Why I'm Not a Fan of Positive Pressure Mechanical Ventilation
We want our homes to be airtight. It saves energy. It helps keep water vapor from getting into places where it can cause problems. It keeps bad air from moldy crawl spaces and contaminated garages out of the house. It just makes sense. Tight homes, though, need a way to replace stale indoor air with outside air. That's the role of a mechanical ventilation system. Of the three types of mechanical ventilation systems, home builders in the Southeast have latched onto one that has a number of problems: the positive pressure system. (The other types are negative pressure and balanced.)
First, a brief review of mechanical ventilation requirements. Paul Raymer, one of the gurus on this topic, wrote a guest post on that topic in the Energy Vanguard Blog last year. The standard that sets the guidelines for how much we need is called ASHRAE 62.2-2010. It specifies a couple of ways that you can figure out how much outside air you need to bring into your home, one of which is by using the table below.
What's wrong with positive pressure mechanical ventilation?
First, let's look at how a positive pressure mechanical ventilation system works. By positive pressure, I mean that the air pressure inside the house is higher than outside pressure, which tends to push air from inside to outside. You can bring in outside air and get positive pressure in the house in one of a few ways, but the main way that home builders do it around here is by using the air handler unit (AHU) in the heating and cooling system, as shown below. (Yes, I've shown the ducts in the attic here because so many are. Putting ducts in an unconditioned attic is still stupid.)
Here's a brief rundown on what's going on:
- When the heating & cooling system is running, the blower in the AHU pulls air from the house on the return side, conditions it, and puts it back into the living space through the supply vents.
- Whenever the electronic damper is open in the ventilation duct, the AHU is also pulling a little bit of air from outside and mixing it with the air from the house. This extra air added to the house is what provides the positive pressure.
- The amount of air that comes through the ventilation duct depends on the size (and other characteristics) of the duct and how the mechanical damper is set (and ideally is based on what ASHRAE 62.2 requires).
- The electronic damper is wired to a controller that determines when ventilation air is allowed to enter.
- The electronic damper can be open or closed when the heating or cooling system is operating.
- The electronic damper also can be open when the house doesn't need heating or cooling, in which case it will run the blower in the AHU independently of the furnace, heat pump, or air conditoner.
- The controller allows the homeowner to set how much ventilation air the home gets.
This type of mechanical ventilation system has been the most popular one installed in high-performance homes in the Southeast over the past decade. It's relatively inexpensive, as the duct, dampers, controller, and installation generally add only about $300 to the cost of the HVAC system. Positive pressure also has the advantage of helping to keep humid air from infiltrating into the house.
Still, I'm leery of this system for a number of reasons:
- You're using the biggest motor in your home, the blower in the AHU, to move a small amount of air. The AHU typically moves 1000 to 2000 cubic feet per minute (cfm) of air, but most homes need only 50 to 100 cfm for ventilation (see table above).
- When the controller brings in outside air at times when the heating or cooling system isn't operating, not only are you using a motor that's way too big, but the outside air comes in at outside conditions: cold in winter and hot and humid in summer.
- A lot of these systems don't bring in as much air as they're supposed to. I was just talking to someone this week who finished a master's thesis in building science on this very topic. She studied this type of system in a multi-family development and found that the systems underperformed because there wasn't enough static pressure to pull in the required amount of air.
The last problem above should be easy to fix. Design and install everything properly and then go through a commissioning process to ensure that it works. One of the nice things about ENERGY STAR Version 3 is that not only is mechanical ventilation required for all homes, but the HERS rater has to measure the air flow to make sure it works.
The first two problems are serious drawbacks that don't have easy solutions. OK, if the AHU has a variable-speed, electronically commutated motor (ECM), that can help a lot with the first problem, but the fact is most blowers still use fixed-speed motors.
The problem of bringing in unconditioned air into the house using the AHU, though, isn't as easy to solve. One potential problem with bringing outside air into the duct system in summer when the air conditioner isn't running is condensation in the ducts. In a place like Charleston, SC, for example, where the dew point pushes 80° F in the summer, it's not hard to imagine condensation happening.
These systems have become popular in the Southeast because they're inexpensive, but maybe it's time to rethink whether we should be installing them.
What's the solution?
For hot or mixed humid climates, which the whole Southeast, what kind of ventilation system should you use? If positive pressure is good for combatting the infiltration of humid air, that means a negative pressure system wouldn't be so good. That leaves the balanced system: a heat recovery ventilator (HRV) or enthalpy recovery ventilator (ERV). Martin Holladay wrote a good article on how to choose between an HRV and an ERV a while back, so that would be a good starting point for learning more.
Do you have a mechanical ventilation system in your home? Which type? How well does it work? If you're a home builder, HVAC contractor, or HERS rater, what's your experience with positive pressure systems of the type I described above? Any problems?
Breathe! - Get Fresh Air into Your Home with ASHRAE 62.2
Dew Point — A More Meaningful Measure of Humidity?
How Much Air Leakage in Your Home Is Too Much?
Photo of fire breather by leepus from flickr.com, used under a Creative Commons license.