HVAC Settings on Smoke Days

Which HVAC setting helps most on wildfire smoke days?

Switch the thermostat's fan from "auto" to "on." CARB's Smoke Ready advice is to set the system to "on" so air is filtered constantly, rather than "auto," which runs it intermittently. EPA's wildfire and indoor air page gives the same instruction for systems with a high-efficiency filter, so the blower keeps pulling room air through the filter whether or not the compressor is cooling.

The fan setting only pays off with a good filter in the slot. Both agencies name MERV 13 or the highest rating your system can handle; the Valley Home Ducts page on MERV filters for wildfire smoke covers whether your blower can take one. Set the cooling temperature as usual. "Fan on" does not cool by itself; it keeps the air moving.

Many wall thermostats put the fan choice on its own button or menu, separate from heat and cool. If yours has no fan setting, it may be an older control. Title 24 requires all thermostats on a system to be replaced with setback thermostats when refrigerant parts are changed, per Energy Code Ace, so a changeout is a natural time to get one. A supply house's thermostat listings (PlumbingSupplyAndMore, same owners as Valley Home Ducts) show the range, from basic programmable units to models with a separate fan schedule.

Should you close fresh-air intakes during wildfire smoke?

Yes. CARB says to close the fresh-air intake so the system runs in recirculation mode and outdoor smoke stays out, and EPA says to set the system to recirculate or close the outdoor intake damper. EPA's first step is simply to find out whether your central system has an intake; many split systems don't.

Signs you have one: a duct or hood from outside that joins the return, a motorized damper near the air handler, or a "ventilation" or "fresh air" setting on the thermostat. If a control exists, use it. If the damper is inside the equipment cabinet or wired to a control you can't find, ask the HVAC contractor who installed it to show you how it closes before fire season. Don't open the cabinet to look; the only switch a homeowner should touch is the thermostat or the system's service switch. If you're unsure, call a licensed technician.

Two other settings matter. CARB says not to run swamp coolers or whole-house fans on smoke days, since both pull outdoor air through the house by design. Keep windows and doors closed, and use the next clean-air break to air the house out, which EPA also suggests.

Do leaky ducts pull wildfire smoke in?

They can, through the return side. DOE's Building America measure guideline on sealing ducts notes that return leaks can draw 130°F attic air into the system, and an attic on a smoke day holds outdoor air. Air pulled in through an attic return leak enters after a grille filter, so it reaches the blower and coil unfiltered.

ENERGY STAR puts typical duct losses at about 20 to 30 percent of the air moving through the system. A duct leakage test tells you how much of that applies to your house, and sealing attic ducts is the lasting fix. Before anyone seals ducts near a gas furnace or water heater, a combustion safety test belongs in the job, before and after, because tighter ducts can change pressures around those appliances.

What about portable air cleaners?

For a single room, a portable air cleaner is the usual next step. CARB says to use a certified portable cleaner and run it continuously on the highest setting, noting such units "can greatly reduce indoor particle levels." EPA adds three points: pick a cleaner sized for the room, place it where people spend the most time, and make sure it does not produce ozone. EPA says units with a known clean air delivery rate (CADR) are typically preferred.

Choose one room as a cleaner-air room, usually a bedroom, keep its door shut and run the cleaner there. EPA's DIY air cleaner research found that box fans with filters attached can effectively reduce smoke in a lab and were almost completely ineffective with dirty filters. CARB's rule for the fan itself: only certified box fans marked UL or ETL, 2012 model or newer.

Why avoid ozone generators?

CARB strongly advises against the in-home use of ozone generators. Under California's air cleaner regulation, electronic air cleaners must meet an ozone emission limit of 0.050 parts per million (50 ppb) and carry a CARB certification label; uncertified ozone generators cannot be sold in California. CARB keeps a list of certified air cleaning devices to check before you buy.

Since October 2020 the rule also covers electronic in-duct air cleaners, so any ionizer or electronic unit a contractor proposes to mount in your ductwork should be CARB-certified. A pleated MERV 13 filter adds no ozone at all. If a product promises to "purify" smoke with ozone or ions, leave it on the shelf.

• 0.050 ppmozone limit for electronic air cleaners in CaliforniaCARB
• 2020year in-duct electronic cleaners came under CARB's ruleCARB
• 2012oldest box-fan model year CARB accepts for DIY cleanersCARB
• 130°Fattic air a leaky return can pull inDOE Building America

Questions from the field

Next in this row

• EAir in the ducts: all three pages
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• F4Evaporative coolers and their ducts
• A1Why attic ducts lose cooling