Thank you to everyone who responded to my recent central heating problem of radiators heating up when only calling for hot water. It does indeed seem like a stuck mid position valve. Can I ask one further question. The valve is a Danfoss Randall HSA3 and even when both CH and HW are turned off and the outlet pipes from the valve are cold, the white plastic top on the valve is really hot. Is this normal and should I get it looked at quickly. Also what is the lever on the side of the valve marked MAN and AUTO for ? Thanks, Lawrie
I don't think the acuator should be hot - but these thinks are a bit of a law unto themselves!
One thing to try is to power it off completely - by turning off the FCU which powers the whole system rather than just turning HW and CH off at the programmer. This can sometimes reset it when it gets 'confused' - and you might find that it works ok when you turn it on again.
The lever enables you to move the valve to the mid position (but not all the way to the CH-only position). It's useful when filling the system, since it allows water to flow into both circuits. You can also use it in an emergency - if the actuator motor fails - to get some hot water to the radiators (but only whilst the HW circuit is calling for heat. It can also give you an indication of the position of the valve. If you can only move the lever slowly against the resistance of the gears and the return spring, the valve is in the rest (HW-only) position. If the lever moves freely with no resistance, the valve is at the mid position, or beyond. [It should definitely be at the HW-only position when you power it down at the FCU. If it isn't, you've got a problem!]
Well at least you know the actuator motor isn't burned out yet!
The explanation is probably in this quote from the UKdiy FAQ:
"In the "both off" state, the system wiring results in grey being live. If the valve happens to be in the A+B or B position, SW1 will have been operated, the motor will be fed with AC, and the valve will wind to B and stay there (although the orange boiler output will not be live). This is a fly in the ointment for this valve configuration: the motor can be left consuming power and wearing out its hot windings unnecessarily (the spec says the valve consumes 6W). This will not happen in the summer though, when heating is never selected: SW1 will be at rest, and the valve will sit at A un-energised."
Here's the full article explaining how the actuator works:
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the HW on should drive the valve away from this mid position and turn the motor off. Until you try to run it with CH on its own: after which it will probably get stuck again.
Still not quite sure if it's really the valve or one of the actuator switches that has got stuck though (The article does actually note the operation "has obviously required quite a bit of lateral thinking to conjure up".).
If, as another contributor said, you can't turn the valve to all three positions with the actuator taken off, then you need a new valve - usually meaning at least partial draining of the system unless you have some handy isolator valves fitted. If you can turn the valve, then you need a new actuator - in my view the most likely thing to fail (And, yes, I have wasted time and money changing a valve, only to find there was nothng wrong with it before now...)
That is heat from the actuator motor, which runs much of the time.
The manual position allows you to bypass the motor system if it has failed and get the heating to work.
These actuators are a poor design and fail regularly. Undo the two screws which hold it down on the valve, after noting which way it goes back. Lift the actuator off and check you can easily turn the valve spindle (the half moon shaped part on top) - It should be possible to turn it 360 and almost manage to turn it with your fingers. If you can, it is fine, so go out and buy another actuator head, then fit it in place of the old one. Just four wires to reconnect, but carefully note what goes where.
It might be worth getting two heads, so you have a spare ready to fit - they are such a common point of failure. Expect failure every 12 to 36 months. They cost £30 to £45 each, but you can buy spare parts for them if the body is sound. You can get replacement motors for £14 and replacement microswitches for £1 each, if you fancy trying to repair a failed on for a spare.
I had endless failures with HSA3s, always the microswitches after not many months. System was re-jugged to use HSA2s (2 port version) which lasted in years what the HSA3s had in months, eventually leaking round the spindle.
Whilst using HSA3s I kept a spare head and swapped, and then changed the micro switches at leasure for the next time.
Can't speak for all possible designs Harry, but the two kinds I have seen either have a ball on a stick inside, or a rubber triangle. Either way the flap or ball will only move less than 180 degree, unless the ports are going in at very odd angles! I would think the shaft would only turn 360 degree if it is broken inside.
We have had two generations of 3 port valves and both rotated for the full 360 degrees. 1 pipe in from either side (horizontal) and the third entering from below (like a T), with the actuator mounted above on the valve spigot.
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