Balancing a domestic hot water system
Test and balance (TAB) firms are occasionally contacted to test and survey large domestic hot water (DHW) systems to diagnose problems. Though some turn out to be fairly straightforward installation errors, more often the issues surround pressure relationships and temperature control at the mixing stations—getting water to the point of use at the proper temperature.
Most systems experiencing problems have repeat offenses such as the following:
- Most DHW systems are not designed to be tested. Few, if any, measuring stations or devices are installed to facilitate any means of measurement of flow, pressures, or temperature.
- The “keep it simple” rule evidently does not apply to DHW systems. Never would you see three sources of water operating at different temperatures, different pressures, and different flows coming into one valve, or worse yet, directly piped to each other on a condenser water or a chilled water (CHW) system.
Larger DHW systems incorporate a variety of end uses and requirements. These uses range from restroom faucets, kitchen sinks, and fitness center showers, to laundry and dishwashing equipment, and even heat exchangers for other equipment. Each of these uses requires DHW service at differing temperatures, flows, pressures, and even different types of water source (city supply, soft water, etc.).
Some uses, such as kitchens, have critical city code requirements for health concerns. Others use sensors and control components to safeguard the end user from high-temperature water that may cause burns, or have main lines that are maintained at certain temperatures to prevent growth of unwanted bacteria. Typically, we see mixing stations with control valves or thermostatic mixing valves (TMV) with an assortment of other associated components. Other possible mixing or control stations have been included in this article for discussion purposes only.
The mixing station
With one large circulation system and several temperature requirements in the building, mixing stations are installed at the nodes of the system to reduce the supply temperature to serve multiple setpoints for varying uses in the building. These setpoints range from a high of 160 F to a low of 90 F. This is one of the main differences between DHW systems and other building hydronic systems such as heating water or CHW systems—in addition to the heating and CHW systems being closed loops. With a CHW system, the chillers are selected to produce a constant supply water temperature to the cooling coils serving air handling units, fan and coils, makeup air systems, etc. Typically, the flow is varied to maintain the desired setpoint. With DHW systems, the means typically used to achieve the required varying water temperatures throughout the building is a mixing station. Large facilities may have 20 or more mixing stations.
Some mixing stations just have a common control valve. Other more complicated stations may have a TMV, circulation pump, aquastats for pump control, high-temperature safety limits (anti-scald valves), multiple pressure-reducing valves, control valves, isolation valves, and monitoring sensors. Still others have two TMVs to act as a modified 1/3 and 2/3 flow control station. Low flow demands, in less occupied periods, are controlled through the smaller valve; higher demands are controlled through both valves or just the large valve.
Mixing valves range from three-way control valves to self-contained TMVs. Some mixing stations also come pre-assembled from the factory. A common mixing station may consist of 160 F water from the main DHW line, a 120 F node return loop line, and a domestic cold water line acting as makeup for the load not returned. Again, flow measuring devices, pressure test ports, and temperature test ports in DHW systems are few and far between, sometimes making “balancing” a moot point. However, these devices are necessary to properly balance a DHW system.
In the DHW mixing stations we have tested, there are some common reoccurring issues. One of the largest problems is that the pressures in the lines entering and leaving the mixing station are different. Water will not travel from a lower pressure to a higher pressure. When the domestic cold water makeup pressure at the line is lower than the warm water return line, makeup water will not enter the system. If the cold water makeup line pressure is higher than the warm water return line, only cold water will enter the mixing station. If, at the TMV, the 160 F water has higher or lower pressure than the other inlet port, then the 160 F water will overcome the cooler water or not be allowed to be introduced into the mixing valve. This will prevent mixing and, in the higher temperature scenario, trip the anti-scald valve shutting down water flow altogether.
Even with pressure reducing valves (PRV) on both sides of the mixing station and with PRVs set at the same pressure, the PRV reaction is slow enough that the pressures will still not match at the mixing valve during certain operations. This may prevent the two water streams from mixing and cause either too hot or too cold of a mixed water stream to the load in the building.
With the configuration in Figure 2, the recirculation pump is in the load loop but is in a position to pressurize the domestic cold water makeup line. If the recirculation pump is positioned in the line leaving the TMV, it may affect the PRV operation. A PRV is not a pressure controlling component, but a pressure-reducing component. In applications where pressure is critical, the distinction makes a big difference.
Other control scenarios
When using heat exchangers to control the DHW system (Figure 3), the makeup water is still at the local node and the load is recirculated by a pump. The difference in this situation is that the load recirculated water stream and the main DHW stream never touch each other. This condition eliminates the flow, temperature, and pressure operating concerns from the main DHW system as is mixes with and affects the load loop. The cold water makeup is simply mixed with the loop return and both are then heated to the desired setpoint by the heat exchanger. No heat exchanger is required if the load loop temperature is the same as the main DHW loop.
Figure 4 indicates another heat exchanger control station but with a very different scenario. The DHW main system pumps the circulation through the mains and the load loop and back into the return main, and the heat exchanger is used to cool the building loop to the setpoint instead of heating the loop water to a setpoint. The cold water makeup is introduced back at the main loop system near the hot water heaters. In this case, the only water stream mixing is done at the main line near the control station node. The warmed water leaving the heat exchanger and the building load return water are piped back to the return main line.
Figure 5 is the simplest of the DHW systems. The advantages of this system are:
- It can be repeated for an unlimited number of nodes in a large facility.
- It has flexible setpoints to facilitate any end-use condition.
- No water streams ever mix together, so fluctuating pressures, temperatures, or flows are not a concern.
- Because there are localized heat sources at each node, if the heater fails or is down for maintainence, only the local loop is down and not the whole system.
- Maintenance is minimized since there are no pumps, control systems, etc.
- Piping and control requirements are minimized.
The disadvantages of the system depicted in Figure 5 are:
- There is no loop recirculation, so in a large facility requiring long loop distances, hot water is not readily available at the faucets. This could be remedied much like a residential system that has a recirculation pump installed and a bypass line at the end of the piping system.
- In large facilities, the main piping is typically in a tunnel or crawl space. Using gas water heaters in confined spaces presents other ventilation, safety, and code issues.
- There are no energy savings to reclaim, such as a mixing station with recirculating unused return hot water (although this could be accomplished by using a return loop back into the inlet piping of the heater).
We do not specifically promote any of these systems or variations of them. Each design has advantages and disadvantages. A rule of thumb is trying to keep it as simple as possible for controllability without sacrificing energy or owner requirements. The reality is that TMVs, PRVs, and pumps all have requirements that are sometimes overlooked in the design phase. We have tested PRVs that have minimum pressure differential requirements to control the leaving pressure properly. The same concerns apply to TMVs and the temperature relationships between the inlet conditions and the outlet controlled condition. Pressures fluctuations or large pressure differentials at the inlets have affected the performance of the TMVs as well.
The actual valve (TMV and PRV) requirements for the valve to operate properly are often overlooked. The manufacturers of TMVs publish a required minimum temperature differential across the valve for proper controllability. PRVs often require a minimum pressure differential for proper control of the leaving pressure. If these parameters are not considered in the design phase and maintained in the building, the equipment will have difficultly controlling the desired setpoints.
Care should be taken when planning what type of mixing or control station will be selected to provide a constant water source at the required temperature. Facilities have encountered major issues with DHW systems that affect not only the temperature at a restroom faucet, but the ability to provide a safe and reliable system to serve laundries, fitness centers, and kitchens.
Miller is the branch manager of Engineered Air Balance. He is a certified test and balance engineer (TBE) with the Associated Air Balance Council, a certified Commissioning Authority with AABC Commissioning Group, an active member of ASHRAE, and has experience with professional TAB services since 1983.