Water Heater Backdrafting: What if you can't reproduce the problem?

Here's a question that was recently emailed to me:

I'm assuming he was reading my blog posts on backdrafting water heaters. I wrote two on this topic a little over a year ago; the first post, titled Water Heater Backdrafting: Why it matters and what to look for, explained what this is all about and how to find signs of a problem. The second post on this topic was titled Water Heater Backdrafting: Why it's happening and how to fix it; pretty self explanatory.

The deformed / melted grommets that were mentioned in his question are the pieces of plastic shown in the photo below.

Note how the melting only occurs on the sides that face the draft hood; this is a dead giveaway that the melting was caused by the water heater backdrafting.

If this is what the home inspector saw, he'd be right to say there were signs of previous backdrafting, but not that the water heater backdrafts. We don't say a water heater backdrafts if we can't see it backdraft. A great way for a home inspector to get visual evidence of a water heater backdrafting is to hold their camera close enough to the draft hood to get their camera lens to fog over. The photo will look like this:

When a home inspector finds signs of previous backdrafting, they can recommend having a second person come out to inspect the situation further, or they can inspect the situation further themselves and make a call based on their professional experience, assuming they're qualified.

If a second person is called in to inspect the situation further, it will almost surely be a plumber or an HVAC contractor. Some of them will be qualified to do this inspection and some won't. If the person coming out to inspect the water heater draft is qualified, they will create a worst-case scenario to test the water heater draft. If you really want to be sure that whoever is coming out to inspect the draft at the water heater is qualified, hire someone who is familiar with BPI's Combustion Safety Test Procedure For Vented Appliances.

Also, if there is a functional whole-house fan installed, I'd argue that this fan should not be turned on. Those fans are only supposed to be operated with the windows open. If a whole house fan is operated with the windows closed, it will cause a natural draft water heater to backdraft every single time. Without exception.

As a home inspector, I already do most of the stuff that BPI lists in their standards, but I don't go around opening and closing doors and measuring pressures in the combustion air zone to figure out the exact worst-case scenario. I just get pretty close. If this test procedure causes the water heater to backdraft, check out my blog post on why it's happening and how to fix it.

If nobody can get the water heater to backdraft again, what should be done? I vote for nothing. There is an inherent danger that comes with owning a natural draft water heater, and this is the risk you take. Today, I call it an acceptable risk because the installation of a natural draft water heater is an accepted residential building practice. Some day in the future it probably won't be; when that happens, I'll change my tune.

Draft is a funny thing, and it's not 100% reliable. This is why powervent water heaters are getting more and more popular, and why energy professionals all seem to hate natural draft water heaters. Sometimes, all it takes is an unusually windy day to cause a water heater to backdraft. I actually experienced this in my own house a couple of years ago. During an extremely windy day, I noticed the tell-tale odor of combustion gases in my basement. I immediately went over to my natural-draft water heater, and sure enough, it was backdrafting.

This only occurred for a few minutes, but the hot gases spilling back into my home were enough to slightly deform one of the pieces of plastic at the draft hood, which now serves as a nice reminder that any natural draft water heater can backdraft. For the record, BPI standards for draft allow appliances to backdraft for up to 60 seconds upon startup.

Author: Reuben Saltzman, Structure Tech Home Inspections