When we purchased our house, it had an existing off-peak electric hot water unit in the roof. We disconnected this as part of the initial renovations and replaced it with a gas-fired external unit. After around 15 years, this unit died, and
we replaced it with another gas-fired unit with a capacity of 300 litres. This was on the basis that this would be our only source of hot water, and we had two teenage boys. Fortunately, Dawn decided to get a unit with a stainless steel boiler
(rather than a mild steel one with a coating) as it would cope with the higher inlet temperatures if we decided to go solar later. Around 2004 we decided to move to solar. We got a three panel unit on the roof (with its own storage tank, but no
booster) and plumbed it into the cold water line to preheat it before it gets to the existing gas unit. In practice, this means the gas unit runs on a pilot light only for much of the year - it still comes on during winter, as the solar doesn't
heat the water sufficiently. This summer (2008) we actually switched the gas unit out of the system and let it go out. This didn't cause any problems, and we will now follow this practice each year.
The solar hot water unit (we went with an Edwards unit) has been one of the most trouble-free pieces of green technology we have ever fitted. We essentially never think about it, and it just does its
thing, providing us with free hot water.
With the benefit of hindsight, it would have made more sense to purchase a small instant gas hot water unit when our first large gas unit died. That would have cost more and probably have been more efficient.
Had we known that the rebates would have got more attractive we might have waited to get the solar unit. But at the time we thought it was a good decision. The payback time on our unit was under ten years, and it came with a fifteen year
warranty, so it was a relatively riskfree choice.
When the current large gas unit dies, we will probably replace it with a small instant unit. When the solar unit finally dies, we will definitely replace it with whatever is the best technology at the time.