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?
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.
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.