Pressurised system hot water issue (Vokera)

1 Feb 2017
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United Kingdom

I have moved into a new build and have a strange problem that the builders have tried to fix for weeks, hope someone can offer a solution!

The system is a Vokera Mynute boiler with pressurised hot water tank and 2 heating zones. The hot water tank also has a solar thermal coil installed within it. The heating works fine (except that upstairs radiators get hot when downstairs heating on and upstairs off, passing motorised valve I suspect?). The issue is that the hot water works every day but gets cold at night.

Every evening this sequence happens:

1. Programmer set for boiler to heat hot water 18:00-23:00 (boiler operates as expected at 18:00).
2. Tank thermostat set to 60 °C (I can see this working at other times of the day as there is a control panel for the solar thermal next to the hot water tank which shows the top and bottom tank temperatures on the screen).
3. Once the water is hot my wife has a shower at ~21:00, after this the water is cold (somewhere between 10 and 30 °C).
4. I would expect the boiler to fire and heat the water as it's before 23:00 and the water is cold, but nothing happens.

The following morning the boiler always operates as normal. There's a +1 hour button that can be pressed to switch the boiler on for an additional hour, this does nothing in the evening but works fine if I wake up early and press it. The builder recently replaced the programmer but it's still the same as before. Any idea what could be the problem?

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Not sure but your probably best employing a heating engineer who is confident working on a s plan.
He will be able to trace the fault with a multimeter
Problem solved. It turns out that the control box for the solar thermal system was overriding all the other controls as it has a default setting that switches the 'alternative' or 'back up' heating source (i.e. the gas boiler) on at 05:00 and off at 21:00.

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