Part 3: Time to Talk about Residential Ventilation – Selecting the system

You’ve accomplished the first steps:

  • You’ve sized the constant, background ventilation system based on the size of the house using table 4.1 or the formula;
  • You’ve sized the local exhaust ventilation system and decided whether they should run intermittently or constantly;
  • Maybe you’ve decided that you could use the same bathroom fan for the whole house, ventilation fan because that is a reasonable approach.

Before going any further you should think some more about the type of system you want to use. Should it be a simple exhaust or supply only system or should it be a balanced heat or energy recovery ventilator?

Exhaust-Only System: These can be as simple as a fan in a central bathroom. This approach puts the house under slight negative pressure, drawing in fresh/make-up air through the myriad of cracks and holes in the house. Bath fans today are exceptionally quiet and energy efficient, and can run with a minimal cost impact even if they are running 24/7. They should be certified for performance by the Home Ventilating Institute (HVI). ASHRAE 62.2-2010 Standard requires the constantly running fan to operate at 1 sone. That’s about the sound a “quiet refrigerator makes in a quiet kitchen”. 

Supply-Only System: These systems work in the opposite direction, pressurizing the house and allowing the air to leak out through the myriad of cracks and holes in the house. This works well in parts of the country that rely on air conditioning because the positive pressure in the house reduces the impact of humidity leaking into the walls. Often this sort of system introduces the outside air into the return side of the central air handler.


HRVs and ERVs: Heat Recovery or Energy Recovery ventilators bring air in from the outside and pass it through an exchanger core across air leaving from the inside of the house. From the second law of thermodynamics, heat moves from the warmer stream to the cooler stream. In an Energy Recovery Ventilator, moisture also moves from the wetter stream to the dryer stream. Air should be drawn from polluting places like bathrooms, and the fresh air supplied back to the most commonly occupied spaces like the bedrooms.

What to choose: There are a bunch of decisions that need to be made in selecting the best system, and I’ll go over them in the next blog.

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