Domestic Hot Water Recirculation Systems
Know the history.
Domestic hot water systems have been installed in buildings for many years dating back to ancient times. Recirculating hot water systems are not quite that old. Gravity hot water circulation began in the U.S. in the late 1870s, right after plumbing moved indoors. During the early years, water and space heating was done in cabins by burning wood in a fireplace or cast iron stove, and water was heated in pots or kettlesfor bathing or cooking purposes. Eventually, coal replaced wood as a fuel source, but there was still no electricity for heating, lights or electrical circulating pumps during these early years. As domestic hot water systems became more sophisticated, cold water was piped to buildings and closed vessels were installed with burners or fire chambers below them for heating the domestic hot water.
In the early years, there were many explosions associated with uncontrolled heat to the water heater in closed piping systems. Eventually, controls were installed to relieve the pressure and temperature, and to control the fuel and combustion air. Coal and wood as a heating source were phased out because of the difficulty of controlling the heat input. Heating oil, natural gas, electricity, solar and geothermal were phased in over many years as heating sources for domestic hot water. The early plumbing fixtures had hot and cold spigots and drain connections to vented drainpipes. As buildings grew in size and complexity, and as the distance from the water heater to the most remote fixture increased, getting hot water from the fixture would take longer because the previously heated water in the pipes had to be drained first.
In the late 1870s, tradesmen were using looped hydronic heating systems to replace steam systems with limited safety controls. Tradesmen learned that hot water rises in the piping system because it was lighter than cold water. They also applied this gravity circulation to domestic hot water systems. Hot water leaving the water heater went up in a pipe vertically through the building and looped back down un-insulated and ran parallel to the hot water riser to the bottom of the water heater. The return riser was not insulated to encourage heat loss, and the cooler water caused gravity circulation. As people on the upper floors of the building used hot water, they only had to drain water from the branch piping until hot water from the riser arrived at the fixture.
The more vertical the system was, the better it worked up to a point. As buildings were built to be about three or four stories in height, depending on the insulation type and thickness, the systems would get too big, and the water would cool down and lose its buoyancy. There were also some other things that were problematic with gravity circulation systems: Horizontal swing check valves resisted flow. Large dips in the piping would allow water to cool off, and the cool water in trapped areas would resist flow. Long horizontal runs with minimal vertical rise had difficulty getting gravity circulation.
The biggest problem to overcome was air trapped in the high point of the system. They addressed this by connecting a regularly used fixture or an automatic air vent at the top of the gravity hot water circulation loop to allow any air to be vented. If air was trapped, a large bubble would resist gravity circulation. A commonly used fixture to the top of the hot water riser vented air and allowed the gravity circulation to continue. Gravity domestic hot water systems were commonly installed before the introduction of electricity and circulating pumps, and some have been installed in newer homes with moderate success. Newer code requirements for water heaters require flappers or a device in the top of the water heater to prevent gravity circulation. This makes the water heater more efficient during efficiency testing, but makes many older buildings that install new water heaters experience problems with respect to gravity circulation. That is when it is time to install a circulating pump.
Since the advent of the circulator pump, many improvements have been made. Early pumps were the same ones used on hydronic systems. The pumps were made of ferrous metals with cast iron and steel parts, and most of them suffered corrosion problems or had rusty water shortly after being installed. Hydronic systems were closed systems with air eliminators to keep air and oxygen out of the piping circuit. Some hydronic systems use corrosion inhibiting chemicals to help prevent corrosion of the ferrous metals. Oxygen contributes to the corrosion process and domestic water systems are open systems with air and oxygen entrained in the water flow. It is for this reason that hydronic pumps and piping can be black steel and cast iron ferrous metals, and domestic hot water systems should be non-ferrous bronze or stainless steel parts with copper piping. Pump manufacturers have continually improved the materials, bearings, seals and efficiency of the circulator pumps.
Code requirements for hot water and temperature maintenance systems
Recently, criteria for temperature maintenance for hot water systems in the models codes were changed from 100-foot distance criteria to a 50-foot criteria. I wrote about this many years ago. I proposed code changes showing a maximum distance of about 25 feet from a circulated main or hot water source would be the ideal maximum distance to allow hot water within a reasonable time, but knowing that would have many industry groups upset with a requirement for recirculation systems in most residences and small buildings, I compromised and proposed a reduction to a maximum of 50 feet. This would allow most residences and smaller buildings to not be required to have temperature maintenance systems. The code change did not pass the first time, but eventually it prevailed.
The 2015 International Plumbing Code section 607.2 has the following language:
607.2 Hot or tempered water supply to fixtures. The developed length of hot or tempered water piping, from the source of hot water to the fixtures that require hot or tempered water, shall not exceed 50 feet (15 240 mm). Recirculating system piping and heat-traced piping shall be considered to be sources of hot or tempered water.
607.2.1 Circulation systems and heat trace systems for maintaining heated water temperature in distribution systems. For Group R2, R3 and R4 occupancies that are three stories or less in height above grade plane, the installation of heated water circulation and temperature maintenance systems shall be in accordance with Section R403.5.1 of the International Energy Conservation Code.
For other than Group R2, R3 and R4 occupancies that are three stories or less in height above grade plane, the installation of heated water circulation and heat trace systems shall be in accordance with Section C404.6 of the International Energy Conservation Code.
607.2.1.1 Pump controls for hot water storage systems.
The controls on pumps that circulate water between a water heater and a storage tank for heated water shall limit operation of the pump from heating cycle start-up to no greater than five minutes after the end of the cycle.
607.2.1.2 Demand recirculation controls for distribution systems. A water distribution system having one or more recirculation pumps that pump water from a heated water supply pipe back to the heated water source through a cold water supply pipe shall be a demand recirculation water system. Pumps shall have controls that comply with both of the following:
The control shall start the pump upon receiving a signal from the action of a user of a fixture or appliance, sensing the presence of a user of a fixture, or sensing the flow of hot or tempered water to a fixture fitting or appliance.
The control shall limit the temperature of the water entering the cold water piping to 104 F (40C).
607.2.2 Piping for recirculation systems having master thermostatic valves. Where a thermostatic mixing valve is used in a system with a hot water recirculating pump, the hot water or tempered water return line shall be routed to the cold water inlet pipe of the water heater and the cold water inlet pipe or the hot water return connection of the thermostatic mixing valve.
Demand circulation controls dilemma
In the 2015 code change cycle, a change was presented to the model codes, and it was touted as saving water and energy along with reducing the time it takes to get hot water at a fixture. The code change was the technology, demand recirculation. I testified against this technology because health and safety should trump water and energy conservation. Many other in the backflow prevention industry have voiced concerns about this technology, but it fell on deaf ears at the code hearings. The code committee voted on this change based on the thought of having instant hot water in their homes, and saving a little water was more important to them than cross-connection. Many of the code committee members voted for this and commented that it would be nice to have for their own home. This code change will allow contaminated hot water to flow into the domestic cold water supply pipes. I have always said that circulating domestic hot water through the cold water pipes is a bad idea, and here’s why:
1. The hot water tank has a magnesium or aluminum anode rod in the hot water tank, which is designed to corrode to sacrifice itself to the steel in the hot water tank. The dissolved metals end up in the hot water. This is why they say don’t cook with hot water.
2. The demand recirculation pumps are supposed to shut off at 104 F. This is an ideal temperature for organic pathogens like Legionella bacteria to grow in the cold water piping.
3. If the thermostat or temperature sensor is not affixed properly to the cold water pipe, when there is a remote temperature sensor, the temperature can overshoot and scalding can become a concern for people using the cold water system.
I was all for allowing this technology in residential applications only, but the code allows it anywhere. So, there will be a condominium or apartment building where someone decides to install one of these demand circulator pumps under their lavatory to circulate hot water. Now everyone in the building will be drinking water with high magnesium or aluminum content and possibly high bacteria content associated with new breeding grounds in the cold water pipes, which will be in the ideal temperature range for Legionella and other bacteria growth. In addition, most of the people in the building will not get clean cold water to cook or brush their teeth with.
I see this as a ticking time bomb and a lawsuit waiting to happen. I prefer minimizing liability and designing hot water systems the correct way with a dedicated hot water return piping system in the original design. The hot water return piping system should be properly sized and balanced. I will not design a building with a demand circulator connecting the domestic hot water to the cold water pipes. Demand circulators are retrofit products for improperly designed systems, that should only be used in single-family homes where the homeowner will live with the consequences of using such a product. Demand circulators should not be designed or installed in commercial or multi-family buildings because of the obvious crossconnection and water quality issues it brings with it.
Designing the domestic hot water recirculating system
Ideally, hot water should arrive at the fixture between zero and ten seconds from the time a faucet or fixture valve is opened. There are a couple of manufacturers that offer fittings and designs to allow the hot water to circulate right up to the fixture, and some manufacturers allow circulation right up to the faucet spout, such as Kemper hygiene systems (bit.do/Kemper) and Viega drinking water systems - Hygiene (bit.do/Viega).
Surveys of how water users showed wait times between 10 and 30 seconds were marginally acceptable and wait times in excess of 30 seconds were considered unacceptable.
The following are a few considerations when piping the recirculated hot water return (HWR) piping:
1. Route the circulated hot water pipe as close to fixtures as possible.
The closer a circulated line is to a fixture, the less time it will require to get hot water from the fixture.
2. Balance the system to have equal flow in the closest and farthest branches.
If the building has multiple hot water mains and branches, each branch should have a balancing valve and check valve before connecting to the hot water return main. Simply installing the valves is not enough; after the system is started up, it must be balanced to assure each branch has the calculated flow rate to maintain the desired temperature in that branch. This prevents short-cycling of the hot water through the path of least resistance (closest branch circuit). I have investigated numerous systems with problems and the problems began because the system was never balanced when it was installed. Untrained maintenance personnel find that there is no flow in the farthest portion of the piping system, so they install a bigger pump. This typically does not solve the problem, but soon after the larger pump is installed, the piping system starts to spring leaks near elbows and valves. Balancing the hot water system is a relatively simple process, but calculations must be performed and flows in gallons per minute must be determined for each balancing valve prior to setting.
3. Minimize flow velocity to prevent erosion in copper piping.
Water flow velocity is very important in domestic hot water pipes with copper piping and brass or copper alloy valves. High water velocities, combined with hot water, can cause velocity erosion issues for the pipe and valve walls. The minimum pipe size I use for the hot water return system piping is ¾-inch pipe. I often see half-inch pipe installed. Smaller diameter pipes create a condition where the velocity increases at the same flow rate, and it also causes system temperature differentials from the supply to the return temperature that exceed the design criteria of 5 F, 10 F or 20 F. In the old days, we would design the return system for a 20-degree temperature differential using the ASPE/ASHRAE sizing method because hot water recirculation systems with older technology like the bi-metallic coil type temperature actuated mixing valves used on master mixing valve installations required at least a 20-degree temperature differential for the bi-metallic coil technology to react properly. Digitally-controlled mixing valves use digital probes with products like the Armstrong “Brain,” which offers accuracies capable of mixing hot water return temperatures with less than 5 F temperature differential and still maintaining a mixing valve outlet temperature setting within 1 F to 2 F of the set point.
The Copper Development Association recommends a maximum flow velocity of eight feet per second for cold water flowing in copper pipes and five feet per second for hot water. It also recommends a maximum velocity of two to three feet per second for hot water over 140 F. These recommendations are sufficiently vague enough to lead you in the right direction, however, I have come up with a more accurate table for pipe sizing and a chart you should refer to in order to assure the flow velocities do not erode the pipe walls. This table has worked well for me and should provide a system that will work without velocity erosion issues.
Domestic hot water above 180 F is not recommended because of the potential for scalding, and as temperatures get higher, the corrosion accelerates. In some unique cases, domestic hot water temperatures can go higher than 180 F booster heaters and steam heat exchangers, or with some types of heat recovery systems or other industrial or institutional piping systems. In these cases, consider sizing the piping to keep velocities lower than two feet per second.
1. Piping hot water return piping in Systems with mixing valves
a. When there is a mixing valve in the system, the tempered water return (TWR) must split and be routed to the cold-water side of the mixing valve and to the cold-water inlet of the water heater. A balancing valve should be placed in the line going to the water heater and the mixing valve for flow adjustments if needed.
b. If the TWR is only piped back to the water heater, when there is no usage in the system and the tempered water circulating pump is running, hot water will leak through the mixing valve manufacturing tolerances, and the temperature of the tempered water system will rise above the mixing valve set point to reach the highest temperature flowing from the water heater.
2. Sizing the circulator
The ASPE Plumbing Engineering Design Handbook, available to ASPE members, has a precise way of sizing the circulating pump based on a 20-degree temperature differential from the water heater out to the farthest fixture and back to the circulator near the water heater. If the system has 140-degree water in the water heater, then the sizing method maintains 130-degree hot water at the end of the system and then back at the cold water inlet to the water heater the temperature would be approximately 120 degrees. The calculation is based on heat loss in the hot water piping circuit. It lists the British thermal unit loss per hour (BTU/Hr) losses for insulated and bare piping based on a 70-degree ambient temperature. A quick and simple way to estimate insulated pipe is to assume 25 to 30 BTUs/Hr per linear foot ignoring the hot water supply and return pipe size. This may simply result in a system where the temperature differential in most cases will be slightly less than 20 F.
If you want to take the time to calculate the system exactly, you can use the Table in the Plumbing Engineering Design Handbook and the BTU/Hr losses can be summed up for the various lengths of different pipe sizes and a total BTU/Hr loss can be calculated. For a 20-degree temperature differential, you would then divide by 10,000 to get the required gallons per minute (GPM) for the branch or pump. This is how the GPM is determined for the pump sizing. For the pump head requirement, the appropriate GPM is assigned to each section of pipe based on the BTU/Hr loss requirements above and from pipe friction loss charts, a total feet of head or pressure drop in pounds per square inch (PSI) can be determined. Remember when selecting pumps to convert from PSI to feet of head, most manufacturers list their pumps on curves listing feet of head on vertically and GPM horizontally. Just remember 1 PSI = 2.31 feet of head and 1 foot of head = .433 PSI.
3. New technology for circulator pumps
Circulator pump manufacturers are coming out with smart pumps with intelligent, built-in controls that can adjust the speed with variable speed motor technology. Features offered on newer circulator pumps include proportional pressure controls. Some options adapt to the changing pressures and flows in the system and adjusts or reduces the speed/pump head to change the efficiency in order to operate at a better efficiency point when water is flowing in the system and the pump does not need to pump as hard. There are flow adapt limits that limit the maximum flow. This is good for minimizing flow velocities in the piping system and can eliminate the need for a balancing valve on the discharge of a circulating pump.
Other circulator pump control methods are constant pressure control methods; the pump will adjust its speed to maintain a constant pressure. Another control method is constant temperature controls where the pump senses the return temperature. As the return temperature rises to the set point, it slows the pump down to prevent overheating when peak usage periods draw hot water out to the end of the system, and then the pump can slow down and save energy. Another option is a constant pump curve mode, which is used when there is a demand for constant flow and constant head. The pump can be adjusted to speed up or slow down to maintain a desired duty point on the pump curve. This setting can allow the elimination of a pressure-reducing valve on the pump discharge. For more information on the new circulator pump technologies contact the following manufacturers:
a) Grundfos: bit.do/Grundfos
b) Taco: bit.do/TacoComfort
c) Bell & Gossett: bit.do/BellGossett
Following these suggestions should keep you out of hot water, but with plenty of hot water.
Ron George, CPD, is president of Plumb-Tech Design & Consulting Services, Monroe, Michigan. Visit www.plumb-techllc.com.