Direct (or open loop) systems circulate the potable water that is delivered to the household as hot water directly through the collector. Direct heating systems have potable water in direct contact with the heating source. In these systems, water quality must be good enough the ensure that the collector heat transfer surfaces are not restricted by impurities depositing on them, reducing the flow and the heat transfer ability of the system. They are typically the cheapest and easiest type of solar system to install and are especially useful for climates where freezing is not a risk.
With a direct heating system, there is no heat exchanger, which allows efficient heat transfer directly into the water. It is simple to add capacity to the system if demand changes and the system operates at standard line pressure. It is easy to combine these systems into current heating systems.
Direct heating systems can, however, have downsides including:
- They have little or no overheating protection systems unless they have a heat export pump
- They don’t have any freeze protection, unless the collectors used are freeze-tolerant.
- Collectors gather scale in hard water areas unless an ion-exchange softener is used.
There are a number of variations to the direct heat system.
Thermosiphon closed-coupled systems are systems where water circulates via the thermosiphon principle. As fluid is heated it expands, reducing its density. This lower density fluid will then rise, driven by the density difference. The thermosiphon action takes the heated water from the collector to the hot water storage tank.
Thermosiphon remote storage system is a system where the storage tank is sited away from the collector. The storage tank can be located beneath the roof cladding and out of site, but must be above the collector for the thermosiphon action to work.
Pumped systems (also known as split systems) use a controlled pump to circulate the fluid between the collector and the storage tank, with the collector and storage tank being some distance from each other. Pumped systems are almost always required when a solar collector is retrofitted to an existing hot water tank. The pump must be controlled by a differential temperature controller that switches the pump on when enough solar gain is available from the collector, and off when there is not.
To see our solar heating cylinders, see our solar cylinders page.