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 features, and there are also various heat sources. Regardless of system setup, energy efficiency is an important aspect to consider: higher-efficiency heaters come with an increased price tag, but the additional cost is recovered many times during their service life 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?

As far as commercial water heating is concerned (apartment buildings, restaurants, hotels, etc.) 99% of the time you’ll see one of two common setups in the boiler room:

• The first is what we call a standard tank-type water heater setup. It contains a 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 tandem with a heat exchanger and hot water tank, commonly referred to as an indirect hot water heater system. In this system, the commercial boiler provides heating for the air in the building, while also providing hot water for the building’s faucets 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 forced to operate at part-load whenever hot water is required during summer months.

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

Operating Principle of Indirect Water Heaters

Indirect water heaters take advantage of a space heating boiler that is already running to keep your building warm. The indirect water tank is located next to the boiler, connected through a closed water loop that runs between both pieces of equipment. When the pipe enters the indirect water tank, it’s twisted around like a coil to maximize heat transfer area and then comes back out. Therefore, hot water travels from the boiler, through the coiled pipe in the indirect tank, and back out to the boiler. This process continues until the water in the tank reaches the required temperature, and it can be supplied to plumbing fixtures.

Subtypes of Indirect Water Heating Systems

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

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

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

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

Sizing an Indirect Water Heater

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

Sizing Formula for Heat Input

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

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

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

• Heat output(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 heat input required for the water is:

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

However, this is heat supplied to the water. Since no system is perfect, it is necessary to account for 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 preventer, creating a “closed system.” As the heated water expands, it builds up pressure in the closed system, and this may cause the T&P (temperature and pressure) relief valve to weep or discharge water. To prevent 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 manufacturer recommendations.

Indirect Water Heater Benefits

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

1. An indirect tank is a fairly inexpensive addition.
2. Indirect tanks don’t require an additional source of fuel.
3. Indirect tanks tend to last longer than a standard tank-type gas-fired water heaters.

Conventional water heaters suffer a faster deterioration because they are exposed directly to a flame at the bottom of the tank. In addition, they must also handle flue gases. Frequent exposure to high temperature causes expansion and contraction, and mechanical stress on welded joints. Eventually, welded seams start to crack open, and that’s when a replacement is required.

With an indirect tank, the temperature differential is a lot less. The pipe with hot boiler water quickly heats the water in the indirect tank, whereas the water in the standard tank-type heater takes longer to heat and is exposed to more drastic temperature extremes. With a coiled pipe inside an indirect tank, there is no frequent thermal expansion, reducing mechanical stress on the tank as a result. Less stress on the tank means less frequent replacements, and that is more money that stays 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 go for a combination system, even if the boiler is already installed in your building, you must assume the cost of extra piping and pumps, plus the electricity to run those pumps, in addition to the indirect tank itself. However, after the initial investment, the combination system should last fairly long.

Indirect water heaters also suffer from lack of redundancy. If your boiler is down, you lose both space heating and domestic hot water. Summer operation is also very inefficient, since the boiler is not providing space heating and must operate at just a fraction of its full capacity. Ideally, the boiler should be able to run at part load during the summer - make sure it is not set at 100% output 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 with precision, to help determine which configuration is the best fit for your application.