Solar hot water systems compared: two tank positions, two collector types
The booster decides what a solar system costs you to run. Before that, two other choices decide whether it fits your roof at all.
Every solar hot water system sold in Australia is built from two separate design choices, not one product. The first is where the tank sits: on the roof next to the collectors (close-coupled). Or on the ground with the collectors alone on the roof (split system). The second is the collector itself: flat plate or evacuated tube.
Sales brochures rarely separate the two, but they answer different questions: what your roof can carry, and how the system copes with cloud and cold. It is worth pulling them apart before you compare anything else. What the booster inside the tank does once the sun has done its share is covered separately in solar hot water and the booster. This page is about the hardware on the roof.
Close-coupled or split: where does the tank live?
A close-coupled system (also called thermosiphon) mounts the storage tank directly above the collectors on the roof. Hot water rises naturally from the collectors into the tank with no pump, which is the appeal. One moving part fewer to fail. The cost is weight: a full tank on the roof frame is a real structural load. And not every roof, especially an older tiled one, is rated to take it without extra bracing.
A split system keeps the tank at ground level, usually next to where an ordinary storage unit would sit, with only the collector panels on the roof and a small pump circulating water between the two. The roof carries almost no extra weight. The trade-off is a pump and a controller that need power and that are one more thing that can fail, plus pipework run between roof and ground that has to be insulated properly or the trip itself wastes what the collectors gained.
| Close-coupled (thermosiphon) | Split system | |
|---|---|---|
| Tank location | On the roof, above the collectors | At ground level |
| Roof structural load | High. Full tank weight when full | Low: collectors only |
| Moving parts | None: relies on natural convection | Circulation pump and controller |
| Typical siting | Roofs rated for the load, often newer builds | Older or lightweight roofs, or where a roof tank looks wrong |
| Access for servicing the tank | On the roof | At ground level, same as a standard tank |
| Common in | Warmer regions with less freeze risk | Cooler regions, where keeping the tank off the roof also keeps it away from frost |
Flat plate or evacuated tube: which collector?
A flat plate collector is a dark absorber sheet under a single sheet of glass in an insulated box. It is simple and low profile, and it is the design most installers default to across most of Australia's climate. An evacuated tube collector is a row of glass tubes, each with the air drawn out to reduce heat loss. That lets it keep working better in cold, overcast or windy conditions than a flat plate of the same area.
The practical difference shows up in climates with real winter cold or frequent cloud, where evacuated tubes hold an efficiency advantage. In the mild-to-hot conditions that cover most of the Australian coast, a well-sized flat plate array does the job at lower cost and with fewer parts that can be damaged. Individual tubes can crack, where a flat plate's single glass sheet is a simpler thing to inspect and replace.
Does the roof direction and pitch still matter?
Yes, and it applies to either collector type. A north-facing roof pitched close to your latitude gets the most consistent year-round exposure; east or west-facing roofs still work but collect less on average.
And a heavily shaded roof (from trees or a neighbouring building) undermines either design regardless of which tank position or collector you choose. This is a site assessment question for whoever quotes the installation, not something a brochure can answer for your specific roof.
What doesn't change between the two designs
Both close-coupled and split systems need a booster, gas or electric, for days the sun doesn't deliver enough. And how that booster is controlled affects the running cost far more than the tank-and-collector choice does; see the booster explainer for that half of the decision.
Both also sit inside the same category as every other hot water system for sizing purposes (see what size hot water system you need) and both are compared against gas, electric resistance and heat pump options in gas, electric, heat pump or solar.
What to ask an installer, whichever type you're considering
- Has the roof been assessed for the load of a full close-coupled tank, or is a split system recommended instead, and why?
- Flat plate or evacuated tube, and what is that recommendation based on for this roof and this climate?
- What backup booster is included, and how is it controlled: see the booster guide for the questions that matter here?
- What is the collector orientation and pitch relative to true north, and what shading was accounted for?
- For a split system, where does the pipework run, and how is it insulated?
What we won't give you is a number for any of this. Installation costs, running costs and any rebate or certificate value change by state, by scheme and by year. And a figure published once on a web page goes stale while still looking current. Get those from a licensed installer quoting your actual roof, and from your state's energy body for anything rebate-related.
Frequently asked questions
What is the difference between close-coupled and split solar hot water systems?
A close-coupled (thermosiphon) system mounts the tank on the roof directly above the collectors and needs no pump, but puts the full weight of a filled tank on the roof structure. A split system keeps the tank at ground level and uses a small pump to circulate water from roof-mounted collectors, which puts far less load on the roof but adds a pump and controller as extra parts that can fail.
Is a flat plate or an evacuated tube collector better?
Evacuated tube collectors hold an efficiency advantage in cold, overcast or windy conditions because each tube has the air drawn out to cut heat loss. In the mild-to-hot climate covering most of the Australian coast, a properly sized flat plate collector performs well at lower cost and with fewer individual parts, such as tubes, that can be damaged.
Do both types of solar hot water system need a booster?
Yes. Every solar hot water system in Australia, close-coupled or split, needs a gas or electric booster for periods when sunlight alone isn't enough to reach a usable temperature. How that booster is controlled has more effect on running cost than the choice between tank designs.
Can any roof take a close-coupled solar hot water system?
Not necessarily. A close-coupled system puts the full weight of a filled tank on the roof frame, and older or lightweight roofs may need a structural assessment or extra bracing before they can take it. A split system, with only the collector panels on the roof, avoids that load and is often recommended where a close-coupled tank isn't suitable.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)