Replies

1. GBA Editor
   Martin Holladay | Nov 07, 2016 08:13am | #1

   Brian,
   If the basement is ever finished, it would be logical to include the basement in your ventilation rate calculations.

   You have two choices: (1) You can plan ahead for that eventuality (and buy the equipment you'll need for the entire house now), or (2) You can install equipment to handle your first floor only, and then change your ventilation system equipment and ducting later, if and when the basement is finished.

   The ASHRAE 62.2 formula already takes occupancy factors into consideration (so that a 2,300 square foot house with 2 occupants is handled differently from a 2,300 square foot house with 5 occupants), so you don't have to reinvent the wheel or introduce fudge factors to account for occupancy variations.

   The ASHRAE 62.2 formula dictates a ventilate rate based on this formula:
   7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area.

   In your case, the ventilation rate with an unfinished basement would be:
   15 cfm + (23 x 3) cfm = 84 cfm

   The ventilation rate with a finished basement would be:
   15 cfm + (46 x 3) cfm = 153 cfm

   I recommend that you install equipment capable of meeting the rate required by the ASHRAE 62.2 formula. Once the equipment is installed, however, you may choose to ventilate at a lower rate than the formula requires. This can be done a variety of ways; one way would be to use a timer that operates the equipment (for example) for 20 minutes every hour.

   For more information on ventilation systems, see Designing a Good Ventilation System.

2. 
   user-4405197 | Nov 07, 2016 08:41am | #2

   Thanks Martin. One question - Is it still permissible to use the old formula that just used 1 cfm per 100 sq ft? I remember Joe Lsitburek arguing the new formula would result in overventilating. I guess I could choose to ventilate at a lower rate after all inspections are done.

3. GBA Editor
   Martin Holladay | Nov 07, 2016 09:01am | #3

   Brian,
   We are rapidly moving towards a code environment where the current ASHRAE 62.2 standard will be mandatory. It's possible to imagine that future home sales might be contingent on the provision of a ventilation system complying with ASHRAE 62.2. In other words, there may be valid reasons to comply with the standard.

   As I noted in my first response, you are not compelled to operate your ventilation equipment at maximum speed, 24 hours a day. You can operate your ventilation equipment any way you want -- including not at all. It's your house.

4. 
   charlie_sullivan | Nov 07, 2016 09:42am | #4

   If you are using equipment like the Lunos units that have ECMs (electronically commutated motors), you will get better fan efficiency operating at low speed all the time than higher speed some of the time (on a timer) and you'll also get better heat transfer efficiency.

5. 
   Jon_R | Nov 07, 2016 11:11am | #5

   > In other words, there may be valid reasons to comply with the standard.

   More importantly, various indoor pollutants are unhealthy. Maybe someday we will stop guessing and identify them more accurately, measure them and ventilate accordingly.

   Personally, I would ventilate at the full ASHRAE 62.2 rate for at least the first year. Longer depending on measured pollutant levels.

6. GBA Editor
   Martin Holladay | Nov 07, 2016 11:23am | #6

   Brian,
   Unlike Jon R, I don't think it's easy or productive to measure indoor air for pollutant levels (other than levels of water vapor and radon). He's right, however, that ventilation -- at some rate that feels comfortable for you family -- is a good idea.