I've had a problem with my heating/hot water system for a few months now. My system uses a 3 port valve to divert the hot water from the boiler to the hot water tank and/or the radiators. The problem is that the valve seemed to be stuck in the mid position, so it always sent the hot water from the boiler to both the hot water tank and radiators regardless of what I wanted – If I wanted hot water, I got heating as well, etc.

Since I have guests coming over Christmas, I thought I'd see if I can fix it.

The valve I have is a "Landis & Gyr" model MAV-322:-

01 - Valve cover

The valve isn't physically stuck, because I can manually force the actuator to move using the lever. Once you let it go though, it just returns to the middle position. So it must be electrical. The valve is pretty simple; there is a motor which turns the valve, and a small control board which has 5 resistors, a diode and two microswitches:-

WARNING – These valves use mains voltages inside – If you intend to dismantle one, make sure it is disconnected from the mains first.

02 - Valve electrics 03 - Control board 04 - Old microswitches

There are just two screws that hold in the motor and the plate to the valve. The circuit board is attached to the plate using two small screws which pass through the microswitches.

Not much to look at, so I tested the microswitches with a meter. The switches are Single Pole Double Throw (SPDT) types, with a normally closed (NC) connection and a normally open (NO) connection.

When I tested them I found that on both of the switches the NC connection didn't work, and on one of the switches the NO connection worked, and the other worked if you really pushed the plunger hard.

 

So after a quick search of the net for replacments, I found some on eBay (where else) for 99p each (plus £2.99 postage):-

05 - New microswitches

I simply unsoldered the two old switches, and soldered in the new ones. Put it all back together and now my system is working properly.

 

 

I opened one of the old switches to see why it wasn't working. Behold:-

06 - Corrosion

Very corroded. I suppose it's to be expected after 20 years of service.

 

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