Last week I came home, opened my front door and was greeted by the smells of a delicious supper cooking, so of course the first words to my dear long suffering wife Kathryn was to say “I think I’ll bump the ventilation rate to 120 CFM”.

Hi, I’m Ross and I’m an energy nerd, welcome to my first blog post.

I had just finished replacing our perfectly good, 10 year old 67% efficient heat recovery ventilation system with a brand new 83% one, something no normal person would ever do with a spare $2400 and many hours of work (of course first the mechanical room needed a coat of paint while that big box was out of the way, which then led to a half dozen other jobs like drywall patching and convincing Kat that we needed to spend a small fortune on a new heat pump water heater – but that’s an episode for another post!).

Considering that we generate more free renewable energy from our solar array than our net-positive energy LEED Platinum home uses in a year, gaining 16% in ventilation efficiency saved me exactly… $0! But hey, those spare electrons go out into the grid where someone else gets some of my sunshine in their power line, so it’s all good. 

Does your house smell funny? 

As a retired Registered Energy Advisor, I have smelled thousands of houses all over Canada, and I can tell right away when I step in your home whether your house has a “fresh air machine” running. If the odour of last night’s fish hits me when I walk in, then I know you don’t have a working HRV or ERV in your house (hang on, I’ll explain the difference shortly). But if I step inside and all I smell is daisies and rainbows, I know you’ve got good ventilation going on. 

So back to where I started this story. I had been obsessing over the “perfect” set & forget continuous whole house ventilation rate for our new Enthalpy Recovery (ERV) unit, and after a great deal of deliberation I had chosen 100 CFM, but apparently my nose disagreed with my math. So how much ventilation does your house need, and why do you need it?

Let me start off by confusing you with a bunch of numbers that don’t match up. Building code generally requires 1 cubic foot per minute (CFM) per 100 square feet (SF) of floor area, PLUS 15 CFM for the first bedroom (assumed as 2 people) and 7.5 CFM for each additional bedroom (person). So our 2,000 SF, 3 bedroom house needs 20+15+7.5+7.5 CFM = 50 CFM, right?

Or, we could go with ASHRAE (62.2), a long debated standard that building science gurus like Joe Lstiburek (another esteemed PhD) and fellow ASHRAE committee members obsess endlessly over, which states we need 3 CFM per 100 SF, plus the same per bedroom as above. In that case, we need 60+15+7.5+7.5 = 90 CFM.

120 CFM of continuous ventilation forevermore

Then again, how about we calculate ventilation by house volume instead? One complete air change every three hours is about right according to most experts, and that was what I was the number I was taught some 40+ years ago, although during COVID some other experts called for 6 air changes per hour. My house has 23,862 cubic feet of air inside it, so 1/3 of that would be 7,954 cubic feet of fresh air every hour, which equals 133 CFM. 

So the experts all agree the right number is 50 CFM, or perhaps it’s 90 CFM, maybe 133 CFM, or at the extreme end to blow all those nasty COVID germs away, 2,386 CFM! Let’s just ignore that last one. So how about we just average 50, 90 and 133?

That brings us back to 91 CFM… so my final decision, based on nothing more than “91 CFM sounds pretty good but I think I might like a bit more fresh air”, came down to 100 CFM. Then my nose said nope, kick the dial up one more click, and since my Panasonic Intelli-balance 200 ERV ratchets up in 20 CFM intervals, that’s why my house will now have 120 CFM of continuous ventilation forevermore. Perfect!