If you think a pressurized solar hot water system fits your needs, you must consider a variety of rules, limitations and requirements, as outlined below.

Array and pipe mounting

Collector arrays should be mounted so that there's a slight slope towards the return inlet, where the solar fluid enters, which should also have a drain for maintenance.

All piping in the system should tilt slightly towards the heat exchanger and, if it runs horizontally, should be supported every 5 to 6 feet (1.5 to 2 meters). Vertical piping needs support every 10 feet (3 meters). If you're using corrugated stainless steel piping, you need to support it every 3 feet (1 meter) on horizontal runs, or anywhere it runs at an angle (non-vertical).

Choosing the right materials

The entire solar fluid loop should be made of copper or stainless steel piping. The only exclusion to this rule is if you're installing a large solar space heating system: in that case you can use Pex for the return (cold) line to the collector, except for the last 10 feet (3 meters) where it attaches to the collector array. Remember to bury Pex piping if it's outside.

The pipe insulation must be able to withstand at least 250°F (120°C) on the way from the collector to the heat exchanger and at least 180°F (80°C) on the return. External pipes should also be jacketed, to protect them from corrosion, weather and inquisitive animals.

Solar fluid maintenance

The industry standard for solar fluid is a mix of simple water and propylene glycol . This mixture can degrade over time and is badly affected by extremely high temperatures. This is not a fatal flaw, but it is very important to make sure that your system circulates the solar fluid whenever the sun is shining and has failovers to cope with the possibility of extreme heat.

If the solar fluid is allowed to reach high temperatures for extended periods, it will degrade much quicker than in normal circumstances, and will become corrosive. This negatively affects every part of the system, so it's important to keep an eye on things. Thankfully, you only need to check the state of the solar fluid every 10 or 15 years, but it's important to remember your maintenance schedule!

Shunt loads

If your pressurized solar thermal system will have down-time when the solar fluid is heated but your domestic water supply is hot enough, you'll need a shunt load (or "heat-diversion load"). This ensures that the solar fluid keeps circulating and doesn't overheat. The best options for this are a buried pipe with no insulation, an outside radiator in a cool, draughty spot (exposed fin-tube setups are popular) or a swimming pool or hot tub.