Worcester over heat on DHW.

[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.

 

[Mastermixlee]

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.

Yeah wasn't going against you so much just trying to stop op wondering where to start with all of it lol.

They mentioned they'd checked pump and that was all ok...unless I'm mixing posts up... Quote possible it's Friday night after all 😀

 

[billski]

Thanks for everyone's input. Most appreciated.
Yes I did see the pump running when it was doing it, and it couldn't be slowed with a screwdriver, but I suppose there is a chance it may have briefly failed for a second, so I'll test it with a voltage pen to see if it briefly looses power. That would explain the kettling and it shutting down. I'm not sure about the oh stat failing as it was over heating. I'll check gas pressure to see if it's modulating. I was thinking it would be a plate blockage as it only does it on DHW and not in max service mode. Surely the only difference there is the plate heat exchanger being used in DHW mode.

 

[billski]

Okay went back to check a few things today,
: Linked out over heat stat and still over heated, linkef out flue over heat stat, still over heated.
: Temps going in and out plate to plate where about 28 degrees different. Got up to 76 in 47 out before it shut down.
: Powe to the pump is constant until it shuts down, then it pulses for a while before the fan comes on and it fires.
: Gas pressure went up and down, but only by 1/2 a mbar so thinks it's modulating. The water flow isn't very high so to me it sounds okay. When the flow is reduced at the tap it does modulate right down.
Surely it can only be the plate to plate do you think??

 

[EvilDrPorkChop]

Okay went back to check a few things today,
: Linked out over heat stat and still over heated, linkef out flue over heat stat, still over heated.
: Temps going in and out plate to plate where about 28 degrees different. Got up to 76 in 47 out before it shut down.
: Powe to the pump is constant until it shuts down, then it pulses for a while before the fan comes on and it fires.
: Gas pressure went up and down, but only by 1/2 a mbar so thinks it's modulating. The water flow isn't very high so to me it sounds okay. When the flow is reduced at the tap it does modulate right down.
Surely it can only be the plate to plate do yoy think??

Sounds like plate is OK to me, but could be wrong.

 

[ShaunCorbs]

Plate not enough difference temp wise

 

[billski]

Plate not enough difference temp wise

What do you mean? Does the plate sound okay or not?

 

[EvilDrPorkChop]

Plate not enough difference temp wise

Surely it's about right though? If your HW is around 55oc then return isn't going to be much less is it.

Plus boiler will modulate at that.

 

[ShaunCorbs]

water incoming temp around 15 dc plus the 28 dc rise give you 43 dc hot water temp

normally in hot water mode the boiler wont modulate eg turn tap fully on should stay on max

 

[billski]

water incoming temp around 15 dc plus the 28 dc rise give you 43 dc hot water temp

normally in hot water mode the boiler wont modulate eg turn tap fully on should stay on max

Sorry so are you saying it sound like the plate needs replacing? Water at the tap was about 53/55. Is it scaled up do you think?

 

[Millsy 82]

Have you got temp clamps for your analyser can you get them as close to the flow of the heat exchanger as possible. See what temp you have coming out of it.

I would be saying pump or plate. Myself. Whip the plate out and have a look inside.

 

[Riley]

Was assuming he had but great shout

 

[EvilDrPorkChop]

Sorry so are you saying it sound like the plate needs replacing? Water at the tap was about 53/55. Is it scaled up do you think?

If water is coming out at that temp doesn't seem much wrong with plate IMO.

I would be saying pump or plate. Myself. Whip the plate out and have a look inside.

Isn't so easy on the Juniors, you have to take the hydraulic block out if I remember correctly. And tbh if you ain't got a new washer pack it's going to leak.

 

[Mastermixlee]

Can get the plates out on them if you cut the hot water outlet pipe. You can then wiggle it out the back down that gap (provided you've got clearance below the boiler to do it).

Though still would of gone the f5 trick personally to do a mini clean on it first.

And obviously check the vanes on the pump too to check no debris in there.

Don't see much else that can be suggested to be honest.

If thermistors are in range, pump voltage is constant when on demand, Ch runs fine, boiler modulates down etc, pump confirmed to not be cutting out etc then there's little else that can cause it.

But without checking and confirming all the above then it'll still be guesswork.

 

[billski]

Can get the plates out on them if you cut the hot water outlet pipe. You can then wiggle it out the back down that gap (provided you've got clearance below the boiler to do it).

Though still would of gone the f5 trick personally to do a mini clean on it first.

And obviously check the vanes on the pump too to check no debris in there.

Don't see much else that can be suggested to be honest.

If thermistors are in range, pump voltage is constant when on demand, Ch runs fine, boiler modulates down etc, pump confirmed to not be cutting out etc then there's little else that can cause it.

But without checking and confirming all the above then it'll still be guesswork.

The only thing that puts me off doing the f3 trick is that wouldn't work on the water side. So if that's scaled up the only thing I could do would be replace it. And yes I've changed a few by cutting a pipe and squeezing it out. I'm always reluctant to replace anything with out being fairly positive it'll sort it, I do hate these kind of faults!

 

[Mastermixlee]

Whilst I've had plenty of scaled plates causing poor hot water performance I've never had scale on the dhw side causing boiler overheating, as the system side would be clear and modulate down based on flow and return thermistors.

Just my experience though.