boiler will not stop pumping and zone valve stays open

[guernseydonkey]

Hi, have a grant boiler for hot water/heating,
the boiler pump keeps running after boiler has shut down on hot water cylinder heating only, zone valve stays open as well, has replaced zone valve head, makes no difference.
Heating side is perfect, could it be the stat on the cylinder?
have had to resort to using the immersion in the cylinder as i dont want the pump continually running !!
Any ideas /help, folks ??

 

[Exedon]

If boiler has shut down and pump still running on a S plan system you have a wireing problem.
Boiler and pump should always be on (and off) together.
If you have a multimeter and are competent to use it you need to be in wireing center valves are wired into and trace where the live feeding pump comes from.

 

[BlueLoo]

Stop replacing stuff without identifying the problem.
If the head was replaced by a plumber, never use them again.

Find the fault, fix the problem.

(simplistically)
The timer/programmer powers the zone valve. The stat interupts power to the zone valve. The zone valve commands the boiler.

Have you changed anything in the system recently?

 

[MeldrewsMate]

If the boiler has shut-down rather than reached temperature, and the pump continues to run, there may be a problem with the wiring (as @BlueLoo says), but that has to have been caused by direct intervention. If the hot water valve or it's end-switch were faulty then the boiler would continue to heat (and the pump run) even though the HW valve was closed, so if your symptom description is correct then these causes must be discounted.

Could it be the simple failure of a pump over-run 'stat within the boiler. I confess I don't know if your boiler even has a pump over-run, though if it's an oil burner I would imagine so due to their high thermal capacity.

You'll need a multimeter and some electrical skills to diagnose properly.

Stop replacing stuff without identifying the problem.

What a great one-liner. How many times do we come across people throwing parts at a problem before they even know what's wrong, or even start looking for the fault?

MM

 

[Exedon]

Op states boiler is a Grant its a typical oil boiler no pump overrun required.

 

[oldbutnotdead]

Control issue of some sort, you need a multimeter. Possibilities- failed cylinder stat, someone has set cylinder stat higher than the boiler output temperature, welded contact on whatever controller you're using for the hot water.