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Motorised Valve For Central Heating System - Fuse?

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carolminniemouse | 09:06 Fri 20th Sep 2019 | How it Works
9 Answers
Hi, I'm feeling very vulnerable re expensive repair required to my central heating system (needless to say I am not a qualified anything). Advised to replace 2-port motorised valve. Before I do this could it be fuse? There is a screw on the bottom of item and I'm tempted to unscrew and open it up. Any advice?

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I'm no expert, but a failure of the diverter valve (the motorised valve) should not cause the boiler to keep switching on and off. The most common symptoms of such a failure (assuming you have a combi boiler, i.e. no hot water tank and there is hot water on demand) are things like the central heating coming on when you turn on the hot tap. A combi boiler will only...
11:49 Fri 20th Sep 2019
Who advised you ? If a plumbing expert they should know the issue.

You've put no details of the problem which doesn't help. Unlikely to be a fuse as the symptoms would be different, I'd have thought.

Undo screws at your own risk. Don't want to flood everywhere.
Carol I wouldnt touch anything if you are not sure. But if you can post make and model here someone maybe able to help.
Some motorised valves have a little lever set in a groove on the square body. This is a manual override if the electric motor has jammed and not diverting hot water where you need it.
Some motorised valve motors can be replaced quite cheaply. It's the plumber's rates that bump up the price. About no more than £10 for a replacement motor and then the sky's the limit for half hours work.
OK. Price is higher now but under £20.
https://www.screwfix.com/p/drayton-synchronous-motor/28670
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Hi re earlier post, problem is boiler keeps switching itself on and off, only stays on around 1 minute but happening several times per hour - so I've switched it off now. I thought it might be an issue with timer/programmer but this checks out. Plumber advised I needed electrician and he recommends changing motorised valve which financial restraints mean that cannot be done before winter. Does this description of problem help?
I'm no expert, but a failure of the diverter valve (the motorised valve) should not cause the boiler to keep switching on and off. The most common symptoms of such a failure (assuming you have a combi boiler, i.e. no hot water tank and there is hot water on demand) are things like the central heating coming on when you turn on the hot tap. A combi boiler will only switch itself off through the application of the timer (which you have already checked); when there's no demand for hot water (i.e. when you turn off the central heating or the hot tap); or when it overheats. Intermittently coming on and going off is known as 'cycling' and was the problem I had, which turned out to be caused by a furred-up heat exchanger within the boiler (this is the apparatus by which the heat from the gas flame is transferred to the cold water coming into the boiler). Because the heat exchanger was furred up, it couldn't transfer the heat quickly enough to the cold water running through it, resulting in temporary overheating and the saftey cut-out operating. Then, as the heat exchanger cooled below the overheat temperature, the boiler would come on again, heat a bit of water, get too hot and cut out. This on/off cycle was repeated and meant that I didn't get meaningful heating or hot water. Unfortunately, a new heat exchanger is expensive and has to be fitted by a Gas Safe registered engineer.

Your problem might not be the heat exchanger, but if it is, I am a bit surprised that the plumber you consulted has referred you straight to an electrician. It could be that the problem is in the printed circuit board within the boiler (hence the electrician), but from my (very limited) experience of combi boilers, the motorised valve wouldn't have been high on my list of things to investigate.

In all matters of this kind, however, you must consult a competent and qualified engineer. My answer above is only intended as a pointer to what might be the cause.
I am no plumber. I had a similar problem with the boiler constantly going on/off every few seconds. It turned out there was little water in the expansion/header tank or an airlock in it . I backfilled the header tank at the draincock of our last radiator in the system by attaching a garden hose and forcing water back up through the system. Did the trick for me.The conventional boiler's thermostat was telling the sensor to shut down as there was no water to heat. (I think)
Should be about £200 to replace it (we've just had ours done and had one done at old house about 7 years ago).
Carol, I've looked at the specifications for several motorised valves, but I haven't found any mention of a fuse.
They are usually connected to a fused 3 amp supply, making another fuse unnecessary.

A stuck valve is only one of many possible causes of "short cycling."
With all respect to the plumber, they are trained in fitting systems, but testing and fault finding is not always within their capabilities.

Same with electricians. He may well be right about the valve, but shouldn't jump to conclusions. You could end up with expense but no solution.

I would honestly advise contacting your boiler manufacturer and getting a name of a local boiler engineer.

I wouldn't attempt to open the valve up. A better way would be to fiddle with any actuator levers on it, to see if anything needs freeing up. They do tend to stick at this time of year, after a season of being idle.

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