Gas water heaters

One big difference between gas water heaters is how they vent. A naturally drafted water heater has a flanged hood at the base of the flue pipe, just above the tank. Older naturally drafted gas water heaters are less efficient and can backdraft or spill combustion gases. Naturally drafted water heaters are still sold, but safer and more efficient ones are available: power-vented models use a fan to push combustion gases out the flue, and direct-vent models use a sealed intake for burning air and a sealed flue for exhaust. With a sealed combustion chamber and a separate air intake, it is almost impossible for these to backdraft harmful flue gases into the house.

Tankless models are another option. A gas tankless water heater has a high-powered burner, a fan and a heat exchanger; condensing models also have a second heat exchanger and a piped air intake to a sealed combustion chamber. Department of Energy guidance says sealed-combustion, direct-vent or power-vented tankless equipment must be used in cold climates. Vent condensation must be controlled as the maker's instructions say, to avoid damaging the water heater.

Source: U.S. Department of Energy

A water heater that takes its air from the room depends on the room. Sealed-combustion appliances draw their burning air from outside, but atmospheric units draw it from the space around them; when there is not enough air, burning is inefficient and dangerous. For atmospheric units, Department of Energy guidance says the space must hold 50 cubic feet for every 1,000 Btu per hour of combined input of all the burning appliances in it. In a confined space without enough air, the area must be vented to outdoors or opened to a larger indoor space.

Exhaust from elsewhere in the house matters too. Illinois' mechanical code says that where a mechanical exhaust removes more air from a room than is supplied, adequate makeup air must be provided. The code also bans manual dampers in combustion air ducts. A technician can tell you whether your water heater has enough air.

Source: U.S. Department of Energy · UpCodes (code text)

Backdrafting is when the gases from burning do not go up the flue pipe and outside, but come out of the hood into the indoor air. Naturally drafted water heaters are prone to it when exhaust fans and clothes dryers pull air out of the home and depressurize it. Very powerful kitchen hood fans, rated as high as 1200 cfm, can easily backdraft a water heater that is not sealed combustion. A backdrafting gas appliance can let carbon monoxide into the home, which can be dangerous or deadly.

Some signs can be seen from the outside of the heater. Melted plastic washers on the pipes and soot on top of the tank are signs that a water heater has backdrafted in the past. A flue pipe that slopes down instead of up makes flue gases likely to back up and spill into the house. A disconnected flue pipe lets carbon monoxide escape into the home, and a plumber should repair it as soon as possible. For ongoing warning, a plug-in combination natural gas and carbon monoxide alarm near the gas water heater can give early notice of a gas leak or carbon monoxide. The safety steps are at the end of this page.

Source: U.S. Department of Energy

Here is what the code says about how a water heater vents. Illinois' mechanical code says every fuel-burning appliance must send its exhaust to a vent or chimney designed for that type of appliance. It also says a naturally drafted appliance may not be connected into a vent or chimney on the outlet side of a mechanical flue exhauster. If a new sealed-combustion direct-vent water heater leaves an unused opening in the chimney wall, that hole must be sealed. The venting and chimneys page explains what can happen when a water heater is left on its own in a shared chimney.

Source: UpCodes (code text) · U.S. Department of Energy

A water heater needs the same yearly care as your other gas appliances. The CDC says to have your heating system, water heater and any other gas burning appliances serviced by a qualified technician every year. A drop in your hot water supply is one of the signs the CPSC lists of an appliance not working properly. Illinois' mechanical code requires water heaters to be listed, labeled and installed as the maker's instructions, the International Plumbing Code and the mechanical code say. The page on which trade does what explains who works on water heaters.

Source: CDC · U.S. Consumer Product Safety Commission · UpCodes (code text)