Vaillant boiler - hot water turning cold mid flow

20 Mar 2015
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United Kingdom
I've got an issue with my Vaillant TURBOmax 828E combi boiler and I am hoping someone might be able to help me understand the likely cause of the problem before I call out an engineer.

The hot water occasionally goes cold mid flow. I first noticed this when taking a shower, so I had a look at the boiler (without turning the shower off) and noticed that the digital temperature gauge had dropped down from around 75 to 42 degrees. Turning the boiler off and on again fixed the issue.

Initially this wasn't happening very often, but now it's happening 2-3 times a week. I also noticed the other day that it happened for the central heating also.

The boiler was in the flat when I bought it in 2007, so I guess it's probably around 15 years old. I am not familiar with the inner workings of gas boilers so I would be really grateful if anyone could let me know what they think the cause is most likely to be...
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