DIY Doctor

Is it Worth 22mm Pipe to Feed new Combo Boiler

Postby dplmartin » Sat Apr 06, 2019 12:10 pm

My mains feed is about 18 l/min at 1 bar pressure. 15mm pipe snakes it’s way from the downstairs toilet (mains entry point) through the kitchen and to the upstairs bathroom before the final tee connects to a 24KW combi boiler (next door to the upstairs bathroom).

When showering there’s a big drop off in water flow if the toilet is flushed or a tap turns on, etc. I know this effect can not be eliminated but I’d like to reduce it.

I’m fitting a new 34KW boiler in the same location. However, I’m looking at cutting off and capping the existing 15mm at the point of the mains stopcock. Instead, I’m going to run a new pipe straight from the mains up to the boiler first. I’ll connect to the boiler and tee off to the existing 15mm from there. Effectively reversing the direction of flow through the existing pipework.

My question is should I run that pipe in 22mm? On the basis that 22mm holds twice the volume of water that 15mm does. If both the boiler and the shower/toilet demand water, there should be less of a drop in the achievable flow rate.

Or do I stick with 15mm because it’s fine in theory but not in practice?
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