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Field Test Best Practices: A Resource for Practical Residential Building Science

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Open Combustion Gas Water Heater

The most common type of gas water heater is a tank-type open combustion gas water heater. In this type of water heater, the gas burner is located at the bottom of the storage tank and is used to heat the volume of water. The flue for the exhaust gases passes vertically through the center of tank, which allows some heat from the flue gases to be transferred to the water. During combustion, air is allowed to freely enter the burner area. Upon exiting the flue that runs up the center of the tank, a vent hood allows air to join the exhaust gases to help the natural ventilation process. Since the combustion process is unconstrained, the optimum ratio of fuel to air cannot be maintained. Also, since air must enter the water heater near the burner and at the vent hood, those areas cannot be insulated, making the flue a source for additional source for heat losses.

A diagram of an open combustion gas water heater is shown. The gas burner is located at the bottom of the tank and the exhaust flue is located in the center of the tank. A vent pipe takes the exhaust outside.

Schematic of a tank type open
combustion gas water heater
Credit: energysavers.gov

Recommended instrumentation for a open combustion gas water heater field test includes: