Indirect fired hot water heater schematicGuide to Indirect-Fired Hot Water Heaters
Indirect calorifier, geyser, hot water cylinder, hot water tank

  • INDIRECT FIRED WATER HEATERS - CONTENTS: Description of Indirect-Fired or "remote" Hot Water Heaters - water tanks heated by a connection to a heating boiler or other heat source.How to obtain more hot water quantity, pressure, flow, at less cost. Options for replacement of tankless coils as domestic hot water source by installing an indirect-fired water heater
  • POST a QUESTION or READ FAQs about indirect-fired hot water heaters

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Indirect water heater properties:

Here we define and explain indirect-fired hot water heaters, describing the characteristics & advantages of this method for making domestic hot water where a gas or oil fired heating boiler is installed.

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How & Why to Install an Indirect-fired Water Heater - get more hot water

Indirect fired hot water heater schematicThanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.

Below we describe some alternative ways to make hot water by using an indirect water heater or on older systems a range boiler water heater, either to replace or to supplement an existing hot water supply system.

After knowing what the hot water problem really is, there are steps we can take to get more hot water or to increase hot water pressure. Here we explain how indirect water heaters and range boilers work.

[Click to enlarge any image]

Article Series Contents

Definitions of Indirect Water Heaters

As we explain at TANKLESS COILS, a conventional tankless coil system imbeds a finned copper tube coil inside of the hot heating boiler water, running domestic cold water through the tankless coil where physically separated boiler water heats the coil and thus heats water passing through the coil.

An indirect-fired water heater also uses a finned-copper tube coil in a different approach to providing more domestic hot water.

In our photo at left, the white Super Stor™ tank is an indirect-fired water heater. Water in this tank is heated by a plumbing loop circulating hot boiler water through a heating coil in the bottom of the Super Stor™ water heater tank.

Indirect fired water heaters: In some other buildings domestic hot water is produced by cycling hot water from a hydronic or steam boiler through a loop inside of a steel tank. The water in the tank is heated by the water in the coil.

Modern systems using this approach use the term indirect-fired water heater and such systems are sold by various companies such as the SuperStorTM unit shown here as the white tank to the right of the heating boiler.

At above-left you may also notice the WellXTrolTM blue water tank off to the left of the boiler as well as in the left foreground covered partly by a cardboard box we have a white-yellow fiberglass close to the thin blue vertical water softener - water treatment equipment in the same photo. And just to be complete, the little tank over the boiler next to our service technician is the expansion tank for this heating boiler.

Plumbing Connections for an Indirect Fired Water Heater

Indirect water heater hot water coil (C) Carson Dunlop AssociatesIndirect fired water heater tanks for domestic hot water, such as the SuperStorTM are usually located close to the heating boiler and will have both cold and hot water lines leaving the tank to supply the building with domestic hot water and a loop of piping that runs between the [usually the] bottom of the tank and a nearby heating boiler.

Follow the pipes to see which pipes are performing which function.

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At left we can see the hot water heating coil found inside the indirect fired water heater. The coil connects to and from the heating boiler while the building's potable water supply enters (cold) and exits (hot) from the indirect water heater's tank.

At the heating boiler or between the heating boiler and the indirect water heater tank, a separate circulator pump is used to circulate boiler water from the heating boiler through the heat exchanger inside the water heater tank.

The water in an indirect fired water heater such as the SuperStorTM unit is heated by a finned copper coil located inside the hot water tank. The internal coil is in turn heated by circulating water inside the coil to and from the heating boiler.

Difference Between an Indirect Water Heater & a Range Boiler Water Heater

Indirect fired water heater plumbing hookup (C) Daniel FriedmanShown in our photographs here and below are two indirect water heaters.

Similar piping and tanks are used to construct a range boiler water heater.

The difference between RANGE BOILER WATER HEATER and Indirect-fired Water Heaters is in the details. The heat exchanger that heats water in the range boiler is in or at the heating boiler rather than inside the hot water tank itself. And typically a range boiler heats water in the hot water tank by circulating hot boiler water through the heat exchanger by convection, not by use of a mechanically-driven circulator pump. .

The range boiler is an old concept in use for about 100 years. Indirect fired water heaters are a modern system and are in current sales and use.

Where an indirect-fired water heater is installed, the coil is inserted not in the heating boiler, but rather in the bottom of a hot water tank. You can see this design in our photo at left (the blue tank is the indirect fired water heater) and also in our sketch at page top, as well as the indirect water heater skecth at above left, both provided courtesy of Carson Dunlop Associates.

Water from the heating boiler (not domestic cold water) is circulated from the boiler, through the coil, and back to the heating boiler. The coil, in turn, heats water stored in the separate hot water tank.

This installation is a bit fancy: the blue indirect fired water heater is heated by the gray Weil McLain™ oil fired heating boiler.

The blue water heater tank is in turn being used in this case to provide a big reservoir of hot water to a water-to-air heating coil installed in the light gray/tan air handler at far left in the photo. This home uses that air handler for both central air conditioning and also for heat.

Alliance indirect water heater from Burnham (C) Daniel FriedmanOther pipes connected to the Weil McLain boiler may indicate that that boiler also provides hot water heat to some building areas.

But the principle is the same. Hot water from the blue indirect-fired water heater tank could have just as well been used to provide domestic hot water for washing and bathing as well as for heating the water-to-air heating system, as well as serving both purposes.

Plumbing Connections & Water Flow Directions at an Indirect Water Heater

At left is an Alliance indirect water heater from Burnham (also discussed below).

The indirect water heater will have multiple plumbing connections and piping loops but it's pretty easy to figure them out.

Domestic hot water supply piping: will include a cold-water supply line into the indirect water heater tank (dark blue arrow at top center of the photo and pointing down) and a hot water supply line exiting the indirect water heater tank to supply hot water to the building's plumbing fixtures (orange arrow pointing up at the top center of the photo).

Indirect water heater's hot water heating coil piping: will include a loop of pipes that bring hot water from the separate hydronic heating boiler (barely visible in the left of this photo) to the inlet side of the heating coil in the indirect water heater (bright red arrows), and that carries water back to the heating boiler to be re-heated (light blue arrows).

It is this second piping loop that heats the hot water coil inside the indirect water heater, essentially transferring heat from the hydronic heating boiler into the hot water storage tank.

Usually this boiler water piping loop will include its own circulator pump that in turn is turned on and off by a thermostat mounted on the indirect water heater.

Controls on Indirect Fired Water Heaters

Extrol water heater, indirect fired (C) Daniel Friedman Indirect water heater check valves on the boiler loop (C) Daniel Friedman

An indirect fired water heater such as the Super Stor™ or Extrol™ water heater (shown at above left) unit will include a temperature control that is usually mounted on the water tank itself. When the hot water temperature drops below the control set point, the control will turn on a circulator pump that will cycle hot boiler water through the heating coil inside of the Super Stor™ tank. The indirect-fired water heater does not itself turn on the heating boiler.

Rather, when temperatures inside the separate heating boiler drop below a low limit (or hot water maintenance level) set on the heating boiler's own control, that system will turn on its burner to re-heat the boiler water directly.

At above right we see check valves on the inlet and return sides of the boiler hot water loop supplying the hot water heating coil on an indirect water heater.

The temperature in the indirect water heater is controlled by a thermostat mounted on the water heater tank. That thermostat turns on or off the circulator pump. Separate, independent temperature controls back in the heating boiler actually turn the heating boiler's burner on or off as needed.

Limit switch on indirect water heater mounted on steam boiler (C) Daniel Friedman Hot water control aquastat on the indirect fired water heater (C) DanieL Friedman

At above left we see a limit switch and a circulator pump mounted on the hot water outlet from a steam boiler whose hot water is being used to heat the hot water heating coil in an indirect fired hot water tank (not shown in this photo).

The circulator is used to circulate hot water to and from the indirect fired water heater but the control at above left is an aquastat and is not the control that maintains hot water in the separate indirect hot water tank. Rather, this control turns the boiler's burner on if needed to re-heat the water in the boiler to assure that it can in turn re-heat the indirect water heater.

At above right, and mounted right on the indirect fired water heater tank we see the aquastat that controls the temperature in the hot water tank itself. This control will switch the circulator (above left) on or off as needed to use boiler water to re-heat water in the indirect hot water tank.

The indirect water heater's aquastat is typically set at 120°F.

See CIRCULATOR PUMPS & RELAYS - home and also See RELAY CONTROL SWITCHES for details about how the circulator pump relay operates.

Watch out: the indirect water heater must have its own temperature & pressure relief valve. You can see this TPR in the close-up photo earlier on this page where we also show the check valves on the boiler piping loop.

Efficiency of Indirect Fired Water Heaters

An indirect-fired water heater is a more efficient way to use a home heating boiler to make domestic hot water. Why?

Because the heating boiler will run less often, and when it does run, it will have a longer "on" cycle (as it has to heat up the tank of hot water). Both of these conditions are a more efficient way to burn home heating oil than using a tankless coil.

In sum, during the heating season, when a heating boiler is likely to be "hot" most of the time anyway because it is heating the home, a tankless coil installed on a heating boiler is a reasonable way to obtain some domestic hot water from the system, though in most installations a tankless coil will not be able to produce as much hot water for as long a time period as what we can obtain from a separate water heater.

But an indirect-fired water heater always makes more efficient use of the home heating boiler, year round. And since the consumer can decide how big a hot water tank she wants, 50 or even 100 gallons of hot water can be kept in reserve.

More than we can normally obtain from a conventional domestic tankless coil on a heating boiler.

Indirect Water Heater Leaks

Leaks at indirect fired water heater and conventional water heater (C) Daniel Friedman Burnham Alliance indirect hot water heater (C) Daniel Friedman

At above left we see water on the floor at two water heaters: an oil fired water heater (white, at left in the photo) and an indirect water heater (blue) at right in the photo. We need a closer inspection to find where this water is actually coming-from as there could be several sources including even the separate hydronic heating boiler (just visible in the far left of the photo).

At above right we illustrate another indirect-fired water heater brand, an Alliance hot water system from Burnham. Here we see water on the floor not at the indirect water heater itself but around the nearby heating boiler.

Leaks in or at an indirect water heater can occur at a variety of locations and are caused by different problems. Any of these leaks, if found and repaired promptly, can salvage the water heater and keep it operating safely. Leaving any of these leaks unattended risks destroying the heater as it can become too corroded to repair, and some of these leaks are also dangerous.

  1. A leak in the indirect water heater tank itself due to corrosion or damage may show up as a puddle on the floor or as wet insulation that steams when the heater is being heated.
  2. Leaks can occur at connecting piping fittings on or around the indirect water heater or calorifier such as where cold inlet or hot water outlet building water supply piping or at the TPR valve mounting fittings or at the connections of the heating piping loop between the separate heating boiler and the heating coil mounted in the indirect water heater.
  3. A leak at the indirect water heaters temperature/pressure relief valve shows up as dripping or spillage at the valve's discharge tube and is typically caused by excessive pressure, by thermal expansion of hot water, by water hammer, or by other more subtle causes.

    Watch out: leaks at a TPR valve are an unsafe condition, as we explain at RELIEF VALVE LEAKS
  4. A leak out of the indirect water heater into the heating boiler can occur if the heat exchanger coil itself develops a leak.

At INDIRECT WATER HEATER COIL LEAKS INTO the BOILER we discuss the water found around the heating boiler at the left of the potograph above, and we explain why a leak in the indirect water heater's heating coil can cause leaks at the heating boiler's TPR valve. There we also explain how to detect this condition.


Continue reading at TANKLESS WATER HEATER INSTALLATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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