Valiant combi boiler HW ok CH not

[greengreengreen]

Valiant combination boiler is playing up.
Hot water is working ok. (But seems much hotter than before, this happened since starting to use CH, could be related)
Problem seen after hot water has been used.

CH turned on, pump is on, burner fires up for 5 seconds, then stops.
This repeats every few minutes. Water stays cold.

It knows that CH is cold so it comes on, what prevents it staying on?

Thanks,

Martin

 

[DP]

With number of models Vaillant make, I will be surprised if anyone can assist you in you plight.

It is akin to saying I am going to by a Ford. Ford is a name not a model.

 

[bathjobby]

Do you have a 3 way motorised valve on your system that directs water to heating, or water, or both water and heating ? If so these symptoms are common to the valve having gone and it is usually the little motor driving the valve that goes. The valve is spring loaded and the spring keeps it in the water only position as this is the most used and therefore doesn't require any power to the motor (the spring just keeps it in that position) so hot water is still available even if the motor doesn't work. When you request central heating the motor is switched on to turn the valve to the correct position (heating only or heating and water). When the valve gets to that position (about 2 secs) it makes a switch (alongside the motor) which signals to the boiler that all is ok and the valve is open. If the boiler doesn't get that signal, it has a safety cutout to prevent it pushing hot water to the valve which is in an 'unknown' position. That safety cutout is usually about 5 seconds.
2 choices, both easy to do, replace the whole motor housing on top of the valve, guessing £30 + for parts, expect £80 + from plumber, or if you can find somewhere that stocks just the little motor, just change that, about £9 +.

 

[namsag]

greengreen ignore everything bathjobby has said as he has described a normal system as you said you have a COMBI boiler so put an answer to DP`s points any you should get some help(sounds like a 242`s servo rocker arm sticking though)

 

[kevplumb]

id agree with namsag check the servo valve

 

[ChrisR]

Praps the impeller's fallen off the pump shaft?

 

[bathjobby]

Sorry folks missed the 'key' combi boiler bit; replied in good faith !

 

[greengreengreen]

OK more information ....It's a Valiant VCW GB242E combi boiler.

Last night it was mild the heating thermostat had gone off, but boiler was still cycling. (Burner on for few seconds, then off for few minutes, pump on all time)

So I turned off the CH, but cycling continued.
There is a switch on boiler to choose HW or HW + CH. I set this to HW only, however cycling and pump continued.
This was with CH off and no water being drawn. It was obviously going to do this all night so I turned it off completely.
This morning turned CH switch on and boiler on. Rads heat all OK.
Then draw HW for baths again OK (but very hot)
After this cycling starts again. (i.e. rads go cold)
So after using HW it thinks that HW is still being drawn perhaps??
So boiler turns on, there is no flow, so it turns off, but all the while CH is off.
So problem is more on the HW side rather than my initial thoughts of faulty CH side. I think this makes sense...

Any ideas??

Thanks,

Martin

 

[namsag]

Bottom centre of your boiler is the water section chrome with 4 pipes, and about 3 inches round directly above this is a black servo valve with 4 little copper pipes to it. Attached and behind this is a rocker arm that operates a microswitch when hot water is demanded . Yours is sticking in the demand position. This is usually caused by the pin in the water section getting slightly scaled and not return to the rest position after use. stripping out water section cleaning pin and greasing it usually does change diaphragm at same time. Very occasionally it will need the servo and if the gland nut has been leeking badly it may need a new bearing plate

 

[kevplumb]

sound advice

 

[ChrisR]

One I'll file, thanks namsag.

 

[greengreengreen]

Namsag's description was perfect. Sure enough the cam wasn't disengaging. The spring wasn't pulling it off the microswitch.

Half the time I turn water on, it sticks. So I guess I need to dismantle it to clean it. There are instructions in the manual which seem quite clear, although it looks like I need to drain boiler to do this...

Thanks very much for your help,

Martin