2 port valve ,cylinder stat query for hot water

[morty1]

hi all,
mother in law calls to say she has no hot water.eventually find junction box for valves in garage loft.when controller calls for hot water, voltage on cable to valve motor(brown) and nothing happens so i'm assuming motor is not driving the valve and making the contact at micro switch to start boiler and pump.I note from various electrical drawings online that it shows the tank stat in series giving the signal for demand to the valve.on hers its open and i can only assume it operates at temperature and signals a relay to cut connection to pump and boiler when temperature is attained.i operated the tank stat manually and voltage passes no problem.my question is that i assume that because the stat operates a different relay it will have no effect on the controller asking the valve to operate because its open and where is this relay likely to be???.
thanks for your help in advance.

 

[SimonH2]

Normally the circuit would be from "HW" connection on system programmer, to tank stat, and then to the motor connection on the valve head (and back to neutral to complete the circuit). So with HW selected on programmer, there should be a live feed to the tank stat. If the tank stat is not satisfied, then there will be a live feed to the valve head motor. The motor then runs and opens the valve. When either the programmer or stat turns off, power is removed from the motor and it spring returns to closed.

Inside the valve head is a small switch which turns on when the valve is open. This is wired to the boiler (and pump if separate) so that when the valve is open, the boiler runs.

Most (all) two port valve actuators have a manual lever - you can work the valve against the spring. As you work it, you'll feel resistance, should hear the gears "whirrr", and at some point should hear a faint click from the switch. When you let go, the valve will wind back - again whirrring and a faint click from the switch. Also, there should be a step/notch that you can push the lever into that will hold it against the spring so the valve stays open.

So diagnostics ?

Manually operate the valve - the boiler should fire up and the pipe to the cylinder coil should get hot. If not, then either the switch is faulty, or there's a fault elsewhere in the circuit. Typically the grey wire should be live and the orange wire powers the pump and boiler.

If the lever won't move, then the valve is probably seized.

Set the programmer to demand hot water, turn the cylinder stat up, the motor should run. If not then check for power across the brown and blue wires on the valve actuator.
Power but motor doesn't run probably means the motor is burned out - either replace the whole head, or with many designs you can replace only the motor.

If there's no power to the actuator, then go back to the tank stat. You should have live on one terminal, and the other connection should go on/off with the stat. If you have live but no output then the stat may be faulty. If no live then work back to the programmer and check there.

There's a system diagram and wiring diagram here :
http://wiki.diyfaq.org.uk/index.php?title=Central_Heating_Controls_and_Zoning

EDIT:
Note, there are no relays in a normal S plan system.

 

[morty1]

Thanks SimonH2, your help is appreciated.