Gas Question (not DIY)

I am about to have a new Combi fitted (Worcester Bosch 30cdi). The installer and I cannot see the existing gas pipe to the current Worcester Highflow Heatslave so are not sure if it is 22mm or smaller. (obviously we will see it when the boiler is removed)
A neighbour in the same type of house found when his Heatslave was replaced that the pipe was only 15mm and had to have a new (ugly external) 22 pipe run installed. This neighbour never had much success with his Heatslave - but mine has always performed well and has been inspected every year by B Gas.
I told the installer I was concerned that the pipe could be 15mm so he offered to perform some sort of drop check. He took a reading at the meter with a manometer with the gas cooker and boiler running - he then repeated the test at the boiler and phoned Worcester Bosch. They confirmed that the pressure was acceptable. I think the drop was 2 milli-bar. (from 21 down to 19)
I realise there is some risk and uncertainly and that I may have to run a new 22mm pipe - but what do you think. Does the 'evidence' point to the hidden pipe being 22mm - or is this irrelevant because the pressure is okay?
Do you think the installers approach is reasonable?
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What do you want, a magic answer?
If the installer has taken readings which WB consider to be acceptable, then either go with professional advice or don't
Nobody else can make your decision for you
--
geoff

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John wrote:

If the pressure at the boiler, with all gas appliances running, is adequate according to the manufacturer, then I don't see that it matters what size the feed pipe is. I would be more interested in whether it is in good condition.
Colin Bignell
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On 13/06/2010 18:25, John wrote:

Is the capacity of the new boiler the same as that of the existing boiler?
If not, the test is invalid because if the new boiler consumes gas at a faster rate, the pressure drop will be higher - and the pressure at the boiler may or may not be within acceptable limits.
--
Cheers,
Roger
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I was assuming that the fitter had told WB that a 30CDi was going to be fitted
--
geoff

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writes

I presume so. I wasn't listening. However the fitter recently went on one of their courses and knew who to speak to.
I take your point about the flow rate. I work in an industry that uses compressed air and understand something of the relationship between mass flow and pressure. (Which is why we have a 17inch air pipe!)
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Thanks for posing that question -
The old boiler gives 2.3 cuM/h at 10 mins after lighting (7 milli bar pressure setting at burner)
The proposed one requires 3.4 cuM/h 10 mins after lighting.
I suppose - I need to expect the worse and expect to have to run another pipe.
Any further info will be appreciated - I like to understand what the issues are.
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---
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Never having CH before in 60 years, I did the same thing plus 10 rads. Now I know why I steered clear, far too expensive to run, but I have it cracked, only use 2 rads and have the stat mobile. Another thing I wouldn't have done is have a 42" plasma, too expensive to run. 100 a month for gas & leccy? Stuff that, I'm happy to stay a tightarse.

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Why do you need rads when you have a 42" plasma heater ?
--
geoff

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Dead right, a third of a kilowatt to watch garbage, and being retired it's always on, almost.
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Is the cooker on its won pipe back to the meter? His test is not valid. In one situation like this, I turned on the cooker and took a reading at the meter dial of what the consumption is in m/h or cu ft/hr. I then from the boiler gas supply fitted a flexible hose and flame trap at the end. This went into the garden. Then turned on the cooker and opened the boiler tap gradually until the combined boiler and cooker reading is enough, and took a reading. It worked. You have to observe the gas cooker.
In another it was obvious that the boiler pipe was just undersized. I figured putting in a 28mm gas meter union and a few foot of 28mm pipe at the meter using a bend (45 degree elbows would have done) would do it - a bit of a gamble. I did not know how much 22mm or 15mm was under the floor. But by turning off the cooker and opening up teh bopioer tap, taking a reading, then look at the charts it is possible to assess how much 22mm is in the run. It did it anyhow.
Each elbow "increases" the length of a run - each elbow adds "length" to the pipe run. The 28mm pipe significantly reduced the run length being a larger bore.
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I forgot to add. The pressure at the cooker must not be more than 1 mb and the same at the boiler. So reading have to be made there as well.

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Thanks the cooker (hob only) must be tee'd off somewhere. The pipe leaving the meter is 22mm. Cannot see any elsewhere.

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On Sun, 13 Jun 2010 18:25:16 +0100, John wrote:

Possibly not, depending whether the inlet pressure was measured at the pipework entering the boiler or within the boiler itself (where it would be lower due to the boiler's inlet connector and pipework). 2mbar is twice the allowable pressure drop. For an existing installation the excess pressure drop is allowable as long as the boiler is working OK, but an installer is not allowed to commission a new installation with excessive pressure drop. (I'm not saying it never happens though.)
There's also the factor of the relative gas consumptions of the old and new appliances: if the new requires a greater gas supply then the pressure drop will be greater.
If the existing appliance's excess pressure drop is caused by something like a few feet of 15mm pipe at the end of an otherwise 22mm or greater pipe run from the meter then it may be simple enough to get a correct supply by continuing the old pipe in 22 all the way to the new appliance. If the existing appliance has anything like the same consumption as the replacement then this is likely to be the case. On the other hand if it's a much lower-rated appliance then replacement of the whole run is likely to be necessary.
As a general comment when I'm looking at replacing boilers for people then the size of gas pipework required is one of the first things I look at when assessing whether I can put in a combi since it can make or break the installation.
--
John Stumbles -- http://yaph.co.uk

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If the existing pipework is under a solid floor the matter is different. If it is marginal installing as much 28mm at the meter can solve matters. But it has to be calculated.
I have seen an illegal work-around. The meter regulator turned full on with another one at the appliance dropping it to the required setting. It worked. Only a boiler was used.
Rail burner boilers are not susceptible to large pressure drops.
Another work-around, depending on pipe runs, etc, is to tee of the cooker pipe and run it to the boiler and tee it into the pipe just before the boiler. This in effect makes the pipe run to the boiler larger, hitting the boiler from two points.
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On Mon, 14 Jun 2010 23:58:47 +0100, "Doctor Drivel"

That's going to be an interesting surprise for the next bloke who adds a cooker or gas fire to that system!
Hope you reported that to the gas company - seeing as they own the original regulator...
--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.
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wibbled:

My contingency is to stump up 400 for new 22 mm gas feed to the boiler in the loft.
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I didn't think gold was suitable for gas pipes.
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I didn't think gold was suitable for gas pipes.
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