Hot water sometimes goes cold

2 Nov 2015
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United Kingdom
Hi, I have an intermittent issue where the hot water to the taps and shower from our combi boiler sometimes decides to run cold. The pressure to the taps has always been low, as it takes ages to run a bath even when the water is coming through hot. Having investigated I think the problem only occurs when the central heating is also drawing from the combi, would that explain the issue and why is this? Do I have a problem with the boiler or the water supply to the boiler?

Thanks in advance
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All I know is it' a Valliant Combi installed around 2001.....
Could be a Turbomax Plus. Is there a drop down front panel? Should be a name badge on it somewhere. If not post a photo.
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A combi boiler only provides hot water or heating and not both at the same time.

So the only effect of having the heating on is that the boiler is already warm.

Thanks Tony, so when the central heating thermostats kicks and fires up the boiler, then any hot water that may be coming from a tap will turn cold as the hot water is diverted to the radiators, is that what you're saying?
The hot water should override the heating. When a tap is open the boiler should divert all its efforts into heating the water. Very difficult to diagnose without the boiler model.

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