Boiler isolation valves - ok?

[Malc]

I want to fit isolation valves in the boiler feed and return. This would be useful if the boiler need to be replaced, the same as with a pump. However it would be before the vent so would this be ok? Obviously closing the valves and firing the boiler would be a BAD idea and there should be a pressure relief valve before the isolation valves in case of idiots or valve faults.

What valves are best, gates, or balls?

BTW mostly this is because I want to get on with replacing my coal fired system before I have to fit a condensing boiler which are not available in wall mount and have other problems with where I want to site it. I need valves so I can run the coal fire to burn the 2 tonne of coal I have, then swap to the oil. Otherwise I'm changing the heating system in winter .

 

[ChrisR]

The BS says no valves or anything between boiler and vent but if you use a full flow valve I don't see it causing any problem. Gate valves would do but they're made so badly and they're not smooth inside, so I'd go for full flow ball valves instead.

The cooler they are the better from the point of view of their seals, which are a nylon or similar. Sizes over 28mm tend to have screwed female ends though I did just use a 42mm compression one. If they're right next to the boiler I would expect the normally available ones to leak if you used them in several years time - like pump valves do. Bes.ltd.uk do some, and you'll find expensive ones at plumbcenter.com. Peglers aren't as good as they used to be but should be OK. For your proposed single use the bes ones would be fine.

 

[oilman]

I see these isolation valves on sealed system combies, but they leak by the time you get to a boiler change, so you have to change the valves anyway. If you change the boiler you are supposed to completely clean the system, so draining it is not unreasonable.