I have been involved in several under floor systems that are effective, even in Victoria. All systems have to be incorporated in the house construction from the beginning as they involve heating a concrete slab floor in winter with sun warmed air. These systems require no further back up. The initial cost is the only cost, except for the electricity (about 4 Watts or less than 1 cent per day) to drive a tiny 75mm diameter fan during the fan in winter. The major cost is in excavating the house site an extra 250mm or 400mm (depending on which system you use) and doubling the amount of concrete. It’s good to find a dedicated builder to do this work as many builders "bung on" excessive extra charges for unusual jobs. Owner builders won’t have this problem.

Another way of using the sun’s heat is to orient the house to the North and build a Trombe wall. Trombe walls have been used successfully in Scandinavia, North America and Australia. The basic Trombe wall is a heat bank. It is part of the house walling of double solid brick and usually 2 metres high. It absorbs heat from the sun in the daytime and emits it to the inside of the house at night. The wall is heated only in winter as it is designed so that no sun falls on it in summer. This is achieved by eaves dimensioned to exclude summer sun after the spring equinox. As winter approaches the wall is again exposed to sun’s heat. Ideally the bricks should be a dark colour to absorb heat. Variants are a hollow cavity brick wall, the heated air from between the two walls being circulated at floor level via vets and a small fan or under the floor via ducts. The amount of roof overhang is critical but is quite easy to calculate. At either of the two equinoxes, (March 20 and September 20) measure how wide a right angle projection has to be at say, 2.5 metres up from the bottom of a north facing wall at midday, before no sunshine touches it. The diagram on the previous page shows the principle. Any of these methods can be used for the whole house or just part of the house. Building from scratch, one would heat the whole house if it were affordable.

Underfloor add-ons at ground level are feasible as long as a north-facing wall is available. A variant of the Trombe wall is also possible.

Underfloor heating systems using warmed air have to be sealed so that the air trapped in when it is finally finished is the only air in it. This is because constantly using outside air and expelling it fills the system with atmospheric dust (which we breathe). At Highett in Melbourne, an experimental house was built in the 1950s which had its heat exchanger on the roof drawing in fresh air all of the time. After about 15 years it no longer worked. The concrete floor was jack-hammered up and the ducts were found to be jammed with dust: hence the sealed system.

I have a small modified Trombe wall which looked at from above is U-shaped. The open side of the U is a pane of glass about 1 metre by 2 metres long. The inside of the 230mm thick brickwork is painted matt black. There are three glass shelves sealed on three sides. They are about 50mm short of full length and slope from end to end. The air to be heated enters at the bottom. As it heats, it loses its density and rises about 50mm to the sloping glass and moves under it to its high end which has the 50mm gap. It strikes the next shelf above and moves (as it warms) up under it to the opposite open end and so on. When it reaches the top it is sucked by fan into the underfloor duct system. Usually the fan is switched on about 10am and off about 6pm.

In this Northern NSW foot-of-the-ranges area, this small Trombe variant heats 60 square metres of floor area to a very comfortable level at night and during the day.

-Jim Knight