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I have a new Worcester 40CDI Regular Boiler that does not come with a built-in expansion vessel. I just discovered that the installer did not install an external expansion vessel either. For HW, I have an unvented cylinder which has a expansion vessel connected to its cold inlet though.

Our radiators don't heat evenly and some remain cold. Called in a plumber and he said that the original installer should have put in an expansion vessel for the boiler and that will solve my heating issues (it seems we've got a pressurized 'Sealed System' - sadly it's also missing a pressure release valve which he said he would install). I'm not convinced as an expansion vessel is essentially only to prevent backflow of water and is a safety measure (right?) and has nothing to do with additional flow of water in our radiators. I reckon it wasn't power flushed properly when the boiler was recently replaced as we've already tried balancing the radiators and that doesn't really help. Some radiators are boiling hot (at 70 degrees set for the boiler) while some are mildly warm.

Any advice? Should I proceed with the expansion vessel anyway (I reckon we still need one for safety reasons but don't think that'll solve my heating problems). Thanks.

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Do you have a backflow preventer? –  User58220 Jan 12 at 23:29
    
I'm sorry for my lack of knowledge but I'm not sure. I'm assuming I do. The house had a working setup before we moved in. We re-used existing pipework, removed the F&E Tank, installed a new boiler, new radiators and a new unvented cylinder. However ever since our heating has been poor and past few days it's been very cold in the house with our radiators partially working at possibly 20% capacity. –  ItsGettingCold Jan 12 at 23:34
    
I should add that although our hot water supply has not much to do with the central heating, we've had no issues with hot water running throughout the house. The house itself is 7 bedrooms detached and has 5 bathrooms. In total there are around 18 radiators. –  ItsGettingCold Jan 12 at 23:37
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A fellow Briton (I'm guessing from your terminology) will have a better sense of what's normal and customary there than I do. Certainly sounds like the installers screwed up.

What sounds like your major heating issue is probably air trapped in the pipes/radiators - particularly the ones that do not heat well - there should be some sort of air vent on those (near the top - possibly on the pipe, possibly on the radiator itself) which you can open to permit the air to leave. Shut when water starts coming out. The expansion vessel is somewhat related, as it makes up the difference in volume from a cold to hot system so that the system does not blow off water (assuming there is a pressure relief somewhere.) If you open a bleeder and you don't reach a point where water comes out, the lack of an expansion vessel would be partly responsible - without one, system pressure will drop rapidly as you remove air, and may drop too low to push all the air out, unless you add more water to the system.

If you really don't have an expansion vessel and pressure relief, and the system is, in fact closed, that creates a significant hazard, as system pressure could rise to unsafe levels as the system heats. I would expect that the boiler would at least have a pressure or pressure/temperature relief. If there really isn't one, you have a potential steam explosion... or you don't have the system type you think you do.

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Many thanks Ecnerwal, your explanation has started to make it clear now, and I see why the engineer said the vessel would effectively make a "80% difference to heating". The remainder he said might be air / gunk in the system. I should mention that the original installer did not put a pressure release valve in, but did put a pressure gauge next to the boiler fill-pipe. Without a valve and the vessel, the pressure does indeed rise to around 3.0 bars from around 1.2 bars (when cold) and I can see how dangerous that can be for a closed system. Will report back after I get the vessel installed. –  ItsGettingCold Jan 13 at 10:22
    
Okay so the vessel did not help at all, not an iota of a difference. Sadly it seems it's a much bigger problem than that. The existing pipework looked as though the flow and return weren't correctly connected, or that there was a short-circuit where flow got attached with flow coming from another side. Either way, we're now ripping up floorboards and relaying all the pipework from the boiler. –  ItsGettingCold Jan 22 at 18:42
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