In the “I heard it so it must be true world”, Exhaust only is bad, bad and bad. Except, not always. Commonly, we hear “Where is the air coming from?” and “but when the hood vent or dryer goes on it certainly cant work” but that falls into another category. What burns me up, I hear this from code officials to the folks who are trained to train these code officials.

Come one folks, stop sipping the coolaid.. I write this from cozy Zone 4A. For those up in 5 and 6 or more, sure I agree Exhaust only may be bad juju… Same with the Zones below me. In my little zone of the world, we have almost as many cooling degree days as heating degree days. Code even says we are vapor neutral. Even though that’s not 100% true, code is close enough.

So, we test an exhaust fan with surprise, an exhaust fan flow meter. Its a box with a big hole and a little hole to attach to our manomitor. The box in use may have an 8 pascal difference (from inside to out) while measuring 125CFM of air flow. The inside surface area of that box is roughly 6 square feet.

SO, a pascal is 1/249th of an inch of water column. That’s a pretty tiny number. Another way to see it (Thanks Google) A pascal unit is equal to only one ten-thousandth of a pound per square inch (1 Pa = 0.000145 psi). So 8 of those bad boys don’t amount to much… 0.00116 PSI for every square inch inside that 6 sq ft box. NOW, get this. A home may have easily 4,000 Square feet of shell area. Go ahead, I dare you, figure out by interpolation what that equivalent PSI is on 4,000 square feet. Im not going to bother because I know its barley anything. While I like to exaggerate for effect, It might not be an exaggeration to say a few molecules at a time through what could be hundreds or thousands of openings (More on this later).

So, that ventilation strategy cant work when the Range Hood or other exhaust is competing ? May be your right, BUT the hood or other exhaust is now bringing in even MORE fresh air, defeating the need for the exhaust to begin with. If anything, may be we are getting too much fresh air. (Remember, the house is not a balloon, when air leaves the same amount MUST come back in to replace what was exhausted). So an ERV wouldnt help your case now because by design when an ERV or HRV is off, the flappers shut and now that hood vent or other exhaust IS pulling air into the home exactly the way you say it cant…

All kidding aside, an ERV or HRV is by far 100% the best way to go. If you can afford it. If your electric rates are low enough. Until recently many HRV’s costed more in electric than the fuel the heating system would have used to heat the cold air pulled in by depressurization.

Our Electric rates are some of the highest in the country. BTU’s from oil are much cheaper and BTU’s from Natural gas are WAY less than oil. My job(s) is not to tell you to do something you have to, if you don’t have to. Code allows exhaust only (because the folks who wrote the code are smarter than the folks who say it doesn’t work) and will always be far better than piping a tube to the air handlers return (the stupidest way to provide ventilation!)

So, the later now. Building science says balanced is best. That’s always 100% true. The house at 2.5 ACH50 with 90CFM exhaust running 45 min per hour is puling bad juju in through those gaps, that’s got to be bad… right ? Yup, but just how bad is it ? Its not as bad as building a house to 4 ACH 50 without any ventilation or GASP the old NYS code of 7ACH. Those houses let way more bad juju in 365 days a year and uncontrollably. It comes down to how much is ok, and is it really going to be a problem ever… If splitting hairs (which you are doing) is not allowed, than the code would read no exhaust only….