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 world.
I had just finished replacing our perfectly good, 10 year old 67% efficient heat recovery ventilation system (ERV) with a brand new 83% efficient one, something no normal person would ever do with a spare $2400 and many, 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 sanding and convincing Kat that we needed to spend a small fortune on a new CO2 heat pump water heater that needed to be installed before the ERV – but that’s a story for another post!
According to HOT2000 energy modeling, we generate more free energy from our solar array than our home uses in a year, so our home is net-positive-energy and our electricity is almost free. According to the same energy model, ventilation is now only 1% of our total energy usage, so gaining 16% in ventilation efficiency with a new unit saves something like zero actual dollars. But on the plus side, the air supply is delivered to the bedrooms at 5 degrees warmer, and it’s a quieter, cleaner (HEPA filtered) and smarter fresh air machine. And fresh air is important in houses.
Does your house smell funny?
As a 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 probably don’t have a working HRV or ERV in your house. But if I step inside and all I can smell is the wind and the sky, I know you’ve got good ventilation going on.
Every house needs ventilation – not through random cracks and holes, but ideally by intentional, measured fresh air coming in and stale air going out. My friend and energy guru Allison Bales, PhD entitled his new book, “A House Needs to Breathe… Or Does It?”, and the definitive answer is that we really don’t want leaky houses, we want to “build tight, ventilate right” (Perera, 1992).
So back to where I started this story, coming home and smelling supper cooking. I had been obsessing over the “perfect” set & forget continuous whole house ventilation rate after finally installing the new Panasonic Intellibalance 200 Enthalpy Recovery Ventilation (ERV) unit, and after a great deal of deliberation I had chosen to set the flow rate at 100 CFM. But then my nose disagreed with my math, and I kicked it up a notch. 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.
90 CFM of continuous ventilation?
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 right back to the ASHRAE standard, which also coincides with the Ontario Building Code. The ERV blower speed controls are in 20 CFM increments, so after far too much deliberation I went for 100 CFM. Then my nose said nope, let’s just arbitrarily kick the dial up one more click, and that’s why my house will now have 120 CFM of balanced, continuous fresh filtered air forevermore. Done!
Got questions? Send them via e-mail or the “contact us’ form. Some ideas for future blog posts – heat pumps and heat pump water heaters, strawbale building, deep energy retrofits, indoor air quality, mold and radon, bubble greenhouses, flying concrete… and the answer to the question about ERVs vs. HRVs.
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