No HW but CH OK on a Gas Rayburn

[ppdave]

Gas Rayburn 480AL, Honeywell V4073A 3 port zone valve, Drayton LP241 programmer.
Switching on CH alone at the programmer :boiler burner ignites and CH operated normally.
Switching on HW alone: burner does not light.
When CH is switched on and running normally, switching on HW cuts off the burner!!

I have tested the programmer switch off line: all seems OK with the appropriate output lines being energised in all setting combinations.

Replacing the V4073A head with a new component leads to the same problem.

Suggestions, please.

 

[Johnmdc]

Could be faulty clock or cylinder stat. Do you have a multi-meter and know how to use it?

 

[mazperks]

with HW & CH off at prog, grey wire (to valve head in wiring centre) should be live; orange dead.

Switch on the hot water, with cyl stat calling; terminal (with orange wire) goes live to boiler. Grey dead.

Heating only selected, room stat calling; white terminal goes live to valve head, and orange wire terminal goes live to boiler.

Heating and HW on and calling; white live, grey dead, orange terminal live to boiler.

If you are not getting the appropriate sw live from room or cyl stat, suspect the programmer.

 

[oilhead]

Has it previously worked or is it fault on the installation of a new component? If it used to work, go to the cylinder stat and check that. It could be it gives no feed on the demand terminal. If it is a fault with the fitting of a new component, room stat or programmer, then make sure it is to the Rayburn diagram, it is different to the Honeywell diagrams as it has to be wired up similar to gravity hot water.

 

[ppdave]

Thank you all for your replies.
Johnmdc:- I do have experience with multimeter and am about to check the unvented HW cylinder thermostat (Codice 542794)
oilhead:- This has been running fine since installation in 2012
I will post results when available.

 

[Johnmdc]

Over heat stat tripped my prediction. Push little white peg to reset it.

 

[ppdave]

Johnmdc
Tried that one first.........no go, unfortunately. Will keep you posted.