The most important consideration when designing a hot water solar system is the freeze protection method. Sounds crazy, but it leads to more failures than anything else. Not just in the freeze damage, but also in other water related problems like corrosion and mineral buildup. Also, the collectors will be colder than the outside air at night, so even in temperate climates freeze damage can occur.
Next is collector "efficiency". This is another mistake people make. Evacuated tube collectors, for instance, can be expensive for their apperature are and also delicate. They can produce very high internal temps when stagnating. These are not characteristics wanted in a hot water system. Just stick with a good quality copper absorber plate with either black chrome (for marginal conditions) or black selective paint, a tempered glass cover and a good aluminum frame. This is the most cost effective design and the easiest to find. The riser tubes in the collector should be large and there should be minimal spacing between them. That's it.
Design the system with a relatively large storage and don't worry about getting the water up to temp. Design it as though the tank was being used to cool the collector. Do whatever you can to "cool" the collectors. Large storage, relatively high flow rate, small differential temps, etc. Remember that efficiency is based on temperatutre. The cooler the panel the less it's losing to the outside. This thinking will deliver the max to the storage tank.
Back to freeze protection and long system life. The best way to design the system is with a method to drain the collectors by gravity when there is no sun or any kind of failure in the system. Do not depend on power to protect the system! In other words, no recirc freeze protection, no electric valves that take power to drain the system, etc. This will make the system much more reliable and more efficient. My favotite is drain back. This is where there is a small holding tank, approximately 4 gallons. that holds the collector water at night. When the collectors warm up, the circulator pushes the water up to the collectors and the air that was in the collectors is stored in the small tank. Then, when the power fails or when the sun sets, the pump stops and the water drains back to the holding tank and the air moves back to the collector. Beautifully simple, no toxic or expensive antifreeze, no mineral buildup, no pressure in the collectors, etc.
Finally, and thanks for your patience, use a differential controller to run the collector pump. Not a timer or a photo switch. And not a photovoltaic collector dedicated to supply the power (spend your money on hot water collectors and use grid power). Place the collector sensor inside the collector against the fin, between the riser tubes, and near the top. The best position is against the back of the absorber where it is out of the sun but sensing plate temp. This can simply be attached with a small sheet metal screw. If you put it in the front it can be fished in near the piping outlet and just lay on the plate. The tank sensor can be slid in near the bottom of the tank, between the insulation and the tank itself, behind the lower cover or near the pipe. No mechanicaL connection is needed, just contact, and not where it is affected by outside conditions. So, try not to put the sensors on the piping near the collector or tank, but in the collector and against the tank itself.
My system is currently (2) 4X8 collectors with painted absorbers set up this way. It has 120 gallons of storage in series with a 50 gallon water heater. My solar tank is simply a 120 gallon electric water heater and my holding tank is a 4 gallon electric water heater. There is a recirc pump on a separate differential controller that recircs the hot water between the gas water heater and the 120 gallon solar tank whenever the solar is warmer than the water heater, giving me 170 gallons of storage and instant hot water at the faucets. With three of us here we can turn off the water heater for about 6 months with no shortage of hot water. For a few months the water heater runs some as needed and, in the stormy winter, it carries most of the load. This is in the Central Valley of CA at near sea level and with poluted air. Higher and cleaner locations can see about 30% to 50% better performance.
