Motorised valve not closing

[F]

The 2-port motorised valve on the central heating isn't turning off once the room thermostat is satisfied and so the boiler continues to fire until I turn the boiler off at the mains.

The actuator is a replacement Siemens RMH on a Boss Therm BZV22 body.

In the fault state, the silver lever below the valve can be freely moved left to right and back with no resistance. If I take the actuator head off the valve the lever springs to the left to the closed position.

When the room stat is calling for heat I assume the valve is driven open and held open but is it driven closed once the stat is satisfied or the hold released and springs are left to close it?

Though the fault has rectified itself a couple or three times, am I right to assume I'm looking for a new actuator head?

[The Natural Philosopher]

springs close it. suppsodely 'failsafe' on power loss

No. Often the problem is scale/corrosion inside the valve body. take the head off and wiggle the valve shaft. If it doesn't move freely you need a whole new valve, and unless you use freeze spray or have isolation valves a drain down of the whole CH or HW primary circuit.

[charles]

my actuator heads are both motor to closed, but spring return ones are also available. You need to determine which kind you have.

[F]

Thanks, if I turn the power off it doesn't close.

The body was installed in December 2006 so it wouldn't be surprising if it's in need of replacement.

What is odd is that with the actuator head off, the lever springs immediately to the off position but with the head on it either doesn't move back at all or, as just now, creeps across very slowly and not quite fully. As though it's having to move the motor's splined spindle as well as the valve?

[F]

I'm suspecting motorised: see my reply to TNP.

[Dave Liquorice]

Not clear to which bit the lever is attached, the acuator (normal) or the valve body (not normal). As it's sort of trying to close my guess would be that the valve is slightly ceased, take the head off and fully operate the valve stub shaft both ways fully a few times and see if that frees it up. Had a honeywell valve do that on me, once free'd off it worked for years with no further problems.

[Harry Bloomfield, Esq.]

F expressed precisely :

Often they use a spring return and the spring fails to close it because the valve is stiff, or the actuator worn. In that design, the motor continues to be powered, but on reduced power whilest ever it needs to be open. If the actual valve is free, then replace the actuator - you should be almost able to turn the D of the valve, with your finger tips if it is free.

Another type is called a MOMO, where there is no spring return involved, the motor runs to position then switches off, then moves to the next position (as required) and switches off. This type only rotates the valve in one direction, rather than back and forth of the spring type.

[The Natural Philosopher]

Mine only did another 5 years after that.

[F]

The shaft on the faulting body was pretty sticky so it's gone. With a bit of luck, the new Drayton will be a long term fix.