Worcester over heat on DHW.

[billski]

Any ideas if I might be missing something? Worcester Greenstar 28i junior, hot water runs for 5 to 10mins then boiler flame cuts out and it over heats. I can hear the main heat exchanger kettling and banging for a few mins, boiler tries to fire 3 or 4 times, then it fires up and hot water runs again for 5 or so mins. Boiler is about 8 years old, cpa all good, fan test point -5.5 mbar, central heating water all seems clear, runs fine on max test mode, pump seems strong. Can it only be the plate heat exchanger and the main heat exchanger blocked or possibly some thing else? Cheers.

 

[Harvest Fields]

Is the water hot out of the taps? Or warm?

 

[ShaunCorbs]

What’s your heat going into the plate and coming out ?

 

[Harvest Fields]

What’s your heat going into the plate and coming out ?

That was my next question.

 

[billski]

Is the water hot out of the taps? Or warm?

. Temp of the tap water 52 constant. Then obviously cold for a while when the boiler cuts out.

 

[billski]

What’s your heat going into the plate and coming out ?

Didn't check that.

 

[Burger]

Has the boiler been serviced? Stepper motor issue?

 

[billski]

Has the boiler been serviced? Stepper motor issue?

I don't think it's been well maintained as the condense trap needed a good clean out. Would a stepper motor fault cause the boiler to kettle and over heat??

 

[Burger]

I don't think it's been well maintained as the condense trap needed a good clean out. Would a stepper motor fault cause the boile to kettle and over heat??

Probably not thinking about it, it was more the 52 degrees, just re-read post.

 

[EvilDrPorkChop]

If you get constant HW at 52oc and boiler works fine for 5 minutes then Plate HEX isn't blocked, does it modulate correctly when it's on HW?

If it was diverter it would go down CH flow so wouldn't kettle.

Pump would be my bet, check to see if it's getting a Live when it kettles. If it is see if it's spinning/operating when it does the issue. Had exactly same issues on a Baxi/Vaillant where CH seemingly worked fine but HW played up. Baxi pump kept cutting out, Vaillant wasn't pumping it round plate quick enough.

 

[billski]

If you get constant HW at 52oc and boiler works fine for 5 minutes then Plate HEX isn't blocked, does it modulate correctly when it's on HW?

If it was diverter it would go down CH flow so wouldn't kettle.

Pump would be my bet, check to see if it's getting a Live when it kettles. If it is see if it's spinning/operating when it does the issue. Had exactly same issues on a Baxi/Vaillant where CH seemingly worked fine but HW played up. Baxi pump kept cutting out, Vaillant wasn't pumping it round plate quick enough.

 

[billski]

I checked the pump and it still seemed to be working when it did it. Had the front screw off and could see it spinning.

 

[Burger]

I checked the pump and it still seemed to be working when it did it. Had the front screw off and could see it spinning.

It could be a weak pump, try stopping with a screwdriver, or test the resistance of it.

 

[Mastermixlee]

Inject some f5 or f3 into the boiler through the bleed point (isolating the ch system isolation valves on boiler) and run the hot water tap for 20 mins to half hour. Will give a good clean out of plate and main heat exchanger and will likely solve it.

Normally would put the temp clamps/probes across plate inlet and outlet as has already been suggested above first though. And a non either some probes from multimeter or non contact voltage detector on the lead to the pump to confirm power supply is consistent to pump.

 

[billski]

It could be a weak pump, try stopping with a screwdriver, or test the resistance of it.

Yeah I did that and it seemed okay.

 

[billski]

Inject some f5 or f3 into the boiler through the bleed point (isolating the ch system isolation valves on boiler) and run the hot water tap for 20 mins to half hour. Will give a good clean out of plate and main heat exchanger and will likely solve it.

Normally would put the temp clamps/probes across plate inlet and outlet as has already been suggested above first though. And a non either some probes from multimeter or non contact voltage detector on the lead to the pump to confirm power supply is consistent to pump.

That sounds promising.
Have you done that your self with the f3/f5 cleaner?

 

[billski]

What temp different should I be getting in and out on a good plate heat exchanger?

 

[Mastermixlee]

That sounds promising.
Have you done that your self with the f3/f5 cleaner?

Many a time. Works a treat. Like a mini powerflush for the heat exchangers. Obviously drain out afterwards.

 

[Millsy 82]

Is the boiler modulating? Keep an eye on the inlet pressure see if it goes up and down.

Remove the motor from the DV and run it round the heating for a bit.

Undo the screw on the pump stick a screwdriver in it if you can stop it pump is shot if it tries to break your screwdriver end off it's all good.

Get it hot and then link out the flue O/H stat.

 

[ShaunCorbs]

around 30-40 dc plus

 

[billski]

Is the boiler modulating? Keep an eye on the inlet pressure see if it goes up and down.

Remove the motor from the DV and run it round the heating for a bit.

Undo the screw on the pump stick a screwdriver in it if you can stop it pump is shot if it tries to break your screwdriver end off it's all good.

Get it hot and then link out the flue O/H stat.

Okay so if it's not modulating what would cause that and you say link out the o/h stat, why?

 

[Mastermixlee]

Okay so if it's not modulating what would cause that and you say link out the o/h stat, why?

Can be dhw temp sensor, PCB or fan PCB that can cause it not to modulate. Would need to find out from manufacturers expected resistance range of thermistor to temperature relation & to find out expected voltages to fan

 

[Millsy 82]

Okay so if it's not modulating what would cause that and you say link out the o/h stat, why?

If it's not modulating then I'd look at thermistor. Generally pcbs on Worcester are not that bad the fans are shocking but if they go they go properly.

Disconnect the FLUE o/h stat. I believe it is linked on the same as the o/h and aps. I've had a couple fail in the past when they start to get hot. They can be tricky to diagnose as they are intermittent.

 

[Mastermixlee]

From your description I'm still of the opinion that it's gonna be restrictions causing the issue. As if it wasn't modulating then the hot water wouldn't likely remain at 52degrees.

And flue o/heat stat wouldn't cause it to bang and crash like you've described.

I'd check the thermistor ranges anyway and then do the old f3/f5 trick.

 

[Millsy 82]

From your description I'm still of the opinion that it's gonna be restrictions causing the issue. As if it wasn't modulating then the hot water wouldn't likely remain at 52degrees.

And flue o/heat stat wouldn't cause it to bang and crash like you've described.

I'd check the thermistor ranges anyway and then do the old f3/f5 trick.

If it was a restriction I would imagine it would happen when the boiler is running on max not so much after it had been running for a while.

My guess would be pump dodgy but I thought I would mention everything else so they can check it all instead of going to the job seeing if its modulating then coming back on here then checking the pump and coming back on here.