Water Heater Indirect Benefits | Domestic Hot Water Systems

Domestic hot water systems are among the most important and basic appliances required by modern society. There are many DHW system configurations available, each offering unique performance characteristics, and there are also various heat sources. Regardless of the system configuration, energy efficiency is an important aspect to consider: higher efficiency heaters have a higher price, but the additional cost is often recouped over its lifetime in the form of energy savings.

The following table summarizes the most common types of domestic hot water systems. This article will focus on the last type: tankless coil and indirect water heaters.

What are the most common DHW system configurations?

When it comes to commercial water heating (apartment buildings, restaurants, hotels, etc.), 99% of the time you will see one of two common configurations in the boiler room:

• The first is what we call a standard tank water heater configuration. It contains one large commercial water heater or several smaller water heaters, appropriately sized for the respective application.
• The second typical configuration is a boiler (water or steam) working in conjunction with a heat exchanger and a hot water tank, commonly called an indirect hot water heating system. In this system, the commercial boiler provides heating to the building's air, while also supplying hot water to the building's taps and fixtures through the heat exchanger. Indirect water heaters work most efficiently during cold months when the heating system is used regularly; on the other hand, the space heating boiler is required to operate at partial load whenever hot water is required during the summer months.

Indirect water heating systems can be powered by natural gas, oil, propane or electricity (any heat source used by the space heating boiler). An indirect water heater can also complement a solar collector, providing any additional heating that cannot be provided with solar radiation alone. The efficiency of indirect combustion water heaters depends mainly on two factors: the effectiveness of the tank insulation and the efficiency of the boiler to which it is connected.

Working principle of indirect water heaters

Indirect water heaters take advantage of a space heating boiler that is already in operation to keep your building warm. The indirect water reservoir is located close to the boiler, connected through a closed water circuit that circulates between the two equipment. When the tube enters the indirect water tank, it is twisted like a coil to maximize the heat transfer area and then returned to the outside. Therefore, the hot water leaves the boiler, passes through the spiral tube in the indirect tank and returns to the boiler. This process continues until the water in the tank reaches the desired temperature and can be supplied to the hydraulic installations.

Subtypes of indirect water heating systems

Indirect water heating systems can be classified into three subtypes depending on how they handle the water flowing through them.

• Storage tank systems are similar to conventional water heaters in their use of hot water tanks. However, instead of keeping the tank temperature high with a direct heat source, water is constantly circulated between the tank and the heat exchanger. These systems are best suited for applications where large volumes of water are consumed, but where demand is intermittent.

• Tankless systems are best suited for applications where the demand for hot water is constant over long periods of time and there is no need to keep the tank full. Because the storage tank is eliminated, this system configuration is more affordable, so it should be considered whenever possible.

• Tankless hybrid systems are suitable when the same building requires two different water temperatures for separate applications. A storage tank system can be used to supply water at one temperature, while the tank water is recirculated through the tankless heater to supply it at an even higher temperature.

Sizing an indirect water heater

To size an indirect water heater, it is important to determine the hot water demand and how it changes hourly. Total demand determines the required indirect water heater capacity, while hourly behavior determines whether it makes sense to use multiple units. If there are certain times when the demand for hot water is especially high, the total required heater capacity may be excessive. In these cases, it makes sense to use a hot water tank and fill it before peak times, thus reducing the required capacity of the heater. Rather than using a large, expensive heater that only runs during peak times, it makes more sense to fill a storage tank with enough hot water in advance, using smaller units.

Sizing formula for heat input

To calculate the heat production of an indirect water heater, the following data is required:

• Required hot water temperature (°F)
• Supply water temperature (°F)
• Flow Rate (gpm)

Once the above date is available, the following formula will be used:

• Heat production (BTU/h) = Temperature rise (°F) x Flow rate (gpm) x Specific heat (BTU/gal°F)

As an example, assume the following data:

• Required temperature = 140°F
• Supply temperature = 40°F
• Flow Rate = 300 gpm
• Specific heat = 8.33 BTU/gal°F

Considering the data above, the thermal input required for water is:

• Heat output = 100°F x 300 gpm x 8.33 BTU/gal°F = 249,900 BTU/hr

However, this is the heat supplied to the water. As no system is perfect, it is necessary to take into account the efficiency of the indirect water heater. Assuming a system efficiency of 80%,

• Heat input = Heat output / efficiency = 249,900 BTU/h / 0.80
• Heat input = 312,375 BTU/h

Sizing an expansion tank

The cold water supply to the indirect heater may contain a check valve, pressure reducing valve, or backflow prevention device, creating a “closed system.” As the heated water expands, it increases pressure in the closed system, and this can cause the T&P (temperature and pressure) relief valve to leak or discharge water. To avoid this, an expansion tank must be installed in the cold water supply line. Size the expansion tank to 10% of the water heater's capacity or according to the manufacturer's recommendations.

Indirect benefits of the water heater

If you already have a large boiler in operation, adding an indirect water heater to establish a combined system is a viable option to consider.

1. An indirect tank is a fairly inexpensive addition.
2. Indirect tanks do not require an additional fuel source.
3. Indirect tanks tend to last longer than standard tank-style gas water heaters.

Conventional water heaters deteriorate more quickly because they are directly exposed to a flame at the bottom of the tank. Furthermore, they must also handle combustion gases. Frequent exposure to high temperatures causes expansion and contraction and mechanical stress on welded joints. Eventually, the welded seams begin to crack and that is when a replacement is needed.

With an indirect tank, the temperature differential is much smaller. The hot water piping from the boiler quickly heats the water in the indirect tank, while water from the standard tank heater takes longer to heat and is exposed to more drastic temperature extremes. With a coiled tube inside an indirect tank, there is no frequent thermal expansion, resulting in reduced mechanical stress on the tank. Less stress on the tank means less frequent replacements and that means more money staying in your pocket.

Points to Consider for Indirect Water Heaters

Whichever water heating system you choose, there will always be different costs to consider. When you opt for a combined system, even if the boiler is already installed in your building, you must assume the cost of extra piping and pumps, plus the electrical energy to run these pumps, in addition to the indirect tank itself. However, after the initial investment, the combined system should last a long time.

Indirect water heaters also suffer from a lack of redundancy. If your boiler fails, you will lose space heating and domestic hot water. Summer operation is also very inefficient, as the boiler does not provide space heating and must only operate at a fraction of its full capacity. Ideally, the boiler should be able to run at partial load during the summer – make sure it is not set to 100% power when only the domestic hot water load is present.

If you manage a commercial property and are trying to decide between combined or separate systems for space heating and domestic hot water, our best recommendation is to get professional advice. Only an expert can analyze your heating loads accurately to help determine which configuration is best suited for your application.