Everything must be made as simple as possible. But not simpler.” — Albert Einstein
Ventilation is not a simple topic. But that doesn’t mean it has to be incomprehensible. In the first article of this series on ventilation, I wrote about the debate over ventilation rates and standards, including a bit of the history of ventilation. This time we’ll get into the nitty gritty, looking at the three different ventilation methods: exhaust-only, supply-only, and balanced.
This won’t be a comprehensive cataloging of systems, though. I’ll help you sort through the pros and cons of the major types and provide some guidance as you choose a mechanical ventilation system for your project. There’s not a single answer here. The best system for you will depend on whether you’re putting it in a new or an existing home, what your budget is, and what the priorities of the occupants are. I’ll also tell you how I prefer to see them installed.
Decide on the ventilation rate. In the first article, I explored the issue of ventilation rates as they relate to standards, codes, and programs. If you’re building a house, you’ve got to figure out what ventilation rate you need for the mechanical ventilation system you install. Of course, I’m assuming you’re reading this article because you are installing a ventilation system. If you’re building homes with the level of airtightness many codes require these days, and especially if you’re insulating with spray foam, we don’t need to argue about the necessity of mechanical ventilation. Just do it.
When picking a ventilation rate, you first need to find out if your building code or efficiency/green building program requires ventilation. If so, you’ve got to install a system that meets their requirements. ENERGY STAR for new homes (and gut-rehabs) and LEED for Homes, for example, both currently require mechanical ventilation that meets ASHRAE 62.2-2010.
If your local code doesn’t require ventilation and the home isn’t going for certification in a program that requires it, you can do what you want. Building an airtight house and installing mechanical ventilation is the smart way to go, and when it comes time to decide how much ventilation you need to install, Building Science Corporation’s BSC-01 is what I’d recommend. I described how it works in the first article, but briefly, it starts with the ASHRAE 62.2-2010 rate and then modifies that number up or down according to how balanced, distributed, and mixed the ventilation air is. Now, let’s look at the types of systems.
Exhaust-only. All right, let’s get this one out of the way first. Using exhaust fans is great for local ventilation of bathrooms, kitchens, garages, and sub-slab areas (for radon control), but it’s not a great strategy for whole-house ventilation. It does have some advantages:
- It’s cheap
- You’re already installing those fans anyway
- It can help prevent condensation inside walls in cold climates
Now let’s look at the disadvantages:
- You don’t know where the makeup air is coming from
- Contaminants from an attached garage or moldy crawl space can be pulled into the house
- Ventilation air probably won’t be mixed or distributed well
- The only filtration that happens is through the building enclosure.
The goal behind whole-house mechanical ventilation is to provide some measure of good indoor air quality. If you take an objective look at those advantages and disadvantages above, it’s hard to conclude that you’ll get that result with an exhaust-only system. But hey, if meeting code or program requirements at the minimum cost is your objective and you don’t care about IAQ, this could be your ventilation strategy.
If you decide to go this route, please don’t do it in a humid climate where the house will be air-conditioned a significant amount of time. Exhaust fans work by pulling air from the house and sending it outdoors. This puts the house under a negative pressure, and the makeup air comes into the house through random leaks. If enough humid air gets pulled into a wall cavity, you could get some nasty microbial growth in there when it finds the cool backside of the drywall.
Also, if you’re building apartments or condos, you’re likely to run into difficulty with makeup air. Compartmentalization is leading to much greater airtightness. You can’t pull air from the corridors or from neighboring units; it’s got to come directly from outdoors. So, you put enough holes in the exterior wall to allow the ventilation system to move enough air. And then the occupants seal up those holes to stop the drafts as soon as it gets cold.
Hmmmm. Sucking on the house isn’t such a great idea.
Supply-only. The next option is blowing. You use a fan in the house to collect outdoor air from a known location and distribute it indoors. This overcomes several objections to the exhaust-only system: You know where the air is coming from; you can filter the air on its way in; and you’re not causing the house to suck in contaminants from the garage, crawl space, basement, or attic. It’ll cost you more than exhaust-only ventilation, though, because you’ve got to spend money on additional controls, ductwork, or fans.
Next, you’ve got to figure out what type of supply-only ventilation system you want to install. You have three options: Standalone supply, central-fan integrated supply (CFIS), or a ventilating dehumidifier.
The standalone supply ventilation system uses its own fan to bring outdoor air into the house. To do it without comfort complaints, you’ll need to temper the outdoor air before introducing it into the house, which you can do by mixing it with indoor air.
In the minimal configuration, you’ll need a box with a fan in it and three ducts. One duct will bring in the outdoor air. A second duct will pull indoor air to the box to mix with the outdoor air. A 2:1 ratio of indoor air to outdoor air is a good mix, so if you need to deliver 100 cfm of outdoor air, you’ll need a fan that moves 300 cfm. You can make the system more sophisticated by pulling indoor air from different locations and delivering the mixture to other locations in the house. That’s a good strategy because the less the occupants notice the system, the more likely it is they won’t turn it off.
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The central fan integrated supply ventilation system is the most common supply-only system, especially here in the humid Southeast. In a house with a ducted, forced-air HVAC system, the CFIS system uses the air handler or furnace fan.
In the barebones configuration, all you need to do is install a duct from the outdoors to the return side of the air handler. Then whenever the heating or cooling system comes on, the air handler will pull in some outdoor air to mix with the return air. One drawback of this configuration is the hole in your building enclosure that sits there 24/7/365.
Although adding a duct only was all some builders did in the old days, now we know it’s critical to use controls like the AirCycler and a damper to close the duct when you don’t want ventilation air. The control can be configured to run the ventilation a certain amount of time, say 30 minutes of each hour, so you get the right amount of ventilation without overventilating.
Ideally, a CFIS system will deliver most of the ventilation air when the HVAC system is already running. In winter and summer, that’s usually not a problem but in the swing seasons or on mild summer and winter days, the house will need ventilation air at times when the system isn’t normally running. In those cases, the controller will turn on the air handler fan to deliver ventilation air when the house isn’t calling for heating or cooling.
Reference: http://www.jlconline.com/how-to/hvac/choosing-a-whole-house-ventilation-strategy_o