If you’re designing a ventilation system, first you have to determine how much outdoor air the house needs. You can use the ASHRAE 62.2 standard or the new BSC-01 standard for that task. Then you have to decide what type of ventilation system to use: positive pressure, negative pressure, or balanced. In many green homes, the balanced system is becoming a popular choice. I’ve seen some installations lately, though, that are missing a key component
The balanced system shown in the photo to the right is one such case. It’s an energy recovery ventilator (ERV) that’s connected to the ducts for the home’s heating and cooling system. The duct with the fresh air from outside is connected to the return plenum of the forced-air heating and cooling system.
If the system is designed to run continuously and factors in the negative pressure created by the duct system it’s attached to, this can work well. A typical ERV, though, can move 100 to 200 cubic feet per minute (cfm) of air. A typical house needs maybe 50 cfm (depending on the size of the house, the number of occupants, and which ventilation standard you go by). What many designers do is set the ERV to run intermittently, say 20 minutes out of each hour, to meet the ventilation needs of the house.
The missing link
If the ERV is designed to bring outdoor air into the house only 20 minutes per hour, there needs to be a way to close the hole to the outside for the other 40 minutes per hour. Such a device is the Turbo Thermo-Encabulator Max.
Oh, wait. Sorry. That’s for something else. What you really need is a motorized zone-control damper, like the one shown below. Every HVAC supply house sells them. The motor is wired to the ERV controller so that it opens and closes when the ERV turns on and off.
The sin of omission
If the motorized damper isn’t there, the negative pressure from being connected to the heating and cooling system’s return ducts will cause more outdoor air to be pulled in through the ERV. (The same will happen for an HRV.) The result is that the house will be overventilated, especially during winter and summer when the heating and cooling system runs more often. If the system is right-sized, it will run more often than a typical over-sized system.
[Editor’s note: The problem described by Allison Bailes only occurs with some brands of ERVs and HRVs — those that don’t include an integral motorized damper. Many ERVs and HRVs include a factory-installed motorized damper (located inside the unit). If you prefer to purchase and ERV or HRV with an integral motorized damper, contact the manufacturer before purchase to find out whether a damper is included.]
Another case where this creates problems is when the home has a ducted minisplit system. Those air handlers run all the time, though they do ramp down to low speed when the thermostat setpoint is met. Still, we’ve seen a system like this that brought in excess ventilation air and made the house uncomfortably humid in summer.
A standard forced-air heating and cooling system can cause problems, too. If the occupants set their fan to the on position, the system will pull in more outdoor air if there’s no motorized damper.
Even if the ERV or HRV has its own dedicated duct system and is not connected to the heating and cooling ducts, you need to be able to close the hole when it’s not running. Otherwise, stack effect, wind, and other mechanical systems in the home can pull in outdoor air when it’s turned off.
If you’re designing or installing ventilation systems, make sure you get to know this little device. It’s an essential component.