22m Gas Supply

[Jupiter01]

My parents require a new combi boiler and we've had a few quotations from local plumbers. Today was the turn of British Gas to provide a quotation. Their plans were broadly inline with the local plumbers except, they insisted that the new boiler requires a 22mm gas supply whereas the current boiler has a 15mm gas supply. They stated that without a 22m gas supply, Worcester Bosch or Ideal would not honour the warranty. Can I please have your thoughts on this as this will require lifting floor boards and carries a heavier price.

Thanks in advance.

 

[Terrywookfit]

Yes 22mm is req.

 

[oldbuffer]

1. The fundamental requirement is that the drop in pressure between meter and any appliance is not more than 1 mbar with that and all other gas appliances running at full blast.
2. To ensure this requires that there is no excessive restriction between meter and appliance.
3. The drop can be calculated, and is a function of length and bore of pipe plus any restrictions imposed by bends, elbows and tees.
4. The restrictions for bends and fittings are calculated as an equivalent pipe length. For example, a 90 degree elbow counts as a further 1/2 metre of pipe.
5. A rough calculation is that a 30 kW boiler would have to be within 3 metres of the gas meter with no bends or other fittings, and no other appliances to get sufficient gas.
6. So it is highly unlikely that 15 mm pipe would be sufficient.

 

[Gasguru]

The pressure drop between the meter and boiler must not exceed a particular limit laid down in the regulations. Depending on the boiler kW size, lenght of pipe and bends determines the pipe size. A short run with a small boiler then 15mm may suffice but I have custoners with large boilers at some distance from the meter where 50mm is required. Calculations (and experience) are used but combis require a high gas flow and 22 or even 28mm is normal. Undersized pipe can be unsafe, non compliant and void the warranty.
Where the existing size may be marginal and difficukt to prove adequate the qoute should carry the caveat that if commissioning shows the pipe to be undersized it must be upgraded at the price qouted.

 

[shambolic]

As said above it’s not the Size of the pipe it’s the dynamic working gas pressure at inlet with boiler on max and other gas appliances on. you are allowed a 1mbar drop over pipe run from meter dynamic pressure.
So if meter is 19mbar with appliances on then you are allowed 18mbar at boiler inlet. If reading taken on gas valve or isolating valve some manufacturers allow a further drop for gas absorption in the valve.
So it’s not straight forward at all.

 

[oldbutnotdead]

Are your parents replacing an existing combi boiler or is the existing boiler heat only and everyone is trying to sell them a combi boiler instead?
The instant hot water feature on a combi is what requires 30kw- worth of gas (and thus big fat supply pipe)- if the heat loss of the house is only (say) 9kw then a heat only boiler supplying that (plus the 3kw for the hot water cylinder)won't need such a fat pipe (the calcs are fiddly and changed around 10 years ago so it might need upgrading from 15mm whatever they fit)