Indirect-fired storage water heaters

In the mid-1970s, a new type of water heater, an indirect-fired storage water heater, was developed. The unit is called indirect-fired because the medium used for heating the domestic water is boiler water, which itself is heated in a separate boiler used for either central heating or space heating. The boiler water circulates through a heat exchanger that is located either within the heater tank or in an external compartment. Figure 16-10 shows one type of indirect-fired storage water heater. This unit has a specially designed heat-exchanger coil mounted in the base of the tank. Hot boiler water flows through the coil, which is surrounded by domestic water. The design of the heat exchanger, which consists of an inner core surrounded by an outer helical coil, is such that it results in a relatively high recovery rate.

The typical capacity of this residential indirect-fired water heater is about 40 gallons. With water heaters, whether there is an adequate supply will depend not only on the capacity but also on the recovery rate of the unit. The actual rate will depend on the Btu rating of the boiler that feeds the heat exchanger. For a typical house with a boiler that has an output rating of about 100,000 Btu per hour, the recovery rate is about 90 gallons per hour.

The indirect-fired storage water heater works off the heating system as a separate zone, utilizing its own circulator pump.

Real Estate Home Inspection photographs of house defects

According to manufacturers, 15–20 gallons of domestic hot water is removed before the zone aquastat calls for hot water from the boiler. Consequently, with the high recovery rate, the boiler firings are kept to a minimum.

In checking the unit, either raise the thermostat setting to HOT or let the hot water run until the boiler fires. You can tell if the pump is operating by touching the water supply to the heater before and after the boiler fires. The pipe should get hot. Look for signs of leakage around the base of the unit. Also, make sure that there is a pressure-temperature relief valve mounted directly on the tank casing. If you adjust the thermostat to activate the boiler and circulator pump, don’t forget to reset it.

In hard-water areas, it is possible that even though the boiler is functioning properly, the domestic water does not get very hot. This is caused by a lime buildup on the heat exchanger, which acts as an insulator and inhibits the heat transfer. In this case, the coil is in need of a cleaning, although it may require replacement.

Checkpoint summary

  • Is hot water supplied by a separate-tank type of water heater or by a tankless coil?
  • If you have a tank-type heater, record the capacity (gallons), the recovery rate (gallons per hour), and the age (from serial number).
  • Are exhaust gases spilling out of the draft diverter?
  • Is tank-type heater adequate for house? (See table.)
  • Is hot-water flow adequate when two fixtures are turned on?
  • Are hot and cold supply lines incorrectly installed (reversed)?
  • Does water heater contain a properly installed temperature-pressure relief valve?
  • Is the relief valve dripping?
    • Look for signs of corrosion or past leakage:
      • Tank type (gas-fired unit): corrosion dust and flaking metal in burner area.
      • Tankless coil: rust and deposits around fittings and gasket.
  • Is there an “instant” hot-water return line?
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