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Mechanical Ventilation: 7 Minutes of BS

February 02, 2017

As houses are built tighter, they exchange less air between inside and outside. This saves energy, but the inside air can get stale if you don't have a fan and a plan

In this episode, we are joined by Jonathan Smegal, M.A.Sc, a Senior Project Manager with RDH Building Science Labs to talk about how yesterday's ventilation problems have flipped.

What it is:

me·chan·i·cal ven·ti·la·tion

məˈkanək(ə)l ven(t)əˈlāSH(ə)n | (noun)

"Mechanical ventilation is the intentional addition of fresh air into the living space for occupant health and comfort."

Jonathan Smegal, M.A.Sc, RDH Building Science Labs

Why it matters: Comfort, health, energy efficiency

Fresh air in a house is very important for the health of the pets and peeps who live there. It also plays a key role in comfort, which translates to fewer complaints for everyone.

Commercially, it improves productivity. But let’s go back to the first one, health. Ventilation cleans the air of pollutants.

" ... whether they are gaseous, particulate, they could just be odors, they don’t have to be harmful, they could just be annoying to the occupants—"

I used to have this dog who was half German Shepard and half Great Dane. And she was a very gaseous pooch.

" ... and other irritants."

"It can add to the comfort, if done correctly, of the occupants—mostly with respect to the interior relative humidity levels. You find that if they get to be extremely low or extremely high the occupants aren’t as comfortable."

And adequate ventilation boosts office productivity.

"Ventilation is important for productivity in a lot of places — in offices—productivity is linked to the ventilation and the indoor air quality ..."

So people who can breathe clean air are more productive. Go figure.

Back in the olden days, ventilation was a completely different problem. Used to be, houses were too leaky. That’s one reason why they invented wallpaper.

"Fresh air in houses was taken care of by the fact that the enclosure was incredibly leaky. But as we’ve tightened building enclosures, the ventilation problem flipped. At the end, Jonathan is going to get into how tight is too tight, but now we’ll look at how leaky is just right.

"ASHRAE 62.1 and ASHRAE 62.2 will tell a builder or an HVAC designer how much ventilation is required either in single family residential or commercial buildings."

How to ventilate right

The American Society of Heating, Refrigerating, and Air Conditioning Engineers has determined how big the holes should be and how many of them are required. And that depends on the building size and the number of people who live there.

"The three most common ways to ventilate a building is to have exhaust only ventilation, supply only ventilation, or balanced ventilation."

—Blow out, suck in, or breathe.

Exhaust only ventilation is probably the most typical and you see it in a lot of residential buildings that have continuously-operated bathroom exhaust fans.

In an exhaust only ventilation system there is typically a constant negative pressure and they are blowing air out of the building and


"Drawing air in from other locations. One of the biggest challenges with exhaust only ventilation system is that you don’t usually know where the air is coming from."

"You’re sucking air through your enclosure, or other locations that aren’t as air tight."

But you do know that it is coming from a dirty crevice full of mouse poop and fiberglass dust.

"It’s not often a good idea to breathe the air that comes through the enclosure. You never know what’s in there."

"The advantage of exhaust only ventilation is that it is simple and often quite cost effective, and it does provide some amount of mixing for a relatively low price. Unfortunately, there’s no energy recovery available on exhaust only ventilation."

"And if you get too much exhausting, you can start to risk back drafting other appliances."

He’s talking about you, commercial range hood in a residential kitchen. He knows you suck kittens across the floor.

"You need to supply a specific source of makeup air so that you don’t start pulling air down through chimneys or through gas-powered appliances."

Which is worse than sucking kittens across the floor. It actually sucks combustion gasses—which kill people—down the chimney and into the living space.

So, yeah. Waaay worse than sucking kittens.

"Supply-only is basically the opposite. You have fans that blow air into your building …"

Called supply fans

" ... and it forces air out through some of the spots in the enclosure and through some of the holes in the building."

"In its simplest form it is literally just a pipe that isn’t really connected to anything, it allows for natural mixing. It might be into the area with the return for the furnace … or they often can be piped right into the ductwork, of the HVAC system and then it is filtered and conditioned and then it’s distributed to the house."

The main downside to pressurizing the house is that during cold weather this can cause moisture to build up inside the wall cavities

" ... if we start blowing warm moist air through the enclosure in a cold climate, we might end up with condensation and moisture issues."

Which can lead to BMR.

So that’s probably the biggest risk of supply only ventilation.

Bugs, mold, and rot.

"In balanced ventilation, you try to avoid putting pressures on the enclosure. So the air that you have coming into the building as fresh air, is balanced by the air leaving the building. Most often that’s done with an HRV, a heat recovery ventilator, or an ERV, an enthalpy recovery ventilator ..."

Ummm, okay, so most people call them ENERGY recovery ventilators. Engineers sometimes like to be precise.


ˈenTHalpē,ənˈTHalpē | noun

“A thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.”

The Google

I don’t even know what that means. Let’s just stick to ERV and avoid the discussion.

"The disadvantages to this system is that they’re more expensive, they’re more difficult to install, but the advantages are they’re balanced, you don’t get pressures on the enclosure, they can be very effective at providing fresh air ...

and they can be very energy efficient—

" ... by recovering the heat that you’re exhausting into fresh air, you can reduce the amount of energy required to condition the air once it gets into the building.

OK. Hold on to your poncho because now we’re going to talk about how tight is right and how tight is wrong.

"Historically, buildings could be ventilated just by the virtue that they were leaky enough. Air would pass through the enclosure in both directions, you’d get mixing of interior and exterior air.

But as we move toward more energy efficient building we require more ventilation—intentionally—because we don’t get as much mixing across the enclosure.

It is important to make buildings as airtight as possible. I don’t think it is possible to be too airtight—you just need to be able to add the ventilation that’s required for the occupants, when your building gets more air tight.

By having an airtight building you can have control over where that ventilation air comes from, and so it’s no longer coming through your enclosure walls, through the dust and dirt and debris and whatever else you have in your walls during construction that’s been there 20 years,

That was kind of a bait and switch. I made you think he was going to say there is such a thing as too tight. But he didn’t. He did say that there is such a thing as under-ventilated.

"So you’re not breathing your walls anymore."

And there’s your bottom line: Walls are meant for holding up roofs, not filtering air.

Now you know a little bit more about mechanical ventilation which is good because you get paid for what you do and what you know. One thing you can do to know more is subscribe to this podcast

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—7 Minutes of BS is a production of the SGC Horizon Media Network.

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