To seal a central heating system or not?

[Jock Spanners]

Good morning everyone,

I have a big old vicarage which needs a new regular boiler. They also want fancy mixer taps in the kitchen . They have two hot water cylinders linked together, a shower pump and curved old radiators in the bay windows. I'm agonizing over what to do. The easiest option is to keep it gravity and simply change the boiler. The second option is to fit a large combi (there's only a short gas main). The third option is the unvented route. My worry is if I seal the system one of the rads will blow or a dodgy pipe joint will leak. I've risked it before and got away with it but I've known it go badly wrong for others. I don't want to suggest re-piping the house. What would you do?

Thanks,

Jock

 

[ShaunCorbs]

Plate the old system and seal the new boiler and maybe an unvented system if possible?

 

[Jock Spanners]

So you'd risk it?
I don't think they can afford to spend much more than the cost of a new boiler instal.

 

[SJB060685]

As shaun says put a plate in. Pressurise the new boiler loop and keep main system open vented. Will prevent any stress on the old system and also protect the boiler and its loop from corrosion etc.
Shaun and I know a number of people who love this idea

 

[Jock Spanners]

This is new to me guys. How exactly do you do it?

 

[SJB060685]

Take your boiler primaries to a suitably sized plate heat exchanger and have the other plate heat exchanger ports to original system. You could pressurise the boiler and its primary loop to PHX and keep the original system open vented.

 

[EvilDrPorkChop]

Personally I’d just seal it and put a combi in. No issues with stored HW, if something does leak it’d probably would do eventually anyway even on OV. If system is big enough put a Low loss header in.
Just advise them accordingly before you seal it.

I may have misunderstood Vicarage - is it a place that's occupied or somewhere like a Church Hall? If it's a residence then Unvented and System boiler and seal it

 

[Chuck]

If it's the big old system I'm imagining you probably want the plate HX in any event because it'll be difficult to impossible to flush the whole thing well enough to pass muster even if it is all leak tight. The cost of the plate will be offset by not having to spend a lot of time power flushing and in a longer life for the boiler.

If the boiler is in or close to the kitchen, it might be worth making it a combi having its DHW supply the kitchen tap with the HW storage tanks heated as a zone on CH circuit. This is worth considering if much water currently has to be wasted before the kitchen tap runs hot.

 

[Jock Spanners]

Take your boiler primaries to a suitably sized plate heat exchanger and have the other plate heat exchanger ports to original system. You could pressurise the boiler and its primary loop to PHX and keep the original system open vented.View attachment 44879

Thanks. Would I need a Pex designed especially for this? I can't picture how it's piped up. Does it have two pairs of ports? I assume this would also work with a system boiler?

 

[SJB060685]

Thanks. Would I need a Pex designed especially for this? I can't picture how it's piped up. Does it have two pairs of ports? I assume this would also work with a system boiler?

Sizing a PHX is not a straight forward process, lots of information is needed to calculate its size, a process im not too familiar with. Your best bet would be to speak to a heat exchanger company and supply them with some information they need. I know a number of people just go with bigger is better but it is quite a complicated process when sizing yourself.
As for tappings, it will be similar to a common PHX on a normal combi. It will have hot in and out and cold in and out.