Valiant running with no command

6 Jun 2021
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United Kingdom
I have a Vailant Ecofit Pure 825 installed in my bungalow. I bought the place two years ago and the boiler had been installed by British Gas two years prior to that.
I'm an electrician by trade and a building management system (BMS) engineer by profession. So, I’m used to fault finding on complex systems. However, this is my first experience with the Vailant domestic boiler and I’m a bit concerned about its controls. So, I’d appreciate some advice.
In the early hours of the morning, I noticed that the towel rail was at full temperature. I was concerned because the system was not programmed to run through the night. The room stat (Hive) was off. The interface unit adjacent to the boiler that picks up the signal from the room stat was off (no green light for heating). All the rads were off except the towel rail at full temp.
The boiler LCD screen had the flame icon in the lower left corner.
The 15mm DHW pipe was cold.
The 22mm CH flow was full temp.
The 22mm CH return less so.
The radiators were just about warm from the previous evening but not hot.
The one rad without a thermostatic valve was cold.
So, this is the second time I’ve noticed this. There have been a couple of times when I’ve thought that the boiler was louder than usual. This was just before it shut off on temperature, almost like it was starting to kettle from lack of flow?
So, is it possible that the boiler could run without the pump?
I would hope that either a flow proving device such as a DP switch or paddle switch would need to activate first?
Any thoughts?
PS yes, I did turn the boiler off and back on again. The boiler reset and sat idle, as expected.
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The boiler has built in frost protection, and a holiday mode with settable minimum temperature, could be one of those switching the boiler on. Take a look in the settings.
Thanks for the quick response Harry.
Very good points, I’ll take a closer look.
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