I have been working in a job on and off for a couple weeks in someone's home.  Keep popping up into the loft. Each time I could here the boiler in the loft firing every so often.  Didn't really think too much of it at first, but realised no heating was on, and no water calling (combi).

Mentioned it to customer, and she said plumber says it normal, it's keeping itself warm and ready, but we do have to turn the rads off on their knobs.  They also mentioned theprogrammable room stat wouldn't ever work, and plumber had replaced it a couple times, and it still doesn't work.

Took a closer look, the boiler is heating the rad circuit via the bypass, as all rads closed.  Looked in the boiler, they left the factory link in across the external controls, causing boiler to fire constantly. Stat was wired to mains side, this link was on the 24v side.  Manual even shows to remove this link in wiring section!

It's been like it for over a year, plumber looking several times before announcing its fine and meant to do it!

Bless him.  

It was all wired fine, it was just the factory link left in which it clearly says to remove when using external controls.  A litte odd as the link was on the ELV side, not the mains side, so I guess you old miss it when you see empty 240v terminals.

Wouldn't have thought so wodsmith, which model do you have.

Our Vaillant boiler has pre-heat for hot water, something I disabled as didn't see the point.  I don't find it too long to wait without TBH.