Central heating fault (Honeywell motorised valve)

diy-in-berks

Recently my central heating played up. My wife complained that the house was getting cold as the heating wouldn't switch on even though the timer and thermostat were set so that it should come on. Strangely when I arrived home the house was warm and the radiators were on. Next day the radiators came on in the morning but later on I was cold. After some fiddling I found that the heating would work perfectly well if the hot water was switched on but with the hot water off it wouldn't switch the boiler and pump on.

My system has a Honeywell 4073A motorised valve to switch the heated water between boiler and radiators. Doing a Google search picked up a number of descriptions of a similar fault scenario and the consensus was that the microswitch in the valve that switches on the boiler and pump was faulty.

A word of warning. The central heating electrics contain mains voltages which can kill. Do not even think of touching this stuff unless you are competent working with electrical equipment.

I took the cover off the valve – loosen the single screw at the end. Do not attempt to remove the screw as it is captive. The cover then lifts off. Inside I could see the mechanism moving fully and hear the microswitches clicking. I downloaded a wiring diagram from the Honeywell web site and using a multimeter checked the voltage between the orange and grey wires at the wiring box. Sure enough, nothing when the microswitch clicked.

The obvious solution was to replace the whole mechanism at the top of the valve but this is expensive (£70 to £100 for a genuine Honeywell) and the local supplier is shut at weekends. I'd also read that some people had replaced just the microswitch so I decided to remove the mechanism and replace the microswitch only. The head comes off after loosening the two screws at opposite corners, then levering off the mechanism upwards. It was quite firmly affixed and needed some effort and twisting to remove.

The microswitch is soldered into a small circuit board which is fixed to the frame with a Torx head screw. My T15 torx driver was too big while my T10 was too small so I figure it is a T12 which I don't have. Fortunately a conventional miniature screwdriver head fitted and it wasn't tight anyway. I also had to remove the motor (one screw) and earth tag to get the board out. The microswitch has three soldered connections. With the board out I unsoldered them, gently levering the microswitch off the board using a screwdriver. It was fiddly but came off more easily than I expected. With the microswitch out I checked it with the multimeter and confirmed that it wasn't closing the connection. The local Maplin had a similar microswitch in stock costing just over £2. When I got it home I found the general dimensions were identical (microswitches follow an international standard) but the pins were too wide to fit the holes in the circuit board. I cut down the pins and filed the edges to make them round and fit the holes. The metal operating arm on the microswitch needed careful bending to match the original microswitch. Although the arm on the new one is shorter it was close enough. I soldered the switch in placed and put it all back together.

To my great delight it all worked perfectly: :j
£150 to £200 saved in parts & labour by not having to call in a plumber.

Next day I was curious about why the microswitch failed. I found the plastic case could be unclipped and it came apart easily. The mechanism itself was moving OK but no current passed. I checked the contacts and found them caked in powdered carbon. I cleaned them by rubbing on rough paper and loads of carbon came off. I put it back together and guess what – it was now passing current. So I could have avoided buying the new microswitch by just cleaning the old one. Only £2 spent on the new one but there was the hassle and time involved. Hopefully someone reading this can learn from my experience.

frisbeej

Good fix.


I have the opposite problem, a valve where the microswitch sticks on sometimes. Need to get around to fixing it or replacing it (about £40) or rigging up a better solution that eliminates the microswitches.