OK, so what are these then?

[MattG]

My central heating system is utterly fubared, so it's all coming out and being replaced.

I've identified everything and worked out what to do where (although it won't be that simple, it never is) but there are two bits that I can't identify.

This:

found on the pipe from the boiler, where the two pipes pointing up go towards the expansion/feed tank in the loft, the pipe on the right is the inlet from the boiler, the left is the feed to the pump.

This:

to a pipe leading away from the pump, after the CH and HW pipes have gone their separate ways, and reattaches to the HW pipe, after it exits the immersion tank. Maybe a pressure bypass for when the TRV's are closed? Although, there were no TRV's until I put them in.

Suggestions on the back of a plain text Usenet post, please.

[fred]

In article , MattG writes

An air separator - for vented systems. Pipes are (horiz) flow in, flow out, (vert) tank feed and tank vent.

I'll go with an automatic bypass too (guess) but I see what you mean about the lack of TRVs.

[MattG]

Ah, excellent. AFAIK, I don't need one of those. Nice shiny new boiler being installed, so remove the vent pipe and plumb in the feed from the expansion, sound reasonable to you?

Yeah, I wonder if maybe it's there as a precautionary measure in case both motorised valves are closed. Although why the boiler would be running if both valves were closed, I don't know. I don't think that's the best place for it either. I think I'll place it on the kitchen radiator pipes, as they are the furthest (in pipe distance) from the boiler, unless anyone else has other suggestions.

[fred]

Do you mean expansion tank or expansion vessel? I'm guessing that you're doing away with the tank and making it a sealed system, is that true? If yes, then I agree it's ok but I would put an automatic air vent at the system high point - mine happens to be right beside the boiler. If no then I know less about what's going on but believe you should retain a separate vent pipe to the expansion tank and buy a new air separator - I hear they're quite good :-).

Yeah, I had guessed at that too. I've used an automatic bypass on my system but I'll leave it to you make the choices to suit your own design needs.

The DIY group faq is a good one in this area and is at:

[MattG]

No, I'm keeping the tank. So it won't be a sealed system. I shall get a new separator and chuck that in will the rest.

Cheers, I think I've got things worked out now.

[Roger Mills]

If the boiler has a relatively small water capacity, it may need to keep the pump running for a bit (via its pump over-run stat) after it has finished firing. In this case, the water has to have somewhere to go - hence the by-pass. Automatic by-pass valves like this are spring operated and only open when a set pressure is exceeded - in other words when the water has nowhere else to go because all the zone valves are closed.

Roger

[MattG]

Ah, that makes sense. Ta.

[N. Thornton]

So how exactly is it fubarred? I ask for a reason.

Regards, NT