Part 2: Time to Talk about Residential Ventilation

ClipartHeaven.com   Whole Building Ventilation: After you get through the Scope and the Definitions of the ASHRAE 62.2-2010 Ventilation Standard, you arrive at “Whole Building Ventilation”.  As I mentioned in the last post, the simplest way to size the fan is to go into the table with the size of the house and the number of bedrooms.  Here’s the table:

Floor Area

(ft2)

0-1

Bedrooms

2-3

4-5

6-7

> 7

<1500

30

45

60

75

90

1501-3000

45

60

75

90

105

3001-4500

60

75

90

105

120

4501-6000

75

90

105

120

135

6001-7500

90

105

120

135

150

> 7500

105

120

135

150

165

(There is another table for figuring this out in L/s and square meters.)

You could use the formula: Qfan = 0.01 Afloor + 7.5(Nbr +1).  The flow of the fan is equal to 0.01 times the area of the floor + 7.5 times the number of bedrooms plus 1.  The thought being that there will be two people in the master bedroom and one in each other bedroom.

It should be noted that Paragraph 4.1.3 of the Standard mentions that the calculation and the table include “a default credit for ventilation provided by infiltration of 2 cfm per 100 square feet of occupiable floor space.”  This means that the standard assumes that there will be some infiltration through the pressure boundary of the home, so that all the necessary ventilation need not be mechanical.  The 62.2-2010 Standard refers to the ASHRAE 136 Standard which can be used to reduce the ventilation rate when the house is leaky.  But let’s stick with the table and formula for now and keep it simple.

Before moving on, however, we should note that the Standard is not particular on the type of system that is used for Whole Building Ventilation.  It can be an Exhaust-only system (putting the building under negative pressure), a Supply-only system (putting the building under positive pressure), or a balanced system like a heat or energy recovery ventilator that draws in the same amount of air that it exhausts.

The fan has be be very quiet, operating at 1 sone or less, and its installed performance must be tested.  (Sones are a linear measurement of sound – 2 sones being twice as loud as 1 sone.  A quiet refrigerator in a quiet kitchen is approximately 1 sone.)

Local Exhaust: The second fundamental section of the Standard prescribes putting exhaust systems at polluting points in the house – the bathrooms and the kitchen – otherwise known as Local Exhaust.  Using the tables, the requirements are pretty simple:

Application

Intermittent Airflow

Continuous Airflow

Bathroom

50 cfm

20 cfm

Kitchen

100 cfm

5 ach

You can run a 50 cfm fan intermittently in the bathroom or you can exhaust 20 cfm continuously.  You can use a 100 cfm range hood intermittently in the kitchen or you can exhaust the space continuously at 5 air changes per hour.  The Standard also allows you to use the bath or kitchen fan as part of the whole building ventilation system.

The local exhaust fan has to be quieter than 3 sones, and they must be measured for airflow once they have been installed.  You can’t just use the airflow number on the ban box or on the label in the fan.

Summary:  So to start with you just need to use the table (or the formula) to size the “Whole Building Ventilation” system and the tables to select the “Local Exhaust” system.  We’ll take on some of the refinements in the next blog.

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