Australian Passive Solar Home

OK, the results for our passive solar house are available for December – the first month of summer.

Note: no mechanical or electrical heating or cooling systems used – zero.

For our Canberra climate zone, it was a reasonably hot month. Logged at the house, the outside temperature was 30 deg C, or more, for 15 days of the month. The maximum outside temperature was 39.6 deg C.

Inside the house, the maximum recorded temperature (middle of the house) was 25.0 deg C.

Overnight outside minima varied widely, from 5 deg C to 19 deg C. There were 16 days where the overnight minimum was 12 deg C or less.

Inside the house, the minimum temperature was 18 deg C, which occurred once. Typically, the overnight inside minimum was around 20 deg C; it sometimes got lower when I chose to open the house at night to help cool the thermal mass. The ’18 deg C’ night was when I went a little too far – opening everything when it would have been better if I’d opened only the windows. Overnight minima with the house opened were typically 19-20 deg C.

The house was opened at night only about 7 times in the month – it didn’t need to be opened every night, as I’d half expected.

On the hottest days, the radiant temperature inside the house was lower than the air temperature i.e. it felt cooler than the air temperature indicated. This is because the body was radiating heat to these cooler surfaces. This effect was most noticeable near the internal water thermal mass tanks.

On a few days, the interior dew point was over 15 deg C; if you were physically working hard in these conditions, air movement from a fan was good.

Carbon dioxide levels were below 750PPM all the time, and usually below 600PPM.

These results are very pleasing. In terms of the NatHERS modelled temperature performance of the house, I was most worried about summer. However, the house has performed considerably better in summer than modelled, presumably because of the thermal mass interior water tanks (total 4300 litres), and the better earth coupling of the slab caused by the deep concrete supporting pillars at one end.

It’s starting to look like we will need very unusual summer conditions to ever need to use the air conditioner – but here comes January!