Central heating not coming on with timer

[johnscales]

Hi.
I've had this problem for a little while where my central heating won't come on when it is timed to.
I can manually switch on the C/H via the mortorised valve (manual lever) but there is obviously something causing the problem. I've also changed the syncronised valve's motor but the problem persists.
Hot water running as per timer settings.
What other things might it be worth looking at?

 

[stem]

What sort of heating system do you have? S-Plan (with 2-Port motorised valves) or a Y-Plan (with a 3-Port motorised valve)

Is the motorised valve actually moving moving when powered? To test, put the lever back in auto. Switch only the central heating 'on' (programmer set on & room stat calling for heat) wait for a minute. Slide the lever on the motorised valve, does it feel loose and floppy, or is there resistance? Now put it back in auto and switch the heating off whilst someone is listening to the valve, it should make a whirring sound when the heating is switched off. does it?

 

[johnscales]

Hi Stem.

It's a conventional boiler system. 2 - port valve. Pretty certain it's an on/off operation switch exactly the same in function, though not in looks to this :

. (2:17 shows the functionality)
I'm not at home at the moment but will certainly give it a try. Since the problem is that the fault is intermittent it might pass the test but then not come on tomorrow so what might be my next steps.? I'm thinking possibly a sticking valve?

Thanks

 

[oldbutnotdead]

The valve bodies are pretty reliable (mine are 29 now, new motors on 2).
I'd be more suspicious of the thermostat or the timeclock, more likely thermostat if its intermittent. Multimetet is the best way to test

 

[stem]

Y-Plan, S-Plan, & Combi's are all conventional. .....So we'll go with an S-Plan which does have a 2-Port motorised valve. (There most likely will also be another one for the hot water cylinder somewhere too)

When the fault is present, if you are competent with a multimeter, check for a 230V across the 2-port motorised valve's brown & blue wires:

If 230V is present and the valve isn't opening, the valve is faulty, I agree with @oldbutnotdead it would be unusual for a valve to be just 'sticking' especially as you have now moved it about with the manual lever a few times. If the live is present, and it doesn't open, and as you have already changed the motor, if you want, you could try a couple of drops of WD40 or similar on the spindle and move it a few times with the manual lever to work it in. The motorised valve also has an internal microswitch that switches the boiler 'on' via the orange wire when the valve is fully open, so it could be that if the valve is opening but the boiler doesn't come on, the microswitch is faulty.

If 230V is not present across the motorised valve's brown and blue wires, then the thermostat, programmer or their associated wiring would be at fault.

 

[johnscales]

Thanks guys.
Thankfully i'm competent with a multimeter so will do that check first. I've also flushed my system a few times so if it turns out that current is reaching the valve I'll move on to take the opportunity in going the whole hog and replacing the entire unit. I haven't done it in around seven years to that's reason enough for me.
Apologies I should have said gravity conventional as it's not a combi.
Also @oldbutnotdead I have a tap stat so shouldn't be the thermostat. By timeclock do you mean the programmer as it had crossed my mind that it may not be sending the 'switch on' signal consistently?

 

[oldbutnotdead]

Intrigued- a tap stat in the heating circuit? Unless the programmer is mechanical then its unlikely to be causing an intermittent fault- the relays in solid state timers tend to fail full stop.
@stem 's diagnostic will work, start at the valve then the room stat then the controller, remember to check both thermostat terminals

 

[johnscales]

Sorry Oldbutnot dead. The tap stat isn't on the heating circuit, it's on the hot water part. There isn't a single point (wall) thermostat for the radiators. All the rads in the house have their own TRV's.
I wasn't clear what you had meant by "I'd be more suspicious of the thermostat" and since the only one in the system that i'm aware of is the tap stat I just called it out.
Btw is it possible to change a tap stat for an electrical cylinder stat? Is it a big job not worth bothering with and just replacing the tap stat? Asking because i'm pretty sure that the tap stat has also had it's day and I can't find any diy's about it. As i'll be draining down the system I thought I could change it out as well.

 

[stem]

If you have a tap-stat for the hot water, you can probably forget the test of the motorised valves orange wire, as it may not be used. It's there for isolation purposes when there is more than one motorised valve in circuit, but it appears that you may only have the one.

So, you don't have a room thermostat for the heating, but you do have a tap-stat for the hot water, both of which are quite rare these days. So, it's not really a 'conventional system' at all

 

[ianmcd]

Try setting your time clock to Gravity HW rather than Fully pumped and see if the heating then comes on, you wont get heating on its own as in the lights on the programmer but the tap stat will close when the cly is hot enough

 

[stem]

Being on 'Fully Pumped' could fit the symptoms, and I wondered about that at one point but dismissed it because originally the OP said the problem has only been present for "a little while" so unless the OP had changed the setting, the 'fault' would have always been there.....hopefully we'll find out.