On 18 Jan, 17:12, "js.b1" wrote:

Beware such backboilers exhibit a winter flood risk when the house is
unoccupied. Cold air plunges down the chimney and over days can cause
the tank &/or pipes to split - the resulting flood from the CW-HW
tanks is either unlimited or still 800L if you have the CW isolated.
I've known 3 fail in this manner, wrecking expensive wooden flooring
and downstairs contents throughout (DG doors meant the water literally
had nowhere to go with solid floors throughout).

Thanks for that. I was aware of the explosion risk ; the freeze &
flood hazard had never occurred to me.

A link to an HSE document on the fatal explosion is below.

http://www.hse.gov.uk/press/2009/coisw61109.htm