Parallel vs. Series Water Heater Installations in Commercial Buildings

During a commercial property inspection, inspectors will have the opportunity to inspect buildings that may have multiple water heaters installed together. These are buildings that have a higher demand for hot water. The best examples of these types of buildings are multi-family and hospitality. The two most common methods for installing two water heaters in tandem are placing them in a series or parallel configuration. Each type of installation has unique characteristics that can provide different recovery rates as well as benefits of hot water distribution in the building. In this article, both types of installations, as well as the benefits or reasons for each, are discussed.

Series

When two water heaters are installed in a series, the first water heater preheats the water, and the second water heater brings it to the desired temperature for use at the fixture. This setup achieves the desired hot water in half the time it would take with a single water heater. Additionally, a key advantage of this configuration is that the two water heaters do not need to be the same size, and the plumbing that connects them does not require specific design measurements.

Series Installation

The installation requires two separate water heaters. These water heaters do not necessarily need to be located in the same location but they often are. The first water heater is installed where the cold water inlet is connected to the potable water supply. This is typically on the top right side of the tank. The top left side of the tank is where the heated water leaves the tank for distribution to a fixture. Instead of being sent to a fixture, like a sink or a shower, it is sent to the next tank and connected to the top right side of that tank. This is where the cold water inlet is typically connected. The water temperature in this tank is warm instead of cold. This allows the second tank to heat warm water to the desired temperature, which leaves the tank through the top left pipe for distribution throughout the building. For this installation to be successful, the water heater temperature of the first tank must be lower than the temperature of the second tank.

In the image below, the water temperatures are represented at the top of the tanks. The blue pipes represent cold water, and the red pipes represent hot water. Note that tank #2 has cold water entering it at 50º F (10º C) and exiting into tank #1 at 80º F (29º C). Tank #1 finally heats the water to 120º F (49º F), which is being distributed throughout the building.

Parallel

When two water heaters are installed parallel to each other, they must be in the same location. Their design and installation must follow certain specific specifications. For example, each pipe at the top of the units must be the same length. Each water heater temperature setting must be set at the same temperature. Without following these installation requirements, one water heater will be working harder or heating water more frequently than the other one.

Parallel Installation

In a parallel configuration, two water heaters are located side by side in the same location. Both water heaters are plumbed to the main cold water supply pipe, and both water heaters are connected to the same hot water distribution pipe that is connected to the various fixtures in the building. The difference is that the cold water pipes that provide the input to each water heater have to be centered and the lengths cut so that they are the same size. If the cold water pipe for either unit varies by even a ½-inch on one side compared to the other, the system will not function properly. The same precise measurements are needed for the hot water side, too.

The image below represents the installation of two parallel water heaters. The cold water is represented by the blue pipes and the hot water is indicated by the red pipes. Note that the distance between the right and left sides of the blue pipes is the same. This allows the 50º F (10º C) water to fill each tank equally. The hot water pipes are the same distance, as well. It allows the heated water to exit each tank at 120º F (49º C), or the same temperature.

Why One Installation Over the Other?

Each unit has its benefits and disadvantages, and the installation preference will depend on the needs of the building and its occupants. A series installation will provide the building with a heated or “hot” water recovery in the tanks at a rate much lower than the tank rating. So, if a tank is designed to recover 45 gallons of hot water per hour and each tank heats the water by 50%, then the two tanks could recover or heat 90 gallons per hour, or 45 gallons in half an hour. This is beneficial if there is a high need for hot water over a longer period. This is important in multi-family properties where there are multiple tenants using hot water throughout the day.

A parallel installation relies on the two tanks to produce the necessary amount of hot water needed, but it will need to have a consistent recovery rate, which is recorded on the label plate. If the two tanks both produce 45 gallons per hour, then together there will be 90 gallons of hot water, but they will then require a complete hour to recover and heat the next 90 gallons of water. This type of installation is beneficial when there is a high demand for hot water at one time, but there is sufficient time between demands to allow for recovery. This is important when a high need for hot water is required at one time, such as washing dishes in a restaurant, and there is sufficient time to provide hot water for future use.

Conclusion

Commercial property inspectors will see both types of hot water heater installations while inspecting commercial properties. A series might be best suited for multi-family properties, where the speed of recovery would make it more convenient for tenants. Multiple tenants can use hot water without the prolonged recovery time. A parallel installation may be found in properties that have a high need for hot water and the installation of two smaller tanks can produce the same amount of hot water as a larger tank, but, in most cases, without the high upfront cost of the purchase of a large tank. Larger tanks typically cost more than two smaller tanks.