Pipe size for combi boiler

Depending on the kw of the appliance then yes you may have to upgrade the gas supply from 22mm to 28mm as 22mm may not get you sufficient supply pressure when appliance is at max gas rate also depends how many gas appliances are on the supply but with out viewing the system it would be hard to say

Hello, there are three main things that come into it. 1. No part of the gas pipe can be any smaller than the connection to the boiler (which comes with the boiler, likely 22mm for a combi). 2.Some boilers may operate down to a low gas pressure like 16mb working pressure without causing a problem with the boiler itself but that takes us to the main point 3. No gas appliance running by itself on maximum or with *all the other gas appliances on maximum at the same time* can have more than 1mb pressure drop/loss between the meter outlet test point and the gas appliance inlet test point. The meter working pressure (when gas is actually running) is set at (millibars) 21mb +-2mb by Cadent on *their* governor (regulator) which has an anti tamper seal and not even gas safe engineers can alter, only Cadent. This means that 19mb to 23mb working pressure at the meter outlet is acceptable therefore -1mb allowable drop would mean 18 to 22mb at any appliance inlet (will all on maximum) would in theory be acceptable unless manufacturers of boilers fires etc. stated that it must be nearer 21mb. The distance from the meter to the appliance means more resistance/friction inside the gas pipe to slow the gas down and lose pressure, the diameter of the pipe also affects this as does the amount of bends as these all create resistance. In your case, I don't know what other gas appliances you have or the length and route of the pipe but I would say that, going by most Condensing combi boilers I see in lofts, 28mm pipe most of the route would be better just to be safe. The boiler may work with 22mm but not get adequate working pressure at the boiler giving intermittent faults at some point and Worcester Bosch would then tell you the gas pipe is not big enough and not come out under warranty for gas pressure related issues until you rectified this. The boiler's gas connection will probably be 22mm so the last part would need to drop down to 22mm but I'd strongly advise 28mm pipe or have the engineer draw out your gas pipe and do a pipe sizing calculation which we all know how to do and have to do it in an assessment every time we re-sit our gas exams to renew our creditations. Your other option is two quotes from the same engineer - one with keeping the existing gas pipe and another for upgrading the gas pipe. If, after installation,the boiler was found to be getting no more than 1mb less than the meter when all appliances on maximum then you don't need a bigger gas pipe and this can be measured accurately and takes guesswork out of it. If not, you'd need the pipework as well. The engineer must state in his boiler benchmark commissioning logbook what the inlet working pressure at the boiler is and remember that 21mb +-2 means 19mb minimum at the meter and -1 applied to this means 22 to 18mb minimum at any appliance with ****all gas appliances on maximum at the same time**** which is the worst case scenario with the highest demand. I hope this was not too much info. All the best!

I agree with what the other Engineer said. It may need larger pipework. Really to get a better answer you could posting the Kw of the boiler and the distance from the gas meter to the boiler, including (best you can) the number of 90degree angels the gas pipe with take. Also, don't buy a Worcester 8000, they are over priced and full of plastic parts. Have a look at either an Atag or high end Baxi (600 or 800) Or even a Baxi -Main (same boiler as the 600/800) but comes with a Main badge instead of a Baxi badge.

I assume you mean the Gas supply! more than likely it will need to be upgraded to 28mm from the meter!!

When we quote for a new boiler we have to calculate what size gas pipe is needed to meet manufactures instructions. To calculate the correct size pipe we will need to take in to consideration the KW of the appliance, the length of the gas run, if there are any other gas appliances in the property and how many/what fittings are going to be used. Any change of the gas pipes direction will restrict the flow rate. Without seeing the job it is not possible to tell you what size pipe you will need, however I would agree that a 28mm gas pipe sounds like the correct option considering the boiler is in the loft. I hope this helps.