I find a fair amount of water heaters in Minneapolis and Saint Paul that backdraft. Backdrafting happens when the exhaust gases from a fuel-burning appliance, such as a water heater, spill in to the room rather that rising up and out of the house through the vent. This is a potential safety hazard that should always be repaired.
The most common question that I get when I find a backdrafting water heater is "How can this be fixed?"
There are many potentials reasons for a water heater backdrafting, but there are several basic things to check - any one of these conditions could be enough by itself to cause a water heater to backdraft.
A missing rain cap. Not only do rain caps help to prevent rain from dumping in to the vent, but they help to downdrafts; these are wind currents that can blow exhaust gases back down the vent. A listed cap is also required by the Minnesota Fuel Gas Code, section 503.6.6.
A backpitched vent connector. A water heater vent connector, often called a 'flue', is required to pitch upwards towards the chimney or vent at a minimum slope of one quarter inch per foot (MFGC 503.10.8). This is a minimum requirement. Just meeting this minimum requirement doesn't mean everything will work the way it's supposed to. The steeper the pitch, the better the draft.
Insufficient combustion air. I start every home inspection the same way - I close up all of the windows and doors, and I turn on all of the 'motors' at the house that remove air. These fans include bathroom exhaust fans, kitchen exhaust fans, and clothes dryers. Besides giving me the opportunity to make sure the fans are properly working, I'm removing a lot of air from the house at one time, which creates a 'worst-case' scenario for the fuel-burning appliances, such as the water heater. If there isn't a proper combustion air duct installed, the water heater vent may end up acting as an air intake. This will definitely cause backdrafting.
A vent connector located directly across from a fan. Most newer furnaces have an induced draft fan that is designed to pull the exhaust gases out of the heat exchanger. This fan ends up 'pushing' the exhaust gases up the vent, and if the water heater vent connector is located directly across from where the exhaust gases are directed, the exhaust gases from the water heater get pushed right back down. The water heater doesn't stand a chance of drafting properly with this configuration.
The water heater isn't venting in to a chimney liner. When a water heater vent connector enters a chimney separately from the furnace or boiler vent connector, there's a good chance that the vent just pokes in to the chimney and the exhaust gases rise up the annular space between the chimney walls and the metal vent liner. The heat created by the water heater may not be enough to heat up this huge area. The water heater vent needs to connect to the metal liner.
An obstructed vent. This one is a curveball that most home inspectors won't be able to diagnose. While replacing a water heater once, I found a dead bird sitting on top of the draft hood. No wonder why the water heater was backdrafting.
If you have a water heater that backdrafts, have it professionally repaired. Your home inspector may be able to give you several potential reasons for the water heater backdrafting, but an obstructed vent is always a possibility, and without taking stuff apart, there's no way to know for sure.