4 with recent persistant v.cold spell.
Open Vented, Hot Water Cylinder.
BG said a British/Scottish Gas 330+ HE (whoever makes that) would be fine, I image the 330 refers to 33kw. I guess 30kw min, prob. 33kw.
BG wanted nearly £5K !!! did inc. tank & few extra's so waiting on more local plumber to quote but no-one seems in much of a rush.
Drain off via pump & stink pipe a few m's away in loft.
BG did not seem to have any problem with a verticle flue going up through roof, it was only the Health & Saftey of trying to work over a conservatory or alternatively a few bends & extra length of flue.
Bit surprised you say nearly all have to be horizontal, looking at boiler adverts they all seem to include verticle flue as option, unless that just means up the wall a bit then out horizontal.
I did not really mean using the existing flue to expel the fumes, more of a conduit which the new flue could slide up inside & just pop out the top so avoiding messing with tiles etc. I guess not the best of ideas anyway & prob. new flue too big to fit inside.
Thanks again.
Thanks for your reply,
Quite large 5 bed house, 16 rads.
Current Potterton Kingfisher II 100,000btu (I think = 30kw) easily copes, usually setting 3-3½ (of 5) perhaps 4 with recent persistant v.cold spell.
Open Vented, Hot Water Cylinder.
BG said a British/Scottish Gas 330+ HE (whoever makes that) would be fine, I image the 330 refers to 33kw. I guess 30kw min, prob. 33kw.
BG wanted nearly £5K !!! did inc. tank & few extra's so waiting on more local plumber to quote but no-one seems in much of a rush.
Drain off via pump & stink pipe a few m's away in loft.
BG did not seem to have any problem with a verticle flue going up through roof, it was only the Health & Saftey of trying to work over a conservatory or alternatively a few bends & extra length of flue.
Bit surprised you say nearly all have to be horizontal, looking at boiler adverts they all seem to include verticle flue as option, unless that just means up the wall a bit then out horizontal.
I did not really mean using the existing flue to expel the fumes, more of a conduit which the new flue could slide up inside & just pop out the top so avoiding messing with tiles etc. I guess not the best of ideas anyway & prob. new flue too big to fit inside.
Thanks again.
glad your getting other quotes, 5k is a tad strong, thought they were trying to be more competitive nowadays?
Just thought I'd post the solution/conclusion to my bioler/pipe size problem.
The installer I used recommended switching to a Greenstar Junior 28i, as he said it required less gas than the Greenstar 30CDi that he was proposing to use. He calulated that it gave us a maximum of 19 metres using 22mm pipe, but that included all the turns and angles, and did not take into account the fact that we have a gas fire and gas oven coming off the same supply. Of the 14 metres, only 3 metres was above ground. He upgraded those 3 metres to 28mm, and also ran a seperate pipe to the cooker, which started almost directly off the the metre.
The upshot of it is that everything works fine, but the fitter was of the opinion that we would have had problems with the planned 30CDi boiler, and that it would have been touch and go with the Junior 28i if he had not upgraded the 3 metres of pipe.
The installer I used recommended switching to a Greenstar Junior 28i, as he said it required less gas than the Greenstar 30CDi that he was proposing to use. He calulated that it gave us a maximum of 19 metres using 22mm pipe, but that included all the turns and angles, and did not take into account the fact that we have a gas fire and gas oven coming off the same supply. Of the 14 metres, only 3 metres was above ground. He upgraded those 3 metres to 28mm, and also ran a seperate pipe to the cooker, which started almost directly off the the metre.
The upshot of it is that everything works fine, but the fitter was of the opinion that we would have had problems with the planned 30CDi boiler, and that it would have been touch and go with the Junior 28i if he had not upgraded the 3 metres of pipe.
Hi,
I'm the guy with the meter at entrance gate 100m away.
I have had a good quote & preparing to go ahead. He's Gas Safe but little worried about promising too much on the Gas Supply because of slightly odd situation. I hope to get a survey from the Gas Company SGN who commission meter moves etc.
Greenstar 30CDi planned.
Could someone advise on I assume a basic principle.
Big mains from road reduced into meter, 40cm out of meter into 60mm big yellow pipe, runs 90 metres to within 1 meter of my house, It is then only 2-3m's to the boiler in 22mm pipe.
If the meter was at the end of my 60mm with 3 or 4 meters & a few bends @ 22mm into boiler gas flow most likely be OK (done pipe size-flow calculations)
My assumption is as it reduces into the meter the pressure will rise speeding flow but of a smaller amount but once out of the meter into the 60mm pipe pressure reduces and true flow rate will be restored. I know I am confusing flow & pressure terms but I sort of know what I mean.
I assume some loss due to friction etc. but in theory will I roughly get the same amount of Gas once clear of the meter.
What would your gut feelings be about pressure/flow at the end of my 90m's of 60mm pipe.
Thanks I/A.
I'm the guy with the meter at entrance gate 100m away.
I have had a good quote & preparing to go ahead. He's Gas Safe but little worried about promising too much on the Gas Supply because of slightly odd situation. I hope to get a survey from the Gas Company SGN who commission meter moves etc.
Greenstar 30CDi planned.
Could someone advise on I assume a basic principle.
Big mains from road reduced into meter, 40cm out of meter into 60mm big yellow pipe, runs 90 metres to within 1 meter of my house, It is then only 2-3m's to the boiler in 22mm pipe.
If the meter was at the end of my 60mm with 3 or 4 meters & a few bends @ 22mm into boiler gas flow most likely be OK (done pipe size-flow calculations)
My assumption is as it reduces into the meter the pressure will rise speeding flow but of a smaller amount but once out of the meter into the 60mm pipe pressure reduces and true flow rate will be restored. I know I am confusing flow & pressure terms but I sort of know what I mean.
I assume some loss due to friction etc. but in theory will I roughly get the same amount of Gas once clear of the meter.
What would your gut feelings be about pressure/flow at the end of my 90m's of 60mm pipe.
Thanks I/A.
I think you are very confused about pressure and flow! Gas flows from high pressure to low pressure. It cannot change pressure upwards in the middle of the pipe!
I would expect it to be OK as you have described it.
But it needs to be tested. If your boiler is working then any competent gas engineer can test the pressures and calculate what it will be with a different consumption.
Tony
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
