On 23/08/2020 11:17:16, Rob Graham wrote:
This is a follow on from a previous post.

I've got to the point of following the boiler re-build instructions
and have come across a warning note saying that I must not fit
isolation valves in an open vented system.

Now I can see why, but in my case I have a boiler pump and a CH pump
- reason is that I have a wood burner with a back boiler, and the
oil boiler and its pump go off when the wood burner is hot. The wood
burner piping is open vented without disruption but the boiler pump
has isolator valves either side of it for replacement - I had to do
that last year and I would have had to drain down most of the system
if the valves weren't there.

Is there a solution to this or is this the boiler manufacturer
protecting himself?

I don't see the issue as long as the valves are open when the pump
and/or boiler are running. They are only fitted for maintenance.

If you have two sources of heat, the usual technique is to use 2 pumps;
each with their own non-return valve to prevent backflow for each system.

You could even use a pipe/cylinder stat to turn off the boiler if the
woodburner is providing sufficient flow temp.