Benefits of hot water recirculation
Add balancing valves for temperature consistency.
Maybe while at home or when traveling, you’ve had this experience: You stand idly by, watching gallons of water go down the drain while waiting for the water to get hot at the shower or sink. Finally, the water begins to warm. Undesirable and annoying, this problem is due to “stranded” water contained in the piping between the heat source and the fixture.
During periods of no demand, this once-hot water gives up heat to the surrounding space and reverts back to ambient temperature. The heat loss rate depends on the type of piping, insulation, air temperature, etc. Longer runs and larger pipe sizes mean longer wait times and more water wasted each time a user needs hot water.
The wait is bad enough, but a cost is associated with the wait. Water use and energy consumption are closely linked. We don’t often make that association, but since energy sources such as gas or electricity are used to heat the water, the more water wasted, the greater the energy costs. Additionally, the water utility company not only uses energy to deliver potable water to our businesses and homes; it also uses energy and resources to treat the sewage.
Due to this source of wasted water and the significant potential in energy and resource savings in preventing it, a growing number of building codes are requiring hot water recirculation or pipe heating (such as electrical tape) to conserve water. Even areas far away from the dry southwest have taken action.
For example, in Wisconsin, where Caleffi’s headquarters is located, the state building code (SPS 382.40 (5)(b)) says: “If the developed length of hot water distribution piping from the source of the hot water supply to a plumbing fixture or appliance exceeds 100 ft., a circulation system or self-regulating electric heating cable shall be provided to maintain the temperature of the hot water within the distribution piping.”
Outside of building codes, various educational bodies also promote water and energy conservation. The U.S. Green Building Council’s LEED requirement REQHEAc2o1-3, “Efficient hot water distribution,” addresses installation of a hot water recirculation system. And Standard 191P, “Standard for the Efficient Use of Water in Building, Site and Mechanical Systems,” is being developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, USGBC, the American Society of Plumbing Engineers and the American Water Works Association.
Hot water recirculation is a highly effective solution to conservation. Figure 1, on page 12, illustrates a simple example. A small return pipe connected to the hot water line at the fixture furthest from the heat source runs back to the heat source inlet. A small circulator in that return pipe, when activated, keeps hot water within a few feet of each fixture, resulting in reduced wait times when there is a demand.
Point-of-use mixing valves, installed at the fixtures (required to have ASSE 1070 certification) assure safe water temperatures for the user at the fixture while allowing the storage tank temperature to be set above 120° F — the minimum temperature to kill Legionella bacteria. Stranded hot water can easily cool down to the temperature range most conducive to Legionella bacteria growth (between 77° and 113°). Recirculation prevents stranded water from cooling to the temperature at which Legionella growth can occur.
Balancing the system
A properly designed and constructed recirculation system will deliver consistent hot water temperatures to each fixture in a plumbing system, regardless of the fixture location within the piping layout. The key to this temperature consistency is balancing the recirculating branch lines.
Water takes the path of least resistance. So in a system without balancing valves, the fixtures at the far end of the piping runs may only see a trickle of flow, while the fixtures closest to the storage tank get more flow than needed. With balancing valves installed as shown in Figure 1, each recirculation riser pipe flow can be set to the intended design flow rate.
One approach to balancing recirculation risers is to install compact pressure-independent balancing valves. These are simple in-line devices that control flow to a fixed rate. They are not field-adjustable, so gal. per min. requirements for each branch circuit must be determined prior to installation and the valve model ordered to match.
Adjustable balancing valves are another popular option. They feature flow control flexibility once installed. As long as the valve adjustable range will cover the installed system flow needs, simply select and install the balancing valve with the appropriate connection sizes.
Traditional adjustable balancing valves have differential pressure ports (aka PT ports) that provide connection points across a pressure drop area in the valve, for use with a differential pressure balancing tool or digital manometer. These tools allow an installer or balancing contractor to adjust the valve flow opening to achieve the design flow rate, while using a chart or calculator to equate differential pressure to flow rate. This process can sometimes be time-consuming and challenging, especially to someone not highly trained in balancing.
New types of balancing valves with direct flow rate readout greatly simplify and speed up the balancing process. No differential pressure readings are needed. Models are available which enable adjustment down to 0.5 gpm, which is a flow rate commonly desired in recirculation lines and not always easily achieved with other types of balancing valves. One of those types of balancing valves (constructed of low-lead brass) is shown on page 10.
Properly designed and installed, a balanced hot water recirculation system will reduce water waste, save time and energy costs, and minimize complaints and frustration for those of us waiting for hot water.
In the United States, the Reduction of Lead in Drinking Water Act went into effect Jan. 4. All devices intended for installation in potable water must be “low-lead” certified. Similarly, low lead is a requirement in Canada starting this year.
Some confusion remains surrounding terminology: low-lead, no-lead and lead-free are used interchangeably. They all mean the same thing when it comes to the new law, which states the lead content must be “… not more than a weighted average of 0.25% for plumbing fixtures …” as determined by wetted area within the fixture. All products that comply will be labeled as such; otherwise they are intended for nonpotable applications only.
Components in a hot water recirculation system must comply with this new low-lead requirement.
About the author: Kevin Freidt is the applications engineering manager at Caleffi North America, located in Milwaukee, Wis. He has more than 30 years of experience in engineering, application, design, product management, marketing, sales and distribution of HVAC controls and hydronic systems in North America.