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?

--
Frank

On 04/10/2020 11:45, F wrote:
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?

springs close it. suppsodely 'failsafe' on power loss


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

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.


--
The biggest threat to humanity comes from socialism, which has utterly
diverted our attention away from what really matters to our existential
survival, to indulging in navel gazing and faux moral investigations
into what the world ought to be, whilst we fail utterly to deal with
what it actually is.

In article ,
F news@nowhere wrote:
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?

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

--
from KT24 in Surrey, England
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle

On 04/10/2020 12:00, The Natural Philosopher wrote:
On 04/10/2020 11:45, F wrote:
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?

springs close it. suppsodely 'failsafe' on power loss

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

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

No. Often the problem is scale/corrosion inside the valve body.
take theÂ* head off and wiggle the valve shaft. If it doesn't move freely
you needÂ* 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.

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?

--
Frank

On 04/10/2020 12:12, charles wrote:
In article ,
F news@nowhere wrote:
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?

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

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

--
Frank

On Sun, 4 Oct 2020 12:33:29 +0100, F wrote:

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?

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.

--
Cheers
Dave.

F expressed precisely :
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?

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.

On 04/10/2020 13:15, Dave Liquorice wrote:
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.
Mine only did another 5 years after that.


--
€œA leader is best When people barely know he exists. Of a good leader,
who talks little,When his work is done, his aim fulfilled,They will say,
€œWe did this ourselves.€�

ۥ Lao Tzu, Tao Te Ching

On 06/10/2020 01:08, The Natural Philosopher wrote:
On 04/10/2020 13:15, Dave Liquorice wrote:
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.
Mine only did another 5 years after that.


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.

--
Frank