is 28mm pipe required to supply gas to 25kw combi boiler?

[Harry Bloomfield]

Some gas engineer don't want to waste time do the calculations and just go for the bigger size (bigger job maybe?)

Last, but one, boiler swap, the engineer was insisting it needed a new pipe, run on the outside surface, all the way from the meter, around two walls of the house. It would look horrible. The new boiler, was a size down from the original, the pipework was all buried, and properly calculated at install. I redid the calcs, sent that potential installer packing, and found another. The 22mm pipe was fine for that install, and the subsequent one too.

 

[fixitflav]

The pressure drop, needs to be calculated, not guessed at. Some gas fitters just play safe and guess. What Kw of combi do you have installed at the moment?

Here's an online calculation. I'd expect it to calculate gas flow if you enter kW and gas type, but oddly, it doesn't and you have to enter a figure. Using a generous 2.5m3/h still gives well below 1mb.

Gas Pipe Calculator

 

[gas112]

A 25kw main combi uses 2.71m3
this can be achieved with roughly 22 metres of 22mm copper so unless you have an excessive amount of elbows in your 10 metre run then your existing supply should be ok
you allow 0.6m for a 22mm elbow .
Also you have to allow for any other appliances coming off the run

 

[fixitflav]

A 25kw main combi uses 2.71m3

Don't know how you get 2.71m3/h. I used a CV (gross) 39.3MJ/m3, and 27kW input, just under 93% efficiency. That gives me 2.47m3/h.

 

[gas112]

Don't know how you get 2.71m3/h. I used a CV (gross) 39.3MJ/m3, and 27kW input, just under 93% efficiency. That gives me 2.47m3/h.

straight out of the main eco compact 25 kw instruction manual

 

[fixitflav]

straight out of the main eco compact 25 kw instruction manual

OK, can't argue with that! From the manual actual max output is ~ 27kW which accounts for it. 22mm still OK though.