Maintaining temperature, pressure, and flow per design conditions will promote laminar flow and ensure adequate service life of the piping system.
by Kevin Wong, MBA, BSc, CAE, and Avishai Moscovich MBA, P.Eng, LEED AP
The following technical overview has been written in response to inquiries regarding corrosion in hot water recirculating systems and velocity effects. The article will discuss factors that may affect the service life of PEX tube and fittings in such systems with specific recommendations for both new and existing multi-unit residential buildings.
A typical hot water recirculating system in a commercial building consists of a piping loop in which hot water, from hot water tanks or boilers, is kept circulating by one or more pumps. This permits hot water to reach most points in the building within relatively short periods of demand time.
In some multi-unit residential buildings, as part of the domestic distribution system, water is pressurized in a high-pressure system and then stepped down in the zones mechanical rooms through an arrangement that includes pressure-reducing valves (PRV), a circulator pump, and a heat exchanger to a low-pressure delivery system to individual suites (see Figure 1). For example, per the Ontario Building Code, with each zone is a group of eight to 12 floors, and we will have hot- and cold-water supply high pressure stepped down through a PRV from 200 pounds per square inch (psi) to an adequate deliverable low-pressure side of 80 psi. (This may vary based on your location and the authority having jurisdiction [AHJ].)
On the low-pressure side we control the zone hot water conditions with a recirculating pump, a heat exchanger, and a balancing valve to collect all of the hot water and put it back into the hot water supply. Typically, this complex arrangement is left to operate with no monitoring or control on system performance. Very often, this system does not even show up on the maintenance schedules given to a service provider by the developer or building owner.
Traditionally, during building commissioning a circuit balancing valve is installed and set to allow design velocity in the recirculation line. Often, within a couple years you will find that balancing valve has been moved and is not in the proper set position anymore, and there is no indication on the flow or water velocity in the system. At the same time, the circulator pump is constantly running without any feedback on hot water demand. Under these unmonitored operation conditions, pressure, temperature, and velocity often creep up to exceed the manufacturer’s optimal working recommendations.
Investigations of PEX tube and fitting samples taken from hot water recirculating systems have identified several factors that contributed to the factors leading to damage and failure of the system:
- Water velocities exceeding 2 feet per second (fps)
- Undersized distribution lines, creating higher apparent velocities
- Oversized circulating pumps with no bypass, creating excessive velocities at times of low demand or during the startup/commissioning phase
- Multiple and/or abrupt changes in direction (i.e., short 90-degree fittings in the system decreasing the energetic of the overall system)
- Failure to flush out debris inside the tube
- Improper joints
- Improper use of throttling valves for system balancing
- Lack of maintenance, monitoring, and control
System performance where temperature, pressure, and flow are maintained per design conditions and meet manufacturer recommendations will promote laminar flow and ensure adequate service life of the piping system.
Excessive Velocity
Excessive velocity in a hot water recirculating system is typically the result of using an oversized pump, undersized distribution lines, or system creep that, along with lack of maintenance, leads to excessive velocities.
Several choices for corrective action to eliminate the problem of erosion corrosion are available. All are based on reducing the water velocity or eliminating the excessive turbulence at connections and fittings. Options include a bypass around the pump to reduce its effective output, a smaller-capacity pump, or a throttling/balancing valve downstream of the pump to restrict the flow, in addition to larger tube sizes in the areas affected.
Temperature Effect
In addition to reducing the flow, it is good practice to limit domestic hot water to an optimal temperature of 140° F (60° C), since increasing the temperature of potable water can substantially change its corrosive effects on materials. Although less common, erosion corrosion could also occur in cold water lines. This is based on the water chemistry of the inlet water from the distribution system or well supply, the efficacy of the water treatment or filtration level, and the resulting corrosiveness of the water itself. It is good practice to observe the recommendations presented here in both hot- and cold-water supply systems.
Erosion Corrosion (for Systems with Brass Fittings)
The pressure loss of a flowing fluid due to friction varies approximately with the square of the flow velocity. As the velocity increases, the abrasive effect on the tube wall increases, and erosion of the tube may occur. The extent of erosion caused by excessive velocity depends on the physical characteristics of the tube material or impediments to flow on the tube wall, such as the interior diameter (ID) of fittings or mineral deposits.
Erosion corrosion occurs at locations where turbulence develops in a system. This turbulence interferes with the normal protective film formation on the inside of the tube or fitting and also erodes the brass surface at that point. Turbulence can be caused by excessive velocity, sudden changes in direction, and flow obstacles such as restrictive fittings (i.e., with a lower ID than the pipe sizing). Erosion corrosion is readily identified from the characteristic appearance of the damaged surface (tear-dropped or leaf-shaped impact craters and scouring in some areas).
While the water is typically deemed potable and thus clean, there are still instances where debris, sediment, and other elements can scour the inside surfaces of any piping material, and excessive velocity increases the risk of erosion corrosion in these cases.
The attack is typically most severe just downstream of a joint or obstruction in the system. The phenomenon of cavitation can occur in systems when the flow velocity is high and the direction of flow is sharply changed. In a fitting, the centrifugal force flowing around a short-bend radius at high velocity causes an increase in pressure at the outer portion of the bend and a resultant lowering of the pressure at the throat.
The pressure in the low-pressure area at the inside of a bend can drop below that of atmospheric, which permits bubbles to form. The bubbles in turn collapse when they flow into a normal pressure area. They collapse with enough force to erode microscopic pieces of metal if they are close to a fitting. This action may continue until perforation occurs through the wall of the fitting. While this can also happen in non-metal fittings, the effects are much less dramatic due to the elasticity/plasticity of the materials and that ability to absorb some of the energetics of cavitation effects.
Lack of Visibility on System Performance
Typically, the hot water recirculation arrangement is left to operate with no visibility on system or fixture performance. Replacing manual valves with actuated connected valves on the hot, cold, and recirculating lines allows the operator to remotely shut down and contain a single zone in case of an emergency instead of shutting down water to the whole building. According to AWWA, every valve should be exercised on a regular basis, but it is never done. Connected valves on the domestic hot water system, which are not modulating on a regular basis like in an HVAC system, can be exercised autonomously to ensure that they operate when needed.
Pressure-reducing valves are complex fixtures requiring periodic service to adjust or replace the diaphragm and springs. When annual or periodic service is not kept, failed PRVs can cause havoc by allowing high-pressure delivery into loops and suites. Delivery of high 200-psi pressure in a low-pressure-rated pipe increases the risk for ruptures, water hammer, and/or temperature imbalances that can potentially lead to scalding. By replacing manual pressure gauges with a real-time connected pressure sensor, we can now understand the operation of a PRV and, most importantly, send an alert to the operator in case a PRV fails and the outlet pressure increases above the design threshold.
Recommendations
When designing hot water circulation systems, it is recommended to keep the following in mind to avoid the above-mentioned field problems and elongate the service life of the piping system.
- The local code and AHJ requirements must be observed when applying the above recommendations.
- Design all hot water recirculating systems to keep velocities below 2 fps. Refer to the 2012 Ontario Building Code, Section 7.6.3.5 (see Figure 2), or to the AHJ in your location.
- The Copper Tubing Institute (CTI) recommends 2–3 fps for hot water recirculation pipe. On ½-inch lines, consider a lower velocity due to potential burrs or imperfections from potential faulty workmanship.
- Plastics Pipe Institute (PPI) test TN-53 recommends following the manufacturer’s recommend maximum velocities in situations outside of typical hot water usage, such as greater than 140° F (60° C) or non-typical applications like constant velocity. Section 6.0 of TN-53 explains how the results of excessive temperatures (conditions beyond F2023) result in brittleness of PEX material.
- PEX manufacturers recommend hot water recirculation velocity to be 2–3 fps.
- For temperatures up to 140° F (60° C), flow maximums should not exceed 3–4 fps or as otherwise recommended by the manufacturer’s guidelines.
- For temperatures greater than 140° F (60° C), follow recommendations by the manufacturer.
- In the piping layout, avoid abrupt changes in piping flow, such as 90-degree fittings to promote laminar flows in the system. Follow the manufacturer’s recommendations for the joining or assembly of the piping.
- Regarding maintenance, monitoring and control, current market Internet of Things (IoT) technologies can easily add visibility and control to hot water recirculation systems, in both new construction and retrofits. By installing a connected flow meter and temperature and pressure sensors in lieu of the manual gauges, you can maintain the recirculation flow rate at optimal position and extend pipe life. Connected valves can be exercised autonomously to ensure that they operate when needed. Furthermore, cloud-connected IoT applications can save energy by adding night setback sequences at low demand periods via throttling down the hot water recirculation flow to a minimum.
Kevin Wong, MBA, BSc, CAE, is the Canadian Codes Manager with Uponor. Prior to joining Uponor, Kevin spent a 12-year tenure as the Technical Manager for CIPH and the Executive Director of CWQA. He has worked with MIFAB, Cimatec, and Jacques Whitford Environmental Engineers & Consultants as a Project Manager. He is actively involved in various U.S., Canadian, and international standards/codes committees including NSF, CSA, WQA, and National Master Spec of Canada. Kevin served as a member of the Standing Committee on Plumbing & HVAC at Codes Canada. Kevin is an environmental sciences graduate from York University and holds an MBA from the Schulich School of Business, with various certificates from University of Toronto and the Canadian Society of Association Executives.
Avishai Moscovich, P.Eng, LEED AP, is a CMO at reed, in charge of the company’s sales and brand strategy. Previously, he was Global Marketing Manager at Armstrong Fluid Technology and a mechanical consulting engineer at AECOM. Avishai is an engineering graduate of Ryerson University and holds an Executive MBA from Kellogg School of Management at Northwestern University.
The opinions expressed in this article are those of the authors and not the American Society of Plumbing Engineers.
