Heatiing pipes - 22mm to 15 back to 22mm

19 Oct 2023
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United Kingdom
HI All,

apologies for the lengthy description, but I'm after your thoughts on the best way to proceed.
I know its hard to give precise information as you have no idea on the size of the house or the size of each radiator

I have just moved in to a house that was built in 1902 which has numerous extensions meaning that the heating system has also been extended from its initial installation.

Whilst doing up the property I have noticed that the heating is setup as follows.

We have a Pressurised Warmfllow Agentis External System 33 Oil Boiler on the ground floor and the main feed is in 22mm pipe.
The pipework goes upstairs in 22mm and then each of the 4 rads, branches off the 22mm in 15mm.

The ground floor reduces down to 15mm pipe and then proceeds to feed 4 rads downstairs, but it then also goes upstairs in 15mm and feeds an additional 2 more rads. Which means we have 6 radiators all fed from 15mm pipe with no 22mm in site.

I'm looking to add an additional radiator downstairs and rather than just feed another radiator off the 15mm branch, I was hoping to replace as much of the 15mm pipe downstairs with 22mm, so the bulk of the main feed is 22mm leaving only the off shoots to each rad in 15mm

However, access to the area that reduces down to 15mm downstairs is under the kitchen and without removing the Kitchen island, it cant be accessed at this time and will have to be done at some later stage.
This means I cant have a main feed of 22mm downstairs

The new rad needs to go in now, as that area is being re-floored and decorated, meaning access is difficult later on without removing flooring, skirting and architrave.

So my dilemma.

A. Do I just feed the new rad off the 15mm pipe
B. Replace as much of the 15mm main feed downstairs with 22mm pipe, meaning that I will have a 2m section that goes from 22 - 15 back to 22, and then replace that section when i come to refurb the kitchen.

Will having a section that goes from 22 - 15 back to 22 cause any issues
With the current setup I'm pretty sure I'm not getting the full efficiency of the boiler
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You've got a 33kW boiler connected to a 10 radiator system? Poor thing must be cycling its tits off. You're not getting the full efficiency because it's massively over-sized, nothing to do with the pipework.

Anyway, there's no theoretical reason why all 15mm shouldn't work. 20kW is about the limit for a 15mm pipe at dT20 and you're almost certainly well below that with your 10 radiators. Over-sizing pipework can lead to sludge buildup due to low system velocity.
Thanks @muggles,

Well that saves me a lot of work

I presume when you say its over cycling you mean the boiler is reaching temp before the rads get hot, thus the boiler will keep shutting down and firing up?

Any way to circumvent this?

Thanks again
Not cheaply, no. A more appropriately sized boiler for the system would be the best solution but obviously that's a high cost option. Oversized boilers are inefficient boilers.
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