Onzo

While most people have a good understanding of energy savings through buying more efficient appliances and insulating their home, there are further options.  A step up from trying to prevent energy leaking from your house is to recover heat energy from air and water that flows out of the house by design.

All houses are designed (and indeed required) to have adequate ventilation to ensure a supply of fresh air and to prevent the build up of moisture. Unfortunately, this ventilation system (implemented in modern houses through trickle vents at the windows and extractors in bathrooms) takes warm air out of the house and sucks in cold air from outside.

If designing a new energy efficient house, it is possible to ensure that the inflow of fresh, but cold, air and the outflow of warm air end up in a single controlled location, rather than small vents all over the house. If this can be achieved, then such a house can use a heat exchanger, which uses the warm air leaving to heat the cold air coming in, recovering significant energy in the process. However, solutions like this are almost impossible to fit to an existing house, and are therefore out of reach of almost everybody.

When it comes to water, there is an even more obvious heat loss. Hot water used in the bath, shower, washing machine and dishwasher are all used for cleaning, and then sent straight down the drain while still hot: literally flushing energy down the drain.

At a technical level, recovery of this heat is moderately simple. A heat exchanger can be built by running the hot waste water through a large vertical metal pipe (where it clings to the outer wall and transmits its heat). Incoming water is then preheated by coiling the inlet pipe around this warm outlet pipe.

A limitation of this system is that it does not usually recover heat when you need it; heat recover from pulling the plug out of the bath would still be lost, as there is not a matching inflow of water unless you happen to turn on the washing machine at the same time. The scenario where this system works extremely well, and recovery of the order of two thirds of the heat is possible, is where this heat exchanger is fitted to a shower. Here, hot water runs down the drain continuously at the same time as there is a demand for more hot water. Heating the water from the shower using the heat from the water running out of the shower is highly efficient.

Commercial systems working on this principle are available, and widely implemented in countries such as Canada. They are particularly easy to fit in houses built with basements, where the heat exchanger can be fitted below the level of a shower very easily. British house design tends to make this harder, but such a system can be fitted when renovating the room below the bathroom.

Energy recovery can work well, but is much more realistic when designed into an energy efficient house.