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What's the likely problem?
There's a 20 year old gas boiler driving central heating and water into a traditional storage tank. CH is fine and water heats OK using the immersion but hot water from the boiler isn't working when the timer/controller tells it to (either via timer or boost). I know nothing about the overall system but it seems to me that some other component that presumably overrules the timer/controller when the tank is at the right temperature isn't doing its thing. The header tank in the loft is full and working.
Thanks.
Motorised zone valve most likely.
You'll have two x two-port valves ( a valve with a motor on top and a pipe running straight through it) or a three port valve( a t-junction in the pipework with a motor on it).
Each valve will have a manual slider to operate the valve - locate the valve that serves the hot water cylinder and move the lever till you hear a click and you should be able to lock it open. If the timer is calling for hot water when you do this, and the boiler fires up and heats the tank, then all that is needed is a new motor - ~£30 on t'internet and an easy DIY.
HTH
Edit if that doesn't work, it might be. Dodgy cylinder stat on the hwc - a cheap component to swap yourself before calling in the professionals. Be aware that it is likely that it is 240v so if that isn't your thing, get a professional out.
You will either have a 2 way or 2 one way motorised valves at the splitter point. One is broken. You might be able to change the actuator. If not its a drain down and replace. You can get them from b&q.
Beaten to it!
Thanks chaps. I pushed the lever on the hot water (Honeywell) motorised valve from Auto to Full Open. There was some resistance and now the switch itself is 'loose' in that it goes freely to either position. Apart from the fact that I may have broken something, the CH now pushes hot water into the tank which suggests the valve is now fully open. It appears to my that this is symptomatic of a stuck valve but the boiler still doesn't come on when the timer/controller requests it. Faulty valve, motor or something else?
yeah, faulty actuator - the top part. definitely replaceable IMO.
There's a microswitch int he actuator that sends a signal to the boiler and pump to come on. basically your timer opens the valve, and the valve calls for the boiler/pump when it is open (to avoid them firing with no-where for the heat to go).
its good that the valve is open and the replacement of the top half of the valve can be done without working on any 'wet' parts.
There was some resistance and now the switch itself is 'loose' in that it goes freely to either position.
Absolutely correct behaviour.
The first time you push it, you're also winding the actuator, hence the resistance. Once it opens and stays open (the actuator is powered open from the controller, the lever is free to move about).
You'll probably find it now works ok as the actuator was sticky and you've unseized it. May well jam again though, so just replace the motor, or the whole thing.
You'll find that the retailer will try to sell you the whole brass valve as well by giving you dire warnings that they won't refund the cost of the motor if the problem turns out to be the valve. However in 99% of cases it's the motor that has failed.
Just spent the day sorting this very problem. I replaced the motor on mine and it burnt out in a couple of months. Either bad luck or a sticky valve. Decided to do a full drain down and replace the valve. (Took 8 hours due to sludge and airlocks. Another story).
The old valve did have a lot of resistance compared to the new one so I'm please to have changed both.
One of mine is still original to the CH going in and the other I replaced probably about 15 years ago. They last longer than the pumps, seem to get 8 years ish out of them...
[url= https://farm6.staticflickr.com/5556/14348623286_5d22950d3c.jp g" target="_blank">https://farm6.staticflickr.com/5556/14348623286_5d22950d3c.jp g"/> [/img][/url][url= https://flic.kr/p/nRWrNE ]Central Heating pump and motorised valves[/url] by [url= https://www.flickr.com/people// ]brf[/url], on Flickr
Problem was a loose wire in the actuator of the motorised valve. Regrettably I needed a plumber/heating engineer to tell me this but I'm not much cop with 240v so fair do's.
Thanks all - spot on advice / diagnosis as usual.