Ducts & air
Whole-house ventilation: the ASHRAE 62.2 formula
Tight houses save energy and trap whatever is in the air. ASHRAE 62.2 says how much outdoor air a home needs, and the arithmetic fits on one line.
Why houses need planned ventilation
Older houses ventilate themselves, badly. Air leaks in through gaps around windows, doors, outlets and the attic hatch, and out through others, driven by wind and by the difference between indoor and outdoor temperature. It is plenty on a windy winter day and almost nothing on a still summer one, and it comes from wherever the gaps are, including the garage and the crawl space.
As houses are air-sealed to save energy, that accidental ventilation falls. The moisture, carbon dioxide, cooking by-products, and chemicals from furniture, cleaning products and finishes that it used to dilute stay inside. ASHRAE Standard 62.2, the residential ventilation standard that many building codes and energy programs reference, sets how much deliberate outdoor air a home should get to keep those pollutants at acceptable levels.
The formula
Since its 2013 edition, ASHRAE 62.2 sets the total required ventilation rate from the floor area and the number of bedrooms:
The area term covers pollutants from the building and its contents; the bedroom term is a stand-in for the number of occupants, assuming two people in the main bedroom and one in each other. If more people live in the house than that assumption, count the actual occupants instead. The earlier 2010 edition used 0.01 × floor area, which is why newer requirements are roughly twice as high for a typical house.
A three-bedroom house with a continuous fan
2,000 sq ft, three bedrooms, four occupants, 8 ft ceilings.
- Area term: 0.03 × 2,000 = 60 CFM
- Occupant term: 7.5 × 4 = 30 CFM
- Ventilation rate: 90 CFM, continuous
- Air changes: 90 × 60 ÷ 16,000 = 0.34 per hour
- Under the 2010 formula: 20 + 30 = 50 CFM
The whole-house ventilation calculator applies the formula and shows the air change rate it amounts to, which the air changes per hour calculator works out for any room or house.
Infiltration credit
The standard allows a credit for air that already leaks in, measured by a blower-door test. A leaky existing house may need little or no mechanical ventilation; a new tight house gets little credit and needs nearly the full rate. Without a blower-door test, take no credit, which is what the calculator does by default. Sizing the fan for the full rate costs little more and protects against the house being tighter than assumed.
Running a fan part of the time
The rate is an average. A fan can run continuously at the rate, or at a higher rate for part of each hour or each few hours, as long as the average over the period comes out right:
A larger house with a fan on a timer
2,800 sq ft, four bedrooms, five occupants, a fan running 30 minutes in each hour.
- Area term: 0.03 × 2,800 = 84 CFM
- Occupant term: 7.5 × 5 = 37.5 CFM
- Ventilation rate: 121.5 CFM
- Fan size at 30 minutes per hour: 121.5 ÷ 0.5 = 243 CFM while running
Three ways to supply it
Exhaust only. A quiet, continuously rated bathroom fan runs at the required rate, and makeup air leaks in through the envelope. It is the cheapest system, and in many existing houses a two-speed bathroom fan does the job, as the guide to bathroom and kitchen exhaust describes. The bathroom fan should be sized with the bathroom exhaust fan calculator for its spot-exhaust job and set to the ventilation rate on its low speed. The downside is that the incoming air comes from wherever the house leaks.
Supply only. A duct brings outdoor air to the return side of the furnace or air handler, often with a motorized damper and a timer. The air comes from a known place and can be filtered. It slightly pressurizes the house, which in cold climates can push moist air into walls.
Balanced. A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) brings in and exhausts equal amounts, and transfers heat, and in an ERV some moisture, between the two streams. It is the most expensive option and the best for tight houses in cold or very humid climates.
HRV or ERV
An HRV transfers heat only, which suits cold, dry winters where the house needs to get rid of moisture. An ERV also transfers moisture, which suits humid summers, when it keeps some of the outdoor moisture out, and very dry winters, when it keeps some indoor moisture in. The ERV/HRV size calculator picks a type for the climate and a unit size with a margin above the required rate.
An HRV in climate zone 5
90 CFM continuous at a 0 °F design temperature, 70 °F indoors.
- Rated capacity needed: 90 × 1.25 = 113 CFM, so a 130 CFM unit
- Heat in the ventilation air: 90 × 1.08 × 70 = 6,804 BTU/hr
- Recovered at 70%: 4,763 BTU/hr; still lost: 2,041 BTU/hr
Without recovery, 90 CFM of outdoor air at 0 °F is a heating load of 6,804 BTU per hour, a noticeable part of a house’s design load. With recovery, most of it comes back.
Ventilation and humidity
In winter, ventilation dries a house out because cold outdoor air holds little moisture. In summer, in humid climates, it brings moisture in that the air conditioner has to remove. Both effects are part of why the guide to humidity, dew point and comfort treats ventilation and humidity together. Ventilation air is only useful if it gets where it is needed: leaky ducts in an attic or crawl space pull in unplanned and often dirty air, which the guide to duct leakage covers.