S Plan and Bypass

The boiler manual will state if it needs one.

No manual!

Adam

What model?

In article , ARWadworth writes

Not necessarily automatic, but yes, some sort of bypass is required and automatic is better. (in t'old days it was a gate valve across the flow and return, a bit from the boiler, set to give just the right amount of flow to allow some bypass flow when the zone valves are shut but without short circuiting the heating loop)

The heat needs to go somewhere, the boiler has some thermal mass so even when it stops firing it needs flow to cool it down or it will (as you've found) overheat, it may make boiling noises or trip the overheat or it may just stew a bit and hate you for stressing it. S-Plan has no flow when both paths are off so you need a bypass to allow flow, simple as that.

No amount of rewiring can stop a boiler overheating if there is no flow to take the residual heat away.

Unless the boiler has an internal bypass you need an external one.

No refs I'm afraid, just common sense. Would a print of the Honeywell spiel help? Would they/could they read it?

S-Plan systems don't *all* need a by-pass - but most do. It depends on the boiler. If the boiler controls the pump and keeps it running (usually called pump over-run) for a bit to remove the residual heat after all demands have been satisfied (and the zone valves have closed) the water needs somewhere to go. With the zone valves closed, it can't go via the normal routes so there needs to be a by-pass. The best sort are automatic (spring loaded) and only open when the pump pressure is higher than normal due to the HW and CH circuits being closed.

Examine the way the pump is wired and see whether it is controlled by the boiler. If it is, the boiler will have specific terminals for the pump connections in addition to live, neutral and earth. It will also have a permanent live as well as a switched live so that it can power the pump when all demands are off.

There is a pump over-run (twas in my original message :-)). But you have answered my question, "not" all S Plans need a bypass.

I wired the system up 5 years ago and it has worked fine until last week. I called and had another look at the job today.

The customer eventually told me that the plumber replaced their cylinder with a second hand one last week and that is when the problems started. So job sorted, the plumber has removed the bypass IMHO.

Adam

The customer has probably been told by the plumber to clear off. Do the same.

Yes, unless it's already in the boiler.