A cautionary tale for those investigating probs with their central heating:
A couple of years ago I finally tracked down a problem that had had generations of visiting plumbers busy, in frenzied and expensive, radiator bleeding / pump replacing exercises, all to no effect, at a friend's house. On careful analysis it turned out to be the motorised valve actuator (Danfoss) that was the culprit all along.
More specifically, it was due to the pair of crap semi-micro switches inside the actuator, which perform an elaborate 'dance' with the motor to wind the valve back and forth between the three port positions. As with all contacts (like the equally crap ones in mouse buttons), they need cleaning from time to time, but as these are fixed inside the actuator assembly they are both difficult to get at, and even more difficult to clean, or replace.
When the main one of these gets dirty and stops conducting, the valve stops in the half-way position, and the room thermostat appears to have packed up, as it 'ceases to turn the 'CH' off' because the water keeps circulating even when it is apparently set at HW only. There is also less pump power to the (wrongly shared) CH side, so people start trying to rebalance/bleed etc. rads, and eventually start blaming the pump. The last thing anyone seems to think of is the valve actuator for some reason (possibly because they are so outrageously expensive!).
Back then, initially, I tried to just replace the switches, and did manage a short term repair after replacing one of them: they are very cheap even at Maplins. The other one soon failed, however, and the way it was mounted led to the ancient plastic mounting plate breaking when I tried to get it off.
So had to grit teeth and face up to buying a new valve/actuator assembly: and thought I'd better do the valve as well while I was at it (which also means more expensive/inhibitor, anti knock down the drain: if only I'd thought to put a stop c*ck on the outlet to the ch system header tank while I was at it...)
After extensive reading of these columns and those of the Screwfix Forum - and some very silly stick from posters who simply would not believe that a semi-micro switch could ever fail - I decided to opt for the more expensive Honeywell V4073A valve and actuator which cost me £60.44 on 15 Nov 2005.
Up until now this seemed to have been a good buy: it had been easy to fit; had what seemed like a clever ball arrangement rather than flaps in the valve itself; and a quiet and smooth switching action.
However, I recently began to notice that the CH seemed to be on when it should not be, so had a closer look yesterday: AND Lo!, inside are just the same crap switches as in the Danfoss! At first, all seemed not lost, as these ones were mounted on their own little circuit board, which could be removed for easy working. Unfortunately, the way they were arranged, they could not be opened for cleaning until one had been removed from the board, but once open they were easy to clean the muck off the contacts.
In the Honeywell case however, there was the additional crapness of a largish 12k resistor which had been dry jointed and was coming in at 20k as a result, and overheating the circuit board to boot. So I had to resolder this along with the cleaned switch.
Then the fun really started! How they assembled the thing I don't know, but the actuating cams/gears/springs etc, are mounted on such a flimsy metal plate that it is almost impossible to put the actuator back on the valve and still have moving parts at the end - the slightest overtightening of a screw, and the whole thing distorts and nothing moves. "Grrr" - and worse, I said many times as I struggled in a corner of a tight cupboard with a red hot water cylinder, for hours to get it to work 'properly'. (Yes I emptied the tank in the end; and yes; the water in the resulting bath was cold by the time I was ready for it!)
Once it became mobile, another problem presented itself, in that, on moving to the centre position from either side, an oscillating motion tended to be set up with the motor driving its cam against the switch arms/springs and being bounced rather than operating the switch that turns it off. Click, click, click, click etc. It may well have been this oscillation tendency which had burned the switch contacts 'prematurely'.
With much fiddling, I managed to - temporarily I expect - bend the switch arms enough to correct the bounce, and get it all 'working'.
BUT what a load of crap for what a load of cash!
With this experience, I can say that, if I had had the old Danfoss (really old) heavy duty gearing, and the Honeywell circuitboard switch mounting method, I could have at least had an assembly that was easy to get on and off and reasonably robust; with switches that could be replaced occasionally. But why are they allowed to get away with still using the same 'failure guaranteed' switches whilst charging prices which ought to come with a lifetime guarantee?!
As I would imagine that numerous CH systems must be in a similar state - unless I have just been extremely unlucky! - with many other people throwing away money trying to solve 'circulation/air lock/pump/thermostat' problems, unaware of the nature of these valve actuators, I thought UK-Diy'ers might appreciate this tip.
With any luck some people reading this might save themselves from wasting cash test-replacing non faulty parts of their systems.
As for a permanent solution: fraid I don't know. Anyone know of any valve actuators that work a completely different and reliable way? At a sensible price?
Take care,
S