Mechanical engineering is an integral part of both HVACR (Heating, Ventilating, Air-Conditioning and Refrigeration) and plumbing. While both disciplines utilize the same science and engineering principles, their application diverges depending on the system under consideration. A prime example of this is the “wet” piping system that moves water through the piping network.

In the HVACR arena, the piping networks all operate in a “closed loop configuration.” In other words, the fluid (typically water with chemical additives) is contained within the confines of the piping and equipment network/system. This fluid, once internal to the system will not be drawn off from that system. Its sole purpose is to move thermal energy from one location to another.

However, plumbing domestic water systems are both “closed” and “open.” In the case of plumbing, the domestic water system is “closed” when there is no withdrawal for the closed piping network. But it becomes an “open” system once a user opens a faucet or flushes a fixture. It remains open until that faucet or fixture closes after its usage.

This would not appear to be a big issue, but it allows “new” water or fluid to enter the system. This allows entrained air to also enter the system. In the HVACR arena, provisions are provided to remove and vent this air out of the system. Through the use of automatic air vents or other means, any accumulation of air will be eliminated from the “closed” piping system.

So why are we concerned about air within the piping system? Air is compressible, while water is not a compressible fluid. Trapped air within the piping will prevent circulation of the fluid as the head pressure produced by the pump is converted into the compressive force absorbed by the entrapped air. Hence the energy produced by the pump no longer can move the fluid as the air has become a block to the circulation of the fluid.

This is well understood by the HVACR design professionals. They configure their piping designs to create “high points” within their piping design. These high points allow for the removal of this entrapped air, allowing the pump energy to move the fluid in a continuously circulated loop moving energy from one point to another.

When designing the hot water return system; it should never come off the top of the hot water piping, creating a high point. The return should tap off of the side, bottom of the horizontal supply line or come off of the drop to the fixture.

But entrapped air in a domestic water system can be a real problem, especially in the domestic hot water return system. This circulation loop (normally referred to as the “recirculation loop” — recirculation simply means that the same amount of fluid continually circulates through the piping loop or circulates over and over again) must be designed to expel the entrained air. The plumbing system is closed, when in a static condition. It becomes an open system when a fixture, faucet or outlet is opened. This opening of the fixture, faucet or outlet changes the hydraulic conditions by allowing the static pressure in the piping to flow toward the atmospheric condition at the fixture, faucet or outlet.

In the dynamic condition, the system pressure will “push” the air along with minimal compression until it is expelled at the terminal point; fixture, faucet or outlet. However, when air is trapped within the hot water return piping, the system pressure along with the pump head pressure will generally not be sufficient to move the entrapped air. It will simply compress that entrapped air and prevent circulation. Any entrapped air within the return system must be removed or prevented from entering that part of the piping or provided with a means of venting that entrapped air.

In the HVACR arena, automatic air vents are commonly used to vent this entrapped air. The fluid within the HVACR, over time, releases entrapped air through these automatic air vents. As the amount of entrapped air is removed, there is less oxygen in the water. This along with chemical treatment lowers the risk of corrosion within the exhaust port from the automatic air vent.

This is not the case in a plumbing system, as fresh water is always entering the system during its dynamic operation. This fresh water brings in more entrained air and minerals. These conditions are problematic for automatic air vents. These vents will quickly corrode shut and stop removing air. Hence, their use is generally avoided in the plumbing arena. Manual vents could be used, but they must be accessible to allow for frequent venting, depending on the amount of entrained air that enters the system.

It is better to design the system to avoid high points within the return system. Generally, this has to do with the way the return system is interconnected to the hot water supply piping. The supply piping should always be the high point of the hot water piping system. That is because the supply piping can use the system pressure to expel the entrapped air out through the outlet.

When designing the hot water return system; it should never come off the top of the hot water piping, creating a high point. The return should tap off of the side, bottom of the horizontal supply line or come off of the drop to the fixture. In this way, the hot water supply expels the air and it becomes much easier to fill the return with fluid without entrapping air. Each return tap should include a balancing valve as well as a check valve. The balance valves regulate the necessary flow to compensate for heat loss and the check valve assure that the flow is in the correct direction. It is also wise to provide a full-size “flushing” point just before the circulating pump. This allows for fully filling the return piping with water and removing any entrapped air during the initial start-up.

Another thing to remember with a hot water return system, it only operates when the hot water system is in a static condition. As the hot water system moves from static to dynamic operation, the hydraulic conditions within the return piping changes. Initially, that first flow from an outlet will reduce the volume of flow in the return loop. As more flow exits the system through multiple outlets, the less the circulating pump will induce flow. This is because of the hydraulic condition throughout the hot water system continued to change. The circulator is selected to overcome frictional losses within the hot water loop and through the associated equipment. It does not care about any changes in elevations or the static pressure within the system. But, as outlets are opened and closed, the suction pressure on the circulator changes due to the hydraulics of the system. As more and more outlets are opened, the suction pressure at the pump will become less versus the pressure on the discharge side of the pump. At some point, circulation will stop and the pump will simply “churn” water until the hydraulic conditions return to a static condition within the hot water system.

The return loop is not intended to induce flow continuously. It is only intended to operate during static flow conditions within the hot water supply system. The return loop simply needs to flow sufficient water to compensate for piping heat loss, nothing more. So, when flow is occurring in the hot water supply system as the various fixtures, faucets and outlets operate, the circulator is not needed. The circulator has done its job, providing hot water to the fixture, faucet or outlet. Flows through these points assure delivery of hot water in a timely manner. The circulator only is needed when the hot water supply becomes static (equal mass in versus mass out) to induce sufficient flow to maintain hot water temperature at the points of delivery.