Ventilation is a great thing. Bringing outdoor air into the home and exhausting stale indoor air improves indoor air quality. Well, most of the time, anyway. Sometimes the outdoor air quality is worse than indoor air. Sometimes you bring in too much humidity and start growing mold. And sometimes you bring in the wrong outdoor air. But the issue of outdoor air vs. indoor air is a topic for another article.
Probably the most common type of whole-house mechanical ventilation system in homes is an exhaust-only system. You put some controls on the exhaust fans that are already in the home and those fans are set to exhaust stale air from the home, either continuously or intermittently. The problem is this type of system sucks. Literally. And if your house is sucking from an attached garage, a moldy crawl space, or dirty attic, you could be making things worse.
One way to avoid having a house that sucks is to do balanced ventilation. You exhaust stale air from the house and you supply an equal amount of outdoor air directly rather than relying on the negative pressure of the house to bring in the outdoor air.
Here are five ways to do balanced ventilation. I’ve put them in increasing order of cost, complexity, and efficiency.
1. Open the windows
OK, technically I shouldn’t include this one because it’s not a real solution for most homes. This one works only if the home is in a mild climate that needs to little to no conditioning. But if that’s your situation, you don’t need a fancy ventilation system. Just open the windows.
2. Pair a central-fan integrated supply system with the exhaust fans
A lot of homes get exhaust-only whole-house ventilation (fans plus controls). One easy way to upgrade is to install a central-fan integrated supply system to complement the exhaust-only side. The two most commonly used controls for this are made by AirCycler and Honeywell.
These systems are integrated with the blower in the central heating and cooling system. They bring in outdoor air when the system is running and mix it with the indoor air circulating through the duct system. It gets filtered and conditioned before being introduced into the home. When tied to the exhaust-only controls, you get balanced ventilation.
The main drawback of this system is energy use in systems that don’t have variable-speed blowers. In addition to bringing in fresh air when the system is heating or cooling, it can turn the blower on when the home doesn’t need heating or cooling. And some blowers use a lot of power. Turning on a 400 watt fan to bring in 50 cubic feet per minute of outdoor air is overkill. If you have a high-efficiency heating and cooling system with a variable-speed blower, you should be able to do this at less than 50 watts.
And another drawback is moisture. In a humid climate, running the blower without the compressor on can evaporate moisture on the coil and put it back into the home. (I’ve shown my data on what happens when you leave the fan on continuously in the summer in an article called How Your Thermostat Can Grow Mold and Make You Uncomfortable.) This wouldn’t be quite as bad as that, but it can make things worse. Curt Kinder, an HVAC contractor in Jacksonville, Florida, wrote recently, “Data and experience in my area of operation, northeast Florida, suggest that operating the system fan independently of the compressor raises indoor relative humidity by upwards of 10%.” If that scares you, consider the next three options.
3. Pair a supply fan with the exhaust fans
Another way to get balanced ventilation is to use the exhaust fans with controls and also install a supply fan. You can do this with a bath fan installed to blow air into the home or you can use a fan made specially for this task. I’m thinking of the QuFresh fan made by Air King.
They have two basic models. One has a sensor for temperature and relative humidity, and the other does not. The purpose of the sensor is to limit the amount of ventilation when it’s really cold, really hot, or really humid outside. It’ll still run 15 minutes an hour so you’ll keep getting some ventilation air.
I like the concept and the features in the QuFresh fan. I haven’t had a chance to try one out yet, but they do a lot of good things. You can adjust the flow rate from 30 to 130 cfm. It has a slot for a 2 inch filter that could be up to MERV 13. It’s quiet (0.5 sone at 50 cfm). And it’s relatively inexpensive.
4. Pair a ventilating dehumidifier with the exhaust fans
The strategy here is to use controls on your exhaust fans, as in the previous two, and supply your ventilation air through a whole-house dehumidifier. Many models allow you to do this by providing two intake ports on the dehumidifier, the smaller of which attaches to a duct that goes to the outdoors. We like Ultra-Aire but you can also find good models from AprilAire, Honeywell, and others. (Disclosure: Therma-Stor, which makes Ultra-Aire dehumidifiers, advertises in the Energy Vanguard Blog.)
One drawback of dehumidifiers is the heat they put into your home. The Ultra-Aire model SD12 eliminates that problem by being a split-system dehumidifier. It removes the humidity indoors but puts the heat outdoors. That means it even provides a bit of cooling (about a third of a ton).
5. Use a heat-recovery or energy-recovery ventilator (HRV or ERV)
This is what most people think of when someone mentions balanced ventilation. The photo at the top of the article shows the inside of a typical ERV, with arrows to show the two air streams. (An HRV looks the same but uses a different material in the heat exchanger.)
The operation is simple. It has two fans, one to exhaust stale indoor air, one to bring in fresh outdoor air. It filters both air streams. The two air streams pass through a heat exchanger, a capillary core in most models. The two air streams pass near each other and exchange heat in an HRV and heat and moisture in an ERV. But the two air streams don’t mix.
This is a great way to ventilate a home. It’s also more expensive than the methods mentioned above. Panasonic does have a small “spot” ERV called the Whisper Comfort, but aside from that model, you’re probably looking at $1,000 or more for an ERV or HRV. The biggest difference between this type of balanced ventilation and the previous ones is the heat exchanger. You get balanced ventilation with recovery, which means you don’t need to do as much conditioning of the outdoor air you bring in.
There’s your quick rundown of the main ways to do balanced ventilation. We’re seeing a lot of creativity in the ventilation market these days because ventilation is a big deal. I think we’ve gotten to the point where we rarely have to fight the battle about the need for airtight houses. The old myth that a house needs to breathe, while not completely gone, has mostly been relegated to the dustbin of bad thinking.
But let’s be clear about what balanced ventilation really means. It doesn’t mean the building is always at neutral pressure with respect to outdoors. Wind, stack effect, and other mechanical systems can always unbalance a building. When we talk about a balanced ventilation system, we mean simply that that particular system doesn’t result in the building going positive or negative because it’s exhausting and supplying equal amounts of air.
Another point here is that my main motivation in writing this article was to show some ways you can convert an exhaust-only system to a balanced system. Methods 2 and 3 above show how you can do that relatively easily and inexpensively. Method 4 would be a good way to do it in the humid climates of the southeastern US.
And one more thing. I was going to make this a list of six ways to do balanced ventilation but decided to save the other one for a separate article. Going a step beyond the ERV, you could go with a souped-up ERV. There are two companies making devices that include balanced ventilation with recovery, a small heat pump, better filtration, and more. One is the Conditioning ERV, or CERV, by Build Equinox. The other is the Minotair by Minotair Ventilation. We’ve got two HVAC design jobs going right now that will be using these devices, so stay tuned for more on these later.
Meanwhile, try to stay in balance. Or at least be positive.
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