How a mid-position valve works
The problem is something to do with the grey wire (terminal 7) not getting 240V when it should. There are two sources: programmer HW OFF; cylinder stat SAT terminal. Here's how you find out which it is:
Measuring AC volts is the same as DC, but make sure the meter is set correctly.
Touch black probe to a neutral in the vertical strip and red to appropriate terminal
Take proper safety precautions, i e, TURN POWER OFF at main connection before disconnecting/reconnecting any wires.
Disconnect the wires from terminals 7 & 16.
Set programmer to HW OFF.
Check for 240Vac on terminal 8
If no voltage possible break in wire from programmer or faulty programmer.
If T8 ok, check T7 - no volts means link 7-8 is faulty.
Disconnect wire from terminal 8 and replace wire in terminal 16
Set cylinder stat to low.
Check for 240Vac on terminal 16 If 0Vac, there is a break in wire from cylinder stat or cylinder stat is faulty.
If 16 is OK, check 240Vac on T8. If none check link from 8 to 16.
Either there is no voltage on valve grey wire (240Vac on terminal 7) or Switch S2 is not moving over (240Vac on terminal 6)If the HW and CH are on and the HW goes off the CH never fires. The actuator doesn't move when the HW goes off. That's the root cause of the problem.
This can only happen if the valve is moving right over. There should be 240V on Terminals 4, 6 and 7.If the HW is on first and then goes off and then the CH comes on everything works fine.
So the valve is returning to HW position OK. The boiler is lit by the HW stat.Likewise if the CH is on first and then goes off and then the HW comes on everything works fine.
So the valve is moving back from CH only to mid-position. Check for 0Vac on terminal 7 and 240Vac on terminal 4.If the CH is on and the HW comes on both heat.
This where we came in!If the HW then goes off, again the actuator doesn't move and the CH never comes back on again.
The problem is something to do with the grey wire (terminal 7) not getting 240V when it should. There are two sources: programmer HW OFF; cylinder stat SAT terminal. Here's how you find out which it is:
Measuring AC volts is the same as DC, but make sure the meter is set correctly.
Touch black probe to a neutral in the vertical strip and red to appropriate terminal
Take proper safety precautions, i e, TURN POWER OFF at main connection before disconnecting/reconnecting any wires.
Disconnect the wires from terminals 7 & 16.
Set programmer to HW OFF.
Check for 240Vac on terminal 8
If no voltage possible break in wire from programmer or faulty programmer.
If T8 ok, check T7 - no volts means link 7-8 is faulty.
Disconnect wire from terminal 8 and replace wire in terminal 16
Set cylinder stat to low.
Check for 240Vac on terminal 16 If 0Vac, there is a break in wire from cylinder stat or cylinder stat is faulty.
If 16 is OK, check 240Vac on T8. If none check link from 8 to 16.