Vokera Compact 24 - hot water on & off

[GH76]

Hi,

My Compact 24 gives hot water for about 2 minutes then cuts out. CH works absolutely fine. Pressure etc all good. Dial turned up full for DHW. Tried reducing the flow into the boiler a couple of turns but no difference.

Measured the resistance across both NTCs giving the following:

DHW - 17k cold, 4.6k after the couple of minutes it runs for
CH - 17k cold, 3.6k after running for 20 minutes

17k is slightly higher than room temperature spec presumably because ambient is colder just now?

http://gasboilerforums.com/thermistoreadingsR-W.html

Any thoughts on whether these readings point to the DHW NTC being at fault? Don't fancy draining the system to replace a part in-spec. Do wonder why the CH NTC isn't going as low as the specified 1.5k though.

Thanks!

 

[GH76]

Hello,

Hoping someone might have some thoughts on this for me. Would be greatly appreciated.

While the system was drained I removed and cleaned the DHW thermistor. Wasn't particulary dirty or anything (Got sprayed in the face you may be amused to hear. Because I'm an idiot and hadn't closed off the water inlet tap Just assumed that because the system was drained . . . . Anyway, moving on . . . .

No change though. Flame cuts off after a couple of minutes of hot water demand, whether CH is on or off. The resistance across thermistor doesn't drop below about 3.5k even if I stop the hot water and immediately take a reading. And I can see it steadily rising the longer the hot water is off. I presume that the hot water should cut off the lower the reading gets, so this 'relatively high' reading means that a 'too hot' signal is not being sent thus can't be the reason for DHW cut off? The thermistor is one of the red types - better I believe, from reading elsewhere here.

Tested the DHW switch for continutity during operation. Circuit still buzing complete while water is demanded but boiler has decided to cut the flame. So I'm right in saying that it's not a fault switch cutting the demand?

CH works absolutely fine. Doesn't cut out at all. Pressure good. No amber or red warning lights. Water nice and clean when drained. (was replacing a rad)

Puzzled. Hope you can help!

Thanks

 

[GH76]

Part 3 of my monologue . . . . .

Turns out CH isn't fine. Cuts out after ~15 minutes too. Just hadn't run it long enough previously to know it was an issue too.

To try to rule out overheating, I took the temperature sensor off the side of the combustion chamber and left it dangling to the open air. Still cut out. So, does that mean it's not shutting down due to an overheat signal?

The Compact doesn't give error codes, unfortunately. But I'm getting no amber or red lights when it cuts out. Sounds like it is still running but there's no flame. Doesn't shut off. I need to manually turn it to '0' to reset.

It won't ignite again for about 20-30 minutes, which is why I suspected overheat. If the primary thermistor was knackered wouldn't that give a red light straight away?

Any input to this issue would be greatly appreciated folks, thank you.