Last month, we covered temperature control valve types, applications and control valve standards. We illustrated how hot water systems should be set up to simultaneously control Legionella bacteria growth by storing and distributing hot water above the Legionella growth temperature. We also showed how to control scalding risk by using and setting temperature-limiting controls at the point of use to deliver a maximum mixed hot water temperature safe for bathing or showering. 

This month, we cover more design, setup, commissioning and maintenance considerations for hot water distribution systems and accessories for a safe hot water distribution system. 

The following are a few very important hot water system commissioning and inspection items: 

Perform a Flow/Time to Maximum Hot Water Temperature Check

1. Document how long it takes to get the maximum hot water temperature to each fixture. 

2. The time to get the maximum hot water temperature to each fixture should be checked and recorded for every fixture. This requires starting a timer and recording the temperature about every five seconds until the temperature reaches the maximum and stabilizes. Make note of three items:

a. How long it takes to reach the maximum hot water temperature; 

b. The maximum temperature reached; 

c. The flow rate in gallons/minute from the fixture.

Check Circulating Pump and Balancing Valves

The model plumbing codes require hot water to be maintained within 50 feet of the plumbing fixtures. The hot water temperature is maintained with circulated hot water and insulation on the pipe or with electric heat tracing cables and insulation. 

Note that for many years, the ASHRAE Handbook recommended hot water be maintained within 100 feet of the plumbing fixtures; the model plumbing codes included the ASHRAE handbook language requiring hot water temperatures to be maintained within 100 feet of the fixtures. 

After the Energy Policy Act of 1992, which significantly reduced the flow of water from most plumbing fixtures, many people realized that the distance from the hot water source to the farthest fixture needed to be reduced because the lower flow rates from plumbing fixtures caused much longer wait times to get hot water at the fixtures. 

However, with the significant flow reductions, getting hot water at the fixture still takes a long time. There may be code changes in the future addressing hot water temperature maintenance even closer to the fixtures, maybe even within a few feet of the faucets and fixture fittings. 

In addition to getting hot water to the fixtures faster, reducing the temperature maintenance distance lessens the amount of heated hot water flowing down the drain. 

1. Record how long it takes to get hot water to each fixture to see if the circulating pump works and if the branches are balanced to provide good flow to each riser or branch. 

2. How long should it take to reach full hot water temperature at fixtures? 

a. Ideally, hot water should arrive at a fixture in less than 10 seconds.

b. It is moderately acceptable if the hot water arrives at a fixture in 10 to 30 seconds.

c. It is generally unacceptable if the hot water takes more than 30 seconds to reach the fixture. 

3. The circulating pump should be checked to ensure it rotates in the right direction and the valves are open.

4. Balancing valves should be checked to ensure they restrict flows on branches near the boiler room. These valves should also be checked to ensure they reduce flows, as required, to prevent erosion of pipe and fittings. 

For example, when a large building’s circulating pump is not working or branches are not receiving hot water flow because of poor balancing, it can take a very long time for hot water to reach remote fixtures (sometimes more than 15 minutes). This is because the hot water must be drawn out through the entire hot water distribution system when the faucet opens at a remote fixture. 

6. A long hot water delay is dangerous! It often leads to complaints of “no hot water” and “not enough hot water,” causing maintenance personnel to increase the hot water temperature setting at the water heater (or increase the size of the circulating pump in response to a complaint).

7. When a circulating pump or balancing valves are not working properly, a bather can receive high-temperature hot water unexpectedly. For example, a bather can start showering and the water will be no hotter than what is comfortable (even with the faucet handle turned to hot). The water flowing from the pipe is the warm water leftover from prior or nearby fixture usage. 

After it is purged from the hot water distribution pipe, full hot water will arrive mid-shower, causing a thermal shock (a sudden temperature change can result in a scald or slip-and-fall thermal shock incident). 

The longer it takes to get hot water to a fixture, the more dangerous it can be without temperature-actuated mixing valves serving the distribution system. 

8. Changes to system temperature flow and balancing from the intended design temperatures can be dangerous. Place signage and instructions for the building operators to follow the design intent of the engineer of record. In my forensic investigation work, I see many successful designs turned upside down when the intended design is not known or clearly stated. 

Check Maximum Hot Water Temperature Setpoints for Limit Stops 

If the maximum temperature flowing from the fixture exceeds the temperature allowed in the codes or a safe temperature, then an adjustment should be made to the maximum temperature-limit stop on shower and tub shower valves and on sink or lavatory faucets with temperature-limiting controls or adjustable limit-stops. 

In addition, for fixtures without faucets or fixture fittings with maximum temperature limit stops, an ASSE 1070-compliant, temperature-limiting device serving the fixture or group of fixtures should be installed and adjusted to provide a safe temperature flowing from the fixture or fixture fittings. Or an ASSE 1017-compliant temperature-actuated mixing valve should be installed, serving the distribution system.

As the showers, tub-showers and other fixtures use temperature-limiting mixing valves, I recommend adjusting the maximum temperature to a safe temperature. I prefer a maximum temperature of 110 F to 115 F, depending on the facility. 

If the facility is a hospital, nursing home or senior living facility, I prefer to set the limit stops on the showers and tub-showers to no more than 110 F. This is plenty hot enough for bathing and showering, and it significantly reduces the risk of scalding. 

Plumbing Codes: No Hot Water Storage or Distribution Temps

I still find that many people mistakenly believe the temperature limit flowing from a fixture is the maximum temperature limit for the water heater thermostat. This is not true! Hot water can be stored at higher temperatures and reduced to the desired distribution temperature for proper mixing at the mixing valve. 

1. Even though the model plumbing codes list maximum hot water temperatures at various fixtures, they do not address storage or distribution temperatures. ASHRAE recommends storing at a minimum of 140 F to control Legionella bacteria growth, and ASHRAE recommends all parts of the distribution system to be above 122 F. 

I prefer to maintain the hot water a couple of degrees above the Legionella bacteria growth temperature, or at 124 F, by using an ASSE 1017 temperature-actuated mixing valve. This also maintains relatively stable hot water distribution temperatures. 

a. In a circulated hot water distribution system, the temperature returning to the water heater (before the cold water inlet and after the circulating pump) is the lowest temperature section in the circulated hot water distribution system. 

b. This is a critical control location where you should have a temperature gauge for monitoring and maintaining the hot water distribution temperature above 124 F. The temperature can be adjusted at the distribution mixing valve (conforming to ASSE 1017). 

For example, with a heat loss of 20 degrees in the hot water distribution system, the outlet of the temperature-actuated mixing valve serving the hot water distribution system would need to be set to about 144 F to maintain 124 F in the return line. 

If the system is designed with slightly larger pipe and more circulated flow, then the temperature drop across the system can be 10 degrees. In such cases, the outlet of the temperature-actuated mixing valve would need to be set to about 134 F. 

2. Some temperature control valve manufacturers generally prefer to see a 10-degree to 20-degree differential in the hot water temperature and the return temperature. During peak flow periods, the return temperature may rise to within a degree or two of the hot water supply temperature. In this scenario, the temperature of the hot water system could creep up to high temperatures. 

Digital mixing valves work well when the return temperature approaches the mixed temperature. Some temperature-actuated valve manufacturers use a thermostatic diverter valve to close the return line to the water heater. It also diverts water to the cold water port on the mixing valve to allow the higher-temperature return water to circulate around and cool down. 

Water Flow Velocity

Another important commissioning item to check when starting up a new system is to verify the maximum velocity of the circulated hot water. Care should be taken not to increase the flow velocity of hot water in smaller piping because high-velocity hot water can erode the inside of the pipe and cause leaks at elbows, fittings and valves where turbulent flow and high-velocity water exist. 

The Copper Development Association publishes a copper tube handbook with the following water velocity limitations (visit https://bit.ly/3SVSCYb to see the “Copper Tube Handbook”):

“To avoid excessive system noise and the possibility of erosion-corrosion, the designer should not exceed flow velocities of 8 [feet/second] for cold water and 5 [feet/second] in hot water up to approximately [140 F]. In systems where water temperatures routinely exceed [140 F], lower flow velocities such as 2 to 3 [feet/second] should not be exceeded. 

“In addition, where 1/2-inch and smaller tube sizes are used, to guard against localized high-velocity turbulence due to possibly faulty workmanship (e.g., burrs at tube ends which were not properly reamed/deburred) or unusually numerous, abrupt changes in flow direction, lower velocities should be considered. 

“Locally aggressive water conditions can combine with these two considerations to cause erosion-corrosion if system velocities are too high.

“Due to constant circulation and elevated water temperatures, particular attention should be paid to water velocities in circulating hot water systems. 

“Both the supply and return piping should be sized so the maximum velocity does not exceed the above recommendations. Care should be taken to ensure that the circulating pump is not oversized, and that the return piping is not undersized; both are common occurrences in installed piping systems.”

Over the years, I have referred to the Copper Development Association Handbook, which gives the maximum water velocities recommended for copper pipe at various temperatures. The recommendations in the handbook had a few gaps for maximum recommended water velocities at various temperatures, so I filled in the gaps and produced the interpolated Copper Pipe Maximum Velocity Table (see Table 1).

 Health/Safety Vs. Water/Energy Conservation 

Health and safety should always be more important than water or energy conservation. Legionella bacteria growth and scalding concerns should override any attempts to save on heat loss energy.

Ultra-low flow fixtures, such as infrared lavatory faucets, can create a stagnant water or aging water condition when the metering faucets are set to extreme lows to save water. In low-flow buildings, there is not enough flow to get the water treatment chemicals (chlorine) to the fixtures and, over time, the chlorine dissipates in the pipe. 

This allows bacteria to grow and colonize on the walls of the pipe in a biofilm; bacteria levels increase in the water pipe. When there is water flow, the bacteria can be swept away and exposed to users in an aerosol. 

High-Efficiency Water Heaters: Is There A Payback?

Most gas-fired, sealed combustion, condensing water heaters are already about 92% to 95% efficient. Most manufacturers would agree that the quest to make a typical sealed combustion gas-fired water heater, which is generally about 92% to 95% efficient, go up to 98% to 99% efficient is a folly. Is it worth the added expense and complexity in a water heater to try and squeeze another 2% or 3% of efficiency out of the current gas-fired sealed combustion technology?

Note that a typical water heater burner only runs 3% to 5% of the day. This figure is arrived at using the following considerations: The burner is “on” for the morning shower period for about 15 minutes, mid-day for about 5 to 10 minutes, and in the evening (for laundry and dishes) for about 15 minutes. 

The burner only runs to make up for these three demand periods, and if you add another 15 to 20 minutes of intermittent burner run time to maintain the hot water temperature, it means the burner runs about 60 minutes per day (out of 1,440 minutes in 24 hours), or 4.1% of the day. 

Assume a model exists that saves an additional 2.5% energy. The actual savings or energy saved would be 2.5% of the 4% of the day the burner is running, which is about 0.0010%, or 1/10th of one percent, savings per day. Now, consider that the lifespan of most water heaters is 8 to 12 years. 

The consensus is that if you try to squeeze an extra 2% or 3% out of an already short period of burner time, you will never likely see a payback on the increased cost of the equipment. 

Also note that a typical water heater burner does not run as long as the burner on a boiler heating a building. A heating boiler operates a significant percentage of the time. During extreme cold periods, a boiler burner can run between 80% to 100% of the time. So, saving a few percentage points on a piece of equipment like that may have a payback. 

Electric water heaters are basically 100% efficient because the heat energy goes into the water; only a small percentage of waste heat goes up the flue. However, the cost of electricity is generally higher than the cost of using natural gas.

Environmental concerns are driving research and development of heat pump water heaters. Heat pump water heaters still have reliability issues in cold weather, although new refrigerants may allow them to work better in cold weather. Electric heating elements can be used as backup heat to supplement the heat pump during peak periods. Note, however, that there are concerns regarding the demands on the electrical grid in the near future.

Next month, we continue with commissioning and troubleshooting domestic hot water systems.