Myson Zone Valve failed in the OPEN position?

Calibos

System controlled by Hive (That I wired and setup myself a few years ago thanks to the great help I got here. Replaced the Original Sunvic controller that was originally installed with system in 2010). Hot Water, Upstairs and Downstairs Myson Zone valves.

Knew it was going to be warm out today but once more than just me started complaining about the heat, I felt a downstairs Radiator and it was lepping off the wall despite the Heating Zones turned off and only Hot Water Zone still on its Schedule. However, the upstairs Zone radiators were all off and cold. Went up to boiler cupboard to find that none of the Motorised Valve Lights were lit. So would I be right in thinking that this means the Downstairs Motorised Myson Valve has failed and stuck Open with the interlock keeping the boiler firing.....but the Hot Water and Upstairs Zone Valves are still working and were closed as they should be atm which is why only the Downstairs zone is heating when it shouldn't be.

I turned the Hot water Zone fully off from my Hive and that shut the Boiler off and the downstairs rads cooled.

Can I remove the Failed Downstairs Zone Motorised Valve and manually close the Downstairs Valve so that I can turn the Hot Water Zone back on without heating downstairs? Theres no resistance when I slide the switch to Manual unlike the other Zone valves. AM I right in thinking that the Manual stetting is effectively 'OPEN' which doesn't help me.

Is it an easy DIY replacement to fit a new Myson? These Myson Valves were installed in 2010 and have never given me any trouble till now.

Irish Steve

I suspect a bit more investigation might be needed to determine if it's the motor section that's failed, or if the valve section is sticking. The 2 can be split without any problems, there are 2 screws in the corner of the valve head that hold it on to the valve body, and the head can be lifted off once the screws are removed.

If the valve body actuator is in line with the pipes, then it's open, and will allow flow through, if it's at 90 degrees to the pipes, then it's closed, though the exact closed position angle is very small, and only a few degrees either side of closed will allow some flow through. It should be possible to turn the valve on the body by hand, or if it's stuck, with a gentle twist from a set of pliers or a small adjustable spanner, and it should turn through 360 degrees without sticking, though it might be tight when you first try turning it. Most valves turn clockwise to open, but initially, just try a gentle twist of a few degrees either way and see what happens.

If the valve moves freely, it's then a case of looking at the actuator, and that might need a bit more time, and if you have to open it up, some care, as there are live wires on the micro switch that's inside the head, which is what will give the boiler power when the motor moves the valve. The actuator should move from closed to open when the electrics (time switch and thermostat) are both on, and as long as the cover of the actuator is not removed, it's safe to test it with power on to see if it moves. If it's not moving correctly, if the head unit has to be split, then this needs to be done with power fully off as outlined below, for safety. Another possible issue is that the spring that returns the valve to the off position may have broken, and from your comments above, the fact that there's no resistance on one valve would suggest either a stuck valve body or a broken return spring.

I don't know for sure about Myson, it used to be possible to get replacement motors for the actuators, and it is still possible to get replacement heads from most plumbers merchants, but it may just be that the head needs cleaning and regreasing, and it will work again, I had a Sunvic stick recently which gave similar issues, but cleaning it and a bit of grease on the gears and bearings sorted it.

If the valve body has to be replaced, that's probably going to mean draining quite a bit of the system, and depending on how it was put in, getting the old unit out may be a challenge, the picture above doesn't give enough above the valves to be able to see how easy it will be to spread the pipes from the valve once the nuts are released. The left hand one in the picture above will not be easy if it's the body, the pump in front of it will make access to the lower nut difficult. Having said that, the most likely suspect is the head, I've replaced a couple of the heads here over the 30 years we've been here, but the valve bodies have not been replaced in that time.

If the head unit has failed, the system can be used for heating hot water with the heating head unit off the valve, and then making sure that the valve body is closed.

If the actuator has failed, and you decide to replace it yourself, don't do anything until you have made sure that there is no power getting to the system, just having the time clock off may not remove all power from the actuator, to be safe, either remove the fuse from the wiring centre if there's one in it, or switch off the circuit breaker at the main panel, as the live wire that's connected to the switch in the valve actuator may still be live if there is power getting into the local wiring panel.

10-10-20

I got a replacement Myson actuator a few years back, and doing a quick Google they seem to be still available.

As Steve said, test the head first. You can remove it easily and safely and watch for the very slow motion of the actuator once the valve is operated.