DIY Doctor UK DIY Forums

I have an old one-pipe system in a small house (5 radiators) and would like to replace the back boiler with a combi. Is this likely to cause any problems? I've received some conflicting advice and vastly different quotes for the work, ranging from one person insisting that installation of a two-pipe system is essential, with others saying that a combi boiler would work fine without making any (expensive) changes to the pipe work. Would it be sensible to see how things go with the one-pipe system or is it essential to get the pipes changed at the same time as the new boiler?

Hi IDC,

I would convert to 2 pipe - brings it into our century- & cos I can't stand 1 pipe systems.

No problem at all with a Combi feeding a one pipe system. Your one pipe loop would satisfy the system pump requirement assuming all radiators have threir flow and returns connected directly into the loop.

I have had both scenarios and both work!

One customer didn't want the expense of ripping up the floorboards to put in an extra pipe so I fitted the boiler on the existing one piper and it worked very well. Remeha boiler so it was bound to!

I would always put a 2 pipe system in from scratch for the same reason a steve says but if the customer is really against a 2 pipe system for the above said reason then the combi will work fine on a 1 piper.

Sometimes when it's just a combi swap, if you have to convert the system to a 2 piper it can mean 2-3 days instead of 1 and ultimately extra cost to the customer.

It'll work, but two pipe systems are preferred.
Personally - I would upgrade to a two pipe system.

New pressurised boiler, old pipework and old rad valves aren't a good combination, for a few reasons - prone to leak, any sludge etc off old system will block the waterways of the new boiler if not flushed properly.

New boiler, new pipework and new radiator valves = no leaks, no sludge etc.


htg