Gas pipe capacity

Is it possible and legal to avoid the use of 28mm gas pipe by teeing off a second 22mm pipe from an existing 22mm pipe? How close to the meter would such a 'Tee' have to be?

I have a gas CH boiler and a completely separate Worcester multipoint water heater which together are close to the limit of the existing 22mm supply.

My intention is to replace the existing Worcester multipoint water heater with a larger capacity model (Rinnai) which will require a larger gas supply, and coincidentally provide some extra capacity for a possible future CH boiler change.

Cic.

Reply to
Cicero
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Cicero wibbled on Wednesday 17 February 2010 20:25

Not a gas person etc...

AFAIK, the main factor is can you meet the max permitted drop of 1 mBar to each device with all devices taking their max consumption simulaneously?

I don't *think* there are any other factors other than that, though if there are, one of the GasSafe chaps who hang around will probably comment.

In my case, I achieved a theoretical pipe layout with a 0.9mBar drop to the boiler (furthest device) and all pipe in 22mm with a tee within 2m of the meter.

That's a bit close, so I would probably do the long single run under the soffits in 28mm and drop down at both ends (the meter pipework is intricate and in good order - I simply don't want to touch it).

Do you have a reference book for doing the calculations?

Reply to
Tim Watts

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Thanks for the reply. Starting at the end. Yes, I have a *.pdf document for calculations (Copper Pipe Association or similar) which agrees pretty well with what you say. Based on their quite detailed calculations it's clear that I must increase the gas flow to achieve the maximum permitted 1 mBar drop as I'm already close to the theoretical limits of my rather convoluted pipework.

The thing that surprises me is that my meter (renewed / upgraded about 2 months ago) appears to have only a 22mm outlet which suggests that fitting a 28mm pipe with reducer will still restrict the flow based on what can get through the 22mm meter outlet.

I'll hang around and see if anybody can explain this apparent paradox.

Cic.

Reply to
Cicero

Gas flow resistance is a function of both diameter and distance (and other things) - so the 22mm outlet will have an effect - but it will be small compared to that of the meters of pipe that follows.

Reply to
John Rumm

In article , Tim Watts writes

Me neither but in electrical installations it is not permitted to use multiple parallel cables/singles to achieve the desired rating so in the paranoid world of gas-safe I could see there being a similar restriction for gas pipework.

One for the gas-safe registered members of the group to give a definitive answer on I think.

If it's ok then the cda pipe sizing guide should do the job fine with the drops on each element being calculated separately and being combined in a series and parallel fashion as necessary.

Reply to
fred

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I assume from this that you're saying that two branches (suggested in my OP) wouldn't make a substantial difference; both pipe pipe runs must be totalled however they're made up?

An interesting point about the 22mm meter outlet. It seems there's likely to be no lack of pressure at the meter itself under most domestic circumstances.

Cic.

Reply to
Cicero

Do you mean...

- one 22mm run to appliance #1

- another 22mm run to appliance #2

- another 22mm run to appliance #3

If so that is fine and quite normal.

The single 22mm off the meter means S.F.A. incidentally. Realise a domestic meter is rated to ?75kW? through that size outlet. It is about ensuring the pressure drop due to appliance when running at maximum can't pull down the pressure in a pipe that also supplies other appliances. Supplying appliances by dedicated pipes is not a bad idea anyway if space permits (at least, it can avoid having to use

35mm or such like).

If you mean two 22mm pipes side-by-side run in parallel to fit within a skirting-cover I suspect there may be no specifically worded exclusion. If you are using Tracpipe anywhere remember 15mm has 22mm o.d., 22mm has 28mm o.d. - just in case you run outside and are suddenly surprised at the "chunky" nature of it.

Each appliance should have its own isolating valve & pressure test point, so you do need to be within the drop limits. The drop limits are in fact a little grey at small pipe sizes (the calculations are a bit optimistic).

Reply to
js.b1

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Yes, this is the general idea. This seems to suggest that I've read too much into another respondent's answer (John Rumm). It seems that as long as the pipe run for each appliance is adequate for that appliance and the total demand of all the appliances doesn't exceed the 75KW (approx.) of the meter then there is no requirement to treat the pipe runs as a single cumulative run.

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I've no intention of putting the pipework behind skirtings and I'm not really planning to run the pipes in parallel so I'm not too bothered about different pipe sizes - plain 22mm is good enough for me.

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Yes, understood, that seems to be the standard.

Thanks,

Cic.

Reply to
Cicero

Basically if you have a meter at A and appliances at B, C, & D then you are interested in the pressure drop across the pipe runs from A to B, A to C, and A to D. It does not really matter whether they are parallel, or tree topology as such - as long as the drop resulting is within limits. However if for example you had a single branch that split to supply two appliances, one would need to include the total flow requirement of both appliances in the calculations of the drop in the part of the run before the split.

There is also a secondary question of whether the meter has enough overall capacity to feed all the appliances.

A U6 meter is good for about 6m^3/hour (tis what the number after the U refers to) and this is about 64kW.

Reply to
John Rumm

Say you have

A B C M ================================= Boiler (2m^3/h) | | | | D ============== Cooker (0.25m^3/h)

The pressure drop at B is calculated for the total flow of 2.25m^3/h through the pipe of length from A to B.

That from B to C is calculated for the boiler's flow of 2m^3/h through a pipe of length B to C. Therefore the pressure drop at C is the sum of the pressure drop at B + Pressure drop at C.

The pressure drop at D is calculated as a flow of 0.25m^3/h through a pipe length of B to D, plus the previously calculated drop at B.

(note the lengths in question are the effective lengths taking account of bends and elbows etc)

The pressure at the meter will normally be somewhat above that of the dynamic flow pressure set by the governor.

(note that the static pressure after governor is also typically higher than the nominal 21 mbar - its not actually regulated to the nominal value until there is some gas flow)

Reply to
John Rumm

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Thanks for that explanation which agrees with js.b1's post. The situation is now much clearer. I hope my plumber agrees as I don't want to rip out existing pipework when a simple 'T' will enable a parallel pipe to be fitted.

I'll check the meter size but I'm sure it should be adequate as it was upgraded only about two months ago.

Cic.

Reply to
Cicero

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Thanks again for that explanation. I've also just checked the meter and it's marked as U6, so now I know my gas limit, for future planning.

Cic.

Reply to
Cicero

Actually, it is. What's difficult is to design parallel circuits safely, and by the time you've added what's necessary to make it safe, you've way exceeded the cost of using the right size conductors in the first place.

Gas pipes don't burn out when you pass too much gas through them!

But there's no point paralleling up when you have multiple appliances at the other end. You can run separate feeds in parallel, but keep them separate at the appliance end.

Reply to
Andrew Gabriel

In article , Andrew Gabriel writes

My mistake, no personal experience on that one but though I had heard it reported here as not being to regs.

That's the rub, hearing running pipes in parallel I actually thought it was truly in parallel ie going to the same appliance but it appears to be to separate appliances so _not_ in parallel.

I figured if the pipes were truly in parallel that someone in the BS institute would have some concerns about purging both limbs or similar.

Reply to
fred

Nothing wrong with it. Take the 22mm pipe back to the meter and run it parrallel. It will be cheaper than running a 28mm pipe and do the job.

If the 22mm pipe is good enough for the Rinnai, you can have a flow switch on the cold mains pipe to the Rinnai. When it draw off DHW it switches out the CH boiler so only one appliance at a time uses the gas pipe. The switching out is by using the room stat circuit.The r'stat and flow switch could be wired parallel.

That is what I would do. It is cheaper (flow switch about £25) and saves a lot of hassle in ripping up the house. Rinnai use this method.Look at how the Rinnai TwinFlow works on their web site:

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are quality !!!!!

Reply to
Doctor Drivel

Size it up from here:

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installing 28mm from the meter use a 28mm meter union. Available from
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You may need some 28mm for a short run from meter. Check it out. Us eteh charts I linked to.

I like the new condensing Rinnai Multi-points.

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can run you CH off one of these using a plate heat exchanger and bronze pump. That will save money on a new CH boiler. It gives an output of 60C so you may need to uprate your rads in some rooms or increase insulation. Increasing loft insulation is probably the best option.

Reply to
Doctor Drivel

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Thanks for both replies. I've already got the Copper Development Association pipework guide which is why I know I need extra capacity for an upgraded multipoint.

Any suggestions for a suitable flow switch?

Cic.

Reply to
Cicero

What I am saying is find out which is the largest appliance, which is the multipoint? And the see if the existing pipe is fine? The flow switch means only one appliance at a time can use the gas pipe. It also mean the capacity of the meter is not exceeded as well. So, it benefits on two counts. If you had, say a 54kW multi-point and a 24kW boiler the flow switch keeps the gas pipe size down and the meter down saving a lot.

If you need to uprate the gas supply use 22mm from the meter and tee in existing 22mm further down.

Rinnai may supply one. It is coloured blue. Try them and price it up.

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flow switch is seen in the picture. The boiler to the right is a rebadged ATAG.

Try and find out what make of flow switch they use. I think I know the make. Then the likes of Farnell may supply one cheaper. Rinnai may give a good deal on the switch as you are buying their appliance.

Reply to
Doctor Drivel

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> If installing 28mm from the meter use a 28mm meter union. Available from

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> You can run you CH off one of these using a plate heat exchanger and

When using a bronze pump and plate heat exchanger on Rinnais for CH, you can use a Smart pump on the CH side and TRVs all around. The units modulate down to around 4 kW. So if less flow through the plate on the CH side the heat will build up in the plate the unit modulate down its burner.

It is cheaper buy a bronze pump and plate heat exchanger than a new boiler and use the same unit. You may need a flow switch to cut out the bronze pump when DHW is called.

Or buy a combis as the CH boiler and connect up the combis DHW as well. If the Rinnai is down the combi gives DHW. Combis are the same price, if not cheaper than system boilers.

Reply to
Doctor Drivel

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>>> If installing 28mm from the meter use a 28mm meter union. Available from

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>>> You can run you CH off one of these using a plate heat exchanger and

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I'm not yet in the market for a new boiler of any kind. I just want to prepare for an upgrade to my multipoint. Rinnai seems to be the best choice provided that I can do things economically.

Cic.

Reply to
Cicero

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