Indirect Hot Water Heaters: A Smart And Reliable Solution For Your Home

Looking for an efficient and reliable way to heat your home’s domestic hot water?

Indirect hot water heaters could be the solution you need.

By using your furnace or boiler to heat the water inside an insulated storage tank with separate fluid pathways, indirect-fired water heaters offer many advantages over other heating systems.

(photo: amazon)

Explore the different types, advantages, and disadvantages, and see how they compare to other systems. Discover the benefits of these innovative heaters and how they can bring added comfort and convenience to your home. Whether you’re looking to save on energy costs or simply improve the performance of your space and water heating systems, indirect hot water heaters are definitely worth considering.

Indirect Hot Water Heaters: What You Need To Know

What Is An Indirect Water Heater And How Does It Work?

An indirect hot water heater is a type of water heating system that uses the heat generated by a separate source, such as a boiler or furnace, to heat water.

Instead of heating water directly, the unit transfers the heat from the primary source to a separate, insulated storage tank containing the water that needs to be heated. The heated water is then stored in the tank until it is needed for domestic use, such as showering, washing dishes, or doing laundry.

Indirect water heaters are known for their efficiency and reliability, as they use a separate heat source that is already in use for another purpose, thus minimizing the need for additional fuel or energy consumption.

These systems are a good choice for many regions, particularly in areas where there is a need for space heating and domestic hot water heating. They are particularly useful in colder climates where homes require a reliable and efficient heating system.

In regions where natural gas or propane is readily available, indirect hot water heaters are a popular choice due to their compatibility with these fuels.

Ultimately, the suitability of an indirect water heater will depend on various factors, including the climate, the type of fuel available, the size of the household, and the hot water usage patterns.


There are different types of indirect hot water heaters available in the market, including:

  • Coil-type
  • Tank-type
  • Single-coil
  • Dual-coil
  • Tank-in-tank

Coil-Type Indirect Water Heaters

Coil-type indirect water heaters are designed to use the heat generated by a boiler to heat the water running through the coils. While these systems are often installed inside the boiler, they can also be installed externally or as part of a separate hot water storage tank. They are sometimes called “tankless coil” or “instantaneous” systems because they do not require a separate storage tank but deliver hot water on demand.

While coil-type systems can be a good option for homeowners with low hot water demand and a high-efficiency boiler, they are typically less efficient than tank-type systems and may not provide a consistent supply of hot water during periods of high demand. Homeowners with high hot water demand or an older boiler may want to consider a tank-type indirect water heater for better performance and longevity.

Tank-Type Indirect Water Heaters

Tank-type indirect water heaters consist of a separate, insulated storage tank and a heat exchanger. The heat exchanger transfers heat from the furnace or boiler to the water in the storage tank and from there to the shower, hot water faucets, etc.

One of the benefits is that they provide a large supply of hot water, even during periods of high demand. They are also more energy-efficient than traditional water heaters, as they use the furnace or boiler’s heat to warm the water rather than relying solely on a heating element or a burner.

One potential drawback of tank-type indirect water heaters is that they may require more space than traditional tank-type water heaters, as they need to accommodate the heat exchanger coil inside the tank. However, they are typically still more compact than tank-in-tank systems or other types of indirect water heaters.

Overall, they are a good option for homeowners who have a high demand for hot water, want to improve their energy efficiency, and have the space and budget for a separate storage tank. They can be a good long-term investment, as they are durable and can provide reliable hot water for many years.

Single-Coil Indirect Water Heaters

Single-coil and dual-coil indirect water heaters are two types of water heating systems that use a heat exchanger to transfer heat from a boiler or other heating source to the water. The primary difference between the two is the number of coils used in the heat exchanger.

In a single-coil indirect water heater, the coil is installed inside the tank. The coil is typically made of copper or another high-conductivity material and is wrapped around the interior of the tank.

The water flows through the coil, and heat is transferred from the heating source to the water through the walls of the coil. As the water heats up, it rises to the top of the tank, where it can be drawn off for domestic use.

Because the coil is installed inside the tank, single-coil systems are typically more compact and easier to install than other types of indirect water heaters. However, they may not be as efficient as dual-coil or tank-in-tank systems, as they can experience some heat loss through the walls of the tank. Additionally, if the coil becomes damaged or corroded, it may be necessary to replace the entire tank, which can be costly.

Dual-Coil Indirect Water Heaters

Dual-coil indirect water heaters, on the other hand, have two coils inside the tank. The primary coil is located at the bottom of the tank, and the secondary coil is located higher up. The primary coil is used to heat the water in the tank, while the secondary coil is used to provide heat to other parts of the house, such as the radiators or baseboard heaters.

However, some dual-coil systems are designed to allow both coils to be used simultaneously with different heating sources. For example, one coil may be connected to a solar panel system to heat the water during the summer months, while the other coil is connected to a boiler or heat pump that takes over during the winter months. This allows the system to take advantage of the free heat provided by the sun when it is available, while also providing backup heating when needed. 

The advantage of a dual-coil system is that it allows you to use the same heating source for both domestic hot water and space heating. Each system can operate independently, without any mixing of its contents, regardless of temperature or operating pressures. This is what makes the system more efficient and cost-effective than using separate systems. However, dual-coil systems are typically more expensive to purchase and install than single-coil systems.

Tank-in-Tank Indirect Water Heater

tank-in-tank indirect water heater is another type of water heating system that uses a unique design to heat water more efficiently and effectively than traditional water heaters. In a tank-in-tank system, the domestic hot water is stored in an inner tank, which is surrounded by a larger outer tank that contains the heating fluid (usually water or a glycol mixture). The small tank’s wall acts as a heat exchanger.

Because the heat exchanger is in direct contact with the stored water, the tank-in-tank system can provide hot water quickly and efficiently, with less heat loss than traditional indirect water heaters. Tank-in-tank systems are also highly versatile, as they can be used with a variety of heating sources, including solar panels, boilers, and heat pumps. They are also available in a range of sizes, from small units for residential use to larger commercial systems.

Indirect vs. Direct Water Heater

Indirect and direct water heaters differ in their design, operation, and energy efficiency.

Direct water heaters, also known as tank-type water heaters, use a gas burner or heating element to heat the water in the tank directly. This type of heater is typically less expensive to purchase and install than indirect water heaters. However, direct water heaters have lower efficiency due to heat loss through the wall of the tank, and they have a shorter lifespan than indirect water heaters.

Indirect water heaters, on the other hand, do not have a burner or heating element in the tank. As we mentioned before, they use a heat exchanger to transfer heat from a separate source, such as a boiler, to the water in the tank. This design reduces heat loss and increases efficiency. They also have a longer lifespan than direct water heaters, as they are not subject to the same thermal stress caused by direct heat.

In terms of installation, indirect water heaters may require additional piping and components to connect to a separate heat source, such as a boiler. However, they are often easier to install than direct water heaters in retrofit applications, as they do not require a flue or venting system.

Overall, indirect water heaters are generally considered more energy-efficient and reliable than direct water heaters, making them a good option for homeowners who prioritize long-term performance and cost savings.


There are several benefits of using an indirect water heater compared to other types of water heating systems:

  • Efficiency: Indirect water heaters are highly efficient as they use a separate heat source, such as a furnace or boiler, that is already in use for space heating purposes. This reduces the need for additional energy consumption, thus lowering the cost of heating water.
  • Better performance: The system can produce hot water year-round in large quantities, with quicker recoveries, and at a reasonable cost.
  • Cost-effectiveness: Since the system uses a separate heat source, indirect water heaters are generally less expensive to operate and maintain than traditional tank-type water heaters. Additionally, some types can be paired with renewable energy sources, such as solar collectors or heat pumps, which can further reduce energy consumption and operating costs.
  • Reliability: Indirect water heaters are known for their reliability due to their simple design with fewer components, which makes them less prone to breakdowns and requires less maintenance than traditional direct-fired water heaters. They also have a longer lifespan compared to other types of water heaters, making them a cost-effective option for providing hot water in a residential setting.
  • Consistent hot water supply: They can provide a continuous supply of hot water, which is especially useful for households with high hot water demand or large families.
  • Versatility: Indirect systems can be used with different types of fuel sources, including natural gas, propane, electricity, oil, and solar energy. This versatility allows homeowners to choose the fuel source that best meets their needs.
  • Year-round use. By continuously using the boiler for heating domestic water, the system can prevent costly deterioration that occurs when the boiler is idle for long periods. During the winter, when the indirect water heater is used for home heating, only a small fraction of the heat is used for domestic water heating. However, in the summer, when space heating is not needed, all heat can be used for hot water preparation. It is tied into your home’s space heating system, which means that it will heat the water even if there is no need for space heating.
  • Less maintenance: Indirect water heaters are designed to be low maintenance compared to other types of water heaters. They have fewer components and do not require an exhaust vent, which reduces the potential for maintenance issues. Most types require only basic annual maintenance, such as checking and flushing the heat exchanger to remove any accumulated sediment or debris. In addition, because the indirect water heater is not directly exposed to the combustion gases produced by the heating system, there is less buildup of soot and other combustion byproducts that can cause problems in the system.
  • Easy to size and install. Sizing and installation can vary depending on the specific model and the heating system they are being installed with. However, indirect water heaters are generally designed to be relatively easy to size and install compared to other types of water heaters. The size is typically determined by the heating output of the boiler or other heat source it is being installed with, rather than the household’s hot water demand. This means that indirect water heaters can be easily sized to match the existing heating system and can often be installed without major modifications to the plumbing or electrical systems in the home. In terms of installation, they are typically installed by a qualified plumber or heating technician, and the installation process can be straightforward.


While indirect water heaters offer many benefits, there are a few potential disadvantages to consider:

  1. Upfront cost: Indirect systems can be more expensive to purchase and install than traditional tank-style water heaters, especially if additional plumbing or electrical work is needed.
  2. Space requirements: While these systems are typically smaller than tank-style water heaters, they do require additional space for the separate storage tank and heat source.
  3. Installation complexity: Installing an indirect water heater can be more complicated than other types of water heaters, particularly if it requires integrating with an existing heating system.
  4. Maintenance: Although they generally require less maintenance than tank-style water heaters, they still require periodic inspections and maintenance to ensure they are operating correctly.
  5. Heat loss: While the storage tank is insulated, there may still be some heat loss over time, which could lead to energy inefficiency.

Overall, the advantages of indirect water heaters often outweigh the disadvantages, but it’s important to consider these factors when deciding if this system is the right choice for your home.


Indirect hot water heaters are a reliable and energy-efficient option for providing hot water in a home. Compared to traditional water heaters, they have higher recovery rates and are less prone to breakdowns due to their simpler operation and fewer components.

One of the advantages of indirect hot water heaters is their ability to use the BTU output from the boiler as a heat source. This means they can provide hot water quickly and efficiently without the need for a separate heating element.

For systems without coils, such as those from Weil McLain, self-cleaning waterways are used to prevent liming and efficiency deterioration. This ensures that the system continues to operate at peak efficiency over time.

Integrated combined heating systems that use the same heat source for both space and water heating are also available. These systems have only one combustion device, reducing the risk of backdrafting and making maintenance easier.

Indirect hot water heater tanks are typically built of stainless steel, which makes them durable and long-lasting. They are also less prone to thermal stress since there is no direct flame or combustion. In fact, some manufacturers even offer a lifetime warranty on their tanks.

When buying a new system, it’s important to consider the tank’s capacity and recovery rate. The most typical capacity of indirect hot water heater tanks is about 40 gallons, and the recovery rate will depend on the BTU input of the unit that feeds the heat exchanger.

Overall, indirect hot water heaters are a great option for homeowners looking to improve the performance of their space and water heating systems while saving on energy costs.

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