combi boiler DHW CH cross connection?

[wmbmrg]

I have an Isar Ideal combi boiler. I recently started draining the central heating system down to remove and refit a radiator on the ground level of a house. The water drained as expected, but after a short while began to run hot, despite the fact the central heating was switched off. Upon checking the boiler, the LCD display showed that the domestic hot water was running. There were no hot water taps on at the time. The problem was remedied, at least to allow me to disconnect the radiator and refit it elsewhere, by turning off the water at the mains. I also switched the boiler off at the electric supply. I thereafter cut the pipes as appropriate, capped off and re-plumbed elsewhere along the same feed pipes, to the new radiator position. Since then the radiators and hot water have worked okay, as they did before I started the work. Does this scenario of hot water coming out of the radiator as it was being drained suggest a cross connection, or is there another simple explanation? I thought the combi CH system was a contained system, and I'm hoping it is otherwise there does not seem much point in adding fernox corrosion inhibitor....? As a further point of note, the filling loop was connected at the time, but in the closed position. Any suggestions and comments gratefully received.
Thanks,
Mark

 

[Kings]

A small amount may have been from an expansion vessel or heat exchanger within the boiler but this needs investigating further.
There shouldn't be a direct link between mains/HW and the CH, only the temp loop. I would have completely isolated the boiler before any work commenced.
I wouldn't put any fernox in just yet!

 

[wmbmrg]

Thanks for your comments Kings.
It is the case that the Isar has a tank for a small volume of stored hot water. I didn't think it was much more than 1.5-2 liters worth, and I think I was beginning to fill my second 5L bucket with water and it was still coming out strong. If it was from the boiler's hot water tank, is there a reason why it would read DHW on the LCD, and also why would it automatically stop running when the mains was switched off, whereas prior to that it was flushing like a good 'un?

 

[Kings]

Another check could be to fill the rads and pressurise the system, turn of cold mains and open a hot tap.
If the CH pressure dissappears then you need to get your system checked.