Y Plan Problem Solving

18 Oct 2007
Reaction score
United Kingdom
The Y plan uses the mid position valve, so first an explanation how it works. The following diagrams show the various states the valve can be in: HW only (top left); HW and CH (top right); CH only (bottom left); CH satisfied (bottom right)

View media item 17313
How the mid-position valve works

When the valve has no power applied, it is in the "rest" position with Port B (hot water) open. In this situation power is applied via programmer HW ON, HW thermostat Common and Call terminals to the boiler. This is shown in the top left diagram (HW Only).

If CH is required at the same time as HW, voltage is applied to the white wire and the motor (M) runs via S1 (top left diagram). When the valve reaches mid position, switch S1 moves over (top right diagram). The motor is now fed via S2, resistor R1 and Diode D. The diode rectifies the AC current so it becomes half wave. The effect of this is to "stall" the motor in mid-position. Resistor R1 limits the current through the motor and consequently the residual magnetism. This ensures that, when CH goes off and 240v is no longer applied to the white wire, the spring in the valve is able to return the valve to its original (port B open) position. The boiler is still supplied via the HW thermostat as in the HW only position. The motor is also supplied via the orange wire and resistor R2 with a lower ac voltage, this also prevents the motor becoming permanently magnetized and ensures the spring return will work

If HW is now satisfied or turned off, 240v is applied to the grey wire, via either the programmer HW OFF or HW stat SAT terminal (bottom left diagram), This applies 240v to the motor via switch S1. This is sufficient to move the motor from mid position to fully across (Port A - CH only) open. The boiler is no longer supplied via HW ON and CALL terminals but, as the valve reaches the end of its travel, switch S2 moves over. 240v is now applied via the white wire, S2 and the orange wire to the boiler.

If CH is now satisfied, or turned off, the white wire no longer carries 240v, so the boiler goes off. The grey wire, however still carries 240v, so the motor is held in the Port A position. Switches S1 and S2 both remain in the "switched over position (bottom right). In this situation resistor R2 now performs a second task, it reduces the voltage applied to the boiler via the grey wire to between 50-150V. This is lower than the gas valve or pump requires to operate, so the boiler does not light.

The valve will stay in the CH position until either HW is called or the power is turned off.

Valve problems

Mechanical: valve sticking or valve passing.

The symptoms of this are pipes getting hot when they should not. This means that either the valve is not moving fully over (sticking) or the valve is leaking when fully over.

The way to check this is to remove the actuator (box on top). How this is done varies depending on valve. On some old valves the actuator cannot be removed - the whole valve had to be taken out. In the case of old Honeywell valves (identified by the lack of a small bump on the actuator) it is possible to get a conversion kit (part no 40003918-007); but you have to drain down to install it.

When the actuator has been removed the actuator can be removed, the valve spindle will be visible. This should turn easily - how far depends on type of valve, in the case of the Honeywell is 10-15°. If the valve is sticking, a small amount of WD40 or similar can be applied to the shaft. If the valve is very stiff a new one will be required. Some manufacturers sell the body separately from the actuator, so this can be installed, provided the actuator is OK. In the case of Honeywell valves, it is possible to get a repair kit consisting of the ball and 'O' rings (part no 272752A/U).

If the valve is not sticking or passing, there may be a problem in the actuator

Checking Actuator

These instructions assume that the Y Plan has been wired as shown in the following "standard" diagram. If your system is wired differently, you will have to make the appropriate changes to terminal numbers. Before assuming the valve is faulty, check that the wiring is correct and all terminals done up.

View media item 70
Most of the tests can be carried out without a meter, but voltages have been given as well.

Check valve for sticking/passing before doing electrical tests.
The actuator must be attached to the valve.
Carry out tests in sequence given.

CH Only Test (Port A open)

Turn Power OFF at main connection for the system (turning CH and HW OFF at programmer is not sufficient) This resets the valve to rest position
Disconnect white valve wire from terminal T5 and grey wire from terminal T7.
Connect white and grey wires to permanent Live (T1)
Turn Power ON
Valve should move over so port A is open and boiler light (check by feeling that only the pipe from port A gets hot)

If the valve does not move, Switch S1 is not moving over.
If the valve moves but the boiler does not light, switch S2 is not moving over. (check 240V on orange)

HW Only Test (Port B open)

Power OFF (valve returns to rest position)
Turn HW stat to lowest
Disconnect grey and white from permanent Live (T1) and temporarily make safe (e.g connect to spare terminals in the wiring centre)
Disconnect cylinder stat Common from T6 and connect to permanent live (T1)
Power ON
Turn cylinder stat to max.
Boiler should light. (check by feeling that only the pipe from port B gets hot)
If it does not light, the cylinder thermostat is suspect (check for 240V at Common and Call terminals of cylinder stat and at terminal:cool:.

CH and HW Test (Ports A and B open)

Power OFF
Turn cylinder stat to lowest
Disconnect cylinder stat wire from permanent live (T1) and reconnect to original terminal (T6)
Disconnect white wire from temporary location and connect to permanent Live (T1)
Power ON
Valve should move to mid position
Turn Cylinder stat to max
Boiler should light (check that pipes from ports A and B both get hot).
If boiler does not light cylinder stat is faulty
Power OFF
Reconnect white and grey to original terminals (T5 and T7)
Turn HW OFF and CH ON
Set CH stat high
Turn Power ON
Valve should move to Port A open, boiler light and Port A get hot.
Turn CH OFF, so boiler goes out.
Boiler should go out but valve stay in CH only position
If boiler does not go out, check voltage on orange wire.
If orange has 240v, the actuator is faulty
If orange has 50-150v, there is a problem with the boiler

If these tests are passed, the problem is elsewhere.

Testing other components

Cylinder thermostat: disconnect wires from CALL (1) and SAT (2) terminals when checking, this prevents false readings due to back feed.

There should be 240V on Call (1) when stat is turned up and 240v on Sat (2) when stat is turned down

Room thermostat: disconnect wire from Switched Live when checking to prevent false readings due to back feed.

There should be 240v on Sw L (3) when stat is turned up and 0V when it is turned down.

Note: you may find that terminals 1 (live) and 3 (Sw Live) of the thermostat have been swapped over. This is not too important for modern digital thermostats but should be corrected if you have the older mechanical thermostat.

Programmer: Check voltages at CH ON, HW ON and HW OFF terminals when programmer set to appropriate positions. Also check the voltages when the timer is used to switch on/off.
  • Thanks
Reactions: DH6
Sponsored Links

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local