Getting Combustion Right
Building codes are nudging the industry away from natural draft appliances.
An HVAC contractor hired to replace an old furnace, boiler or hot water heater in an existing home might sometimes install another natural draft model rather than a more efficient direct-vent. This is usually to save money. But if any remodeling or weatherization work has been done on the home, the replacement may not comply with the code. It could also pose a health hazard.
The International Residential Code (IRC) requires that any room with a fuel burning, natural draft appliance be supplied with combustion air in an amount specified by the manufacturer and the code. The International Code Council (ICC) added this requirement, in part due to changing construction practices.
HVAC contractors didn't always have to worry about combustion air when homes were drafty enough to supply it without help. That's obviously not the case with a new, tightly built home. However, an older structure that has been modernized may not be able to supply the needed air either, as it probably has fewer drafts than it used to.
Modernization work includes everything from major additions (whether second stories or rooms that extend the home's footprint), to the replacement of windows, storm windows or exterior doors. It could also include caulking, weather stripping and other weatherization work.
A second reason for the ICC's combustion air requirement is that today's fuel-burning appliances have less tolerance for negative air pressures in the home. That's because they use more of the heat they generate to actually heat air or water, and send less of it up the chimney or exhaust pipe. Since the strength of the exhaust draft depends in part on the temperature difference between the supply and exhaust air streams, a cooler exhaust means a weaker natural draft. Even a slight negative air pressure in the combustion zone could in some cases overpower that weak draft, pulling carbon monoxide and other combustion products into the home.
When specifying a natural-draft combustion appliance, it's wise to always perform a "worst case" Combustion Appliance Zone (CAZ) test. This means running every exhaust fan in the home at high speed—dryer, air handler, bath fans—while using a CO detector and a smoke pencil to see if the appliance backdrafts.
If it does, the appliance will need a dedicated air supply, which could mean running a duct from the outside to the mechanical room. Depending on the home, this could drive installed costs higher than that of a direct-vent model.
Yes, natural draft appliances can be installed safely as long as they're provided with enough air. If you find yourself questioning whether they're worth the effort, you're not alone.
In fact, code-writing organizations actively discourage using natural-draft appliances because contractors don't always get combustion air right. During negotiations for the 2018 IRC, a proposal was considered that would require CAZ testing anytime a home undergoes remodeling work, even if the appliance isn't being replaced. The proposal failed to pass, but could re-appear in the future.
Of course, you don't have to worry about any of this with direct-vent combustion appliances because these appliances take their combustion air directly from the outdoors. This invites the question: If you're not defaulting to direct-vent, why not?
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