Why an ERV is the Best Way to Ventilate
Ventilating your home is smart. By replacing dirty or stale air with fresh, healthy air, you can keep your head clear and breathe a whole lot easier.
And by "ventilating," we're not talking about opening windows or using a bath fan. We're talking about the consistent exchange of indoor air with outdoor air. Filtered, of course.
But other than opening windows - and we've already discussed why that's not a great idea - how should you ventilate? We recently explored why mechanical ventilation is a great indoor air quality upgrade for just about any home, no matter how "tight" or "leaky" the home is. What we didn't cover is the type of of mechanical ventilation you should select.
Here in the Atlanta area, there are two types of mechanical ventilation systems to choose from:
- Positive pressure ventilation systems, typically a whole-house dehumidifier with an integrated fresh air damper
- Balanced ventilation systems, which almost always rely on an energy recovery ventilator (ERV) in our climate zone
Long story short: Positive pressure ventilation is great for indoor air quality, but balanced ventilation is even better.
Here's the lowdown on positive pressure ventilation.
In most green grass climates around the U.S., it gets super humid during the summer. It's so humid, in fact, that many people opt to install whole-house dehumidifiers. These machines remove humidity when air conditioners aren't running, improving overall summertime comfort and indoor air quality.
Ventilation is a valuable add-on to these dehumidifiers. Wire in a damper, duct it to the outdoors, and set it to open at a frequency that makes sense for the home. You're set! The blower fan pulls in fresh air, which is dehumidified and filtered before it enters your living space.
We call this "positive pressure" ventilation because it involves pushing air into the home.
In most homes, the dirtier air seeps out on its own as the clean air is pushed in. This is the opposite of "negative pressure" ventilation, which involves pulling air out of the house so that less clean, unfiltered air from the outdoors naturally seeps in. That's what happens when your run a kitchen fan, FYI.
Positive pressure is a great thing - especially for "normal" homes that aren't particularly airtight or that haven't received significant energy efficiency improvements.
Balanced ventilation is different.
When a home has an ERV - the most common type of balanced ventilation system for warm, humid climates - clean air doesn't push dirtier air out of the home. Instead, the equipment mechanically introduces and removes air from the home at the same time.
We call this "balanced" ventilation because equal amounts of air are exchanged by the ERV.
For many years, ERVs have been a mainstay of high performance, energy efficient homes. Those homes are built "tight," so they don't allow a lot of air to enter or escape. A tight home is good for your energy bills, but it's almost impossible to ventilate without a mechanical system for getting old air out and bringing new air in.
An ERV solves that problem. It sucks the indoor air outside while also pushing outdoor air inside, filtering it along the way.
The diagram below demonstrates how this works. The red arrows represent "old" air being pulled from the house. The blue arrows represent the fresh, incoming air.
An ERV also transfers humidity from the incoming air to the outgoing air. It's not a dehumidifier, but it does introduce fresh air during the summer without increasing humidity in a significant way.
For indoor air quality, an ERV is the best choice.
Before going any further, we just want to say that positive pressure ventilation is a great choice for many homes. We're about to sing the praises of ERVs, but make no mistake! A whole-house dehumidifier with fresh air is still an excellent indoor air quality upgrade.
Ok. So why are ERVs better?
For starters, positive pressure systems generally rely on your HVAC system's blower fan. An ERV doesn't need to. While the ERV contains a fan that's less powerful than your HVAC fan, it's still powerful enough to ventilate your entire home.
That might seen like a minor difference, but it's huge. Here's why:
- Consistency: If your home needs, say… 60 CFM of ventilation for 24 hours per day, the HVAC blower fan will be too powerful to perform that sort of air exchange over a 24-hour period. Instead, it will cycle on and off to provide the same amount of ventilation in intervals. That's not ideal. If you're inside, it's better for air to be ventilating all the time. The ERV fan can run all the time, providing more consistent ventilation.
- Efficiency: Compared to an HVAC blower fan, your ERV's fan is less expensive to operate. It's less powerful and cycles on less frequently, so it ventilates your home without making a big dent in your utility bill.
- Control: With a positive pressure system, it's hard to be sure you're bringing in enough fresh air. It's all tied into your HVAC, so you're relying on two fans (there's one inside the dehumidifier) with different power levels. The ERV operates independently, so you can easily verify that it's exchanging the right amount of air.
In sum, an ERV offers more consistent ventilation and better control over how much fresh air you're getting. It's also easier on your utility bills.
Another ERV benefit: year-round humidity control
The big ERV benefit is ventilation. A smaller, but still significant, plus is humidity control.
We've already mentioned that ERVs transfer humidity from incoming air to the outgoing air. That's great during the summer. But during the winter, an ERV also transfers moisture from the outgoing air to the incoming air!
Compared to positive pressure ventilation, which basically dumps dry outdoor air into your home during the winter, the ERV lets you ventilate and maintain a more comfortable humidity level.
That way, you can better avoid the misery of dry skin and static electricity when it's cold outside. You're also less likely to need other mechanical systems, like humidifiers, when you ventilate with an ERV.
ERVs sound awesome! I want one! What next?
We come to your home and install the unit. There are two ways to do it, and neither is necessarily better than the other:
- Separate from HVAC: If you've got the space for new ductwork, we can install the ERV separate from your HVAC system. Typically, we'll set it up to remove air from bathrooms and introduce air into areas where you spend more time, like your living room. It won't tie into your HVAC in any way whatsoever.
- Connected to HVAC: With this installation method, the ERV still removes air from bathrooms via dedicated ductwork. The difference is that it pushes the fresh air into your return plenum. The air passes through the ductwork into the home whether the blower fan is running or not.
Regardless of the installation method, be sure to work with a home performance pro before selecting an ERV and having it installed. You might even decide to go with positive pressure ventilation, especially if you need serious dehumidification during the summer.
We just heaped tons of love on ERVs, but positive pressure ventilation is still far better than no mechanical ventilation at all. It's a great option for many people!
Ultimately, your home and your preferences are unique. Keep that in mind as you weigh your mechanical ventilation options.
Image Credit: Craig Payne