CH Pipe Sizing for new extension (and original house)

[GCarnegi]

Building a new extension and am planning to run CH pipework myself if possible.
At present, it seems like both upstairs and downstairs is on 10mm pipe, splitting off from 22mm pipe from boiler, under the house.

I'll be putting 2 more rads upstairs and 2 more downstairs, but am guessing I should probably extend 22mm pipe from boiler, then 15mm to everything.

Question is, should I replace existing 10mm pipework with 15mm? Would rather not rip up floors everywhere and have the expense of new TRVs, but would appreciate to know what you would do in your own house (not in the hypothetical ideal world...)

Probably doesn't matter as I'm coming direct from boiler rather than extending existing circuits, but also worried that I'm going to "steal" flow and thus heat, from existing circuit. As such, maybe better stick to 10mm new pipe, to make flow same as in existing system.??

Hope boiler is up to it, it's only three years old!

What's the worst that can happen with an undersized boiler? Cool rads? (serious question)

 

[Agile]

Its boiler power not age thats relevant!

I doubt its a problem but you have not bothered to give boiler power and heat loss of the house so I cannot comment.

Generally as long as you use 15 mm for up to 4-5 kW of rads there shopuld not be a problem.

Properly balanced (!) oversized pipework should not be any problem.

Tony

 

[GCarnegi]

It's a 27kw Worcester combi. I know technically age is not relevant, but financially it is... ie I wouldn't mind replacing it so much if it was 15 years old!

Actually, just realised that new extension is replacing old extension (which had the two downstairs rads already, so all I'm really ADDING is two upstairs.

Boiler was sized for:
living room 2ft dbl
kitchen 3ft dbl
utility 3ft sng
wc 2ft sng
bed1 2ft sng
bed2 2ft sng
bath. towel rail.

I'm adding:
bed3 3ft dbl
bed4 2ft dbl

Heat loss of house (no idea, sorry, but it's always toasty, and half is new extension built to regs)

 

[ServiceNumpty]

It's probably the WB 27CDI, so heating shouldn't be a problem. If it's 10mm they might come off a manifold somewhere, you might have spare ports.