This month, we continue with the checklist for commissioning and troubleshooting domestic hot water systems. Our focus is on verification that temperature controls are working, and we will continue later with discussions on solar systems, heat recovery systems, thermal expansion tanks, temperature gauge locations and types, temperature effects on Legionella bacteria, hot water temperature maintenance cables, hot water balancing valve types and locations, and temperature-actuated balancing valves. 

In addition, we will discuss system water temperatures, pressure-temperature relationships, velocity effects on piping materials, flexible connectors in large-diameter hot water and heating hot water piping systems, pressure-reducing valves in hot water systems, and more. 

As always, document, document, document. This requires preventative maintenance scheduling, and you should look at the manufacturer’s literature for each piece of equipment and component in the hot water system to determine service requirements. Usually, these can be found online; however, verify that the literature or manual is up to date. 

At a minimum, document any information about pipe material and its pressure and temperature limits; temperature-actuated mixing valves (TMVs), if present; circulating pumps; balancing valves; water heaters; and any temperature-limiting valves and temperature setpoints in the hot water system. 

TMVs: Generally 

TMVs must conform to the standards listed in the codes for each application/location. 

They will maintain distribution temperatures within a tight distribution temperature range as prescribed in the ASSE/CSA Temperature Actuated Mixing Valve and Temperature Limiting Valve Standards. 

A TMV conforming to ASSE 1017 can stabilize distribution temperatures slightly above the Legionella bacteria growth temperature and prevent temperature variations that can cause scalding incidents. 

TMVs work well for stabilizing system temperatures and with point-of-use mixing valves to provide safe temperatures at fixtures used for bathing and washing purposes. 

Consider if a storage-type water heater does not have a mixing valve installed on the hot water distribution pipe leaving the water heater and there is no hot water circulating pump. The hot water distribution temperatures can periodically rise to as much as 30 F higher than the water heater thermostat setpoint after short, intermittent hot water usage conditions, which cause short-cycling of the water heater burner. 

As short, intermittent draws of hot water are made, the cold water enters the bottom of a storage-type water heater, triggering the burner to cycle “on” to heat the few gallons of cold water introduced to the bottom of the water heater. This can cause water overheating near the top of the water heater. 

Also, consider that same situation with the water heater’s burner control thermostat set to approximately 120 F; the temperature can rise to 150 F or higher due to short, intermittent hot water usage conditions, which are common in all building types. 

This will cause an instant scalding condition at all fixtures, including showers and pressure-balance-type tub-showers, if the limit-stops were set when the water temperature was 120 F. The limit-stops would be nearly wide open at that temperature. This is why having a TMV downstream of a water heater is important.

TMVs need maintenance; they should be inspected every six months after installation to see if there is any scale buildup on the moving parts due to hard water. Note that there are often seasonal differences in water quality.

In hard water areas, scale and sediment can build up on a temperature control valve’s moving parts, seizing them up. When seized, the TMV becomes not much more than a tee in the piping; or, if the TMV is seized in fully open or fully closed on one side or another, the temperatures and outlet pressures could swing significantly. 

In many valve designs, it is possible to remove and clean the thermal motor or moving parts of the valve. If the moving parts show signs of scale, remove them from the valve body and place them in a plastic bucket of a 50% vinegar and 50% water solution. Allow the scale and mineral deposits to be dissolved by the mild acid of the vinegar water. The vinegar water solution should neutralize as minerals dissolve. If the solution stops dissolving the mineral buildup and becomes milky looking, a fresh solution may be needed. 

Consider installing a water softener for the hot and cold water in the tempered water system in hard water areas.

Ideally, the TMV should be set in circulated systems to maintain the hot water circulated loop a couple of degrees above the maximum Legionella bacteria growth temperature of 122 F. (See Table 2.)

Note on Table 2: The temperatures are based on laboratory tests. Field conditions may vary due to differences in water quality, insulating properties of host amoeba, biofilm, scale and sediment. 

According to laboratory tests, in temperatures above 122 F and up to 131 F, Legionella bacteria can survive but will not grow. Legionella bacteria goes into a cyst stage at temperatures above 122 F; while in the cyst stage, the bacteria does not reproduce. 

Above 131 F, the bacteria die. However, there have been recorded instances where Legionella bacteria survived at higher temperatures when it was imbedded deep within mineral deposits covered with biofilm. 

The scale, minerals and sediment combined with a biofilm that grows on the walls of water distribution pipes can cause a slight insulating effect and a variation in growth and disinfection temperatures from the temperatures recorded in controlled laboratory conditions. 

Legionella bacteria can also be sheltered inside a host amoeba, which also provides a level of insulation from hot water temperatures. 

TMVs: Check, Set or Adjust Temperature 

There should be a temperature gauge near the outlet of the TMV or an adjacent fixture from where temperature readings can be performed. 

The adjacent fixture (i.e., sink) should be located about 10 feet of the TMV’s outlet. A calibrated thermometer should be kept at the sink for regular checks while flowing water from the sink faucet. 

There must be flow through the system to allow the valve to mix while you are reading and adjusting the TMV outlet temperature.

At least one remote hot water faucet downstream of the mixing valve must be flowing to read, record and then adjust the TMV outlet temperature to the desired temperature.

1. Low-Flow Temperature Control Test 

This low-flow test is designed to see if the mixing valve will safely and accurately control the temperature at the fixtures during low flows. In some installations, the control valve (TMV) is so big it cannot react properly during low flows.

This test should be done with one hot water faucet flowing. Use a two-handle faucet if one is available; if not, a single-handle faucet can be used where it is opened to the full hot position. 

To see how long it takes to get hot water to the fixture at the first draw, you want a cold start. Do not use hot water from adjacent fixtures (or other fixtures on that branch) for 20 minutes prior to conducting this flow test. 

If hot water has been flowed, allow the water to cool for 20 to 30 minutes before conducting the test. 

The test will record the fixture outlet temperature every 10 seconds for about four minutes.

If the distance from the TMV to the fixture is more than 20 feet or runs through a large uncirculated branch main, determine how long it will take to purge the hot water (at least two times the volume) from the branch main and fixture branch piping from the TMV, water heater, circulated hot water line or hot water source to determine and record the maximum temperature flowing from the TMV. (See Table 1.)

Record the temperature every 10 seconds to establish a temperature rise profile. Document: 

Time to highest temp: ____ F 

Time to high temp from cold start: _______ 

Record flow rate after peak temp is reached: _____ gpm 

During or after the temperature test, measure the flow rate using a bucket with a one-gallon mark and determine how long it takes to flow one gallon of water into the bucket at full flow. (Note: Divide 60 by the number of seconds to get 1 gallon to determine the gallons per minute flow rate.) 

Flow water and record temperatures for a longer time if the hot water temperature has not stabilized. 

If the hot water takes more than 30 seconds to get to the fixture, check the plumbing system. You may consider a temperature maintenance system, which will save energy by reducing the amount of pre-heated water (that has cooled to ambient temperature) that is dumped down the drain. 

If there is a circulating system, check to see if the circulating pump is working properly. The circulated hot water may need to be maintained closer to the fixtures to reduce the time it takes to get hot water flowing from a fixture. If a recirculation system is not maintaining temperature properly, check to see if it is piped correctly and if a balancing valve or shut-off valve might be closed. 

If there are heating cables, they should be located within 50 feet of the fixture.

2. Flow Modulation Test

After the low flow test with one faucet flowing, open enough faucets to achieve 50% of the rated flow through the valve and record the temperatures. 

Then, switch back to using one fixture and repeat the temperature recordings. 

This test should verify if the TMV is controlling the temperature within a few degrees in each condition. (See Table 3.) 

3. Recording Temperatures Over Time

To record and log the temperatures over time, a temperature data logger must be installed with temperature leads under the insulation on the piping in the appropriate locations to record the following: 

TMV hot water inlet temperature; 

TMV cold water inlet temperature;

TMV tempered water outlet temperature;  

TMV tempered water return pipe, before it connects to the cold water inlet or the tempered water return connection on the TMV. This temperature will typically fluctuate as usage in the building pulls hot water out to the end of the system. 

These inlets and outlets should be monitored, and temperatures recorded continuously at increments so that personnel can look at the data over 24 hours to verify that the TMV is working to control the hot water distribution system temperatures as intended. 

Record the high TMV outlet temp = ___ F at TMV outlet, Date/Time: ___

Record the low TMV outlet temp = ___ F at TMV outlet, Date/Time: ___ 

Digital Mixing Valves

Digital mixing valves (DMVs) are often used in larger buildings where temperature control accuracy is critical. 

They generally react quicker than TMVs and are generally more accurate. Also, they can display and record system temperatures at selected increments of time. 

DMVs use digital temperature sensors in the mixed water outlet of the valve to control cold water and hot water valves several times a second to maintain very accurate temperatures. 

Many DMVs can monitor, control and document temperatures throughout the hot water system. 

With new monitoring, data logging and analysis functions, DMVs can monitor temperatures and readjust the valve setpoints with a remote login. If temperatures do not respond and system temperatures are out of the desired temperature ranges, the DMVs can send text or email message alerts to facility engineers or maintenance personnel.

New DMVs, digital balancing valves and digital monitoring (temperature sensors) in the hot water distribution piping system can allow precise control of the hot water distribution system. 

This digital technology allows hot water system controls to be connected to the building management system, allowing facility engineers or maintenance personnel to monitor and respond to temperature anomalies. 

Before this technology, personnel would have to manually walk to the valve to check the temperature gauge and then check them at the end of the branch or the top of the riser. Using a tool, the technician would manually adjust the temperature setting of the TMV, either higher or lower, by a “guesstimated” increment, and then monitor the temperature gauges to see how the system reacted to the incremental adjustment — repeat until the desired result is achieved. 

These new types of digital systems allow digital monitoring, control and digital recording of system temperatures every few seconds. This can be done from a computer console without running around the building. However, all system components are subject to scale, sediment, corrosion, erosion and chemical attack. Therefore, all components should be inspected at six-month intervals to see if there is any scale buildup on the moving parts of the DMV due to hard water or other issues preventing the DMV from working properly. 

On the market, there are new DMV technologies that make it possible to additionally monitor and control the flow of water in domestic hot water systems. Some systems can eliminate stagnation in dead-legs, where bacteria grow in dead-end branches. Digital hot water systems can monitor, record and control the entire hot water system temperatures and minimize scalding with accurate temperature control and alarm conditions, and prevent Legionella bacteria growth by thermally cycling to a disinfection temperature at predetermined off-hours in the system. 

When using a DMV, it is easy to document the highest temperature and the lowest temperature on the circulated hot water system by recording the tempered water outlet and the hot water return (inlet) temperatures on a DMV. 

DMVs can allow the temperature to be recorded as often as every second up to every minute or so. Ideally, recording the temperatures every 2 to 5 seconds will give a good indication of what the system temperatures are doing over a long, peak usage period. Plotting the system temperatures and heat recovery cycles can show where stacking or other system conditions can cause higher system temperatures under certain usage conditions.