Ultimate List of 35-50 Gallon Indirect Water Heaters!

A new indirect water heater can both keep you warm in the coldest winter months and save you money during the summer. Investing in a high-efficiency water heater with a more energy-saving design can improve your living conditions as well as help you with utility expenses in the long run.

Modern indirect water heaters come in several sizes designed to suit a great variety of households. Since every household comes with its demands concerning performance, application, and water output, the choice of the best appliance for any given setting depends on the heater’s inherent power, the size of its tank, and how it uses generated heat.

A list of top-rated Indirect Water Heaters

PhotoProductDimensionsContainer sizeDetails
SuperStor SU-45SuperStor SU-4552.5” H x 19.25” D45 gallonsView on Amazon
Amtrol CH41ZAmtrol CH41Z48” H x 22” D41 gallonsView on Amazon
AB IWH401C Indirect Hot Water HeaterAB IWH401C Indirect Hot Water Heater54” H x 21” D40 gallonsView on Amazon
Crown MS2-50Crown MS2-5049.75” H x 23.5” D50 gallonsView on Amazon
Triangle Tube SMART80Triangle Tube SMART8061” H x 26” D70+14 gallonsView on Amazon

Things to Consider

Whether your home needs a new water heater or a replacement, it is important to outline the primary needs of your family and the amount of power your household requires to be adequately sustained.


Usually, it denotes the size of the inner tank, and modern heaters come in a wide selection of tank sizes, from 30 gallons to 125 and up. The standard household would benefit from a residential heater with a 35-45 gallon tank, but larger families might go up higher.

First-hour rating

This describes the ability of the water heater to deliver fully heated water in the first hour of usage. The heater’s max capacity and recovery rate should be considered when looking at this rating.

Every family uses water differently, so a “good” first-hour rating is only as good as the needs of each individual. To estimate which heater would provide an optimal ratio of energy-saving efficiency and hot water output, consider the amount of water generally used in your household daily.

For better reference, you may use a table describing approximate consumption of water (in gallons) per single use:

Review of the Best Indirect Water Heaters

SuperStor SU-45 Indirect Water Heater




The HTP SuperStor SSU-45 is an indirect water heater with a stainless steel tank and improved insulation that can considerably increase the recovery rate and reduce hourly heat loss.

Designed for indoor installation, this SuperStor Ultra water heater can deliver commercial-grade performance in residential settings, increasing the hot water delivery by at least 50% compared to regular residential heaters of similar capacity.

The durable stainless steel tank ensures consistent operation at high temperatures and resists the effects of corrosion, prolonging the overall service life as well as requiring minimal maintenance.

At 45 gallons of total volume, the tank is both relatively compact and capacious, capable of maintaining large amounts of water supply regularly. Installed in the cool water circuit, the buffer tank protects the heater and the exchange system by eliminating common problems such as short cycling and improved the efficiency of the overall performance.

Made from corrosion-resistant stainless steel, the internal outlet tube is designed to draw hot water from the dome of the tank, extracting between 5% and 7% more hot water than standard heaters.

The aquastat well and cold water inlet are also reinforced with stainless steel, while the commercial heat exchanger is manufactured with finned copper-nickel alloy, amounting to 20 square feet of total surface and featuring 11 fins per inch along the length. As such, the exchanger design helps the heater reach a higher BTU output for a tank this size.

The water-blown foam insulation features thick 2-inch thick walls, reducing the hourly heat loss to less than 0.5ºF per hour, while its chlorofluorocarbon-free construction ensures all operation is environmentally safe.

A recommended flow rate of this model is 10 gallons per hour with the average pressure drop of 7.9 feet. Depending on the hour rating, the heater varies its pressure-flow accordingly. For example, at 140ºF it puts out 212 gallons per hour and at 115ºF it draws 292 gallons per hour.

Lead-free and NSF-certified, this HTP SuperStor heater comes with a 1-inch boiler supply connection and 1-inch boiler return connection for a more effortless installation. Completed with a smooth silver finish over galvanized steel, the outer layer of this model resists corrosion and structural damage such as denting, making it highly durable and easy to look after.


  • Container size: 45 gallons
  • First-hour rating: 212 GPH
  • Working pressure: 150 psi
  • Electric heater output: 141,000 BTU
  • Shipping weight: 72 lbs
  • Finish: Silver gray
  • Dimensions: 52.5” H x 19.25” D


  • Reduced heat loss
  • Improved insulation
  • Durable commercial heat exchanger
  • 50% more hot water output
  • NSF-certified operation
  • Environmentally safe construction
  • Superior recovery rate



Amtrol CH41Z Indirect Water Heater



The Amtrol BoilerMate CH41Z is an indirect-fired water heater with the energy-efficient operation and high gloss blue finish.

This high-performance model is equipped with a large 41-gallon bimodal tank made from high-density polyethylene—or HDPE—making the base construction exceptionally durable.

As such, there is no need to install anode rods, and the tank resists the effects of hard water, chlorides, and corrosion. In addition to its improved rust protection, the poly tank stores and delivers only clean water, while at the same time supporting high temperatures reaching up to 180ºF.

The tank is placed in the steel stand with low-lead composition and protected by the deep-drawn pressure shell. The shell is made of steel, making it two times stronger than standard designs, and its rugged base is welded with extra precision.

The entire construction is insulated with molded urethane to provide a tighter and safer seal on both the shell and the tank. This way the appliance reduces the heat loss by the hour, dramatically improving the energy efficiency of the entire operation.

For added flexibility of usage, this BoilerMate model uses the existing energy in its boiler system to lower the power consumption while increasing the overall output.

This heater also provides a more accessible maintenance process, as its coil can be removed with ease and cleaned when needed. The heat exchanger’s top mount construction and finned design promotes a more effective transference of temperatures, ensuring faster delivery of hot water.

Top construction also saves you the need to vent or add other utility connections, making the heater more energy-efficient than traditional electric heaters. The double-wall option is also available as long as it complies with your regional code.

With its easy-to-use mechanical aquastat, this water heater can be freely adjusted to suit the needs of your family. The storage buffer can service a large number of people, and the exchanger system acts fast and prepares the required water volume on time.


  • Container size: 41 gallons
  • First-hour rating: 89-170 gallons
  • Working pressure: 150 psi
  • Electric heater output: 100,000 BTU
  • Shipping weight: 124 lbs
  • Finish: High gloss blue
  • Dimensions: 48” H x 22” D


  • Good continuous flow rating
  • Durable tank materials
  • Reduces energy expenses
  • Easy to clean
  • Supports high temperatures




AB IWH401C Indirect Hot Water Heater




The AB IWH401C is an indirect water heater with 38.6 gallons of net volume manufactured by Alfa Heating Supply and designed for cold or freezing winter conditions.

This water heater offers an impressive hourly output of 2.5 kilowatts or 8530 BTU, making it highly effective with delivering large volumes of water in one go. Able to heat to 40 gallons of water, the single-coil of this tank suits closed-loop residential systems located in harsher conditions.

Supported by the low-density Incoloy 800 immersion type electric element, the system delivers consistent performance with the average recovery rate of 21 gallons per hour. Both inlet and outlet threads measure at ¾ inches as per the National Standard Taper Pipe chart.

The enamel steel inner tank helps prevent corrosion and buildup that may usually result from contact with hard water. The main construction combines an enameled steel inner tank with a galvanized steel outer tank to enhance the durability of the entire unit.

One of the main advantages of enamel tanks is their superior thermal resistance, making this particular model more durable when applied with high temperatures. Moreover, the enamel coating keeps the interior protected from scaling and corrosion as well as the effects of oxygen that could be trapped in the exchange system.

Located between the outer tank and the inner tank, the insulation layer creates the necessary shield for preserving heat which, in turn, helps reduce heat loss as well as energy consumption. Made from CFC-free polyurethane foam, the insulation encases the inner tank while ensuring the safety of the outer tank even at high temperatures.

For added safety, the heater includes two relief valves: one for pressure relief and one for temperature relief. In the case of system overload, the heater releases excess pressure or temperature through the dedicated valves and down the drainpipe.

Another safety feature comes in the form of the magnesium anode supplied with the heater. The rod is utilized to protect the unit from corrosion and other water-related damages.


  • Container size: 40 gallons
  • Working pressure: 101.5 psi
  • Electric heater output: 8530 BTU
  • Shipping weight: 176 lbs
  • Finish: White
  • Dimensions: 54” H x 21” D


  • Protection from corrosion and scales
  • Enamel steel design
  • Supports high temperatures
  • Resists surface damage



Crown MS2-50 Indirect Water Heater




The Mega-Stor II-50 is an indirect-fired water heater with a vertical construction and adjustable Honeywell aquastat.

This Mega-Stor II model offers 50 gallons of tank volume able to provide hot water for large families at faster rates. Designed for residential use, the heater is more energy-efficient as it uses the power generated by the Velocity boiler to heat the water in the stainless steel tank.

With the help of the smooth stainless steel coil located in the bottom half of the unit, the heat is distributed efficiently throughout the tank with variable recovery rates that surpass standard heaters powered by gas, oil, or electricity.

The standby loss of this model amounts to only 0.9ºF per hour made possible by the heater’s 2-inch thick polystyrene foam insulation. This way you can leave the unit on standby for longer without worrying about the heat levels.

With its 444 stainless steel composition, rugged design of the coated steel jacket, and durable polymer caps, this model suits all your heavy-duty needs as it effectively resists structural damage such as corrosion, surface abrasions, or interior cracks.

The cover can be removed for easier inspection and maintenance, while the adjustable feet accommodate different surfaces and uneven floors.

This model is AHRI certified, ensuring its performance ratings are compliant with the standards of the Air-Conditioning, Heating, and Refrigeration Institute.


  • Container size: 50 gallons
  • First-hour rating: 162-497 GPH
  • Working pressure: 150 psi
  • Electric heater output: 121,000 BTU
  • Shipping weight: 100 lbs
  • Finish: Multicolor
  • Dimensions: 49.75” H x 23.5” D


  • Durable materials
  • High continuous flow rates
  • Heavy-duty application
  • Reduced heat loss



Triangle Tube SMART80 Indirect Water Heater




The Triangle Tube SMART80 is an indirect-fired water heater with large domestic capacity, improved energy efficiency, and one of the lowest pressure drop ratings in the industry.

This capacious heater comes with a tank-in-tank construction meant to store domestic water at high temperatures while protecting the contents from various bacteria growths. The interior tank consists of durable stainless steel that saves you the hassle of installing extra anode rods.

Surrounded by the boiler water stored in the outer tank, the inner tank provides superior heat exchange qualities typical of wrap around exchangers. When the outer tank draws boiler water and circulates it around the inner tank, the process heats the domestic water inside the inner tank in a highly energy-efficient fashion.

Once the exchange is complete, outer tank water is transferred back to the boiler so it can be reheated. Boil the boiler and circulator shut down when the inner tank thermostat reaches the preset mark.

With the approximate total volume of 84 gallons, this heater provides a large domestic capacity of 70 gallons and adds 14 gallons to its heating capacity, maintaining a consistent water supply.

A self-cleaning heat exchanger is essentially the outer stainless steel surface of the inner tank that amounts to 28 square feet. Thanks to its corrosion-resistant composition, the exchanger can self-descale and protect the surface from rust and other water-related damage.

Also, the 2-inch thick layer of polyurethane foam insulation retains desired temperature levels for longer periods when the appliance is on standby, while the continuous hot water flow feature reaches 400 gallons per hour at a temperature of 200ºF.


  • Container size: 70+14 gallons
  • First-hour rating: 460 GPH
  • Working pressure: 150 psi
  • Electric heater output: 300,000 BTU
  • Shipping weight: 271 lbs
  • Finish: Multicolor
  • Dimensions: 61” H x 26” D


  • Efficient tank-in-tank technology
  • Recovery rate is 3 times faster than standard heaters
  • Low-temperature loss on standby
  • Durable tank materials
  • Self-cleaning heat exchanger surface
  • Continuous hot water flow



In Conclusion

What are indirect water heaters?

Indirect water heaters use the boiler or main furnace to heat the water that is then circulated through the interior heat exchanger and stored in the inner tank. This type of heater draws the energy retained in the tank to minimize the usage of the boiler which can considerably reduce the overall energy intake.

Are indirect water heaters more efficient?

Even though indirect water heaters can be rather bulky contraptions on the account of storage tank required for their full operation, most models of this type are more efficient in their performance when it comes to residential heating. Compared to standard heaters, indirect-fired units conserve more energy and retain heat for longer periods.

Are indirect water heaters worth it?

Households that service a large number of people would generally see considerable benefits with the use of an indirect water heater. This type of heater comes with a dedicated boiler as well as better insulation for the inner tank which reduces temperature loss and therefore requires less energy on energy support. This means less re-heating and more temperature consistency, especially when the high-efficiency boiler is working in the cold start mode.

Why are indirect water heaters so expensive?

Most indirect water heaters are equipped with expensive materials such as durable stainless steel tanks and cupronickel heat exchanger coils designed to last for 20-30 years. Inner insulation also includes high-quality materials and a more flexible design compared to heaters with glass-lined tanks.

What does an indirect water heater do?

Indirect water heaters differ from direct water heaters, in that they heat water without using direct electricity, but instead employ heat transfer. This is accomplished by a heat exchange system that consists of a tank, plumbing connections in the flange, and a ring gasket design that has an in-built seal. At its heart, an indirect water heater incorporates sustainable design, with its innate capability to reduce carbon footprint, energy usage, and eventually increase ROI.

How does an indirect water heater work?

The system you may find in most indirect water heaters doesn’t include the usual electric heating or gas heating elements. Instead, their heat exchange system uses a fluid that absorbs heat from one area and releases it in another.

In an indirect water heater, the coil located in the tank wraps around it to provide enough surface area to efficiently radiate heat throughout the entire water tank. This exchange of heat from the coils to the tank leaves the coils cooled. By this time, the fluid is already traveling back up, out of the tank, and towards the heat source. The loop then continues.

When it comes to the heat source itself, modern indirect heaters use a furnace or a boiler. Older furnaces usually cannot be linked up to an indirect water heater because of incompatible HVAC integration. However, newer furnaces can fit without a hitch.

Boilers are generally a better choice as they have a higher efficiency rate with regards to heating via steam. As for power, the boiler can use gas heating and solar heating to power the main process. The heater itself does not generate heat, and so it does not require any venting system in its design.

Why use an indirect water heater?

Indirect water heaters can save money in the long run (as mentioned before, the ROI takes shorter to obtain). This is largely because the indirect heater will draw heat from the boiler throughout the year, while the boiler itself will only function when necessary.

This is what allows for continued energy savings regardless of the season. Indirect water heaters also consist of fewer parts, resulting in less maintenance.

Indirect water heaters use readily-available heating systems which presents an additional advantage: no need for extra installation for a heating system means it won’t take up extra energy, maintenance or cost.

This especially applies to gas-reliant boilers, but the one exception is electrical boilers. Statistically, efficiency can reach up to 96%, though the average range is between 60% and 70%.

An inevitable aspect of efficiency is heat loss, and indirect water heaters experience decreased heat loss since they are better insulated.

Another aspect of efficiency is performance. Since indirect water heaters are larger and usually connected to high-performance boilers, the performance of an indirect water heater can be between 200% to 300% of a standard water heater.

Finally, the size of the indirect water heater referred to previously, can be used to store water. Storage can be anywhere from 20 to 300 gallons.

How long do indirect water heaters last?

The average indirect water heater comes with a lifetime warranty. This is because their tanks are usually made from stainless steel, which doesn’t react with chemicals and erodes. Moreover, the tanks of indirect water heaters are not put through the circumstances and conditions that direct water heaters are put through.

How much does an indirect water heater cost?

Including purchase and installation (specifically labor and transport), an indirect water heater may cost between $800 and $1,500. Since water heaters do come in different specifications—tankless, with tanks, and so forth—the price can vary significantly. A tankless heater maybe twice as expensive, going so far as $3,000, with the cheapest model priced around $1,000.

Do indirect water heaters save money?

Indirect water heaters tend to have a shorter return on investment than direct water heater systems largely due to the higher energy savings derived from the boiler’s running time and the usage of easily accessible heating systems (as opposed to direct water heaters’ need for dedicated heating system installation).

This combination positively impacts energy consumption, makes maintenance easier and reduces overall cost. It also boosts efficiency by about 60-96% which means there is less input of energy and therefore less strain put on financial resources to obtain wanted heating results.

The efficiency coefficient relies on performance and heat loss, and the performance itself can be twice or thrice the standard direct water heater’s performance, while heat loss is considerably reduced. All these factors work together to allow indirect water heaters to save money.

What size indirect water heater do I need?

An indirect water heater with storage of 30 gallons would sufficiently provide hot water for approximately a dozen people since a boiler consumes a high amount of BTUs with consistently held high temperatures that can keep up with demand without major issues occurring. This considered a smaller storage indirect water heater can handle single-person usage.

What size of indirect water heater suits a family of 4?

Medium to large households can be sufficiently serviced by a 65-75 gallon indirect water heaters with a good first-hour rating requirement.

How to install an indirect water heater?

The process of installation for the indirect water heater is not much different from the direct water heater.

That said, before the installation of the heater itself, it is required to arrange the furnace or boiler placement. The boiler must be set up in a separate, previously designated zone that will connect it to the heat source. There is an exchange pipe that contains non-potable water, in case there is no other exchange fluid that can absorb and release heat.

This exchange pipe is what coils inside the tank and radiates heat throughout the tank. This non-potable water pipe is separate from the tank’s actual water supply that is to be used and heated.

The installation of the heater begins with the power source. Firstly, make sure the power source is disconnected, and that the installation of the water heater itself must be close to the power source, that is to say within visual access. If it is not within access, then a disconnect switch or other alternative means must be implemented while installing, to ensure the safety of the process.

Secondly, electrical wires must be protected with appropriate conduits. However, in the case that the water heater is not installed near the power source, or if perhaps it’s in another compartment, then conduits are not necessary.

Thirdly, access should be ensured for the water heater. Although indirect water heaters have a lifelong warranty, there is the off chance that maintenance may be needed. For these cases, access should be physically and visually clear to enable ease of maintenance. Safety standards in your area must be followed and met in creating this access.

For example, if the heater is placed in the attic, there is a required width and length of the room in which the heater is located in, as well as the platform it should be installed on.

Discharge piping location is also an issue to consider. If not done well, it can cause leaking and other related problems. Discharge piping should face downwards, so that heated water that may be up to scalding temperatures can safely run away from contact with those who may be handling the piping.

The piping cannot be PVC as the diameter has to be larger than PVC pipes allow. Traps cannot be built into the pipes either.

Finally, it is also necessary to check supply power with a circuit meter, making sure there is no excess of power that is fed into the heater.

How to wire an indirect water heater?

Writing is a vital step in setting up an indirect water heater. First, the main power switch must be turned off before commencing the installation and wiring.

  • The main water supply should be switched off so water does not enter the boiler while the installation is ongoing.
  • Any remaining water should be drained out through the draw-off valve present in the boiler. Preferably, it should be drained towards the outdoors through a hose, or into a utility sink.
  • The level of the new water heater should be stably set, using shims.
  • The boiler tap should connect to the indirect water heater through the coil where the heat exchange takes place. It should be positioned close to the waterline, preferably under it, to stave away the possibility for impurities to gather and solidify at the bottom of the boiler.
  • A 1-inch copper pipe connection should run from the boiler to the beginning of the heat exchanger. There should be a ball valve at the tap point for shutting off.
  • Below the ball valve should be a filter and a point of flushing out unwanted remnants of sludge or filtration. This can be met by placing a Y-strainer. All connections should be level.
  • The copper pipes should be sanded and fluxed on all connections before soldered with a propane torch.
  • Following this, the connection with the draw-off valve should be broken.
  • Now, the output for the coil to the input of the boiler should be formed. The second 1-inch pipe should be run as well. It should consist of a circulator pump, a valve for purging unnecessary remnants and impurities, a ball valve, and another purge valve. These connections should also be soldered with a propane torch.
  • The main water supply connection to the boiler should be broken with a pipe cutter. Excess water should be caught immediately. This is commonly achieved with a bucket.
  • A new copper pipe should be installed with connections soldered. This time with a ½-inch diameter. This pipe requires adapter fittings, dope and plumber’s tape for connections. The pipe should have a tee-connection as well, linking to the new pipe previously mentioned in the boiler.
  • There should be a range of valves connected to the pipe specific to each model. This should be in the installation manual of the product. The pipe should then be connected to both the cold input of the indirect water heater, as well as the thermostatic valve that helps achieve wanted temperatures by mixing.
  • Another ½-inch pipe needs to be dry fit from the output of the indirect water heater to a relief valve and the aforementioned thermostatic valve, as well as the main hot water supply that supplies the home.
  • Next, a temperature sensor should be attached to tell the temperature of the heater. Along with this, an electronic relay should be fed to the boiler. The wirings between the temperature sensor, relay on a boiler, and pump should all be connected to each other with care.
  • In the final step, the main power source should be turned back on again, then the water supply should be activated. This ought to lead to the purge valves pushing out excess air that might have gotten trapped in the system. When the air exits the system, some dripping might occur which should be removed before the unit starts running properly.

Do indirect water heaters have anode rods?

Some models do indeed come with anode rods, but many modern models bypass this feature. Both the presence and the quantity of anode rods depends on the manufacturer as well as the make and model of the heater. For example, some models have only one anode rod, while others have two, all of which can be positioned to the sides or in the center of the tank.

How to clean an indirect water heater?

While any cleaning agent deemed safe for cleaning facilities that deal with potable water can be used to clean an indirect water heater, it must be noted that these products have a certain level of acidity that will eventually wear away at the foulant and at the metal itself.

This implies that measures must be taken to neutralize the pH of the cleaning substance, perhaps by implementing a routine where the cleaner is followed with a more alkaline substance to balance out the pH for water and tank safety.

Additionally, in the case of hard water which tends to contain increased rates of calcium that reduces water output for users, the coil should be pulled to check for any impurities, after which a filter should be installed in the system to prevent hardening from happening again.

There is also a chemical-free way to clean the indirect water heater. It involves draining the tank, and once the tank is empty, raising the temperature to around 400o F. The hot coil will then be introduced to cold input water, shocking the coil and causing the calcium to fall off in flakes.

How to replace an indirect water heater?

The replacement of an indirect water heater requires serious consideration: first, the replacement needs to be of the same make, model, and size. Then there is the matter of permit fees, tanks for expansion, and the breaking and repairing of carpentry to be able to first access the tank, then seal it off visually. The carpentry will most likely consist of framing a new wall, breaking and cleaning off space, or installing drywall.

Additionally, there may need to add lines and pipes for access to electricity or other means of fuel, from the current system to the indirect water heater. A water line installation (if not previously installed, or not sufficient for the replacement) will need to be looked at.

Indirect water heater vs on-demand water heater

Indirect water heaters are powered by a furnace or boiler to generate hot water and as such, they require water storage.

On-demand water heaters, often called tankless water heaters, provide hot water on the spot as they generate it only when the hot water faucet is opened. The water then runs through the heat exchanger in the furnace or boiler without being stored in a tank.

Both heater specifications come with their considerable upsides that are best utilized in certain settings and weather conditions.

Indirect water heater for radiant heat

When applied for generating radiant floor heat, indirect water heaters can often be more cost-effective than regular boilers. Radiant floor heat is maintained through the application of warm water supplied for the underfloor piping system. Since boilers are designed to create extremely hot water for traditional radiators, many boiler models fail to operate efficiently when running on lower temperatures. This can result in expensive hardware upgrades to avoid possible damage to the system.

Indirect water heaters, on the other hand, are designed for an efficient run at lower temperatures. In contrast to boilers that are engineered to boil, water heaters are designed to support the process of flue gas condensation. The heat exchanger is an indirect water heater cools the flue gas below its water dew point and releases the heat through the subsequent water condensation which it recovers as low-temperature heat.

Leading industry brands recommend using only high-efficiency water heaters for radiant heat. A high-efficiency indirect water heater manufactured with durable materials is both energy-efficient and more environmentally friendly, helping you reduce utility expenses as well as carbon footprint.

About the author

Lou Anderson

Lou Anderson

Lou Anderson is a keen home and garden innovator with over 25 years of hands-on experience in, home improvement, landscape gardening and all things appliance related to making life easier and more interesting.

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