Worcester 24i overheat stat?

[hi1]

Following problems with the hot water running cold and then only tepid i changed the heat exchanger( we live in a hard water area). DHW resistor done prior with no change to water temperature. Flow through taps good. I had to take the overheat stat from its location when changing the exchanger, there was next to no thermal paste on it and i have no way to determine whether it was making enough contact to operate. On refitment i flooded the locating hole with paste, the overheat stat now operates when the heating has been on for a period, the radiator temps don't appear to be any hotter than prior to the change. Rads bled, Accumulator repressurised. New air vent fitted to exchanger during change.
So is it likely that the overheat stat is u/s
Is it possible to check that the air vent is working, air still trapped in exchanger causing hot spot?
Pump shaft rotating presume impeller is working otherwise upstairs rads wouldn't heat?

 

[hi1]

As a bit of extra info, just been stood staring at the thing when a demand was made for hot water, there was a horrible gurgling sound followed by a clunk and the pressure gauge dropped from 1.5 bar to 0.5 bar swang back up to 2.5 bar back below 1.5 and then settled back on 1.5 bar in a second. I thought the gauge gave a reading of pressure in the sealed i.e central heating part of the system whereas the hot water side was mains which passes through the exchanger. Could this new exchanger have an internal defect and be letting by? Its the second new one, the first was warped along its length so that one of the connecting unions was 10mm off a flat surface with the other 3 touching whereas the one which came off had all 4 unions resting on a flat surface, whats happened to quality control?

 

[hi1]

Bumping You can tell by the number of posts that the heating is being put on again after the summer hibernation.

 

[hi1]

Saga continues with pictures hopefully

Still continues to make horrible gurgling noises when hot water selected and when demand finishes so

Disconnected flow switch, place ohmeter across leads, electrics off, no circuit made until hot water tap opened, this means switch good?

Put ohmeter across terminals of DHW and central heating sensors both give same reading

Drained water out through PRV, already won't reseal, until flow stopped. Checked pressure of accumulator, 10psi as suggested on boiler common faults.

Removed AAC, new, replaced with old one which promptly leaked. New one reinstalled.

System repressurised to 1.5 bar. Hot water demand made makes horrible noise like air trapped in system, pressure gauge bounces about abit overheat switch triggers.

Have a look at pictures of exchanger that came off. DHW seems to be connected to tubes that run through matrix but heating pipes only appear to connect to plates/ is this the only surface where heat exchange for the heating takes place or are some of the tubes connected to some to DHW and some to heating and if so can there be letting by cold water into the heating circuit?

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• hi1
• 5 Nov 2010
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[Agile]

We would not have advised you to replace the heat exchanger yourself as thats classed as part of the combustion process.

If the temperature control circuit is working properly these should never give an problem.

I had to deal with one where BG had knocked the sensor cable off and the DHW tube had lime scaled to the point where there was little flow.

Tony

 

[boilerdoktor]

you can't check this but your gas valve may not be modulating down and thus on "high fire" all the time on hot water this causes kettling and overheating!

Is your pump running well and free??

It does sound like a h/exch problem but the above mentioned should be checked! worcester do a standard £210 all in fee, may be worth a call! your boiler will need parts and they cost so this call may work out to be a sound investment! Personally if i was fixing your boiler I would take temps at certain parts of the boiler and this would tell me if the h/ex was blocked! check g/v etc.

 

[hi1]

Thanks to Agile and boilerdoktor for their replies.

As an update to this post

Refitted old heat exchanger, noises and gauge jumping about cease.


Return new exchanger for refund. They test it for pressure, pressurise it to 1.5 bar using hot water connections. Pressure holds so it can't be letting by? yes

They said heat exchanger is pipe within pipe type, heating water gets heated in outer pipe which conducts heat to inner pipe containing dhw. Because old exchanger would probably have magnitite lining the pipe then it was less efficient at heating dhw. They also said when demand for hot water ceases pump runs to clear the hot water in the heating pipe, though where it goes i know not, because water isn't getting that hot the old pump can cope with carrying this hot water away and there are no problems however with new exchanger water gets hotter and old pump can't remove water fast enough resulting in the kettling and pressure rise.
Replace pump and all will be well. Replaced pump head with new one, refitted new heat exchanger and lo and behold its back to making the noises, pressure gauge swinging about.
Is it possible that there is some sort of fault in this new exchanger, pipe touching or brazing forming a bridge which could give a localised hot spot which could give rise to water boiling. The only common factor points to this exchanger not being right but i can't prove it.
I appreciate the point regarding the gas modulating valve but wouldn't that give rise to the same symptoms with both exchangers?
All i want is a hot bath.

 

[Agile]

VERY rarely there can be something partly blocking the HE when new.

But thats unusual.

The pump does not run during DHW demand as far as I know on your model.

Tony