Hi,
I have a Rinnai gas water heater, and I live in Perth Australia which has 30+C days. I have a fairly large roof, so was wondering if a solar hot water system like this one: 32 SQ Pool Solar Heating | eBay (http://www.ebay.com.au/itm/32-sq-Pool-Solar-Heating-/331425315635)
Could be used to 'pre-heat' the water going into the heater.
I have a couple of concerns regarding this:
1. If the water isn't flowing constantly, it may get too hot... Not sure it's a plumber question.
2. Assuming my temperature is set to 23C, and lets say the water coming in is also 23C, does that mean that the gas heater would not fire up?
Thanks in advance!

I can't say whether that system would work. I suppose you'd have to experiment with it a bit to find out. It would really depend on at what rate you are using the water, and at what rate the panels are heating. It would definitely add energy to the system, but it is hard to say how much. You would also have to be sure the system is designed your household water pressure and the flow rate you intend.

1. It would probably not get too hot if it is not flowing. As long as the heater can take the sun, I doubt it would be capable of even boiling the water.

2. I'm guessing that your water heater has regular thermostat built in. In that case as long as the water temp meets or exceeds the set temp, it will not have reason to fire.

A general setup for solar water heaters may involve a second tank, and a recirculation pump or two. Often a daylight sensor and/or water temp sensor. Either way the water in the solar panel gets recirculated through a tank allowing for it to heat up a bunch of water while there is no flow.

It may be the hot tank or a second tank that feeds/recirculates with the hot tank. It still adds energy to the system during high flow, reducing fuel consumption.

I just googled 'solar hot water heaters in Australia', and there is much you should read. There is the solar hot heater ACT for Australia which gives you rebates, so I think it is all worth looking into.

I doublt though you are taking the right approach asking us, we are an international site which makes me doubt that you are heading in the right direction.

There are also many websites for companies in Australia who handle this product. So the problem you are siting, the temp. of the water from the outside temperature being a problem, has already been dealt with I would imagine. Good luck.

Sorry for not providing a link, but there were so many to choose from in Australia.