What pipework is required for combi boiler?

Mimacat

Our current combi boiler is fitted to 15mm central heating pipework.

British Gas have advised that all of the pipework in the house should be changed to 22mm.
Another plumber has told me that if I increase at least 4m of pipework to 22mm, then that should help to ease the flow.
And the third quote, the plumber reckoned running a 22mm pipe from the boiler (approx 10cm in legth) to 15mm pipework will help.
One plumber said that we don't need to change any pipes!!

confused, so if you can help that would be great!

We are considering Worcester Bosch 30si / Viessman / Glow Worm Ultraicom 30ci.

BaileyB

Mine is 22mm from the boiler and then there are 15mm coming off to feed 4/5 rads. The another feed coming off the 22 to feed the other 4/5 rads i have.

trogg

I think you may be a little confused,

Combi boilers usually need 22mm of GAS pipework to it, but if the boiler is near to the gas metre and no other gas appliance is connected to this gas pipework ,then, sometimes the last few meters can be reduced to 15mm.
In my experience, most engineers will run 22mm gas.

As for central heating (water), usually 22mm from the boiler, reducing to 15mm or sometimes 10mm to radiators.

Both of the above require calculations to be done .

I would go back to all the engineers and ask them for some more information.

ormus

the gas feed should be in 22mm.
all other pipework is usually ok in 15mm. that goes for normal smallish 2/3 bed houses. where the boiler is not far from the first rad.
a correct CH system should be designed by a proper CH engineer. that takes a good hour or two.
rad sizing alone takes a good while.
in practice most plumbers just wing it, based on experience.

robv_3

Its all based on velocity of water in the pipes. To fast and its noisy, to slow and need bigger pipes. Get to wrong and you end up with a few cold rads or rads that heat very slowly etc. Turning the pump up can overcome bad design but this will result in higher energy usage and pumps failing more often.

To wing it work on 15kbtu (~3 radiators) for 15mm pipe and 35kbtu for 22mm but with condensing boilers it would be more due to they being designed for a higher temperature differential.