Water Heater Backdrafting - How to fix and prevent it, Part 1 of 2: Why it matters and what to do about it.
Potentially hazardous exhaust gases are coming back into the home if a water heater backdrafts.The easy part is to determine if a water heater is backdrafting.The difficult part is figuring out why it's happening and fixing it.I'll cover as many possible causes of backdraft as I can think of.It's often a combination of conditions.
The exhaust gases come from the top of the water heater to the vent.The potential for the water heater to backdraft increases if a vent is not properly installed.
Improper pitch is one of the most common defects with a vent connector, and it should be avoided if you can.
Backdrafting can be caused by insufficient rise in the vent connector.If there is a quick 90 degree turn right at the top of the draft hood and the vent connector only has the minimum pitch, there's a good chance that it won't draft properly.There needs to be more of a rise in the vent.
Tall and short water heaters are the most atmospherically vented.Short water heaters are almost a foot shorter than tall ones.Replacing a tall water heater with a short will increase the potential for the water to draft properly.
The greater the potential that it won't draft properly, the longer the vent connector runs horizontally.Section 503.10.9 of the Minnesota Fuel Gas Code states that a vent connector should be as short as practical and the appliance located as close to the chimney as possible.The water heater is installed far away from the chimney opening than it should be.The water heater could be moved closer to the chimney.
One thing that can be done to help with a long horizontal run is to replace a single-wall vent connector.This will help the vent to heat up faster.
The draft fan is designed to pull exhaust gases out of the furnace's heat exchanger.The fan pushes the exhaust gases up the vent, and if the water heating vent is located directly across from where the gases are directed, they will get pushed back down.This configuration will not work for most water heaters.
One way to fix this is to re-arrange the common vent so the water heater vent is not visible from the furnace.
An old 80% efficient furnace is replaced with a new, high efficiency furnace that will vent through the side of the house, leaving the water heater vent an orphan.This illustration was provided by the Minnesota Department of Commerce, Division of Energy Resources.
The small burner on the water heater may not generate enough heat to warm up the chimney or vent, which can lead to backdrafting.A new vent liner is usually installed to allow the water heater to vent into a smaller space.
Adding a power vent to the water heater is mentioned in the image above.The powervent conversion kit and chimney stack assist kit are made by the White Bear Lake man.
Backdrafting can be caused by an obstruction of a chimney or vent.I once found a dead bird blocking the draft hood of a water heating system.A listed cap should be installed at the vent terminal to help prevent that from happening.
The Minnesota Fuel Gas Code requires a cap to prevent debris from falling into the vent, keep rain out, and prevent downdrafts caused by strong outdoor winds.
There needs to be plenty of air for a water heater to draft.All it takes is a single bathroom exhaust fan to pull enough air out of the house to make the water heater backdraft.Throw in a big kitchen hood fan and it's almost a guarantee that the water heater will backdraft, even if it drafts very well.
A difficult condition to diagnose is insufficient combustion / makeup air.They head down to the basement to diagnose the problem after being called out to fix a backdrafting water heater.There are no signs of previous backdrafting, the vent is all installed to code, and there is a nice rise before the chimney.They have the water heating up and it drafts perfectly for them, so they say there are no problems.
According to Steve Schirber, the steps that I mentioned in my post last week are very similar to the ones that the standards for testing water heater draft are based on.I won't go into all of them, but they require closing windows and doors, turning on exhaust fans, and running the water heating system.
You can see what happens if you open a few windows or a door.The problem with insufficient air or makeup air is if the water heater starts drafting properly after doing that.
There is a fix for this, it is to hire a savvy plumbing or heating and cooling contractor.If the duct is already installed, they'll make sure it's not blocked and the intake is clean.They might install a makeup air duct if that all looks good.They can use Minnesota Mechanical Code Section 501.3 to find a formula for the amount of makeup air needed, or they can guess at the size and do a performance test.I might write a post explaining how to use Section 501.3 to figure out a formula for makeup air.