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i recently had an old back boiler replaced by a combi. there was a 28mm pipe between the backboiler and the hot water cylinder and the pipes from the cylinder to the rads 15mm
when the combi was installed the plumber left the 28mm pipe in the middle of the system and reduced it at both ends to 15mm to join to the existing 15mm pipes.
is this arrangement likely to affect the performance of the system and should the 28mm have been replaced by a 15mm to match the rest of the system?
any thoughts or suggestions greatly appreciated.

I don't understand why he would use the 28mm pipes,

Have you left old radiators and pipeowork in ? has he removed old boiler ?

Normally the 28mm pipes would be redundant or removed and the 22mm heating flow and return would be used. Using the 28mm pipes doesn't make any sense, unless your central heating pipes went from the old boiler doen under the floor - and the only pipe to go up to where the new boiler is was the 28mm ones. But you will loose quite abit of heat from the 28mm pipes especially if they're not lagged.

thanks for the reply htg
sorry if i didn't make things clear.
the 28 mm pipe was part of the old system and it ran between the backboiler and the hot water cylinder which have both been removed and a new combi installed. unfortunately it is a one pipe system (which the installer didn't tell me about until the new installaton was complete) and so this old 28mm pipe is in the middle of the loop so i have a 15mm run joining onto 3 metres of 28mm and then back to 15mm to complete the loop. this is the loop feeding the rads and the 28mm pipe doesn't go anywhere near the new boiler. hope this makes sense.
thanks

forgot to say in last post
the old rads and original pipework have been left in

Not the best way to install a new boiler to a system.

Did he flush the system, and add inhibitor ?
If not you're going to have problems in the future, not only because of the build up of debri's in the old pipework and radiators but the worst part of a system for limescale build up is in the primaries (28mm going from boiler to cylinder) which you say have been left in.

With a pressurised system and the better heat output from the boiler the limescale will come away from the pipes and cause problems to the new boiler heat exchangers, diverters, pump etc etc.

Ideally and i know it's not always an option when it comes the cost - but a new boiler, new radiators if required - but at least new radiator valves and new pipework are best.

If I was you I would save up and have the system re-piped (2 pipe system) and new radiators if required, and like i say if not new radiators then new valves.

the system was flushed and inhibitor added
this one pipe system has given me nothing but trouble since the new boiler was installed. i think eventually i will have to change to a 2 pipe system.
thanks again