water heating system

Indirect Water Heaters: A Boiler Briefing

Wondering about the difference between indirect water heaters and standard tank-type water heaters? It’s all about the boiler!

Your business needs hot water, and both a tank-type heater and an indirect water heating setup will have benefits and drawbacks. Adding an indirect tank to a heating boiler to create a “combination system” can provide extra power and save space, but a traditional water heater setup may create fewer headaches in the long run. We’ll run through some key factors to consider, including the main differences between the two systems to help you decide if your boiler should also work as part of your commercial water heating system.

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When it comes to water heating in commercial applications (aka businesses such as restaurants, hotels, and apartment buildings), most of the time, you’ll still see what we call a standard tank-type water heater setup in the “boiler room.” You might be familiar with the way a tank-type water heater works from experience in your business or home, but there are many variations among the types of commercial water heaters, making this type of setup an extremely versatile option for various applications. Regardless of size or configuration, the classic tank-type water heater setup basically consists of one or multiple commercial water heaters, in standard or high-efficiency variations, heating and storing the domestic hot water for your building.

While still the most common setup, it’s not always the only option available for getting hot water to your faucets. Another type of commercial water heating setup that has gained popularity is an indirect water heating system or a combination system, as it’s more commonly known because it combines an indirect tank with a heating boiler (hydronic or steam) to also provide domestic hot water for a building in addition to the heat.


The Heating Boiler and Its Loyal Sidekick, The Indirect Tank

To understand how this system works, it’s helpful to have some basic knowledge about boilers. Like water heaters, boilers also produce hot water, but they can’t store it, and the hot water (or steam) they produce can be used for various purposes—the most typical use being space heating (aka hydronic heating). Hydronic heating boilers used only for space heating don’t need a place to store hot water because it’s sent out into the system to heat the building instead, usually using radiators to distribute heat throughout the building (known as radiant heat). In a building with radiant heat, water is heated by a boiler to a point just below boiling and flows in a constant loop from the boiler, to the pipes, into the radiators, and back to the boiler to be reheated (this is known as a closed-loop system).

When a boiler is used for BOTH hydronic heating and water heating, it’s known as a combination system. A combination heating system features two main components:

  1. A hydronic (hot water or steam) heating boiler. The primary part of this system, providing heating for the building. Commercial boiler sizes can range from 2’ by 2’ to over 5’ tall and more than 8’ long, depending on your application and configuration.
  2. An indirect water heater tank for the domestic hot water for the building. An indirect tank is visually similar to a standard water heater but without the mechanical guts required to heat the water alone, so it needs the boiler to be useful.

This means that a heating boiler is a critical prerequisite when considering an indirect water heater as an option for your domestic hot water. An indirect water heater is kind of like a sidecar on a motorcycle, piggy-backing off the power of the primary appliance to gain a secondary bonus function.

Indirect water heaters are named quite literally—they don’t directly heat the water like standard water heaters, so they don’t have their own controls, but they are also more than just storage tanks. Like a standard water heater, an indirect water heater has its own cold-water inlet and is filled with fresh potable water from the municipal water system. But the indirect heater is entirely controlled by the boiler and connected to it through a closed-loop water pipeline running between them. A pipe runs from the boiler into the indirect tank, where it connects with a heat exchanger coil that’s basically a metal pipe twisted around like a corkscrew inside the tank, surrounded by the domestic water to be heated. The other end of the coil connects to another pipe that runs back to the boiler, creating a closed loop. (See diagram.)

indirect Water Heater System DiagramTo be clear, in these types of combinations systems, the water that is heated directly by the boiler to heat a building and the hot water that comes out of the faucets or runs through the dishwasher are not the same—those two different streams of water never come in contact with one another. There are two closed loops of non-potable water continually heated by the boiler, referred to as the primary loop and the secondary loop. The primary loop of water is used for the hydronic heat, but that loop also connects to the secondary loop, which is used to heat the domestic, or potable, water stored inside the indirect tank. The common element is the boiler and connected piping—not the resulting water.

So, the water in this secondary loop is continuously heated by the boiler to near boiling, runs into the indirect tank, and through the coiled corkscrew-like heat exchanger pipe inside the tank. Heat is then transferred from that super-hot water in the coiled pipe to the surrounding cooler water within the indirect tank, while the water within the coil loops back to the boiler to be reheated. The coil design provides more surface area, maximizing the heat transfer to ensure the water held in the indirect tank gets HOT enough to provide plenty of domestic hot water for sinks, dishwashers, and faucets. (Although most of the time, the term “hot water heater” is redundant, in the case of the indirect tank, it’s very true! An indirect tank is actually a “hot water heater” using hot water to heat cold water!)


The Requirements of a Combination Heating System

The advantages of an indirect water heater system mostly come down to energy efficiency, cost, and a space-saving footprint, but the practicality of a combination system really hinges your current system. Earlier, we used the analogy that adding an indirect water heater to a boiler was akin to adding a sidecar to a motorcycle to gain additional functionality. Let’s take that further to help explain why indirect water heaters are still such a niche fit in today’s market. If you don’t even have a motorcycle, then having a sidecar would be useless and silly. Similarly, if you don’t have a hydronic heating boiler, then an indirect water heater has nothing to connect to for its heat source, making it pointless to consider.

If you do have a motorcycle, then adding a sidecar may be an option as long as your existing vehicle can handle the addition and do so efficiently. So, if your building is already set up with a boiler for hydronic heat (often the case in apartment buildings), then adding an indirect water heater to establish a combination system may be a cost-effective and efficient option for your water heating. But just having an existing boiler for hydronic heat doesn’t necessarily mean your boiler is right for this kind of system.

Simply put, not all hydronic boilers are efficient enough to be worth using for multiple purposes. Technically, indirect water heaters can be retrofit onto older, standard-efficiency boilers (the atmospheric kind with efficiencies between 78-86%), but doing so can reduce the lifespan of your boiler by making it work all year to provide hot water, and this doesn’t usually provide any benefits over a standard water heater that can achieve the same or higher efficiency levels.

Realistically, the best time to start looking into a combination system is when you’re shopping for a new higher efficiency hydronic boiler, whether to replace an older existing unit or just for a new build from scratch. So, if you’re starting with a clean slate and are installing a new boiler system, an indirect tank is a fairly inexpensive addition that may save you money going forward by increasing the efficiency of your water heating operations.

The Benefits of Multitasking

Adding an indirect water heating system to an existing setup requires investing in extra piping and pumps, the electricity to run those pumps, and the indirect tank itself. However, once you make the initial investment, a combination system can be relatively long-lasting. Boilers are designed for long-term operation, especially the newer modulating models that offer variable settings to avoid being over-worked—so you’re already getting a lot of life out of that piece of equipment. Additionally, indirect water heater tanks tend to last longer than a standard tank-type gas-fired water heater. Here’s why…

A standard atmospheric tank-type water heater pulls in air from the environment to generate a flame that burns the fuel (whether natural gas, propane, or oil). The heated gases flow up through multiple exhaust flues inside the water heater. Baffles in the flue pipes slow the hot air down as it travels upward, heating the water surrounding those pipes as it continues upward towards a collection hood, out the top flue through the chimney, and outside. When enough hot water is used in the building, the water heater tank is refilled with fresh cold water from outside to be heated up for the next round. This process exposes water heaters to consistent temperature changes from cold to hot, causing the standard tank to constantly expand and contract, putting stress on the welded joints. Eventually, those high heat exchanges degrade the water heater tank and those welded seams bust open—and that’s when replacement is required.

With an indirect water heater, there’s no direct heat or combustion within the tank—the water within is heated passively, when very hot water from the boiler travels through the coiled pipe in the indirect tank, transferring heat to the cooler water held within the indirect tank. The hot boiler water travels through the coiled pipe rather slowly, but that heats the water around it (inside the indirect tank) very quickly. When an indirect tank is replenished with cold water, the water flows in just above the heating coil and begins warming up immediately, so the temperature differential, or Δ (delta) T, is a lot less. In turn, the thermal expansion is less drastic, resulting in less stress on the tank itself. This process is also what makes indirect water heaters naturally more efficient than standard atmospheric water heaters. (Though many high-efficiency water heaters gain their efficiencies by using similar technology like coiled heat exchangers, so again, it’s important to review all of your options to make the best choice for your building and needs.)

If you happen to have limited space in your boiler room, an indirect tank can be a great space-saving option because there’s no additional venting. Plus, you can often get away with a smaller indirect tank for hot water than you’d need with a typical water heater because the high BTU input of the boiler provides the benefit of quick hot water recovery, which makes up for a lack of storage.


All that said, it’s important to clarify that indirect water heaters really do tend to be most beneficial when working in conjunction with the newer, high efficiency modulating boiler models, for a couple of reasons. First, these high-efficiency boilers achieve efficiency ratings between 90-99%, making them efficient enough for multi-tasking to provide both heat and hot water, and still save you money on your energy bill. But the modulating aspect of these boilers is another huge factor in getting the most out of a combination system. Modulating boilers have the ability to provide varying levels of burner firings, so they can determine if they need to fire a full flame or low flame. Specifically, that means that during warm weather when you wouldn’t be using your boiler for heat, the boiler can stay on low when only providing the hot water.

If you plan to update an existing boiler system to include an indirect water heater, there are additional considerations such as wear on the boiler, as well as the efficiency of the boiler. Boiler efficiency definitely impacts whether it’s worth choosing indirect over a standard water heater. Using an inefficient boiler for multiple functions doesn’t increase its efficiency.

Generally speaking, when boilers are used only for hydronic heating, they’re off during the warmer months of the year. So, it’s important to consider that adding an indirect heater for domestic water use would lead to the boiler working year-round (and the associated costs).  And a boiler without a modulating burner control means the boiler can’t tell the difference between needing to fire for heat or the hot water, so it fires the same amount either way and therefore, will fire more often when also used for hot water. This could seriously shorten the lifespan of the boiler because it’s being used more frequently than originally intended.

One downside to any combination system is that the hydronic heat always takes precedence over the domestic hot water. Whenever the building requires heat, the primary loop kicks in, and if tenants are showering or using domestic water at the same time, the building heat still takes priority. So, if another polar vortex hits, that boiler may frequently need to satisfy its heating needs over the showers, dishwashers, etc. Most systems are set up to not “ignore” the domestic needs for longer than 30 minutes, but what if that happens to be the timeframe when half the building is getting ready for work and taking showers?

Also, much like when you choose one big water heater over several smaller units, when your boiler goes down, your entire system is DOWN. If you rely on one boiler for hydronic heating and water heating and it goes down, your entire building will lose both hot water and heat, all in one shot; there’s no redundancy or backup. It’s important to plan accordingly and assess your needs before choosing a water heating system.

Another consideration is the quality of the water used in your combination system. As with tank-type water heaters, it’s important to ensure your boiler and indirect water heater tank are using softened water. You may think your water isn’t hard enough to require a water softener, but if you want your boiler and water heaters to live long, healthy lives, it’s critical. When hard water is heated, the natural minerals in that water get separated out and left behind to stick to whatever they land on. The hotter the water gets, the more sediment gets shaken out and left behind to build up inside your equipment and pipes. Did we mention that boilers make really hot water?

Sediment in the heat exchanger tubes of a boiler (the most expensive part of a boiler) causes a ripple effect that can eventually clog the tubes and inhibit the free flow of water. As minerals and sediment pile up inside the tubes, little spaces in between the deposits create pockets where water vapor gets stuck and pops out, creating little vibrations. This can cause the boiler to bounce up and down, eventually causing the heat exchanger to fail. This will eventually result in a leak from the heat exchanger that will extinguish the pilot assembly, putting a stop to all heating capability and end up as a giant puddle on the floor of your boiler room. Sediment build-up problems will develop gradually, but by the time noticeable symptoms appear, the damage is difficult to reverse. Since hard water can lead to the catastrophic failure of your equipment, prevention by softening is a worthy investment to preserve your boiler and indirect water heating system for as long as possible.

If you’re at the beginning stages of a commercial water heating setup or you’re ready for a total overhaul, you may be faced with the decision between a standard tank-type water heater or a combination system that uses a boiler and an indirect tank. With the right boiler, a combination system is a great option to consider for heating your commercial water and your commercial space. An indirect water heater tank can be a fairly inexpensive addition to your hydronic heating system, which may save you money going forward by increasing the efficiency of your water heating operations.

While there are many benefits of the boiler and indirect water heater tank system, there are also cases where it may not be an ideal fit for your business. It’s important to understand the basics of boilers and water heaters and weigh the cost/benefits before making a change. A professional, trained water heating system technician can help you weigh your options and determine which commercial water heating set up best fits your commercial application.

Don’t know where to start? Review our water heater information to make the best decisions about your commercial heating and domestic water setup.  Or contact our trained professionals at Reliable Water Services (800-356-1444). We’re happy to help you choose the best water heating system for your commercial application.