Why are my ceilings raining?

You know the feeling. It's December and cold outside, but dry at least. You arrive home, open your front door, and find that your ceilings are raining all over your sofa, bed, cooker...

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My modern combination gas boiler (for hot water and central heating) had sprung a severe leak in the loft, and was sprinkling cold water liberally around most rooms in the house.

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Why was the boiler in the loft in the first place? The 'old' boiler had been on the ground floor, in a corner of the kitchen. Just over a year ago, the installer of the 'new' boiler had advised tucking it away in the loft, where it would get on with its work quietly and unobtrusively. Nice theory; shame about the practice!