Flue Water Heater

Smoke leaving a wood stove still carries much of the heat the fire made, and it goes up the chimney and away. A flue water heater is a heat exchanger, a pipe or jacket carrying water, fitted into that path so the passing smoke warms the water on its way out. It costs no fuel of its own: the fire was going to burn anyway.

It makes nothing by itself: there has to be a solid-fuel stove, it has to be alight, and the water is hot only while the fire is. It must also leave the chimney working. A chimney pulls smoke upwards because the smoke is warm and rises, so the warmer the chimney the better that pull. Taking heat out of the flue takes it out of the thing that makes the stove run. It also has to fit a connecting run that should be no more than about 3 m (10 ft) between stove and chimney, at the same 50 mm (2 in) clearance from anything that can burn as the pipe.

The main failure is designed in, not worn in. Creosote, a tarry deposit that can catch fire inside the flue, forms from wet wood, incomplete burning and cool surfaces, and water absorbs heat so strongly that the exchanger makes one on purpose. The same cooling weakens the draw, and a weak draw burns less completely, so one fitting causes two of the three at once, worst in the slow overnight fire it looks most attractive for. The chimney's routine therefore tightens: inspection before the season, checks at least monthly through it, and a daily brisk fire with the damper open to burn deposits off while thin, with one more surface to watch. A fire cannot be switched off, so the water side must keep shedding heat rather than rely on a control, and because the fire sets the temperature, a mixing valve downstream stops the tap scalding. It pairs with a solar water heater, which works in the months nobody lights a stove; between them hot water costs no dedicated fuel. No approved source gives a service life.