Rather than running water down the drain, residential hot water circulating systems provide quick hot water to the user, providing more convenience for the user.
The piping connecting from the heater outlet to the fixtures, and then back to the heater, forms a loop in traditional circulating systems. Heat is circulated through the loop using a pump. Despite providing near-instant heated water, these systems are extremely expensive to operate. This decreases the life of the heater and wastes a lot of energy keeping the pipe hot. These aren’t suitable for tankless heaters. Grundfos, Taco, Bell & Gusset, and other companies offer traditional circulating systems.
The Autocirc system from Laing can be retrofitted into existing homes without a dedicated return line. Basically, the pump is mounted under the sink, and it pumps the hot water into the cold water pipe, and when the temperature reaches 95 degrees, the pump shuts off. The pump turns back on after the temperature reaches 85 degrees, and the cycle repeats. If the plumbing arrangement is correct, the pump can handle several fixtures. Keeping the plumbing filled with above-average temperature water wastes a lot of energy, and the cold water line ends up with tepid water instead of cold. Tankless water heaters are not compatible with the Laing autocirc.
The Grundfos company makes a similar circulating system that can be retrofitted into homes without a dedicated hot return line. Each fixture in the home has a thermally controlled valve that connects to the outlet of the heater. This is done by connecting your water lines to the valves. Since the water heater outlet pump provides pressure, water flows from the hot line to the cold line when each valve is cold. Once the valve has been heated, the valve closes. Using this system, heated water can be kept close to all of your home’s fixtures all at the same time. However, it delivers only lukewarm water, and it replaces cold water with warm. It is not as good as the Laing Autocirc. Tankless heaters aren’t compatible with it.
A “demand” hot water system is manufactured by Metlund and Chilipepper. A demand hot water system is a heated water circulation system used in residential buildings that does not require a dedicated return, so it can be retrofitted into an existing home. In addition, the return line is connected to the cold water pipe. In the same way as the Laing pump, demand pumps connect hot and cold lines under a fixture. Pumps move water directly from hot water lines into cold water pipes when they are activated by pushing a button, and shut off when hot water reaches the pump. Water is moved more quickly when a demand system pump operates.
It saves the same amount of water as a traditional system, but has several advantages over it. It uses less energy than a traditional system. Due to the short duration of the hot water demand system…usually less than $2.00 a year…it consumes very little energy to pump hot water. The water stops flowing when the fixture is reached, which means the system does not use any more energy than it would without the circulation system in place. Additionally, the installation cost is considerably lower. Any house can be retrofitted with this system.
With the Demand system, at least one of the large Metlund pumps will work with tankless water heaters because the pumps are more powerful than those in the other systems. Tankless heaters can’t be activated by most circulating pumps.