Indirect Hot Water Heater

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Explore the indirect hot water heaters for domestic hot water heating; explore the types, how it compares to other systems, see the advantages and what benefits can bring to your home.

Known as the indirect-fired system, the indirect system uses the furnace or boiler-based space heating for domestic hot water heating inside the insulated storage tank, while fluids are kept separate.

Thanks to the insulated storage tank, energy inside is kept longer while the furnace or boiler operates less often.

The indirect system permits hot water production year-round in great quantities and at a reasonable cost. The indirect system is tied into your home's space heating system and it will heat the water even if there is no need for space heating.

Continuous use of the boiler for heating domestic hot water is very good for the system as it prevents costly deterioration which occurs when the boiler is idle for long periods.

During the winter, when the indirect water heater is used for the home heating, only small fraction of the heat is used for domestic water heating. In summer all of the heat is used for hot water preparation.

Indirect hot water heater systems can heat water by using natural gas, propane, electricity, oil or solar energy. Heat pumps, gas or oil fired burners are used as the source of the heat.


The most typical indirect hot water heater system is a coil inside the tank as it provides a large surface area with an excellent recovery rates.

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Two types or copper tube coiled systems are used: internal and external submerged. Internal submerged is screwed into or bolted onto the boiler.

The external submerged type is consisted of the copper tubes encased in a steel or cast iron casing. An external heat exchanger requires two pumps for water and heat transfer.

Indirect fired heaters can be designed, constructed and purchased as the integrated systems, which incorporates the boiler or furnace as the heat source, storage tank and heater controls into one; or as the separate components.

The boiler water circulates through the heat exchanger by using the pump when there is a call from thermostat. The heat exchanger coils located inside the storage tank heat the surrounding water without mixing the boiler water with the potable water supply. Another way to heat is to use a combination of the insulated storage tank and the external heat exchanger.

The boiler should be configured to shut down, or keep the heat on a lower level when there is no call for space or hot water heating.

All the piping between boiler, heat exchanger and heater should be insulated to minimize the heat loss.

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Furnace based indirect hot water heater system is less common, and water in a heat exchanger coil circulates through the furnace to be heated, then through the storage tank.

Indirect-fired systems are also called combined systems whose heat source is used for both water heating and space heating. Indirect heating system efficiency is indicated by the combined appliance efficiency rating or CAE. Higher efficiency comes with the higher CAE rating, and can be founded from 0.59 to 0.90. Recommendation is to look for systems that can produce 0.85 or higher.

What is great with this indirect type of heating is when combined with high efficient boilers it provides one of the least expensive methods of heating. These indirect systems are more efficient than if used separately, as they eliminate the extra standby loss of another tank or unit.

The price for these combined and integrated heating systems cost more than if you buy furnace and storage tank separate. On the other side installation and maintenance cost less as the system is very simple. One of the important parts for heat transfer is the pump that circulates hot water between the heat exchanger and the tank.

Another type of the indirect fired heaters is a tank-within-a-tank design, like Weil Mclain indirect water heaters, which has a corrugated stainless steel inner tank and steel outer tank. This design produces twice the peak flow of a tankless coil, at least 50% more than a comparably sized direct heater and three times as much as an electric unit.


  • The greatest advantages of the gas fired indirect water heaters is the significant reduction of flue losses.

  • These units are more efficient, with less standby heat loss, when compare to the standard tank type.

  • Less maintenance - no additional burners to maintain.

  • Less cost - no additional venting required or fuel lines to run.

  • Better performance - more available hot water with quicker recoveries.

  • Easy to size and easy to install.

Indirect hot water heaters generally have higher recovery rates than typical tank type heater as they use the BTU output from the boiler as the heat source.

On the systems with no coils, Weil Mclain for example, large self cleaning waterways are used, which are preventing liming and efficiency deterioration.

As said before, integrated combined heating systems are using the same heat source for both space and water heating. One of the advantages of the integral systems over separate unit systems is the presence of only one combustion device that reduces the backdrafting risk of one appliance, reduced regular service needs and one venting system used.

Storage tanks used in indirect fired heating systems are usually build of the stainless steel, and with the absence of the direct flame or combustion, thermal stress is significantly reduced, making them very durable with a long life span. Some manufacturers are providing a lifetime warranty.

The most typical capacity of the indirect hot water heater tanks is about 40 gallons. Capacity is an important element when selecting to buy a new indirect water heating system, but also the recovery rate. The actual recovery rate will depend on the Btu input of the unit that feeds the heat exchanger.

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