Balanced ·mechanical ventilation with heat recovery (MYHR) offers, in principle, a way of reducing carbon dioxide emissions from dwellings. In order for such systems to produce a clear reduction in C02 it is necessary for the emissions from the low temperature heat saved to exceed those from the electricity used to drive them. This condition places a lower limit on the coefficient of performance (COP) of MYHR systems, which in the UK is around 3. The major variable in system performance is the electrical input to the fans.

Across the world, the dominant form of building construction is heavy, load-bearing masonry or poured concrete, not timber- or steelframe. It is possible to make these buildings very energy-efficient, but they present very different problems from those associated with timber-frame buildings.

You have been using HOT2000 to determine if house designs meet the R-2000 energy target and have always believed that it does a good job. One day you decide to model your own home and compare the results to the utility bills. You find that the predicted and measured energy usage differs by a significant amount! Does this sound familiar? What's going on? Why the difference? Does this mean the utility meters are no good (unlikely); that you don't know how to perform take-offs (shudder); or that HOT2000 is no good (shriek)? Don't panic.