My son-in-law had tidied up the airing cupboard and had caught the leaver unlatching it. Result was when domestic hot water was required it motored open, but it did not open when central heating was required, however if the central heating was required then once the hot water was run, it would stay working until the thermostat turned it off again. I had problems with my daughters old house, typical new baby so critical and a Sunday so had to find it parts required before shops closed at 4 pm. As an electrician I thought it would be easy, I found a wiring diagram for Y plan and tried to follow it. I have tried looking at past threads but have been unable to find an answer to my dilemma, so I apologise if this question has been asked in past. Anyway having diagnosed this far I'm going to leave it with the central heating and the water heating on full time to see if I can get the temperature in the bungalow up to the expected level. That is, there is a shared (flow) valve which has to open to allow any water to flow, then a second 3 way valve which decides which way the water will flow.

When there is a CH demand but no HW demand (tank stat satisfied or HW off at programmer) the valve moves to the CH position. A microswitch in the valve actuator switches a live supply to the orange wire - which then powers the boiler and pump. Now some heat is getting to the cold radiators. The new (compared to the others) double radiator is doing something I haven't encountered before. The rear part is heating up but the front part is staying cold. It doesn't have air in because I have bled it (amazingly after a long search I found a bleed screw) and it was full of water. Digging so far indicates that if the controller calls for heating (with thermostat set above ambient temperature) nothing happens. One year ago (almost to the day) the Drayton MA1 mid-position actuator was replaced (4 years old) on my central heating system as it wasn't operating correctly. It was replaced with another MA1 and the problem was solved. However, I have just switched on my central heating for the first time this year and find that I always get CH with HW even when the CH timer is off. I looked at the MA1 and can see that the valve position is in the middle (HW+CH). If I unsnap the actuator from the valve I can turn the valve manually. I then tried the following with the MA1 removed:

Given that, if the flow valve is permanently open and the 3 way switch is in the middle then water should always flow around the central heating. Just recently had to change my ACL Lifestyle mid position valve as a result of the rads getting hot when HW only was called for, which was due to old age, crap in the system and port A not fully shutting off. The new valve I fitted is a Drayton MA1, like for like swap on size and wire colours.

I assume if the 3 way valve was working properly and the heating side wiring wasn't requesting heating then the 3 way valve would be set to Heat and the central heating wouldn't work. However although I heard the main valve move and watched the plastic lever move, I haven't heard anything from the 3 way valve and the plastic lever seems to be floating. The manual lever only moves the valve to the mid position, and is useful when filling or bleeding the system to make sure that all water paths are open. It will flop around when the valve is powered to the mid position or beyond - this is normal.The mid position is done by limiting current to motor and switching off/on as limit switch is activated, and full over by giving full current and the motor stalls so yes they get hot.