Yes, I also have a tiled roof in Canberra, have observed how wonderfully warm it is in the ceiling cavity in winter, and have seriously thought about installing ducts and a fan to circulate the cavity air through the main living area, which is about 65 sqm.
However, we now have a solution which is better IMHO, though not suitable for everyone. Quite independently, we recently installed a large (6kW) PV system. My wife had been asking for airconditioning for years, which I resisted for years because I don't approve of burning fossil fuels to produce electricity used for aircon. However, as we now produce greatly more electricity than we consume, I happily relented and we recently intalled aircon in the main living area. We don't use it much, but it is nice on those small number of stinking hot days, and the power consumption hardly even makes a dent in the power generated by the PV system.
That is all by way of historical background. We have gas heating, and when I bought the aircon I didn't think too much about the reverse-cycle (R/C) heating capability. However, it slowly dawned on me that my previous plan of circulating warm air from the roof cavity is now completely obsolete, and makes no sense at all for my situation. In reality, the R/C aircon in conjunction with the PV lets us heat the house all day for no cost, and with no CO2 emission. For those that already have or are planning a largish (say >3kW) PV installation, R/C heating is a very efficient and elegant way of using sunlight to heat your house during the daytime. As a bonus, you get cheap and emission free aircon in summer, if you want it. As a further bonus, you make money and save emissions by selling the power generated when you are neither heating nor cooling, which is probably most of the time.
The numbers work out very well. As you probably know, R/C systems typically operate with a COP of around 3.8, meaning that 3.8kW of heating is produced for every kW of electrical power consumed, which is about as close to a free lunch as you will ever get Our split system Mitsubishi Electric MSZGE71VA is rated at 7.1kW cooling and 8.1kW with COP=3.83, but we would virtually never need 8.1kW of heating. On most winter days, 3.8kW of heating would be (more than) enough, consuming 1.0kW of electrical power, which is easily met with even a small PV system, remembering that the PV system generates electricity ALL of the time, not just on those cold winter days when you want heating. I like playing with numbers, so here are some more FYI. The efficiency of solar panels is around 15.5%, but with a COP of 3.83, the effective efficiency is 15.5x3.83 = 59%, which is remarkably good. To look at it another way, standard solar panels are rated at 190W each, but with a COP=3.83, each panel can (in good sunlight) produce 190x3.83 = 727W of heating in your house, not bad at all for a panel measuring only ~1600x800mm.
A thorough cost-benefit analysis would be interesting. For me, the environmental benefits of heating in this way are more inportant than the financial cost, in which case PV-RC heating is hard to beat. However, with the cost of solar panels falling dramatically to <$1/watt, PV-RC heating may also make good financial sense. If you invest money in a hydronic heating system, or in fans, ductwork and filters to use the warm air in the ceiling cavity, then that investment is saving energy, emissions and money ONLY on the relatively few cold winter days when it it being used. In contrast, if money is invested in a larger PV system to run R/C heating, then the additional electricity produced is usefully used ALL year, not just when heating is required.
My apologies for straying slightly off-topic.
Posted Thursday 12 Jan 2012 @ 6:16:59 am from IP #