Boiler constantly cutting in and out?

Hi, for a while now my boiler has been constantly cutting in and out for around 10 secs ("short-cycling"?) once it has been on for about an hour. The Boiler is a Thorn Apollo. It is starting to drive me crazy ... I've had a CH engineer round a couple of times (reliable bloke) who has no idea what the problem could be (he measured the difference across the 2 pipes into the boiler and said it was fine).

By the tank upstairs there is a valve called a Honeywell 3 Position Y-Plan Sundial Divertor - it does not appear to work (the CH engineer said it should be automatically moving, but it stays in the center position).

It could be unrelated, but there is also a lot of cracking in the pipes and in one of the bathroom radiator joints.

The living room thermostat is new, but whatever temp I set it to it makes no difference to the short-cycling.

I don't have a clue about anything to do with CH ... anyone have any ideas before I go mad?

He may be a reliable bloke ( whatever that means ) but not being able to diagnose a simple fault is not much use to you.

If the 3-port valve is faulty then it should be replaced as the contacts inside drive the pump and boiler! Quite apart from controling the CH and HW temperatures.

Tony

hmm, he did say he could replace it but wasn't sure it would solve the problem, and as we're moving house in a few months time I said "don't bother if you're not sure it will solve it".

Is there an easy way I can check if the Y-plan thing is or isn't working? Will I run into problems if I put the switch in either the "heating" or "DHW" positions?

The really odd thing is, this all started about 4 months ago - it then suddenlly started cutting in and out as would be normal - a couple of weeks later and it is back to not working properly again

As I say, I don't really understand any of this stuff, but i would have thought that something somewhere in the system is telling something else that a certain temp has been reached ... and then something else is saying it hasn't???

P.S. I say he's reliable becuase he always turns up when he says he's going to and doesn't charge much (if he charges anything at all).

I'm seeing a similar problem with an Apollo Fanfare sometimes cycling on and off at about 30 sec intervals.

We already suspected the 3-way valve was dodgy because sometimes the rads heat up even if only hot water is selected on the programmer.

Is the 3-way valve definitely the most likely cause of this behaviour then? Is it straightforward to replace?