THE WORD "COMMON" in cental heating
- Thread starter reg7
- Start date
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To common, say, all neutrals or earths into one terminal.
For switching it is the terminal that switches between normally open and normally closed. i.e. it is common to them both. Is this an attempted Nest installation?
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Just asking! 
Hi Echo the husky, you helped me before, but it seems another problem has happened. Only using hot water at the moment but it has been getting red hot and the boiler even started to come on some times without the programmer coming on. I have checked the Cylinder Thermostat it still clicks but I can hear the 3 port valve humming so I have to turn the stat up to about 70/80 to turn off the valve. Any ideas please.
Thats quite common
When you say turn off the valve what do you mean? If you are only using the hot water the valve should not be humming. If the programmer really is off, the boiler should not come on at all. (unless there is a frost stat wired in and calling for heat, very unlikely in this weather!)
Here is how a Y plan system operates with HW only:
From first switch on at mains isolator:
Valve motor will have no power and will be held at HW only by internal spring.
When HW is selected at programmer it puts power to the cyl stat common terminal.
If the water is not hot enough, the cyl stat sends power directly to the pump and boiler to heat the water.
When the cyl is hot enough to operate the cly stat, it's contact clicks over, so power is no longer going to the pump and boiler and they stop, and power is then sent to the grey wire (HW off) to the valve, though in this condition nothing happens, the valve stays where it is.
When the cyl cools down again the cyl stat clicks back and powers the pump and boiler once again.
That's it - end of. The valve should not move at all.
The valve will only move if the heating has been or is currently being used. If the heating was the last to shut off, the valve will start off in the heating only position, and will spring return to HW only, when hot water is next required. After this, it will stay in the HW only position forever, until heating is next used.
The test:
Turn HW on at programmer.
Turn cly stat up until it clicks, boiler and pump should start.
Turn cly stat down until it clicks, boiler and pump should stop.
Valve should not be humming at all and should be in the HW only position.
My goodness I don't half waffle on.
Here is how a Y plan system operates with HW only:
From first switch on at mains isolator:
Valve motor will have no power and will be held at HW only by internal spring.
When HW is selected at programmer it puts power to the cyl stat common terminal.
If the water is not hot enough, the cyl stat sends power directly to the pump and boiler to heat the water.
When the cyl is hot enough to operate the cly stat, it's contact clicks over, so power is no longer going to the pump and boiler and they stop, and power is then sent to the grey wire (HW off) to the valve, though in this condition nothing happens, the valve stays where it is.
When the cyl cools down again the cyl stat clicks back and powers the pump and boiler once again.
That's it - end of. The valve should not move at all.
The valve will only move if the heating has been or is currently being used. If the heating was the last to shut off, the valve will start off in the heating only position, and will spring return to HW only, when hot water is next required. After this, it will stay in the HW only position forever, until heating is next used.
The test:
Turn HW on at programmer.
Turn cly stat up until it clicks, boiler and pump should start.
Turn cly stat down until it clicks, boiler and pump should stop.
Valve should not be humming at all and should be in the HW only position.
My goodness I don't half waffle on.
When I installed a Y plan system in my last property , if the heating was the last thing on then the coil was still energized and using power when the heating went off, we used to flick the DHW on momentarily to bring it back to mid position. If the solenoid on the OP system is dodgy then could that be the source of the humming?
litl
litl
Oh dear.
Solenoid?
I didn't know where to start with that one....
The actuator in the mid port valve is a synchronous motor. Fed with AC voltage it will rotate the valve spindle against a spring return. When it reaches the end stop it stalls but continues to have AC voltage fed to it. (*) Turn off the AC and the spring rotates the spindle ( and the motor ) backwards to the other end stops.
To hold the spindle at the mid position ( HW and CH ) the motor is driven by AC to the mid point where a micro switch operates. The micro switch inserts a diode into the feed to the motor which means the motor now has a DC supply. This DC supply holds the motor stationary at the mid point. Without the DC supply the spring return would rotate the spindle back to the end stop.
(*) some actuators use DC ( micro switch and diode ) to hold the motor close to the end stop but wear and tear on the micro switch can mean the motor is held before the mechanical end stop and the port that should be closed at that end stop doesn't fully close
To hold the spindle at the mid position ( HW and CH ) the motor is driven by AC to the mid point where a micro switch operates. The micro switch inserts a diode into the feed to the motor which means the motor now has a DC supply. This DC supply holds the motor stationary at the mid point. Without the DC supply the spring return would rotate the spindle back to the end stop.
(*) some actuators use DC ( micro switch and diode ) to hold the motor close to the end stop but wear and tear on the micro switch can mean the motor is held before the mechanical end stop and the port that should be closed at that end stop doesn't fully close
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Hi Echo the husky, thanks for your reply, I have rechecked the wiring an I had the make on rise and break on rise in 6 & 7 in junction box instead of 7 & 6 both wrong way round. I have corrected them and checked the junction box wiring against the "Y" plan and all is correct, and it all seems OK now. The only thing I'm not sure of is with the programmer in between "ON" times if I turn the cylinder stat down from 65 to 45 and below the boiler fires up the 3 port valve comes on and the pump comes on. But as you don't make a habit of turning the cylinder stat up and down is this RIGHT or WRONG. By the way you don't waffle on your very helpful.
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