3 port valve

[kaboom]

Dear Gurus,
Please enlighten me as to how you can tell if the motor has gone or the valve itself? Potterton profile cut out on overheat. With programmer demanding CH and HW no flow to rads past 3PV. Flow to both if set to manual and no overheat. Thought there should be flow whatever the valve was doing- so why overheat? Something else perhaps?

 

[ChrisR]

Well yes, the boiler shouldn't get to overheat, it should cut out on its normal control thermostat.
If the valve isn't moving from start position (probably HW only) then it's robably a dud motor - especially as you can move it to MAN manually.
Obviously you coulcd get in there with a voltmeter if you know how to use one.
Does it overheat on HW only? If so look for an air bleed point.

 

[kaboom]

It gets worse. I was able to get it going with the valve on manual but the boiler locked out again shorty afterwards. Got it going again the next morning but same after a while excpt that now the reset button is in the 'on' position but nothing happens. Programmer showing call for heat and hw (although immersion on and very hot). If I take the 3pv off off manual and put it back the pump runs for 3-4 seconds - should that happen? Frankly I'm baffled.

 

[felix]

If your immersion heater is on then it's quite likely that the tank stat is over-riding the controller and saying you DON'T want hot water. It might be doing this anyway if it's faulty.

Now here's the problem. On a normal three port valve setup, the boiler and pump get their power from the controller/tank stat ONLY if there is a demand for HW. When the demand is for both, power from the CH demand and room stat drive the valve to mid-position but the boiler still relies on the HW demand.

So what happens if the demand is CH only? Well, if - and only if - the valve has reached mid-position, an internal switch connects the motor to No-HW and it drives to the CH only end. Note that No-HW is a real wire with volts on it, not just an absence of HW demand. When - and only when - the valve reaches the CH only position another internal switch connects the power coming from CH demand/room stat to the HW demand at the controller. This back feed of power makes the boiler go.

What does all this mean? Suppose your tank stat is faulty (or just doing it's job) so HW demand has gone. The boiler won't start. The CH demand should drive the motor to mid-position but that won't happen if the motor has failed.

If the motor works but No-HW isn't getting to the valve, it can't drive the motor to the CH end and so no power comes back down the HW wire to start the boiler.

There are a lot of possible fault scenarios here but once you understand the operation of the valve you can start making meaningful tests instead of clutching at straws. Incidentally, that manual lever can be very useful for fault finding as well as filling and bleeding.

 

[kaboom]

Thanks Felix. Let me see if I grasped that?
With the valve set to manual (mid pos?) the boiler won't be getting a call for rad heat if the cyl stat is satisfied even though the programmer is showing demand? So the boiler may not have tripped out (this time), it could just be idle because the cylinder stat is satisfied by the immersion. So I shouldn't expect to get rad heat with a knackered valve motor unless the cylinder is cool enough? So change that 3pv and see what happens?

 

[MANDATE]

This may help you to understand it! When hot water required, power is supplied through cylinder stat direct to boiler/pump. With no power going to 3PV the valve rests in the first position ie HW.
When CH required power supplied through room stat and white wire of 3PV and micro switches and turns motor and actuator to mid position. If HW is not on, or is on but has been satisfied power is applied to the grey wire of the 3PV and the motor /actuator continues on to the final positon. the power holds it there until HW is required again. The original feed to the boiler from the cylinder stat is cut off and replaced with another feed from the orange wire of the 3PV.
When there is demand for both HW and CH the valve needs to be in mid positon. Full power was used to drive the motor to mid position at which point the micro switches cut off this power and replaces it with with a reduced power, not strong enough to drive it forward but enough to prevent the spring from returning it to first position,
In my case one of the microswitches was faulty so although the valve moved to the CH positon no power went to the boiler. It functioned ok when both HW and CH was called for, but The HW was soon satisfied and then no CH, unless you washed up or had a bath. A replacement actuator head cured the problem.

 

[kevplumb]

mandate e kev

 

[felix]

Kaboom, your analysis is correct but don't condemn that valve without a few more tests. They aren't cheap. When the demand for CH reaches the valve the motor should drive it to mid position. You should be able to hear thiis and also feel it through the manual lever. If nothing seems to be moving, check that the demand is actually getting to the valve.

I can't confirm or refute Mandate's claim that this will be the white wire but, whatever its colour, only one of those three wires should go on and off with the room stat. If you have a CH demand at the valve and nothing is moving, THEN you need a new valve.

PS: You might be able to fit a new valve to the old body for a bit less than the price of a whole valve or, if you fancy some electromechanical microsurgery, a new motor will be a lot cheaper.