HVAC · Lesson 8 of 13Beginner6 min read

Ventilation and Indoor Air

A furnace and an air conditioner mostly recirculate the same indoor air. Bringing in fresh air, getting rid of moisture and keeping combustion gases out are separate jobs, and they matter for your health.

Why houses need fresh air

Cooking, showers, cleaning products, furniture, people and pets all add moisture and pollutants to indoor air. EPA says the most effective fix is usually to remove or reduce the sources. The next is ventilation: replacing indoor air with outdoor air.

Most home heating and cooling systems, including forced-air furnaces, don’t mechanically bring fresh air into the house, EPA notes. Older homes get theirs through open windows and doors and through leaks in the building. As homes are sealed better to save energy, those accidental leaks shrink, so newer homes add planned ventilation instead.

Spot ventilation: exhaust fans

Bathroom and kitchen fans that exhaust to the outdoors remove moisture and pollutants right where they’re made, says EPA. Run the bathroom fan or open a window when showering, and use the kitchen fan while cooking. EPA also recommends venting clothes dryers, stoves and other appliances that produce moisture to the outside where possible. A fan that only blows into the attic moves the moisture to a place where it can cause mold.

Whole-house ventilation, HRVs and ERVs

Whole-house systems ventilate the entire home on purpose. Some simply run an exhaust fan and let fresh air leak in to replace it; others blow in filtered outdoor air. A balanced system does both at once, with matched fans.

Balanced systems often use a heat recovery ventilator (HRV), also called an air-to-air heat exchanger. Stale outgoing air and fresh incoming air pass each other through a core of thin plates without mixing, and heat moves across the plates. In winter the incoming air is warmed by the air leaving; in summer it’s cooled. An energy recovery ventilator (ERV) also transfers moisture, so it can help keep humid outdoor air from making the house damp in summer, and keep winter air from getting too dry.

A balanced ventilator with heat recovery, shown in winter. An energy recovery ventilator (ERV) looks the same but its core also passes moisture. Sources: U.S. EPA: Improving Indoor Air Quality; Wikipedia (secondary): Energy recovery ventilation.

Humidity and mold

EPA’s rule is simple: the best way to control mold is to control moisture. It suggests keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, which you can check with an inexpensive hygrometer. Air conditioners and dehumidifiers lower humidity; exhaust fans remove it at the source.

If you see condensation on windows, walls or pipes, EPA says to dry the surface and reduce the moisture source quickly, and to insulate cold surfaces such as cold-water pipes. Keep air conditioner drip pans clean and drain lines flowing (Lesson 5). If you suspect the heating and cooling system itself is contaminated with mold, EPA advises not running it until it’s cleaned.

Carbon monoxide

Carbon monoxide (CO) is an odorless, colorless gas that kills without warning, says the CDC. Gas- and oil-burning furnaces, boilers and water heaters, along with portable generators, vehicles and charcoal grills, can produce it when they aren’t working or aren’t vented properly. The most common symptoms are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, often described as flu-like.

Prevention, from CDC, CPSC and the U.S. Fire Administration:

Have your heating system, water heater and any other gas-, oil- or coal-burning appliances serviced by a qualified technician every year.
Install CO alarms on every level of the home and outside sleeping areas, avoiding spots near heating vents or where furniture or drapes can cover them. CPSC recommends alarms that meet the current UL 2034 standard.
Test CO alarms monthly, and replace them as the manufacturer directs (CDC says every five years if no instructions are given).
Keep the vents for the furnace, dryer, stove and fireplace clear of snow and debris.
Never run a generator inside the home or garage, even with doors and windows open.

StopIf the CO alarm sounds

From the U.S. Consumer Product Safety Commission:

Never ignore it. Don’t try to find the source of the CO.
Immediately move outside to fresh air.
Call your emergency services, fire department or 911, then do a head count to check that everyone is out.
Don’t go back in until the emergency responders give you permission.
If the source is a malfunctioning appliance, don’t use it again until trained personnel have serviced it.

Practice the plan with your household (Test Smoke and CO Alarms).

Ventilation and air-quality parts
Part	What it does	Signs of trouble
Bathroom exhaust fan	Removes shower moisture; should vent outdoors.	Mirror stays fogged; mildew on the ceiling; loud or weak.
Kitchen range hood	Removes cooking moisture, smoke and odors.	Grease-clogged filter; recirculates instead of venting outside.
Dryer vent	Carries hot, moist dryer air outdoors.	Lint buildup, a crushed hose, a stuck outdoor flap.
HRV or ERV	Swaps stale air for fresh while recovering heat (and, in an ERV, moisture).	Dirty filters or core; frost on the core in very cold weather.
Dehumidifier	Pulls water from damp air with its own small refrigeration loop.	Full bucket, iced coil, musty smell.
CO alarm	Warns of carbon monoxide you can’t see or smell.	Chirping (battery or end of life); past its replace-by date.
Hygrometer	Measures relative humidity.	Readings over 60 percent mean too much moisture.

Check yourself

Tap an answer to check it, or open “Show the answer.”

According to EPA, what do most home heating and cooling systems do about fresh air?
Show the answer
They don’t mechanically bring fresh air into the house. Most forced-air systems recirculate indoor air; fresh air comes from windows, leaks or a separate ventilation system.
How does an ERV differ from an HRV?
Show the answer
An ERV also transfers moisture between the airstreams. Both recover heat; an energy recovery ventilator also moves moisture.
What indoor humidity does EPA recommend?
Show the answer
Below 60 percent, ideally 30 to 50 percent. Below 60 percent, ideally 30 to 50, to keep mold and dust mites in check.
Your CO alarm sounds. What does CPSC say to do first?
Show the answer
Immediately move outside to fresh air, then call 911 or the fire department. Get everyone outside, call for help, and stay out until responders say it’s safe.
Where should CO alarms go, according to CPSC?
Show the answer
On every level of the home and outside sleeping areas. On every level and outside sleeping areas, away from heating vents and anything that could cover them.

Go deeper

U.S. EPA: Improving Indoor Air Quality EPA’s guide to improving indoor air quality.
U.S. EPA: A Brief Guide to Mold, Moisture, and Your Home EPA’s short guide to mold and moisture.
U.S. Consumer Product Safety Commission: Carbon Monoxide Questions and Answers CPSC’s carbon monoxide questions and answers.
Being an Adult 101: Test Smoke and CO Alarms Step-by-step alarm testing.

Sources

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A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.