Vaillant ecoTEC plus will not switch off the heating

[thegarbage]

Hi, I have a Vaillant ecoTEC plus boiler (in garage), Honeywell ST9400C controller and a bog standard Honeywell "dial" thermostat (in hall). The problem is that this boiler has a mind of its own and will switch the heating off and on when it likes and will not do what the controller and thermostat tell it to do. Please note this is an intermittant problem but it will occur every few days or so or after I have made a change to the controller.

Typical scenario1:
Cold day, heating comes on at 15:30 as per the controller, thermostat is set at about 18, the house heats up nicely and then the heating goes off about 6:30 ish. The house starts cooling and everyone is getting cold, then about an hour or an hour and half later the heating comes back on and stays on until 22:00 when the controller switches it off. During the time when the heating is off, the controller has the heating light on, the thermostat is calling for heat (I turn the dial and can hear the click at about 14 degress, then turn it back to 18 and hear it click again). The boiler however is well and truly off, no pump, radiator or flame indicator. I switch the whole thing off/on, turn the temp dial on the boiler up and down to no avail, still no heat. I have to wait for the boiler to decide.

Typlical scenario2:
Heating comes on and stays on, controller swicthes the heat off at 22:00, the light goes off on the controller, but that damn boiler keeps going and going sometimes past midnight whilst we're all sweating in bed. I have to either say a prayer for it to switch off or go into the garage and just manually switch the thing off.

The wierd thing about this, is that it can operate for days with no problems and then decide to start playing games with me. I've also noticed that if I make a change to the normal pattern, say extend the heating by an hour one night or change the timings, then it will start to do it's own thing again. It will take 2 or 3 days to settle then it will operate normally.

I've searched previously for this problem and noticed a few with the same issue. Someone mentioned that it could be controller wiring?

 

[D_Hailsham]

Hi, I have a Vaillant ecoTEC plus boiler

Which EcoTEC plus?

 

[thegarbage]

Hi, I have a Vaillant ecoTEC plus boiler

Which EcoTEC plus?

It's a 418, cheers.

 

[D_Hailsham]

It's a 418.

OK, an open vented boiler. Which means you have a hot water cylinder and one or more motorized valves, something like these:

View media item 5946 View media item 11762
Which have you got?

 

[ChrisR]

Take the lid off the wiring junction box, which is hopefully a proper one and not a spider's web stuffed into a tiny box.

Look for an Orange wire, There may be two, and that's the connection which will go to the boiler to turn it on.
So use a meter to see if it's live or not when the boiler should be off or on.
A Neon screwdriver would do to tell, at a pinch, as long as you test it before and after you use it.

The switched live "on" signal comes via orange from the valve(s), and they can stick open.

 

[thegarbage]

It's a 418.

OK, an open vented boiler. Which means you have a hot water cylinder and one or more motorized valves, something like these:

View media item 5946 View media item 11762
Which have you got?

I've got the 3-way valve (about 2 years old) and looks identical to your picture. I'm not upto messing with wires (I'll have to call an engineer for that) but ChrisR mentions above about checking if the orange wire is live or not when the boiler is supposed to be on or off. Does this ring true for my scenario when most of the time the boiler does what its supposed to do but on other occasions completely ignores what my Honeywell programmer tells it to.

 

[D_Hailsham]

I've got the 3-way valve (about 2 years old) and looks identical to your picture. I'm not upto messing with wires (I'll have to call an engineer for that) but ChrisR mentions above about checking if the orange wire is live or not when the boiler is supposed to be on or off. Does this ring true for my scenario when most of the time the boiler does what its supposed to do but on other occasions completely ignores what my Honeywell programmer tells it to.

The symptoms you give suggest that the valve is either sticking or has an electrical fault. You can easily check if it's sticking as follows:

Turn off the power to the heating system (setting the timer to HW OFF and CH OFF is not enough). This resets the valve to the default position.

At the end of the valve there is a metal lever in a slot - it should be at the AUTO end.

Move the lever to the MAN end; you should feel some resistance.

If there is resistance, release the lever and it should move back to the AUTO end with a whirring noise as the valve resets. Check a few times.

If the valve opens and closes easily, the fault is in the electrical side.

If there is no resistance (lever moves easily) the valve is stuck open for CH.

You can often clear this by removing the actuator (box on top) and lubricating the spindle with a small amount of WD40 or similar. You should be able to rotate it by hand - about 20° between stops.