Skip to content NREL Buildings Research National Renewable Energy Laboratory (NREL)
Field Test Best Practices: A Resource for Practical Residential Building Science

Main menu

Gas Water Heaters

Gas water heaters make up the majority of water heaters in homes in the United States and are generally more efficient in terms of source energy consumption and operating cost. There are number of different types of gas water heaters, all with different functionality and intended uses.

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.

Sealed Combustion Gas Water Heater

Sealed combustion gas water heaters, also known as direct vent gas water heaters, are similar to open combustion gas water heaters, but with restrictions on the air flow. In these water heaters, the inlet air flow to the burner is controlled to ensure the proper fuel-to-air ratio and the exhaust vent is also sealed between the water heater and the outside. This typically means that the system requires a fan or blower to pull the exhaust gases outside. The advantages to a direct vent system are that the combustion process is optimized and is more efficient. The entire system is sealed, so it can also be better insulated, which in turn improves overall system efficiency. There are higher installation costs with this type of water heater and the energy use of the blower must also be factored in.

Condensing Gas Water Heater

Condensing gas water heaters come in two forms: a storage tank type and tankless. The principle behind both is the same: The hot exhaust gas produced during the combustion of natural gas is used to heat the water. This is done by making the exhaust pipe in a shape with more surface area, thus, promoting heat transfer. A diagram of a tank-style condensing gas water heater is shown in the figure below.

Gas Tankless Water Heater

Gas tankless water heaters use natural gas combustion to deliver hot water when it is needed in the house, rather than maintaining a tank of hot water. Tank type water heaters need to supply enough energy to heat the tank of water and to compensate for heat transfer losses through the tank. Tankless water heaters do not have tank losses and so can be more efficient. Gas tankless water heaters have larger gas burners, needed to quickly heat water from the mains water temperature to the desired delivery temperature at the required flow rate. Gas tankless water heaters can sometimes have trouble with low flow rates. Very low flow rates will not trigger the burner to come on. Also, tankless water heater, both gas and electric, can create a "cold water sandwich." After one person in the house is done with their demand for hot water, the tankless water heater will turn off. When the next person asks for hot water, there may still be hot water in the pipes. The tankless water heater cannot immediately start heating water, so there will be a small amount of cold water in the lines, followed by newly heated hot water. This is not a problem in a tank style water heater, since demand for hot water will always be supplied from a preheated tank.

Boilers

The residential boiler is a cast-iron, steel, aluminum, or copper pressure vessel heat exchanger designed to burn natural gas or oil and to transfer the released heat to water (in-water boilers) or water and steam (in-steam boilers). Boilers are most often used to provide heat for space conditioning but combined systems can be used to provide heat for water heating as well.