Many homes in America do not have electrical outlets under their sinks. So, they unknowing consider the recirculation type systems which have the pump installed on top of the water heater because there is almost always an outlet near the water heater. CAUTION: These types of recirculation systems utilize thermal actuation type comfort valves / bypass valves to control the crossover of water from the homes hot water line into the homes cold water line. Serious inefficiencies exist with these systems and strongly recommend reading the following before making a final decision for your project.
Crossover Prevention is critical to a recirculation systems ability to prevent water from bleeding into the cold water line from the hot water line as well as water from the cold water line bleeding in to the hot water line. Preventing crossover between hot and cold water lines is critical to a homes efficiency as well as comfort. Imagine the amount of energy a home would waste if when every time cold water in the home was used, water from the homes hot water line bled over into the homes cold water line. To an extent that the water coming out of the homes cold water faucets were no longer cold but at room temperature or higher. It's easy to understand how critical positive crossover prevention is to the efficiency of a hot water recirculation system.
WHY AVOID THERMAL ACTUATION TYPE VALVE SYSTEMS?
Thermal Actuating Valves (bypass valves / comfort valves) open and close based on the temperature of the water contacting the valve. Engineers have identified these valves as the main cause of a homes loss of cold water comfort. These valves open / close based on the temperature of the water which contacts them and rely on the systems "heater mounted pumps" pressure to overcome the opposing pressure of the cold water line at the bypass valve. The problem is that anytime cold water within the home is used, the pressure in the cold water line drops allowing water from the hot water side to easily pass through the comfort valve / bypass valve and into the cold water line. During this inefficient bleedover, cold city water enters the homes hot water heater to replace the water which bled through the bypass valve. This inefficient bleedover continues as long as cold water is being used / faucet is open or water hot enough to close the comfort valve reaches the comfort valve. This is a significant waste of valuable thermal energy and is the cause of a homeowners loss of cold water comfort in their home. Depending on how frequently cold water is used in the home (faucets, toilets, sprinklers, etc.), this inefficiency can significantly raise the homeowners water heating bill. Recirculation systems which have their pumping mechanism attached at the water heater and have bypass / comfort valves connected throughout the home utilize these inefficient temperature actuation type valves.
1st Inefficiency inherent with these valves is they cannot maintain positive closure when >20 pounds of opposing water pressure is forced on them, as when a faucet or tap is open. Thus, leaching between hot and cold water lines occur every time the water lines pressure drops, i.e. laundry, dishwasher, icebox refill, toilet flush, sprinkler system, etc.
2nd Inefficiency inherent with this type of recirculation system is that during scheduled-on periods the water heater mounted pump runs continuously consuming valuable electrical energy even when hot water has filled the hot water pipes. This also causes undo wear on the pump shortening it's lifespan.
Homeowners unknowingly have these type systems installed in their home because they just don't have an electrical outlet under their fixture / sink that's farthest from the homes water heater, believing they'll be saving money by not paying to have an outlet installed. But, in reality, they'll be wasting a great amount of energy and their homes cold water comfort every month. These major flaws have resulted in homeowners complaining of loss of their homes cold water comfort making them waste water down the drain in order to get cold water from their homes cold water line when they should be addressing the wasted thermal energy that their paying for each month.