Gas Experts Required - Is Supply Pipe Size OK?

[thescullster]

Hi all

Looking at a replacement central heating/HW boiler and reckon I will need something like W/Bosch 24Ri. This states a requirement of 2.67cu.m/hr of gas.

I have a 22mm gas pipe running right across the house to the location of the current boiler. Assessment of length and addition of effective length for bends etc gives

23m equivalent length of pipe. From R D Treloar, this looks to be right on the limit for 22mm dia pipe.

Also we have a gas fire tapped off this main (approx 4 m from gas meter). This is an occasional use decorative affair claiming 6.5Kw gross heat input required.

Does anyone know enough to comment on the viability of this install? What would the effect be if the boiler were running and the fire turned on?

As we have concrete floors throughout and the boiler location is as far from the meter as geographically possible, what are the alternatives (please don't suggest moving)?

Also I note that previously it would have been likely that this main would serve: Valor Homeflame gas fire Landing gas heater Gas cooker CH boiler 50,000 Btu

Surely this lot would put as much if not more load on the supply? Final question - if the gas supply pressure as measured is high, can I take this into account in calculation? I believe 20mBar is the datum pressure on which these figures are based?

TIA as ever

Phil

[Andy]

Have you seen this site?

need to convert BTU and kW into cu.m/hour, but you can do that from your bill, I believe 1cu.m natural gas = 1,117 kW/h, or 1 unit ( =1 cu.foot ) =31.4 kW/H, give or take a little ( it varies with the gas content and atmospheric pressure ).

As for your gas pressure, the usual quoted spec for an appliance at full kilter is 20mBar +/- 1mBar, so if your gas governor gave a higher pressure than that yes, that could work to your advantage. Remember though, the governor attached to your gas meter only works when there is a load on i i.e. someting is drawing gas. It will read perhaps 26mBar or higher if you measure its output when nothing is drawing gas.

Andy.

[Gav]

the easiest way to see if the installation pipework is to find the 'standing' pressure at the current appliance, then if it is 21 mbar - 1 mbar for pipework, it should be ok, any less then you may need to resize but as yet, i think, 22mm is the standard maximum pipe size, although i think it will be moving to 32 or 35 with the advent and realisation of new technology

[John Stumbles]

22mm copper, not 3/4" outside diameter steel, right? The latter is 1/2" bore.

If it's 4m from the meter I'd guess (without doing the calcs for you, which you seem capable of doing yourself, with Treloar's help) that it won't make too much difference.

In practice, none.

Ahem! you say 22mm pipe, suggesting copper rather than steel. Now you say concrete floors. Errr, it _is_ adequately protected where it's laid in the concrete, isn't it?

... and the boiler location is as far from

If you replaced accessible lengths with 28mm and/or 22mm elbow bends with swept bends you could reduce the effective length by perhaps a few metres.

Maybe. Maybe it was under-specced for that lot. Quite possibly they added appliances without calculating & checking (shouldn't have, of course).

No. It has to be < 1mBar drop in the pipework. (If the meter regulator is high when you install the appliance and subsequently gets re-set correctly are you going to shut down the appliance because the pressure is now below spec?)

In practice one sees a lot of installations in which the drop is >

1mBar. If I came across one I wouldn't be worried as long as the pressure is still reasonable, and far enough above the boiler's required burner pressure not to affect operation. However in installing an appliance one can't take such an accomodating attitude but should do it by the book.

[TheScullster]

First of all, thanks to all respondents for useful comment

"John Stumbles" wrote

Yes original 22mm copper in 1970s build.

Again the installation is original. Haven't had need to expose the gas, but the heating pipes are wrapped in hession type stuff, laid in a sand filled channel and concreted over. Does this constitute adequate protection?

Unfortunately, nearly all is buried.

From the various responses and Treloar, I believe that the installation should be OK. Will put the onus on the installer to confirm and check pressures though.

Thanks for the comprehensive response

Phil

[John Stumbles]

No, unfortunately not. At least, not by current standards - I don't know what was required in 1978 but if I came across an installation like that now I'd have to label it At Risk since, for all I know, the last thou of copper might be about to corrode through and spout a leak.

The pipework should be protected by suitable wrapping tape or by being coated with a bituminous paint. At the time of your installation a suitable tape would probably have been grease-filled cloth ('Denso' tape). (In the current British Standard this is no longer allowed and PVC tape should be used instead.)

Can you carefully expose a bit of the buried pipework to see if it's properly wrapped?

[Doctor Drivel]

Phil

It is standard practice to have the boiler gas supply dedicated to the boiler only and no tee offs. Then you are certain there will be no problems. Even doing what the sizing charts say cannot be fully guaranteed that one appliance will not affect the other. With the crap meter regulators Transco fit and the introduction of pre-mix burners on boilers it is best do what I advise. Keep the boiler supply separate right back to the meter. Been there, done it, seen it all.

[Andy Hall]

On Fri, 2 Jun 2006 15:18:36 +0100, Doctor Drivel wrote (in article ):

Been doing a spot of fitting professionally?

[Doctor Drivel]

Matt, as you read, I said "Been there, done it, seen it all.". Do you need a pro in your place to put things right?

[John Stumbles]

What 'standard' is that, then?

In the UK the current standard is BS6891:2005 "Installation of low pressure gas pipework ... in domestic premises". Appendix A of this standards document explicitly shows how to size pipe to supply multiple appliances _with_ tee offs.

[Doctor Drivel]

De-facto standard. I know no one who know his stuff not to have a dedicated gas supply directly back to the meter. Many of the old area Gas Boards would not have it any other way. Now the privateers are in charge anything goes.

Read what I wrote and take note. If you want no problems at all have a dedicated supply.

[Andy Hall]

On Fri, 2 Jun 2006 15:34:59 +0100, Doctor Drivel wrote (in article ):

There's nothing that needs to be put right, but why don't you post your contact details for future reference.

[Andy Hall]

On Fri, 2 Jun 2006 18:02:31 +0100, Doctor Drivel wrote (in article ):

What a load of old nonsense.

Liberation of the gas industry from government control was in 1986.

The requirement for high capacity gas supplies to individual appliances is largely a phenomenon of the combi era, and combis had nothing like the market share then that they have today.

Clearly you know better than the industry professionals and standards bodies....

[Ed Sirett]

By the book it usually possible to correctly design the pipework. In practice the pipework in _most_ houses and flats (that I come across) drops more than 1 mBar between meter and appliances.

The installer can't usually take the risk to reuse undersized (but functional) pipework because the monitoring and inspection process has real teeth.

By the book the meter supplies gas at 19-23 (used to be 20-22) mbar. The pipework should take no more than 1 mbar so leaving 18mbar + at the appliances.

The W/B will insist on 18mbar minimum inlet working pressure in the manual. IIRC this is a forced premix burner so will not readily tolerate poor inlet pressures.

In practice the meter will almost certainly be giving 22 to 20 mbar at the outlet depending on the rate of gas flow. This means that you will almost certainly get your 18 mbar even with the fire on at the boiler. So you have a functional but irregular installation just almost like everyone else.

[Doctor Drivel]

Matt, so you do need your place sorting out. Yopu are just not doing it yet.

[Doctor Drivel]

[TheScullster]

"Ed Sirett" wrote

Thanks for the concise, informative response Ed. Of course, the onus will be on the installer to check all this, but I didn't want to go too far down a blind alley with my planning process.

Phil