Secondary returns are a type of plumbing system used in both commercial and residential water heating applications to ensure that hot water is always available at every point of use, or at every faucet. When a particular area of the plumbing system calls for hot water that is not available from the primary water heater, the hot water is drawn from the secondary return system instead.
Secondary plumbing returns are a hot water recirculation system that typically consist of a copper pipe and tee system. Secondary returns connect to your household water supply lines to recirculate hot water back to the water heater or point of use. This recirculation by the return ensures hot water is always available at the point of usage. In some systems a pump is also used and is programmed so that it operates during periods of maximum hot water usage, which in most homes is usually during the early morning and evening hours.
How it Works
In a secondary return system, the pipes are arranged in a loop that relies on the natural tendency for hot water to rise upward and cold to fall down. In a typical heating system, hot water pipes are connected to your hot water heater and rise up, running to each fixture in your home, ending at the farthest fixture from the water heater. This creates half of the loop needed for a secondary return. By installing another set of piping a minimum of 2 feet from all high points in your hot water system, you create the other half of the loop. This secondary piping is directed back down to the water heater where the drain valve is located. In a home where the hot water lines drop below the water heater, a recirculating pump is connected into the loop here as well to circulate the water through the secondary pipes. The flow through the loop is slow, even with a pump, but any amount of movement ensures there is always hot water at each fixture.
Secondary returns can help prevent overheating of your water heating system by ensuring that sufficient water is circulated through your plumbing system at all times. In larger applications, such as commercial plumbing, it can also aid in preventing choking points, which occur when the heating system has plenty of power and hot water, but the inlets and outlets are too small to circulate the amount of water required. In a secondary return system, hot water isn't drawn from the water heating unit, but from a secondary loop of pipes in which heated water can flow at a high volume.
Secondary Returns and Heat Pumps
Secondary return systems may not work effectively with heat pumps, which are often used with boiler systems. This is because heat pumps are low temperature devices, so the water stored there is typically cooler than it would be in a system without a heat pump. You can resolve this by using a secondary return loop connected to a water pump and a flow boiler, but according to Kensa Heat Pumps, this type of secondary return system can be costly. Another option is to wrap the pipes with trace heating tape, which is an electrical plumbing tape that helps to maintain the water temperature in the pipe by heating it. Trace heating tape is often used in water supply systems to prevent freezing of your water pipes. In a secondary return loop, the tape is linked with a timer to operate only during the maximum periods of hot water usage.