gas meters and pressure

[Nelson Longflap]

As the collective knowledge here exceeds that of any individual, I'm optimistic somebody can tell me about gas meters and associated gas flow / pressure issues.

The context is our London flat, halfway up a tower block needs its antique boiler replaced.  All such work requires landlord approval.  The existing gas feed is a half inch steel pipe that pops up from the concrete floor behind our (electric) cooker; the steel pipe has had a 15mm copper pipe grafted on to feed the existing CH boiler.  I'm told modern boilers require a 22mm feed and obviously a half inch bottleneck isn't acceptable. This seems convincing and I'm happy to go along with the need for a complete new gas feed from meter by the front door to the boiler in the kitchen.

The landlord, aka Corp of London, is by no means the worst of landlords, but is a bit of a faceless bureaucracy operating by its own arcane rules and precedents.  The landlord says the only way to run the feed is at ground level(ish) involving gas pipe into a public void full of drains and ventilation stuff through the bathroom and outside wall, along the balcony, back into the kitchen then up to the boiler.  It's a lot of work, and the route through the external void space requires asbestos testing (just in case). So a considerable extra expense, and a particularly naff pipe run. 

My alternative route is up an internal cupboard from the boiler, popping out at ceiling height, running round the hall and into the kitchen behind wall cupboards then dropping about 900 mm to connect to the boiler. The company we chose to do the fitting ran this proposal past the landlord who said it's not acceptable because the extra height up to the ceiling demands more pressure, and a drop in pressure could starve the boiler. 

I'm not an expert, but I believe that as well as measuring consumption gas meters regulate mains pressure to suit the demand from the devices within the building.  Given that there are another nine floors above I assume there is more than enough mains pressure to handle the extra 900 mm we need to run pipe at ceiling height.  Am I wrong?

Plan B would be to dig up the floor and replace the steel pipework with 22mm copper, but it's a lot of work and I don't want to destroy a nice original quarry tile floor that covers the hall, kitchen and bathroom.  Plan C is to abandon gas altogether and do something electric, but I don't know anything about the effectiveness of instant water heaters for baths, etc. 
 
I personally haven't approached the landlord, so could present them with my preferred scheme as a new project. Any advice gratefully received ... TIA.

[rogerzilla]

These days you're not allowed to run gas pipes inside cavity walls, so I'm not sure about the safety of having one in a roof void., even if permitted.  A slow leak can go undetected and cause an explosive gas/air mix that will easily blow a wall apart with overpressure.

The drop in pressure from floor to ceiling, for gas, is the square root of sod all.  We're not talking about water here, we're talking about stuff that weighs a few kg per square metre when under pressure.  It sounds like a rather dubious excuse.

Can the pipe be run externally (e.g. if there is a balcony)?  There is no issue with gas pipes freezing, at least not on Earth.

[Wombat]

Most retrofit gas feeds are run externally, we have loads of low rise blocks with external gas feeds, up to the 10 storey ones.  not sure about the taller ones.

Its mostly about flow rate, not pressure.  These damn modern combi boilers need a much higher flow rate of gas when they are heating the house and then you want water too, thus requiring the larger gas main.  Your gas supplier or the gas transport company (Southern Gas Networks round here) dictate the supply pipe and meter type/size.  I've just been involved in a scheme to lay on gas supplies to deprived areas, what a bloody nightmare that was....

[a lower gear]

The boiler manufactuers' websites will specify the diameter of the incoming gas supply and the min/max pressures. Every bend in a run of gas pipe, and every metre of straight pipe decreases pressure due to friction, so the longer the run and the more bends (elbows are worse than pulled bends in this respect), the larger the diamter has to be to achieve the minimum required pressure. There are standard tables in plumbing and gasfitting manuals to enable calculation of supply pipe diameters. Both gasfitters that have worked on my house merely guessed; I had to work out the diameters for them  - they clearly hadn't done so since leaving technical college decades earlier and their guesses of required diameter were consistently and significantly too small. The situation is clearly rather different in a slarger, very tall multiple-occupancy property such as your block of flats. I imagine (usual caveats) that there is a bloodly big supply pipe ruinning the height of the building with a largeish shared pipe along each corridor and that the (rather small to my mind) 22mm feed is merely your flat's supply tee'd off the aforementioned larger pipes. Most domestic boilers of moderate size to my limited experience (usual caveats) need a 22mm final feed - if the pipe is long with lots of bends then 'upstream' it may have to become of a larger diameter for reason outlined. Get Treloar on plumbing and gasfitting out of your public library; he gives very clear worked examples and straightforward tables of pipe sizes and progressive presssure losses from various kinds of bends. At least then you can satisfy yourself that the calculations are correct. I've no prior experience of issues potentially surrounding a vertical loop of the type you enviage but can't, myself (usual caveats) any problem with it as gas pipes aren't prone to 'air-locks' of the type associated with water pipes.

[rafletcher]

I'd be absolutely amazed if a 22mm pipe were required for a domestic CH boiler - assuming an individual feed, which may or may not be the case in your flat. Certainly for an average domestic dwelling a 15mm feed will comfortably do boiler and cooker and several gas fires.

[numbnuts]

In my block of flats we had to have meters fitted outside now all the pipe work to boiler are 22mm

[David Martin]

There are, as has been mentioned, standard tables for working out th epressure drop with m equivalent (takes into account bends) from the meter. Up to the meter will be a standard (and potentially higher pressure?) that you shouldn't have to worry about. Our end point is some distance from the meter so I laid in 28mm pipe with 8 elbows which the fitters were quite happy with [1] once they had inspected and tested. The previous run when I didn't know what I was doing was 15mm with more bends and this dropped the final pressure to around 11cm WP at the boiler. That was why we had issues (as well as all the crap that comes along the gas pipe clogging it - unless you have seen it you wouldn't believe it.).
You should be fine with a 22mm pipe run - do not use 15mm - and reduce it at the last moment if necessary.  You'd probably get away with 15mm on a short run (ie round a flat) but 22mm is standard, or 28mm for runs more than about 15m equiv (each bend adds a certain distance equivalence). There are rules as to where you can run pipes - you will need to check those as you will need them to be suitably ventilated, and in many cases the pipe should be sleeved, especially if they are embedded in walls.

*Get proper qualified professional advice* and ignore everything I have said except as a scene setting.

[1] They were even happier at not having to lay the pipes as is it was between Christmas and New Year and an absolute pain of a job to sort out.

[rogerzilla]

Normally, swept bends made with a pipe bender can be ignored as they have little impact on flow.  Elbows and tees have an equivalent length calculated as a large multiple of pipe diameter, which I'd look up if I were at home.

[Kim]

I'd be absolutely amazed if a 22mm pipe were required for a domestic CH boiler

Depends on the boiler, I expect.  A high-power combi can eat a lot of gas.

Ours has a 15mm inlet, but the run from the meter is (sensibly) 22mm or the imperial equivalent, with a reducing thingy just before the boiler.  Presumably there's a limit to how much 15mm you could get away with.

[rafletcher]

That kinda defeats the object as I would imagine the 22>15 reducer has a much higher pressure drop than a straight 15mm feed with no reducer, but hey, when did fluid dynamics matter. And we're talking low velocity gas here, not high velocity liquid, but rules is rules I guess.

[Kim]

Well, it does give the option of upgrading the boiler without having to replace the gas pipe, which strikes me as sensible.  The pressures are presumably in-spec at the boiler.

[Wombat]

I'd be absolutely amazed if a 22mm pipe were required for a domestic CH boiler - assuming an individual feed, which may or may not be the case in your flat. Certainly for an average domestic dwelling a 15mm feed will comfortably do boiler and cooker and several gas fires.

'fraid they are... its those combis again...  my 5 yr old modern condensing boiler is on a 15mm supply, but then its not a combi.  I hate combis, but then I think you've already worked that out!

[rogerzilla]

An elbow is the same as 32 pipe diameters, so about 2/3 of a metre of 22mm pipe. However, this is for big pipes, not copper tube.

[David Frank]

The modern boiler should use less gas than the existing, assuming that the output is the same, because it will be more efficient - around 100% for a condensing boiler against 75% for an old boiler.
Is the proposed boiler the  correct kW rating?  It needs to be no larger than the sum of the outputs of all the radiators plus a small allowance, say 10% for heat up, and an allowance for hot water.
Some people ignore the hot water cylinder because the boiler will only need to provide full heating to the radiators when it is at the lowest winter temperature - this happens for only a few hours per year. the rest of the time there is always spare capacity to make hot water.
 You can check by measuring the rads and using manufacturers sizing tables off the web (Myson, Stelrad). Some installers like to go large, but there is no point using a boiler larger than the output of the rads.
There are no pressure losses associated with vertical gas pipe - as Rogerzilla says it is the same weight as air.
Some heating installers like to oversize pipes - it saves them the bother of calculating the correct size and the risk of probs if they get it wrong.

[David Martin]

The problem with gas is that there isn't very much of it in any given volume. That is why frictional losses on the pipe walls are so important (and as soon as you have the crap that comes out of old cast iron gas mains clogging it up - that is another turbulence source).

The book of rules is there because of people who second guess it and go "I can't believe it could drop pressure like that, it's only gas", forgetting that actually it is rather a lot of gas moving relatively fast. You know how fast gas burns? It is flowing faster than that.

[a lower gear]

I dug out my plumbing textbook: R.D.Treloar Plumbing and gas installations, Oxford, 2nd ed, 2000.

PP.158-161 "Pipe Sizing of Domestic Gas Pipework"

"Relevant British Standard BS 6891"

"When pipe sizing hot or cold water pipework, if the pipework is undersized, the worst that can happen is that an appliance  becomes starved of water flow, which results in inconvenience and inefficient usage. There is no dange rto the occupier of the building. With gas supply pipework on the other hand, undersized pipework may result in a dangerous situation. If the pressure is insufficent at the burner, not enough primary air will be drawn in to achieve complete combustion, which may result in the production of carbon monoxide. In very bad cases a flash-back into the pipework may occur which could lead to an explosion. The maximum pressure drop between the  meter and the furthest applicance, under maximum flow conditions, must not exceed 1 mbar."

"An allowance must be made for frictional resistance through fittings. Each time the gas passes round an elbow or tee fitting, 0.5m should be allowed, and 0.3m in the case of pulled 90deg. bends."

The required flow rate "is found by adding the total kW rating for all appliances being supplied and undertaking the following calculation:

kW x 3.6 divided by Calorific value

(Note C.V. = 38.5 MJ/m³)"

Basically you use the total gas consumption of all applicances in the property from the meter to the first junction in the pipework, then the consumption of the appliances on each leg of pipework thereafter.

As an example, Chez Gears is a smallish three-bed semi with a ground floor extension; our appliances are:

Cooker 14.5kW = 1.356 m³/hr

Combi boiler 30.5 kW = 2.852 m³/hr (22mm inlet fitting ; the manufacturer, Baxi, stated "Ensure that the pipework from the meter to the appliance is of adequate size. Do not use pipes of a smaller diameter than the boiler gas connection, 22mm")

Fire 3.0kw = 0.281 m³/hr

The maximum lengths of copper pipe for the above flows are:

15mm diam pipe:
0.281 m³/hr = 30.0m
1.356 m³/hr = 11.0m

22mm diam pipe:
1.356 m³/hr = 30.0m
2.852 m³/hr = 21.0m
Cooker + boiler 4.208 m³/hr = 10.5m
All three appliances 4.489 m³/hr = 9.5m

28mm diam pipe:
Cooker + boiler 4.208 m³/hr = 30.0m

So you measure the length of the supply pipework (the "gas carcass"), and add on an allowance of 0.5m for each elbow or tee, and add on an allowance of 0.3m for every pulled 90 deg. bend, and then use the following table:

"Flow discharge of natural gas in m³/h from Table X (BS 2871) copper tube with a 1.0mbar pressure differential between each end.

Pipe diam (mm) 15: length of pipe in metres / discharge flow rate (m³/h) 3 / 1.5   6 / 1.9   9 / 1.5   12 / 1.3   15 / 1.1   20 / 0.95   25 / 0.92   30 / 0.88


Pipe diam (mm) 22: length of pipe in metres / discharge flow rate (m³/h) 3 / 8.7   6 / 5.8   9 / 4.6   12 / 3.9   15 / 3.4   20 / 2.9   25 / 2.5   30 / 2.3

Pipe diam (mm) 28: length of pipe in metres / discharge flow rate (m³/h) 3 / 18.0   6 / 12.0   9 / 9.4   12 / 8.0   15 / 7.0   20 / 5.9   25 / 5.2   30 / 4.7

"The progressive pressure loss is the sum total of the pressure losses for each section preceding the section in question."

"In conclusion, one estimates a suggested pipe diamter and completes the table for the section to prove its suitablity for use. If it proves undersized one simply goes back to ... choose a larger pipe diameter. It may be that one has to increase the diameter of the first section [of the gas carcase]... Sometimes one opts for large rpipework to allow for possible extensions to the gas pipework in future years, e.g. a larger boiler or extra gas fire."

Hope this is useful!











 

[Nelson Longflap]

All that information is exceptionally useful ... and all collected in one thread too 
Absolutely brilliant, thanks so much.  I'll get out my tape measure and count joints, but at first site my preferred route should yield less of a pressure drop than the landlord's naff route, and given less materials required, a bit of a time saving and no asbestos testing I'm seeing some sparkly wheels complete with tyres and tubes before my eyes! 

It is a combi boiler, so likely to have high demand from time to time, but with only three rads to supply I can, based on the above come up with a good estimate of the capacity required too. Great result

I'll need to play the politics carefully so as not to antagonise the landlord.

[David Martin]

Your preferred route has to abide by the other regs regarding the routing of pipework through unventilated voids.

[Nelson Longflap]

Thanks David - my route goes through no voids. It's the landlord's route that dips into a (I think ventilated) void full of shared drains and nasty stuff.  As RZ suggested a slow leak in there would never be detected, unless it all went bang one day.