Faulty switch inside mid position valve?

Hi guys.

I have a question regarding the old ACL lifestyle mid position valve in my (Y plan) central heating / hot water system. The mid position valve refuses to go from hot water only -> hot water and central heating - > central heating only, unless pushed over 'manually' using the switch on the front. However, it is perfectly happy to stay in the right place once moved there, the boiller/pump fire up correctly in every position, and the mid position valve moves on demand from central heating only -> central heating and hot water -> hot water only correctly.

I've been inside the valve, and the valve itself moves freely (not stuck on anything), the springs are in perfect condition, and nothing else looks broken. I also replaced the motor, with no change in the symptoms. From my understanding of this (extremely helpful) wiring diagram:

http://www.gasheating.co.uk/Images/Internal_Wiring_of_Mid_Position_Motorised_Valve.png

... this points to a problem in either the wiring between switch 1 and the motor, or a problem in switch 1 itself. The circuit board itself appears fine, so I'm guessing its the switch. My reasons are as follows:

The motor stalls correctly, and the boiler is energised by the orange wire correctly, so all the connections in/around switch 2, and the white wire must be OK. Theoretically, there may also be a problem with the grey wire, but this would prevent the motor moving from mid position to fully open only, and not from closed to mid position as it is doing. I should point out that I haven't been inside the valve with the power on (I'm liable to kill myself here I think), so I can't say that this is definitely true, but it's the only explaination that fits everything. Have I got any of this wrong? Feel free to point out my obvious mistake; I'm only a lowly vet with no real experience of either plumming or electrics....

Anyway, my question is - how likely is it that the switch is not passing current from either input to the output terminal (which it seems to be doing here), can the switch be replaced, and where can I buy one? I'm keen to avoid replacing the whole valve, since I don't fancy draining down the system etc (these old lifestyle valves don't have removable heads I think). Replacing the circuit board would be fair enough though, if anybody knows where I could get one. The switch says 5(1)A over 250V, and has a small square button on the top that I assume is the switch part. Maplin have miniature microswitches which appear identical, but they're 3A not 5A. Does this matter?

Any help greatly appreciated!

Thanks,

Matt

I haven't eviscerated one of those heads for detailed analysis, but if you went in with the power off and £10 multimeter (with a continuity beeper and Ohms range) you should be able to probe the switches and check that the contacts switch over.

My suspicion is though, that the valve mechanism has in fact become too stiff for the motor to operate as well as compressing the spring. That's the way that make fails. I did lubricate one to get it going, only for it to fail again shortly after.

Ah; that sounds like a distinct possibility. Maybe time for a new valve then... Are the newer Drayton valves the best to replace with, i.e. a direct replacement for the plumbing side of the valve? How big a job is it?

I'm thinking of putting in a power shower at the moment as well, so it might be an idea to get a plumber in to do the lot.... Any idea of cost?

Thanks for your help,

Matt

the switches on acl valves can fail, ive replaced quite a few but they genrally get stuck, arc across the contacts and set themselves on fire until the 3 amp fuse blows.

ive not changed many for just not working but it has happened.

i had a supply of the switches for a while so you can get them if you shop around.

Spent a fair bit of the afternoon disassembling the valve, and I've come to the conclusion that it's the valve itself that is sticking. I lubricated as much as possible and removed 1 of the springs, so now it will move to/from the mid to open positions OK, but it still needs 'encouragement' to move to/from the closed position. I suppose at least it works as long as hot water isn't called for on its own, but I'm going to have to replace it sooner or later.....

Should I replace it with a Drayton or Honeywell valve do you think? Is one more reliable than the other, or easier to fit (same connections as the ACL lifestyle one I have) etc? Any comments welcome! Either way, I'll make sure the replacement has a removable head.

Thanks for your help so far,

Matt