2.3 Drainback Hot Water System
The fluids that are circulated into the collectors are separated from the heated water that will be used in the home by a double-walled heat exchanger.
A heat exchanger is used to transfer the heat from the fluids circulating through the collectors to the water used in the home. The fluids that are used in the collectors can be water, oil, an antifreeze solution, or refrigerant.
The heat exchangers should be double-walled to prevent contamination of the household water.
The controller in these systems will activate the pumps to the collectors and heat exchanger when design temperature differences are reached.
The heat exchanger may be separate from the storage tank or built into it.
2.4 Guidelines summary for solar domestic water heating systems:
A well designed system will provide 50-80% of a home's hot water needs (less in winter, more in summer).
There should be 10-15 square feet of solar collector area for each person in the household.
The storage tank should hold 20-30 gallons per person.
There should be no shade on the collectors during the hours from 9:00 AM to 3:00 PM.
The collectors should face south and be tilted at a 30 degree angle (slight variations noted above will not significantly harm performance).
The collectors and storage tank should be in close proximity to the backup system and house distribution system to avoid excessive pipe losses. The pipes need to be well insulated.
Mixing valves or thermal shutoff devices should be employed to protect from excessively high temperatures.
Select systems that are tested and certified by the Solar Rating and Certification Corporation (SRCC).