The GRC Drain Back System
The way to make the drain back concept work was to rethink all the features of the system. Minimize the problems and maximize the efficiency. This approach was the origin of the Gravely Research Corporation drain back design.
The original concepts were:
- Non-pressurized operation – No expensive ASME-rated tanks. No code requirements. No pressure safety devices needed.
- No heat exchangers between the tank and the collectors. Maximum the heat delivered to the tank. Catch all the energy possible!
- Maximum efficiency in the delivery of heat to all applications. Use no heat exchangers where possible, such as some space heating loops. Only one domestic hot water exchanger is required.
- Minimize evaporation losses from the non-pressurized tank. The tank vent design that emerged prevents ordinary evaporation losses while maintaining atmospheric pressure.
- Simple corrosion control. A non-toxic, food-grade boiler corrosion chemical was selected that scavenges oxygen from the water, prevents galvanic corrosion, and helps clean the piping.
- Unified tank system with multiple energy inputs and multiple outputs. This is referred to as “Grand Central Station”, where all the energy is routed into and out of the tank storage system.
- Simplify controls with no prioritization of energy output among applications. All applications have equal access to the energy. This prevents wasting stored energy by having one application holding off another.
- Maximize thermal energy conservation. Enclose all pumps, exchangers, and controls within the thermal insulation of the system, where feasible. Use excess heat from pumps, for example, to heat the tank. Minimize line losses by including local plumbing inside the insulation shell. . Some classes of pumps are water-cooled. They will work very well inside the insulating shell. Larger pumps are air-cooled and will not operate within the thermal shell of the tank.
The result is a system that is the simplest possible, the most economical to build, the highest efficiency, and the most durable. Many are still running after 25 years with only routine maintenance.
In operation, when the collector pump turns off, all the water drains naturally back into the tank from both the supply and return lines. If there is a power failure, the water drains back in the same fashion as a normal shutdown. Neither heat dumps or antifreeze are needed to protect the system, since the water is not in the collectors when the pump turns off, for whatever reason.
There is only one failure mode for a GRC drain back system. It occurs if the solar control activates when it should be off. Of course, this can’t happen in a power failure.
The “power on” failure mode has never been observed in the wild, but it is possible. A simple override control can be used to prevent the system from running when the tank is warmer than a certain temperature, or colder than a certain temperature. It simply interrupts power to the collector pump and requires human reset to start again.
This failure mode has been observed, however, due to operator error, so its effects are well known. In one case, someone left the control in the “Manual On” mode in cold weather. In another case, the factory controls were replaced by unauthorized heat pump controls in the field.
If the “power on” failure occurs at night during freezing weather, and the system runs long enough to dump the tank heat and freeze the collectors, the result is a rupture of a collector pipe and dumping of the tank contents on the roof. When the tank contents are gone, the process stops. Since the water solution is non-toxic, the run off is no more dangerous than rain water on the roof.
If the control failure turned the system on during a hot day, the collectors might boil, sending steam out of the atmospheric vent, thereby preventing excessive pressures. If this condition lasted long enough, the tank water would be depleted and the steaming action would stop as in the freezing case.
Current remote monitored controls are programmed to eliminate tampering and to report failures immediately.
With only one obscure failure mode, and no dangerous results, the GRC drain back system is far more “fail safe” than a glycol system under any climate or operating condition.
There are two elements to the GRC Drain Back design: the system concept and the resulting implementation into a family of products.
The product consists of a Fluid Handling System with all the components built in under the insulating shell. The electrical controls are mounted in a cabinet attached to the outside of the tank. A patent was granted for the drain back product design.