Backdrafting Water Heater Vent

Most of the issues I find at a home inspection are both important and easy to fix. Old smoke alarms are a good example, because replacing them is not hard.

A backdrafting water heater is different. I cannot underscore the importance, because carbon dioxide is deadly. The issue is compounded by how hard it is to cure.

But when we talk about backdrafting, what are we taking about?

You know what natural draft is, right? In a natural draft water heater, there is a little hood at the top, right above the center. While the water heater is running, exhaust from the water heater is entering the vent at that place. The exhaust, being hot, heads up the vent. This is called “stack effect“.

The problem with natural draft is that it is not too strong. A slight vacuum in the house can actually pull the exhaust out of the vent. Vacuums are caused from bathroom and kitchen exhausts, wood burning fireplaces and clothes dryers.

Moisture and toxins, anything in that exhaust, is now in the air that you are breathing and not leaving the house.

There is also a thing called “orphaned vent“. When an older furnace is replaced, the new one is usually a sealed combustion unit. You’ve seen them; where there are one or two PVC pipes that come out the side of the house. The newer furnaces are so efficient that the heat in the exhaust is very low, so the old vent is not needed.

But the water heater is still there, still using the vent that the old furnace used to use. Problem is, the water heater vent is too small for the furnace vent. In the winter, that big vent is full of cold air that won’t move and the water heater pours exhaust into the room.

To test for backdrafting, I create a “worse-case scenario” by turning on all of the fans and turning down the furnace. With the water heater running, I hold my hands at the vent. It’s easy to see if the exhaust is going up the vent or warming my hands.

What to do?

It’s possible that the installer of the new furnace serviced the old vent to accommodate the smaller water heater vent. If not, that can be done fairly quickly by an HVAC specialist. A lot of folks simply replace the water heater with a direct vent, if the original is more than 10 years old.