Our Heating Bill Doubled — Three Engineers Missed It, A Candle Found It In Four Minutes

The test itself

You need a candle or a stick of incense, and a windy day. That is the whole equipment list.

Close every window and external door. Turn off any extractor fans and, if you have one, the mechanical ventilation. You want the house still, so the only air moving is air that is getting in on its own.

Light the candle and hold it about ten centimetres from the surface you are testing, moving it slowly — slower than feels necessary, roughly the pace of a second hand. Watch the flame rather than the wick. Where the flame leans consistently in one direction, air is moving through. A flicker is a draught in the room; a steady lean is a hole.

Mark every spot with a strip of low-tack painter’s tape as you find it. Do not stop to fix anything yet. The point of the exercise is a map, and people who fix as they go almost always stop after the first one and miss the larger gap two metres further along.

Work the whole perimeter of each room at three heights: skirting level, handle height, and above the frame.