Well, winter arrives and a sensible man's mind turns to the central heating system (apologies to Alfred, Lord Tennyson). As local temperatures were dropping it was time to fire up the boiler and restart the central heating (CH). It had been working properly before it was put to bed in the spring so there should have been no problems.
And getting it started was, indeed, no problem. I just repressurised the system and switched the boiler back on. What was a problem was the dripping noise coming from behind me. The zone valve controlling the hot water loop was steadily leaking from the head unit. Buggrit.
At least diagnosing the problem wasn't going to be too difficult. The CH was switched back off and a bucket placed under the offending valve, while the system water pressure bled back out via the valve. As, given the temperatures, this was a bit of a time pressured situation, I didn't take any pictures so what follows is just a quick report. Also, whilst I am comfortable working on a 240 V system, I don't want to provide too much detail just in case (ie don't do it unless you know what you are doing).
Generally, the head units of motorised valves cause the most problems and that is why they are replaceable separately from the valve bodies. However in this case it was obviously a problem with the valve body itself. But as it had leaked through the head unit I decided to replace the entire assembly.
The first job was to clear a space and get access to the valve, which took longer than it should have. Too much crap dumped in the utility room (mostly by me though, so hey-ho). Next was to isolate the CH system so that, A) it can't turn itself back on while the system is dry and B) there was no power to the junction box where the valve electrics were hooked in.
Once that was done it was time to drain the CH system. There is a stopcock just underneath the boiler that is below the level of the valve, so I opened it and let it run for 10 mins. Then went upstairs and opened all the bleed valves on the radiators and had lunch while the system drained out.
With the system emptied the next task was to remove the duff valve. First, I disconnected the wiring after A) confirming with a multimeter that everything in the box was dead and B) taking a picture so that I could re-wire to the same pattern (the wiring layout is generally common between these valves). With that done I was able to remove the head unit and then out came the spanners and the valve body was removed, leaving a gap for the new valve.
The replacement valve is a Drayton 27100 Motorised Valve (ZA5/679-2); as I had no time to look into what was the better option I just went down to the nearest plumber's trade counter and asked them for a replacement. Fortunately the spacing between the inlet and outlet connections for these valves is also pretty standard. So, after checking the olives on the 22 mm pipes were OK and dabbing on some new jointing compound, the new valve body was fitted into the gap whilst taking care to hold the body and not twist the existing pipework when I tightened up the connections. The new valve head was then attached, manually set to Open and was wired back into the junction box.
The final job was to cross my fingers and refill the CH system from the filling loop. Happily no more leaks. The system was then repressurised and each upstairs radiator bled in turn, repeating the process until all the air had been removed and the pressure stabilised.
Now everything was ready for a test and, switching the boiler back on, all worked as it had been.
Hurrah!
For approximately eighteen hours…
Time taken: Most of a day (including tidy-up prep and going to get new valve).
Cost: Drayton valve, £62.78; 0.5 g jointing compound, £0.03; 100L of water, £0.40 - Total cost, £63.21.
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