Combi Boiler Problem

[marctwo]

My Combi boiler (Vaillant) has a problem. When you run the hot water you can hear the boiler fire up. The display on the front shows the water temp going up. You then hear a kettling sound the the boiler shuts off. The water temp drops and then it fires up and the whole cycle starts again.

We have had it checked and been told it's a problem with the heat exchanger. Can anybody confirm that this sounds like the problem?

[cjs]

Does the central heating work?

[marctwo]

I think so. It's not been on much lately!

[cjs]

Well I would be testing the CH so that you now exactly what issues you have.

Does the boiler actually produce any HW?

[marctwo]

Yes, it produces hot water. From a tap it runs hot then cooler, then hot again. The shower is not so noticible as it's thermostatic and the boiler fires up again before it gets too cold.

I will test the CH tonight, thanks for your help.

[eskidavies]

diverter valve or actuator if its like the vaillant i repaired a few yrs back cant remember model

[Leebo77]

eskidavies said:

diverter valve or actuator if its like the vaillant i repaired a few yrs back cant remember model

Just had mine replaced on my vaillant due to the same symptoms (only if OP's CH works)

[Aydogflipper]

Had one not producing hot water but central ehating was fine at worked.

diverter valve fault was the issue as has been suggested.

Regards

[johnthegas]

It's the plate to plate heat exchanger blocked up. the boiler is kettling, then overheating and cutting out, then resetting and firing again. Hence the hot-warm-cold-warm-hot water.
Check for either scale build up (from hard water) or debris in the primary side of the plate to plate.
Then fit an in line softener in the cold feed to the boiler and / or a Magnaclean Twintech into the primary return to the boiler.
Problem solved.

[marctwo]

johnthegas said:

It's the plate to plate heat exchanger blocked up. the boiler is kettling, then overheating and cutting out, then resetting and firing again. Hence the hot-warm-cold-warm-hot water.
Check for either scale build up (from hard water) or debris in the primary side of the plate to plate.
Then fit an in line softener in the cold feed to the boiler and / or a Magnaclean Twintech into the primary return to the boiler.
Problem solved.

CH works just fine.

This is more-or-less what the heating engineer said. We live in a hard water area so it's likely to be scale buid-up. I assume, in addition to the suggested water softener, we'll need a new heat exchanger? What kind of money are we talking here?

Thanks to everyone for their helpful replies.

[Arthur Jackson]

If it's scale the hot flow will generally be noticeably reduced. More likely the heating side full of debris....but as John says, could be either.

[johnthegas]

Marc the CH will be fine because it's the plate to plate heat exchanger that has scaled up and this is what produces hot water.
From memory, a Vaillant plate to plate will be in the region of £140 plus vat, expect to pay £70 odd to have it changed (London rates!)
Hope that helps,
John.

[john_p]

Yeah this is exactly what happened to our Worcester-Bosch. New heat exchanger cured it right away. CH unaffected, DHW would randomly cut out, which made showers fun

[Simon Brooks]

Sounds like you have a blockage in the heat exchanger, I've had the same problem a few times, have been informed it's quite often caused by heated towel rails corroding and breaking down internally

Edited by Simon Brooks on Sunday 9th October 17:34

[sa v8 mate]

Yep, defo the plate heat ex, where I work one of the lads has a sonic cleaner, like a mini bath that scaled up heat exchangers go into. Sonic sound waves clean all the scale out, sounds far fetched but it works, we've saved a fortune doing this proceedure with contract customers. Fit a Magnetic filter on the return at the same time or you'll have the same problem in the future unless you're selling up in the next 2 years.

[marctwo]

Are there any heating engineers covering SE London on here or any recommendations?

[marctwo]

Anyone?

[marctwo]

Now have a new heat exchanger and all seems well. Thanks for all the helpful advice everyone.

One more question. The plumber is recommending we fit a Magnaclean Twintech filter. Is this worthwhile?

[Chrisgr31]

marctwo said:

One more question. The plumber is recommending we fit a Magnaclean Twintech filter. Is this worthwhile?

Was JohntheGas your plumber as thats exactly what he recommended further up the thread!

[eskidavies]

filter on the heating circuit is good ,ive just been on a course for main boilers and seen a ,plate exchanger cut in half ,i recon you'd av hell ov a job pushing any ste out of that,also like has been said pending on where you are could be domestic side scaling up-:fit a scale reducer,just to prevent your current problems airing themselves in the futre.any boiler with both got to ok on hydralic side.then you got electrical probs, but thats another matter.