Replies

1. GBA Editor
   Martin Holladay | Nov 13, 2015 11:10am | #1

   Scott,
   It isn't necessary to provide a makeup air source for a clothes dryer. If you want a clothes dryer that doesn't require an exhaust duct, you can purchase a condensing clothes dryer.

   As long as your range hood fan is less than 400 cfm, most codes do not require a dedicated source of makeup air. You may need to crack a window open, though, if your house is really tight. For more information on this issue, see Makeup Air for Range Hoods.

   There are lots of ways to provide fresh air for occupants. To learn more about ventilation systems, see Designing a Good Ventilation System.

2. 
   ntisdell | Nov 13, 2015 01:49pm | #2

   To get an idea of makeup potential in the home you can add up your total square footage of windows, multiplied by the leakage specification from the manufacturer. In many cases the leakage will do a decent job of equalizing pressure/airflow. Add to that door leakage ratings...

   Also each of your exhaust fans, water heater, etc (when off) will also provide small amount of makeup air leakage in most cases.

3. GBA Editor
   Martin Holladay | Nov 14, 2015 05:25am | #3

   Nick,
   Your calculation method implies that most air leakage is at windows and doors. It isn't. There are a thousand locations in the typical thermal envelope where air can leak in.

   Your basic premise, however -- that most exhaust fans (except for large range hood fans, wood-burning fireplaces, and in some cases wood stoves) don't need a dedicated source of makeup air -- is correct.

4. 
   ntisdell | Nov 16, 2015 12:44pm | #4

   Yeah, no doubt about there being more leakage. In many out of sight locations.

   However doors/windows seem to be the easiest to quantify being that there is a specification on it. Which over the many windows/doors in a home the average spec should be close enough for makeup air calcs. As kind of a minimum check, instead of using 'assumed leakage' by sqft + year built (per MN code for instance, which gives higher values)

   Personally when I turn on the dryer or couple exhaust fans, I feel better knowing that the makeup air is being pulled in through windows/doors... then being pulling into wall cavities :-/

   However, i'm only kidding myself, since stack effect forces, wind, and generally leakiness of my nice code built house, likely isn't effect much when i turn on my fans. :(

5. 
   fitchplate | Nov 17, 2015 11:38am | #5

   The proverbial debate and not everyone agrees.

   Make up air is another word for infiltration and infiltration is an accident, not a plan. Air will be drawn in from the closest point of lowest resistance to supply any location where there is low pressure. We built so we can manually increase or decrease the volume and rate of flow of make-up air to fit comfort issues associated with season and weather patterns.

   For example, I use an exhaust only timed per hour bath fan for ventilating my small house. We have 4 manually controlled fresh air intakes (Aldes and Panasonic brands). At this time of year, since the bath is close to the bedroom, we find the need to close the bedroom vent so the cool incoming air comes from a different location and is not pulled across the bed.

   If your house is very well sealed with caulking, tapes and gaskets; you will draw most air in the form of infiltration around the windows and doors (path of least resistance). This air is drafty and uncomfortable and may not be the best location for the makeup air stream. If you have manually controlled, thoughtfully placed wall vents, you can control the airstream and so affect occupant comfort.

   I want to control the air intake and reduce fugitive infiltration and also any associated discomfort from the makeup vents. I therefore have a vent close to my wood stove (the wall behind it), I have an intake vent using two in-line back flow preventers (dampers) in the washer-dryer closet (we isolate the washer-dryer space), and I have a vent in the ground floor utility room (where the boiler and hydronics distribution system warms the air) so I get winter season warming of incoming fresh air as it diffuses into the house. But in the summer, that boiler provides DHW so I close the utility room vent in favor of one that does not warn the incoming air.

   If I close the manual vents, the draft under the entrance doors and around windows increases. Once cannot eliminate the need and physics of air flow into a building but one can manage its impacts on the occupants and the humidity levels.