Note: This is Part 2 in a series. To see Part 1: Clearing the Air, click here. To see Part 3: Commissioning a Ventilation System, click here.

"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.

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.

This system can work well, but what we've found is that many CFIS systems have been thrown in with the assumption that if there's a duct, there will be air. Just as most bath fans don't move enough air, however, CFIS systems that were never commissioned often don't deliver on the promise of better indoor air quality. It turns out that placement of the outdoor air duct on the return plenum can have a big effect on the amount of air delivered. Always commission! (Which means, measure the air flow through the duct and adjust the duct as necessary.)

The ventilating dehumidifier is a great system for humid climates, especially in low-load homes. It works just like the standalone supply with one important difference. There's a box with two ducts bringing air in from outdoors and indoors. Then there's a duct sending the tempered ventilation air into the house. The difference is what's in the box isn't just a fan; it's a fan with direct expansion dehumidifier.

Since ASHRAE 62.2 got rid of the default infiltration credit, new homes trying to meet the 2013 version will probably need more ventilation air. Small, energy efficient homes have low cooling loads, which means the air conditioner won't run as much. As a result of these two things, many of these low-load homes in humid climates will need some kind of supplemental dehumidification. The ventilating dehumidifier is a great solution, and it's probably even better in mixed-humid than in hot-humid climates because of their already lower cooling needs.

Balanced. Better than either sucking or blowing ventilation strategies is the sucking and blowing strategy. If you do both at the same time with balanced air flow into and out of the house, your ventilation system doesn't affect the house pressure. That's a good thing.

Everyone always thinks balanced ventilation is synonymous with using an energy recovery ventilator (ERV) or heat recovery ventilator (HRV). It's certainly true that ERVs and HRVs are ways of doing balanced ventilation (or can be when they're commissioned properly). But those would both more accurately be described as balanced ventilation with recovery.

What they do is to have the two airstreams pass near each other without mixing (an ideal that isn't always met). As the two airstreams go through the ERV or HRV, they also exchange heat (ERV and HRV) and moisture (ERV only). In winter, then, the outgoing warm air gives up some of its heat to the incoming cold air. The warm air also has more moisture and in an ERV, some of that moisture will migrate to the cold, dry air coming into the house. So ERVs and HRVs are balanced ventilation with partial recovery of heat and moisture.

Another type of balanced ventilation system would be a simple combination of exhaust and supply fans, set up to come on and go off together and move the same amount of air. The Lunos fans exploit this concept and even add heat recovery. They don't have to be in the same part of the house, and in fact it's better for mixing if they aren't. Then the positive pressure where the supply fans are can push air into the lower pressure areas near the exhaust fans.

So those are your options for balanced ventilation. You can blow equal amounts of air into and out of the house while recovering heat and moisture, or you can just blow equal amounts of air into and out of the house. If you're looking for the Cadillac of ventilation systems, look no further than an ERV or HRV. There are some very sophisticated models out there with really high efficiency. The two most popular with the Passive House community are those made by Zender and Ultimate Air, but the new Air Pohoda has a clever new way of recovering heat and moisture. It even lets you dial in the amount of latent recovery you want and can eliminate the need for defrosting the recovery core. [, such as …. can you suggest one or two examples?]If you're in a fairly mild climate without too much latent load, balanced without recovery could be just what you need.

Distributing the ventilation air. Once you've chosen a ventilation rate and a strategy – exhaust, supply, or balanced – you're almost there. You still have another important decision to make, though: How will you move the ventilation air around the house?

On this topic, I agree with Henry Gifford, who said, "Give me an H. Give me a V. Give me an AC. Don't do HVAC to me." What he meant by that was that he likes to keep those three functions separate. If you're dealing with forced-air systems in homes, separating heating and air conditioning isn't important or practical. But using a separate duct system for ventilation can help.

The argument in favor of using the heating and air conditioning ducts for ventilation is that those ducts are already there, so you might as well use them. It also prevents getting walls, ceilings, and floors cluttered with extra vents and allows you to use the air handler fan to bring in your ventilation air.

The arguments against using the heating and air conditioning ducts are: (i) It can be difficult to get the right amount of air flow; (ii) you can balance an ERV or HRV connected to the return plenum or supply trunkline for when the air handler is running or when it's not running, but you can't balance it for both cases; (iii) using the air handler fan to ventilate the house isn't the most efficient way to ventilate since it's usually the biggest fan in the house; and (iv) the heating and air conditioning ducts are designed (we hope!) to move a lot more air than the ventilation system so trying to ventilate through them without the air handler running (as with an ERV/HRV, standalone supply system, or a ventilating dehumidifier) may not result in ventilation air getting distributed well.

In the end, to get the best ventilation possible, it's best to use a duct system dedicated to the ventilation system. Obviously that doesn't work for exhaust-only ventilation, but then why would you want to do that if you care about ventilation effectiveness. It also doesn't work for central-fan integrated supply because it only works when you use the air handler and heating and air conditioning ducts. But either type of balanced system (with or without recovery), a standalone supply system, or a ventilating dehumidifier will do better with its own duct system.

What should you do? If you care about good indoor air quality and whole-house ventilation that works, the first thing to do is avoid exhaust-only systems. Supply-only systems are a step up but have a slight possibility of creating condensation problems in cold climates if the building does not have exterior insulation. This type of system could create a slight positive pressure inside the building, forcing warm, humid, indoor air into wall cavities. If that air finds cold sheathing, it can wet the materials and eventually grow mold or rot the wall. Balanced is best when it comes to ventilation, and balanced with recovery is the best of the best.

Beyond picking a system, though, there are three critical steps to getting a ventilation system that really works: Design properly. Install well. Commission completely. (Commissioning is nothing more than a quality-control check to make sure what you did performs as it should. We'll explore this in detail in the next part of this series.) Get those three things right, and you'll have a system that should do its job, provide good indoor air quality, and, perhaps most important, not be disabled by the occupants who don't like the noise or comfort problems that too many ventilation systems have.