USA Water quality is supported by its audience. If you buy something through our links, we may earn money from our affiliate partners. Learn more

The Best Indirect Water Heaters for Modern Families

Fact checked by Stephen Conklin

best indirect water heater

Experts have spoken! Only the best indirect water heater can provide families with existing space-heating appliances a more energy-efficient and cost-effective way to enjoy hot showers and other hot water needs on-demand.

Modern indirect water heaters work using the heat coming from boilers through efficient heat transfer. New indirect water heater systems are cost-effective and energy-efficient technologies that rely on boilers’ quick-heating and high-temperature capabilities. Many modern indirect water heater systems can heat water faster than tankless water heaters.

Choosing an indirect water heater for one’s home can be challenging because there are several considerations one must think about before buying. We deemed the three following factors crucial in picking the top-rated indirect water heaters for the home.

  • Capacity: All water heaters must supply a family with their hot water requirements, especially during peak use. Hence, an indirect water heater with a higher hot water capacity is better than a unit with a lesser volume. For example, a 40-gallon device is more suitable for most families than a 30-gallon tank.
  • Insulation: Indirect water heaters rely on an external heat source to heat the water. Hence, it would be best to get a product with high-quality insulation to retain as much heat in the tank as possible.
  • First-hour Rating: An indirect water heater must deliver as much hot water as possible within the first hour of its operation. Hence, a product with a 200-gallon first-hour rating (FHR) is better than an indirect water heater with a 150-gallon first-hour capability.

One can also consider other indirect water heater attributes before buying. We shall discuss these additional factors in the following indirect hot water reviews and the succeeding buying guide.

Top 1

Burnham AL35SL

Weight 178.35 Pounds
Capacity 35 Gallons
Material R-16 PU Foam

Jump to Review

Top 2

Triangle Tube SMART40

Weight ‎135 Pounds
Capacity 36 Gallons
Material 2″ PU Foam, Stainless Steel

Jump to Review

Top 3

Amtrol BoilerMate WH-41Z

Weight 128 pounds
Capacity 41 Gallons
Material HDPE

Jump to Review

Table of Contents

Top 8 Indirect Water Heater Reviews

1. Burnham AL35SL Alliance SL Indirect-fired Water Heater

Many households trust Burnham indirect water heaters because of their reliability, performance, and efficiency. The Burnham ALS5SL Alliance SL continues the brand’s tradition of excellence by offering families a cost-effective way to use some of their space-heating requirements to heat their water.

Although I found mixed Burnham indirect water heater reviews, I still find this product exceptional in water-heating capabilities. It comes with a 35-gallon tank and 192-gallon first-hour rating, sufficient to deliver hot water for a family of five to six, depending on the existing boiler’s heating performance and energy efficiency.

This indirect heater also provides 162 gallons of hot water from the second hour onwards, ensuring a peak flow rate of 6 gallons per minute. I appreciate its overall design, contributing to 30 percent energy savings compared to traditional direct water heaters.

This hydrastone-lined inner tank’s material has exceptional strength and wear resistance, making it suitable for hot water tank applications. My family never complains about the water temperature dipping to uncomfortably low levels because the tank retains heat like a charm.

Moreover, I find the polyurethane R-16 foam insulation sufficiently thick to prevent unnecessary heat loss while ensuring better heat retention. It maintains the ideal water temperature for everyone to enjoy.

Unfortunately, I find this indirect heater heavy for a one-person installation. Weighing in at 178.35 pounds, one might need two helpers to mount the indirect water heater onto its place.

Still, this is an indirect water heater worth buying if one already has an existing boiler in the home. It is efficient, quick to heat the water, and retains heat better than other indirect water heaters.

  • 35-gallon indirect water heater for a small to medium-sized family
  • Premium-quality hydrastone inner tank lining
  • Thick R-16 polyurethane foam tank insulation
  • 30% better energy savings from traditional water heaters
  • 192-gallon first-hour and 162-gallon continuous flow ratings
  • Has ultra-fast hot water recovery for heavy consumption
  • Heavy (178.35 pound) for one-person installation

This indirect water heater also has an ultra-fast hot water recovery, making it suitable for families with heavy hot-water consumption.

2. Triangle Tube SMART40 Phase III Indirect Water Heater

The Triangle Tube SMART40 Phase III is a good-quality indirect water heater for modern families. It delivers hot water when one needs it, regardless of volume and without experiencing substantial water pressure drops.

I appreciate this indirect water heater tank’s 36-gallon capacity, which should be sufficient for a family of five or six. We can connect this indirect hot water heater to an existing boiler to optimize thermal energy and ensure efficient heat transfer.

I also love this indirect water heater’s FHR of 180 gallons and 150-gallon continuous hot water flow. I believe its performance is due to the indirect water heater’s tank-in-tank design. The system makes it more efficient to heat the water, raising the water temperature faster.

The tank itself comes with durable stainless steel construction, increasing its heat retention capability and heat loss prevention. Wrapping the outer tank is a two-inch-thick polyurethane foam, adding to the indirect hot water heater’s exceptional energy efficiency.

Besdies, this indirect water heater has a self-cleaning and self-descaling heat exchanger. Users will never fret about complicated maintenance procedures because this indirect water heater keeps itself in pristine condition.

Sadly, several Triangle Tube indirect water heater reviews underscore reliability issues, especially leaks. I am unsure whether these leaks are inherent product defects or a result of faulty installation. Additionally, at 135 pounds, this indirect water heater still calls for a two-person setup.
  • 180-gallon first-hour recovery and 150-gallon continuous flow rating
  • 36-gallon capacity for medium-sized households
  • Two-inch-thick polyurethane foam insulation for avoid heat loss
  • Self-descaling heat exchanger for minimal maintenance troubles
  • Premium-quality stainless steel tank construction
  • Some leaking issues after a while
  • A bit heavy, requiring 2 people to set it up
Nevertheless, this indirect hot water heater remains a favorite of many energy-conscious families. They can maximize their boiler’s space-heating capabilities by diverting some of it to this product to heat their water. It is an efficient system that should give households all the hot water they need.

3. Amtrol BoilerMate WH-41Z Indirect-fired Water Heater

Families who want an energy-efficient way to heat their water using a boiler should consider the Amtrol BoilerMate WH-41Z. This indirect-fired water heater has one of the best insulating materials and energy transfer efficiencies.

According to top Amtrol BoilerMate reviews, this water heater model is perfect for small to medium-sized households with its 41-gallon heating chamber. The top-mounted heat exchange coil has a 40% greater surface area than conventional indirect water heaters, allowing the appliance to heat more water and faster, too.

Households often worry that their indirect hot water heater will lose more heat than intended. This product ensures optimization of the boiler’s thermal energy. It enables families to enjoy an endless hot water supply for their various activities.

I appreciate this indirect water heater’s high-density polyethylene tank construction with exceptional corrosion-proofing. Never again will people worry about rust in their hot water tanks because this appliance can keep corrosion in check.

The heater coil’s finned design also enhances the appliance’s water-heating capabilities. The heat exchanger is also removable, allowing anyone to maintain its integrity and clean the tank’s interior. No one will call a plumber to clean and flush the indirect water heater again.

I also love the electromechanical Aquastat because it raises the temperature faster than other indirect water heaters. It ensures a faster recovery rate, allowing everyone to enjoy hot water.

Sadly, I found some worrying BoilerMate indirect fired water heater reviews, underscoring its durability. Although this product features a deep-drawn pressure shell, some users still complained of tanks not lasting several years. Additionally, installation can be tricky, and most users prefer getting professional assistance.
  • 41-gallon capacity for medium-sized families
  • Corrosion-proof bimodal HDPE tank construction
  • Top-mounted removable heating coil with finned design
  • Fast-acting electromechanical Aquastat
  • 40% greater energy transfer efficiency than other brands
  • Not the most long-lasting device
  • Professional assistance is required for installation
Nevertheless, I find this indirect hot water heater exceptional for its energy transfer efficiency. It heats the water quickly and recovers it equally fast. If one can get a good quality product from a reputable retailer, this indirect water heater’s performance and overall quality are spot-on.

4. Crown MS2-50 Mega Stor Indirect Water Heater

The Crown MS2-50 Mega Stor is an energy-efficient indirect-fired water heater that features exceptional first-hour and continuous hot water draw ratings. Not only does it guarantee an unlimited hot water supply. This indirect water heating system also comes with third-party certifications to keep users at ease.

I am astonished by this 50-gallon appliance’s 236-gallon FHR. Most water heaters I know cannot deliver even half that, making this product a worthy choice for families with heavy hot-water consumption. Not only is its FHR remarkable, but its continuous water flow is also commendable. People can use up to 193 gallons of hot water per hour.

Another thing that surprised me was the heater’s standby loss. I know of products that lose up to 1.5 degrees Fahrenheit per hour. This product only decreases its water temperature by 0.9 degrees Fahrenheit per four. Hence, if the temperature is 130 degrees Fahrenheit, it would be a comfortable 121 degrees Fahrenheit after ten hours.

Part of this product’s charm is its heavy-duty stainless-steel tank construction with high-density polymer for the top and bottom sections. It also has two-inch-thick foam insulation, retaining heat and reducing heat loss. Many Crown Mega Stor indirect water heater reviews highlight stainless steel design, one of the product’s most important attributes.

It also has the nod of the Air-conditioning, Heating, & Refrigeration Institute (AHRI) and the Electrical Testing Labs (ETL).

This product is not suitable for people who want a space-saving indirect water heater because it measures 49.75 inches tall and 23.5 inches in diameter. Families with small boiler room spaces might find it challenging to install this water heater. Moreover, it is pricier than other brands.
  • 236-gallon FHR and 193-gallon continuous draw rating
  • Only 0.9-degree-Fahrenheit standby loss per hour
  • Heavy-duty stainless-steel and polymer construction
  • Two-inch-thick foam insulation, retaining heat and reducing heat loss
  • 50-gallon indirect water heater tank capacity
  • AHRI and ETL certification
  • Not space-efficient (49.75″ tall and 23.5″ in diameter)
  • Pricier than standard products
I do not mind paying a handsome price for this indirect hot water heater. This water heating system’s first-hour rating, capacity, and continuous water flow rating are unbeatable.

5. Triangle Tube SMART80 Phase III Indirect Water Heater

Large families and households with exceptionally high hot-water consumption can rely on the Triangle Tube SMART80 to deliver an unmatched hot water supply for different activities. This product does not only have an excellent first-hour rating, but its continuous flow and peak flow rates are also astonishing.

Capable of handling 70 gallons of hot water at a time, it would be safe to assume this indirect water heater can only deliver as much hot water. However, this is not true. It provides 460 gallons of hot water in the first hour of operation alone. Families can enjoy up to 400 gallons of hot water in subsequent hours.

Every household member will have access to an unlimited hot water supply, perfect for hot showers, dishwashing, clothes washing, and other hot water-related chores. The system also ensures a maximum hot water flow rate of 12.5 gallons per minute (GPM). I find this surprising since most American households only need 7 to 10 GPM.

Although there are no indirect hot water heater ratings for this product on some online retailers, it has favorable reviews on other e-commerce sites. Many users also commend the indirect water heater’s self-cleaning and self-descaling heating coil inside the tank. I can only imagine the people’s sigh of relief when they learn this product does not require complicated maintenance.

Its two-inch-thick foam insulation also contributes to the water heater’s energy efficiency. It minimizes heat loss and improves heat retention.
  • 70-gallon indirect water tank capacity
  • 460-gallon first-hour and 400-gallon continuous flow rating
  • 5 gallons per minute peak flow rate
  • Self-cleaning and the self-descaling heating coil inside the tank
  • Thick polyurethane insulation for energy efficiency
  • Not for small homes due to its large design
  • Expert installation recommended
Unfortunately, this indirect water heater comes in at a hefty 271 pounds and requires floor space. Although it can be a worthy DIY project, I highly recommend having professional plumbers install the system.

6. Westinghouse Electric Water Heater

Many homeowners consider the Westinghouse Electric Water Heater the top-tier indirect hot water heater on the market. It is perfect for households without space-heating appliances, such as furnaces and boilers. This product only requires 4,500 watts of electricity to heat the water for the whole family’s enjoyment.

Although its 112-gallon FHR might seem modest compared to the other indirect water heaters on this list, it is sufficient for a small family. What I like the most is its energy and thermal efficiency, rated at 0.94 and 98%, respectively.

Families can enjoy better savings every time they use their hot showers, dishwashers, washing machines, and other hot water-using appliances. Not only will bath times and washings be more pleasant. These activities will be more convenient, too.

I admire the electric water heater’s stainless-steel tank design, eliminating the need for anode rods. Stainless steel has this uncanny ability to repel substances, preventing them from sticking to the surface.

It should make maintenance a breeze while ensuring overall indirect water heater longevity. Scale buildup will never be a cause of concern for families using this indirect water heater.

If that is not enough, the company offers a limited lifetime warranty. One only needs to register the indirect hot water heater online to enjoy such a privilege. I can think of a few brands that provide such a guarantee. I know I will feel more confident having this appliance in my home than another indirect water heater.

My only concern is that 80 gallons are too much for a family of three or four. One must also dedicate a 240-volt system to accommodate the indirect water heater’s 4,500-watt power requirement.
  • 80-gallon indirect water heater
  • 112-gallon FHR, sufficient for small families
  • 94 Uniform Energy Factor and 98% thermal efficiency
  • Stainless-steel tank requires no anode rods for better corrosion resistance
  • Offers a lifetime warranty to ensure good quality
  • Requires a dedicated 240-volt connection
Still, I recommend this electric water heater to families without boilers or compatible space-heating technologies. Although installation can be tricky, the system’s performance and energy efficiency should compensate for the initial inconvenience.

7. HTP SuperStor Ultra SSU30 Indirect Water Heater

Families looking for a small indirect water heater can buy the HTP SuperStor Ultra SSU30. Although this indirect water heater has a space-saving design, it never compromises its ability to deliver sufficient hot water supply to a family of three to five.

I love this indirect water heater’s small footprint, requiring only 19.25 inches in diameter and vertical space of only 39.5 inches. Other brands can be as wide as 24 inches and as tall as 70 inches, making installation cumbersome for households with super-tight boiler rooms. This product will never demand too much space from one’s basement.

I agree with many SuperStor indirect water heater reviews highlighting this product’s suitability for small families. It has a unique water-blown, environmentally safe, and CFC-free foam insulation. Heat loss is never an issue while guaranteeing exceptional heat retention.

The heat exchanger unit is also different from other brands. It features an advanced cupro-nickel coil with a surface area-increasing finned design. Heating the water is never a problem because this water heating system can accomplish the task in less time. People taking turns in the shower will never complain about the water being too cold.

Like other trustworthy indirect water heater brands, I admire this product for its limited lifetime warranty. However, if one uses the company’s commercial model, the guarantee is only good for five years. Still, it is better than a two-year warranty or no guarantee at all.

While I admire this product’s compact design, I find its 154-gallon FHR a few tens of gallons lower than a similar 30-gallon unit. When one considers its price tag, it is a bit more costly.
  • 30-gallon water storage tank and 154-gallon FHR
  • Unique cupro-nickel heat exchanger coil with finned design
  • Two-inch-thick, water-blown, CFC-free foam insulation
  • Space-saving design (19.25” diameter and 39.5” vertical space)
  • Comes with a limited lifetime warranty
  • Slightly lower FHR (154 gallons) than the leading brand
  • Priced higher than other 30-gallon units
Nevertheless, this indirect water heater remains an excellent choice for small families and tiny homes with prohibitive living spaces.

8. Rheem MR50245 Marathon Tall Electric Water Heater

Rheem is one of the undisputed leaders in water heating technologies. The MR50245 Marathon Electric Water Heater is the company’s entry into the competitive indirect water heater market. This product has sufficient features to make it a wise choice for modern families.

I love this indirect water heater’s lightweight and durable tank design. While most water heaters on the market feature polyurethane and steel tank construction, this appliance has a polybutene tank. It features multiple layers of fiberglass filaments to give the system exceptional heat retention.

The tank is also corrosion and rust-proof, eliminating the need for additional treatments. The company also offers a limited lifetime warranty for its indirect water heaters, saving buyers the hassle of replacing or repairing their units if something goes wrong.

This indirect water heater has a modest 61-gallon FHR and a 20-gallon hourly recovery rate. While these numbers might seem dismal, they are sufficient for small households and families with low to moderate hot-water needs. More pleasant hot showers await everyone in the home.

It also has a Uniform Energy Factor of 0.90. Although it is lower than Westinghouse’s 0.94 UEF rating, I still find it within industry standards. At least, the energy efficiency is not lower than 0.80. Families can still enjoy substantial energy savings using this indirect water heater.

If one is looking for a friendlier indirect water heater price, it would be best to consider other products. Despite its lower FHR and energy efficiency than other indirect water heaters, this device has a prohibitive price tag that can shoo away potential buyers.
  • 61-gallon FHR and a 20-gallon hourly recovery rate
  • Rust and corrosion-proof polybutene tank with a lifetime warranty
  • 90 Uniform Energy Factor, within the standard
  • Allows substantial energy saving
  • Pricier than other indirect water heaters
  • Less energy-efficient than other brands
However, this is a good indirect heater that is perfect if one does not mind paying this much. It is suitable for small households and families with modest hot-water requirements. Its lifetime warranty and tank durability are also worth considering.

What to Look for When Buying Indirect Water Heaters


Buying indirect water heaters is almost similar to choosing a conventional direct water heater system. After all, the only difference between the two is the absence of a heat source built into indirect heater systems.

Although indirect heaters also have storage tanks, they do not have gas heating elements, unlike direct water heaters. Instead, these appliances rely on an external heat source to heat the water. Cold water enters the water tank, where the heat exchange system transfers heat to the water, resulting in heated water.

Ideally, indirect water heaters draw heat from a furnace or a boiler, although some indirect heating systems work with electricity or solar energy. They should also have minimal impact on pressure flow, unlike traditional water heaters.

Although the disadvantages of indirect water heater are several, its benefits outnumber them. Here are some items prospective buyers must look for when choosing an indirect water heating system.


Indirect-fired water heaters that rely on boiler systems or furnaces do not need a large-capacity tank to deliver sufficient amounts of hot water.

The reason is simple. Boilers are efficient space-heating technologies that produce intense heat quickly, too. Since an indirect water heater connects to a boiler system, it is faster to heat the water than tankless water heaters.

Experts say a good indirect water heater system with a 30-gallon tank capacity mated to a high-efficiency boiler is sufficient to supply hot water for 12 to 14 people. It would be wise to remember that the higher the indirect water heater’s capacity, the higher is its price.

First-hour Rating

All water heaters must deliver hot water at the ideal temperature within the first hour of use. That is why potential buyers must always look at the device’s first-hour rating or FHR. The number reflects how many gallons of hot water the indirect water heater can produce in the first hour of operation.

Some indirect water heaters have an FHR of 460 gallons despite having only a 70-gallon capacity. On the other hand, some 30-gallon units have an FHR of 190 gallons. The ratings depend on boiler BTUs and overall energy efficiency.

One can calculate the peak hot water use by adding all hot-water fixtures’ flow rates. For example, showers have a flow rate of 1.5 to 3.0 gallons per minute, while dishwashers require 2.0 to 4.0 GPM. One must determine the household’s maximum hot water use to identify the most suitable FHR.

For example, three people taking 10-minute turns at the shower will consume 45 to 90 gallons of hot water. Hence, the 190-gallon FHR unit is more than sufficient.


Indirect water heaters are dependent on boiler or furnace function to keep the water at the ideal temperature. Unfortunately, boilers and other compatible space-heating systems are not always on. Hence, there is the risk of heat loss.

That is why it would be best to check the indirect water heater’s insulation. Most products come with polyurethane insulation, although some will have polybutene materials. The foam’s thickness matters because it aids in heat retention and heat loss prevention.

Internal Tank Surface

Most indirect water heaters have glass interior tank lining to prevent limescale and corrosion formation. Other products feature stainless steel construction. It would be best to research the materials used in the indirect water heater tank’s interior construction.


One can expect indirect fired water heater prices to be anywhere between $600 and $4,000. While price is almost always a reliable indicator of quality workmanship and performance, it would still be best to research the product one wants to buy.

Not all expensive indirect heaters are worth their price tag in the same way that affordable options can also offer exceptional value.


Some indirect water heaters come with a limited lifetime warranty, giving users peace of mind. It is also an indirect measure of the product’s performance and quality construction. After all, no company will want to stand by its indirect water heater for countless years without being confident about its quality.

How Do You Size an Indirect Water Heater

Sizing an indirect water heater is as straightforward as determining a family’s peak hot water requirements. It would be best to check the average water flow rate (gallons per minute) from all water fixtures during heavy hot water consumption.

For example, mornings are typically the busiest part of the day when everyone takes turns showering. If a three-member household uses 2.5 gallons of hot water per minute, the family needs 7.5 gallons per minute. If each member showers for 10 minutes, the total hot water output is 75 gallons.

One must also check the indirect water heater’s FHR and the boiler’s BTU rating. A 50 gallon indirect water heater mated to a 50,000-BTU boiler has an FHR of 85 gallons. It should be sufficient to supply the three-member household’s hot water needs.

In most cases, a 30-gallon indirect water heater is sufficient for 12 to 14 people. The reason for this is the boiler’s high efficiency, high-temperature, and fast-heating capabilities. It can keep up with increased hot water demands.

How Long Do Indirect Water Heaters Last


The average indirect water heater lifespan is 20 to 25 years if one observes meticulous care. Tanks lined with high-quality glass materials tend to last longer. These devices require little maintenance. However, one must maintain the system’s boiler or whatever device heats the indirect water heater.

How Much Does It Cost to Install an Indirect Water Heater

Installing an indirect water heater can cost families $400 to $2,000, although the total cost can reach $1,200 to $6,000. The installation expenses depend on the boiler or furnace type and distance to the indirect water heater tank. In general, installation cost increases with the distance between the two components. Unfortunately, the greater the distance, the lower the energy efficiency.

The cost does not include the unit’s purchase price, which is often higher than other water heater types. A budget-friendly indirect water heater cost $600 to $800, while high-end products cost up to $4,000. It can bring the initial indirect water heater setup cost to $1,800 to $10,000.

Is It Worth It

According to the US Department of Energy, indirect water heaters are a cost-effective means of heating water. These systems rely on a building’s space-heating appliances.

One of the most common disadvantages of an indirect water heater is its initial startup cost. However, this system pays for itself within a few years, making it more cost-effective in the long term.

It is also more energy-efficient than other systems, allowing the boiler or furnace to supply heat to the hot water storage tank, delivering hot water to everyone. The system allows the boiler to turn on and off less frequently, further saving energy for the home.


The best indirect water heater works like an on-demand tankless water heater in supplying limitless hot water to everyone in a household. It is a cost-effective system that relies on a home’s existing furnace or boiler, maximizing its thermal energy output to heat the water.

Capacity, first-hour-rating, insulation, internal tank surface, warranty, and price are some of the most important considerations one must think about when choosing the most trusted indirect water heaters.

5/5 - (3 votes)