Diagnosing a zone valve body

[paul798]

Good morning!

We have a pressurised system boiler with two zone valves, controlling the hot water and heating circuits. We noticed that the radiators are hot when only the hot water zone should be open. The room thermostat is fine and I've narrowed the issue down to a fault in the zone valve controlling the heating circuit (Honeywell 272848, 7 years old). The fault seems to be mechanical with the 'manual' lever slipping on the cogwheel and then getting stuck, preventing the motor from doing its job. I was planning to simply replace the whole head but thought of checking the actual valve body too.

I noticed that the valve turns freely and easily (good) but:
1. No matter which way I turn the valve stem, hot water still enters the heating circuit
2. The way the valve stem points (using the flat sides as position indicator) in the open / closed position seems to not be symmetrical. If you look straight at the valve body it perfectly points to the far corner (as per the example image below, not our actual valve) in one position but doesn't turn enough to fully point to the other corner.

Is 2. normal and is the actual valve faulty too then? Is there anything else I could try to either narrow down the issue or fix it or is it time to call the plumber and get the whole valve replaced?

Thank you!

 

[CountryFan]

If it's letting by no matter which way you turn it then the ball could have perished or have debris on it. Probably time for a plumber to change whole thing or get a maintenance kit if it's that type

 

[paul798]

Managed to fix it. For anyone who finds this later:

* The issue was that the gear and pinion had become worn to the point where they were slipping. This in turn prevented the valve to fully open, the microswitch never engaged and the boiler didn't get the signal to fire
* The valve itself was fine. I believe the reason it let by some hot water was that the messed up gear prevented the spring from fully closing the valve. When I took off the head it still let by some hot water but this was without me putting any pressure on the valve like would be the case with a functioning head. When holding the valve closed with an adjustable spanner no more hot water got past. I gave the valve stem a few gentle taps for good measure but this may not have been necessary.
* I ended up buying a replacement valve and swapping out the heads. Worked a treat and I still have the valve body if I need to swap out the rubber ball in the future.