Vaillant VCW Sine 18 T3 Overheating & Tepid Hot Water

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I have a VCW Sine 18 T3, which apart from problems with the diaphram and diverter valve (both replaced in last year) has functioned pretty reliably for the last 6 years.

I recently had a system leak due to the failure of a section of plastic pipework (peeled open like a banana skin) installed by the previous owner. I replaced the failed section of pipe (with copper) and mopped up, refilled the system and bled the rads, and heat exchanger and cranked up the boiler. The system was not working properly after this, heating seemed to be overheating and hot water was tepid (max burn not being triggered). I ended up bleeding the pump and the system sprang into life and has been working okay ever since, until last night. I am away and the missus came home to an overheating boiler and tepid hot water. I instructed her to bleed the pump, which seemed to temporarily fix the problem, but it's back this morning. Boiler also seems to be losing pressure and requires refilling, although no obvious leaks. Any suggestions.

Is it repairable or does it need replacing?

Graham.
 
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Sounds like a flow issue you have. Either you still have air in the system or maybe the pump is on its last legs. Have you tried increasing the pump speed to see if that helps?

Are your radiators also cold or is it only the HW side of things?
 
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No the rads are blisteringly hot, (which leads me away from the a pump problem) even though the boiler is only on 1 (no room stat installed). Boiler temp approaching 100 deg C when missus turned it off.

Problem seems to be that if there are 3 types of flame:
1. Pilot only.
2. meduim.
3. Full (normally high hot water demand or heating starting from cold).

Then when the system turns on I am only getting medium, but it doesn't seem to turn itself off, it just keeps on heating. Type 2 also isn't enough to heat the shower above tepid and having checked the water section / diaphram I have eliminated this, especially since the problem was temporarily cured by bleeding the pump last night.

For info, how tight should the dust cap on the pump's automatic bleed valve be?
 
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The sensor may be failed! You can measure the resistance which should be around 1300 ohms cold and far less hot.

These are getting very old now but they are still totally repairable!

However, the number of engioneers who understand them is getting very small now.

A further problem is that most of them have multiple faults and dealing with just the cause of the immediate failure would leave a boiler likely to fail again later soon.

That problem is exacerbated by owners who just want a cheap repair and dont want a proper overhaul. Us engineers dont like to do a "quick fix" to keep it struggling on. If its to be repaired then we like to repair it to an as new standard.

Tony
 
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For info, how tight should the dust cap on the pump's automatic bleed valve be?
It should be completely loose. If it lets water out it needs replacing.
 
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Bearing in mind the comment about there being few engineers who still understand these boilers. Does anyone know of one in the Bristol area?
 
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A chap advertises in your local paper; Post or Observer. He has posted there for a few years now, claims to be an ex vaillant engineer with 25 years experinence.
 
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I expect that Chris Hutt will be very good with them.

I have lost track of how to contact him though but try local searches or through the CORGI listing.

Tony
 

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