Completing the set: we now have solar hot water

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A shiny new set of 30 evacuated tubes on the roof
A shiny new set of 30 evacuated tubes on the roof

When the feed in tariff for solar electricity was put in place, we were one of the early movers, installing a 1.6kw solar PV system on our roof. With the limited space on our roof, we had to pick between solar electricity and solar hot water, and we chose the former.

Once we finished our renovation, however, we had 99m2 extra of flat roof to play with.

Nonetheless, we’d already purchased a super-efficient instantaneous gas system to replace our 1988-era electric hot water tank. We’d hardly be saving the environment if we threw that away, so solar hot water remained on the long-term to-do list.

With the likelihood of rising gas prices over the next few years, I recently looked back into solar hot water options.

What I discovered is that it was possible to reuse our instantaneous gas system as part of a solar hot water installation.

It’s simple really: the evacuated tubes on the roof heat up the water in a storage tank (shown below). The output of the tank goes through the instantaneous gas system. If the water is already hot, the gas system does nothing. If it’s only warm, it boosts it as required.

So no manual boosting or fiddling around, with a 100% guarantee of hot water.

We ended up purchasing a system with 30 evacuated tubes, plus a 300L storage tank. This is far in excess of what we need for two people, but with the roof partially shaded in winter, the extra tubes were chosen to ensure we take maximum advantage of the morning sun.

We’ll be getting back $1300 from the government for the STCs (solar credits).

On current gas prices, the payback period is 8-10 years, which isn’t great. But I’m expecting the gas prices to rise, which should seriously shorten the payback period.

Anyway, it’s good to complete the set of environment-saving technologies 🙂

PS. after 3+ months of no rain, we had 2 solid weeks of rain immediately after putting in the solar hot water. Not great for our solar generation, but good for the garden I guess 😉

The 300L tank gets squeezed into the side passage of the house.
The 300L tank gets squeezed into the side passage of the house, alongside the instantaneous gas system.

Installing solar-powered roof ventilation

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Some time back we installed a R3.5 batts in the roof of the original half of the house, along with reflective foil. Despite this, the roof space still gets very hot in summer, and after a run of really hot days, we can feel the heat radiating down into the rooms over night.

For this reason, it’s highly recommended to ventilate the roof space during summer. While traditional ‘wheely birds’ are an option, what I’ve read suggests that they simply don’t draw through enough air in an hour to make a real dent on a typical roof.

We therefore focused on an active ventilation system. There are heaps of different options, but my search narrowed down to two products, both solar-powered:

In the end, we went for the Solar Star, which seemed like the better fit for our needs. For the size of our roof, a single Solar Star RM 1200 model was the recommended option, and we  bundled in a thermostatic control.

The Solar Star unit, sitting on our table before installation.

Installation was simple enough. It comes with a plastic flashing suitable for a corrugated roof, plus the necessary screws and instructions. We further simplified the process by fitting the unit directly under the ridge capping, which allowed us to skip a lot of the more fiddly waterproofing steps.

The flashing installed for the Solar Star, looking through to the reflective sarking underneath (which we cut a hole through at the end of the job).

All up, the job took about an hour, most of which involved getting the tools onto the roof and generally stuffing around.

The Solar Star installed, just under the ridge of the roof. Not pretty per-se, but far from ugly.

Ideally, at this point I’d be able to report (with graphs!) the roof temperature before and after installation, compared to the outside temperature. But life has been busy!

So I can report that the fan runs steadily and quietly, and I’ll post later with a purely qualitative assessment of the impact.