Assess the Home for Ventilation Needs and Limitations
Walk through the house to identify the presence and condition of all current HVAC equipment.
- Does the kitchen have a range hood? Is it ducted outside (not into the attic, basement, or crawlspace)?
- Does each bathroom have an exhaust fan? Is it ducted outside (not into the attic, basement, or crawlspace)?
- Test each fan for adequate air flow, if you have flow testing equipment available.
- How loud are the fans? Few contractors have sound testing equipment for the field, but if a fan is annoyingly loud to your ear, occupants are unlikely to use it regularly.
- If you have access to the ducting for the fans, check the duct type (smooth or flex), diameter, length, and condition.
- Are the wall caps and roof jacks unobstructed and firmly connected to the ductwork? Do fan backdraft dampers work? Or are they stuck?
- Does the home have a ducted heating or cooling system? What is its condition? Does the space conditioning ductwork deliver air to all rooms? Is the ductwork well sealed?
- Is the clothes dryer ducted to the outside?
Assessing an apartment building? View additional assessment questions.
In most cases, existing fans are unlikely to meet current sound and air flow requirements. Usually the best option is to remove and replace existing fans to meet ASHRAE Standard 62.2 requirements for local exhaust ventilation. Remember, even when the project budget is tight, a simple ventilation improvement such as a single upgraded bath fan can meet the requirements for both local exhaust ventilation and whole-building ventilation.
However, if you want to determine whether an existing kitchen range hood or bath fan(s) will meet the requirements for local exhaust ventilation, use a flow hood or other airflow measuring device to test the airflow installed fans. Make sure the ducting meets the ASHRAE 62.2 Prescriptive Duct Sizing Chart.
Are Combustion Appliances Present?
Title 24 Requirement
ASHRAE 62.2-2007 & ASHRAE 62.2-2010
When natural-draft appliances are present, the combined exhaust of the two largest fans in the house cannot exceed the equivalent of 15 cfm per 100 sq ft of living area (0.15 cfm per sq ft).
Best Practice Recommendation
When natural-draft appliances are present, use the combustion safety test found in the Building Performance Institute (BPI) Building Analyst Professional Standard to test for combustion gas spillage from all combustion appliances in a worst case situation. Remediate if necessary. If required by the Authority Having Jurisdiction, also calculate the combined exhaust of the two largest fans in the house to show that the limit of 15 cfm/100 sq ft has not been exceeded.
- Are there natural-draft (atmospherically-vented) combustion appliances within the home's envelope, such as a natural gas or propane heater or water heating equipment?
- Is there a fireplace or a gas log?
Usually, adding ventilation equipment improves indoor air quality in homes, but harm can be done if operating the ventilation fans pulls dangerous combustion gases into the living space.
The chimney for a fireplace, heater, or natural-draft (atmospherically-vented) water heater is designed to carry toxic combustion products to the outdoors. If a home is under enough negative pressure, the chimney can spill toxic combustion products into the room where the appliance is located. This dangerous situation may occur often or only under certain circumstances, such as when multiple fans operate simultaneously or when larger fans (clothes dryer, range hood) are operating.
The safest way to avoid backdrafting problems is to replace natural-draft combustion appliances with direct-vent, sealed combustion appliances. (Power-vented appliances do not offer the same margin of safety that sealed combustion appliances do, but they are a close second.)
When it is not possible to replace natural-draft HVAC equipment with sealed combustion equipment, ASHRAE Standard 62.2 provides a prescriptive requirement to avoid backdrafting:
When naturally-vented appliances are present, the combined exhaust of the two largest fans in the house cannot exceed the equivalent of 15 cfm per 100 sq ft of living area (0.15 cfm per sq ft).
Since the dryer and the range hood are normally the largest fans, their combined airflow at the highest setting cannot exceed 15 cfm/100 sq ft (or 0.15 cfm per sq ft). For example, for a 2,000 sq ft house, the two largest fans operating on highest speeds must not exceed 300 cfm of airflow (0.15 cfm x 2,000 sq ft = 300 cfm).
Many utility rebate programs in California require the use of the combustion safety test found in the Building Performance Institute (BPI) Building Analyst Professional Standard as a performance approach to combustion safety. This is the most comprehensive approach for existing buildings, since it involves testing for combustion gas spillage from all combustion appliances in a worst case situation (with dryer and all exhaust fans operating at once).
How Tight is the House?
In California, low-income weatherization and many utility programs require a blower door test before and after weatherization measures to verify that the home has adequate ventilation, to document energy savings, and to increase ventilation, if needed.
The tightness of a house or apartment varies with its construction, size, height, and local climate. ASHRAE Standard 62.2 assumes that, on average, a new house will leak at the rate of 2 cfm per 100 sq ft of floor area, which is built into the required airflow formula for whole-building mechanical ventilation. If air sealing techniques reduce average air leakage to less than 2 cfm per 100 sq ft, the ASHRAE ventilation rates may be inadequate. If a home's average air leakage is greater than 2 cfm per 100 sq ft, the ASHRAE ventilation rates may be too high.
Building airtightness is commonly measured with a blower door test. ASHRAE Standard 62.2 DOES NOT require blower door testing of the house. However, when the goal is to build tight and ventilate right, it's useful to know how tight the building is for these reasons:
- Some contractors use a blower door during the alteration or energy upgrade process to identify and seal air leaks.
- Using blower door test results allows the contractor to better match the ventilation system with the home's ventilation needs (by using a multi-speed fan setting, setting a damper, or using a speed control or timer).
- Some contractors use blower door test results for an existing house to reduce the amount of whole-building ventilation required to meet ASHRAE Standard 62.2. If you think your house will have an air leakage rate of more than 2 cfm / 100 sq ft when the energy upgrade is complete, you can use the predicted leakage from a post-airtightening blower door test to reduce the amount of mechanical ventilation you will need when you calculate the whole-building ventilation rate in Step 3.
- Many utility energy upgrade programs and all federally-funded weatherization programs require blower door testing.
Blower Door Testing
Houses are tested for air tightness with a blower door: a calibrated fan and controller that places the house or apartment under positive and/or negative pressure and measures the cfm required to reach a certain pressure difference between the inside and the outside of the building. This test pressure is usually 50 Pascals, or about 0.2 inches of water column. Software compares the airflow and the pressure and calculates how many cubic feet the blower door fan would pull in an hour. This is expressed as the Cubic Feet per Minute at 50 Pascals (CFM50) or Air Changes per Hour at 50 Pascals (ACH50). The software then calculates an annual average leakage rate for the house, based on the building height and exposure to wind. This ACH Natural is used as a measure of how much unintentional ventilation occurs on an annual basis. The reduction in CFM50 before and after air sealing measures are complete gives a measure of how effectively air leakage was reduced. It also provides an annual estimate of the leakiness of the house that can be used in an optional calculation to reduce the required amount of mechanical ventilation. However, the annual estimate is an average over all seasons of the year. Reducing the whole-building ventilation rate based on this average means the home is likely to be seriously underventilated during mild weather.