Ductwork and Combustion Vents: Dangerous Interactions

The company I work for, Edge Energy, does weatherization, home-performance contracting, renewable-energy contracting, and energy-code-compliance verification. As an energy auditor and the Director of Building Services for the company, I’m involved in the whole range of our projects. I’m certified as an energy auditor by the Building Performance Institute (BPI), and as a Home Energy Rating System (HERS) rater by the Residential Energy Services Network (RESNET). I conduct energy audits on existing homes, make recommendations for improvements, and manage the related weatherization work. I also manage more extensive insulation and air-sealing projects on new construction and home rehabs. (We often do that kind of work before passing customers to our solar division, to help get the homes closer to “net zero” when the alternative generation comes on line.) In addition, I’m responsible for our energy-code-compliance testing, and I certify new homes for Energy Star labeling. With this wide range of work in various stages of construction, I’ve been exposed to concerns with comfort, moisture, health and safety, and heating and cooling in both existing and new homes.

One of the trickiest problems for a non-specialist to grasp involves the interaction between combustion appliances and the heating and cooling ductwork in a home. In this story, I’ll take a look at two examples where it took a little detective work to get to the bottom of a water-heater backdrafting problem. In one case, I was called in to troubleshoot by an HVAC contractor we work with, and in the other, I went to the house to conduct an audit for a homeowner who wanted help solving comfort issues and lowering energy bills.

In both cases, I immediately suspected that the air-conditioning ducts were the cause of the backdrafting problem; but in one case, there turned out to be another culprit.

Case One: Process of Elimination

When the HVAC contractor described this backdrafting water-heater situation to me over the phone, I was pretty sure I knew what I would find at the house. The basement had been recently remodeled, and there was a large return grille in the finished space connecting through the wall of the mechanical room, with a large, short run of duct straight to the air handler. I expected to find out, as I have in other, similar cases, that the oversized return was depressurizing the basement enough to backdraft the water heater.

To find out, I started by conducting a standard “worst-case” depressurization test. We measured the air pressures throughout the house. If a room had positive pressure, we closed the door to shut it off from the rest of the space. If it had negative pressure, we left the doors open. We turned on every fan that could create a negative pressure in the house: dryers, bath exhaust fans, and the recently installed kitchen downdraft vent. Under those conditions, the air pressure in the mechanical room dropped to -3.8 pascals (Pa)—and sure enough, the water heater began to backdraft.

Next, I tried to eliminate my chosen suspect: the return grille. I taped off the grille with 6-inch-wide tape from Energy Conservatory, expecting the problem to go away (this is the tape we commonly use for airtightness testing—it holds well enough, but you can pull it off without damaging paint).

But I was wrong. Basically, nothing happened: The water heater continued to backdraft, and the pressure in the mechanical room increased by only about 0.2 Pa.

So next I asked the HVAC contractor to go upstairs to the kitchen and turn off the downdraft fan on the range. “Don’t tell me before you do it,” I said. “Just do it, and I’ll watch the manometer.”

Sure enough, a minute or two later, the manometer reading changed to about -0.6 Pa. Of the -3.8 Pa depressurization, about -3.2 Pa was caused by the kitchen fan.

Still, I was glad I had gone to the trouble of investigating all possible causes, even if some turned out to be dead ends. The obvious solution when you have a backdrafting problem is to replace the natural-draft appliance with a power-vented unit. And in the end, that’s what I recommended. But it was worth it to look more closely. If the duct return grille had been to blame, the problem could have been fixed by just reducing the size of the return in the basement, and perhaps increasing the size of the upstairs returns, which would have been a less expensive fix.

Case Two: Out of Balance

My second example was part of a typical home-performance job. The customers called us because they were uncomfortable in the house, and they wanted to add some attic insulation. But the first principle of home performance is “do no harm.” So as always, I did a routine walk-through of the house to find out whether insulation and air-sealing could cause any kind of health or safety issue—or in particular, create a combustion safety situation.

And in fact, a worst-case depressurization test of the mechanical room did induce obvious backdrafting of the water heater. Photos in the slideshow at left show me using a smoke pencil to reveal the backdrafting, but the problem was also evident when I held a mirror next to the flue, and the moist exhaust fogged up the glass. In fact, I could feel the gas like a hot breeze against my face without any equipment at all.

In this case, the air-conditioning return ducts and grilles—the usual suspects—did turn out to be the root of the problem.

Because the supply registers in the house weren’t delivering enough heating and cooling (especially at the furthest reaches of the duct runs), the homeowners had sealed up the supply ducts in their basement with tape and mastic. But they didn’t know enough to seal up the return ductwork also—they didn’t realize that by sealing only the supply ducts, they were changing the air pressure in the mechanical room and creating a negative pressure that would affect the water-heater combustion vent.

It’s not clear who added a whole return grille to the system right in the mechanical room, when the return outside the mechanical room door would have been sufficient. But in combination, the two return vents created a suction that completely overpowered the natural draft in the water-heater flue.

It’s interesting that I was able to address these issues with materials that were actually sitting in the mechanical room, just a few feet from the problem. The homeowners still had some good-quality foil tape on hand and some mastic. I taped up the slot for the air filter with that tape—I find this to be a practical way to seal that component, and it’s easy enough for homeowners to change the tape every few months when they change the filters. (You can even write the date on the tape with a felt marker as a reminder to change the filter when it’s time.)

I had taped over the return grille with Energy Conservatory tape as part of my pressure testing, and I advised the homeowners to make that fix permanent with the same tape and mastic they had used on their ducts (and to seal the return ducts too).

This still leaves the homeowners with supply ductwork installed in the 1940s that doesn’t deliver enough heat or cooling to their rooms. I recommended an Aeroseal job: a thorough cleaning, followed by sealing the entire duct system with latex aerosol blown into the ducts. This provides air-quality benefits as well as energy savings, and it’s pretty much the only way to seal this home’s duct system, which is mostly inaccessible. But an Aeroseal franchise would charge a couple thousand dollars to do the job, and this customer may choose to apply only envelope air-sealing and insulation upgrades.