The two most commonly used systems in colder or freezing climates are Closed-Loop Pressurized (Glycol) and Closed-Loop Non-pressurized (Drainback). All major parts are interchangeable with some additional items for the pressurized systems. These would be considered ACTIVE systems as pumps, valves, or other controls are used to circulate the water within the system. Domestic hot water is never directly heated or comes into contact with the solar collectors. These systems use separate heat exchangers to transfer heat from the solar collectors to the domestic hot water.


This type system will almost always be referred to as containing food grade Glycol. A Glycol and water mixture helps keep the system from freezing. After the solar collector and heat exchanger are plumbed, they are charged with the glycol mixture and pressurized.

Either the flat panel or evacuated tube type collector can be used. Since the system is always full of liquid it is subject to freezing in the winter and overheating in the summer. Since the system is pressurized it will need an expansion tank and one way valve to prevent thermosyphoning or heat loss.

The glycol mixture prevents freezing in the winter, however it is subject to chemical breakdown from over heating in the summer. Heat build up at the collector can cause the mixture to thicken and become acid. Every several years it is necessary to open the system, check the quality of the mixture and replace if necessary. Occasionally it is recommended to put an external radiator on the system to discharge the excess heat away from the solar collector. This solution has several wasteful components to it. Additional parts, maintenance, and expense are required because of the pressure and excess heat potential.


The Closed-Loop Drainback systems are proving to be very reliable and efficient. The Drainback is a non-pressurized closed-loop system that uses distilled water (no glycol) as the heat transfer agent. Distilled water is the most efficient liquid transfer agent. A small reservoir located inside the utility room contains the distilled water. When the solar collector is hotter than the storage tank (domestic hot water) the controller will activate a pump which charges the collector system with distilled water. The collector heats the circulating water in the reservoir and transfers this heat through the heat exchanger to the hot water storage tank. When a pre-determined temperature or the storage tank equals the collector temperature the controller shuts off the pump. At this time all the distilled water DRAINS BACK into the storage reservoir in the utility room. Until the collector is hot enough and the storage unit calls for heat, there is never any water in the collector. The system design eliminates any freezing or overheating concerns.

Installation inside the utility room is easier as fewer parts are needed. Outside installation is more precise as the system needs to be sloped such that all piping drains back into the reservoir. Collector lift kits are available making installation much easier. Either flat panel or evacuated tube collectors can be used in a drainback system. One needs to check with the collector manufacturer to make sure the design of the collector will allow it to completely drain if used in a drainback system.



Freeze tolerant

More Installation options

Uses smaller collector circulation pump

System always under pressure

Uses pressurized heat exchanger tank

Needs more parts-expansion tank, check valves

Demands more maintenance

Subject to summer system overheating


Eliminates system freezing concerns

Eliminates system overheating concerns

Uses small non-pressurized tank with heat exchanger

Needs larger collector circulation pump

No required maintenance

Components all systems need include: collectors, differential controller (tells pumps when collectors offer heat gain), tempering valve (located on house hot water to reduce/prevent scalding), circulation pumps (normally two needed), solar storage tank (usually 85 to 120 gallon, may contain heat exchangers), back up water heater (normally existing home water heater, activates if water temp falls below preset level, back up heater and solar storage units can be combined for a one tank system-has disadvantages). Depending on the system selected, valves, drainback tank, expansion tank, pressure relief valves, temperature gauges (at least 2), check valves, high temperature insulation for copper piping (must be able to withstand temps above 200 degrees F), and all systems must be plumbed with copper piping because of the high potential water temperatures.

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