We bet you have thought about air quality a few more times since 2020 than you ever did before! The recent pandemic has made so many of us aware of the particles in the air we breathe that can make us sick. Those of us in Passive Home design have always been aware of air quality and how we can make the air we breathe inside of our spaces as healthy as possible. It’s always been front and center in our design approach. That’s why we focus so much on Energy Recovery Ventilators in our conversations with customers and partners.
Energy Recovery Ventilators (ERV) are a vital piece of equipment for passive or high-performance homes. Most existing homes are super leaky and get plenty of “fresh” air through the holes and cracks in the building envelope. Newer homes are getting more and more airtight and need more fresh air. ERVs are designed to provide ventilation that removes stale air and sometimes humidity, while delivering fresh air into the home. That “stale” air contains CO2, humidity, and toxins. We breathe, creating humidity and CO2, and cook, creating humidity and toxins. That new couch with the new couch smell – well – that smell is usually not good for you. The smell is generally toxins off-gassing from the materials the couch is made of.
At the very core, ERVs have blowers and a heat exchanger. Their primary objective is to improve indoor air quality by bringing in fresh air. In the winter months, ERVs help recover warm air that may have otherwise been lost, especially in leaky, older homes that get plenty of fresh air through the holes and cracks in the building envelope. ERV systems capture some of the warmth from the exhaust or stale air stream and transfers that warmth back into the fresh air stream it’s pulling from the exterior. Likewise, in the summer months, ERVs help keep the heat out and the cool in by reversing the process. They put the “warmth” coming in via the fresh air stream into the exhaust air stream. In doing so, ERVs improve the HVAC system efficiency of a home as a primary benefit!
What’s the Difference Between an ERV and an HRV?
When you start looking into ventilators, you will see the acronym for Heat Recovery Ventilator (HRV) pop up. At first glance, it may feel like ERVs and HRVs are the same. The primary difference is ERVs manage or transfer humidity where HRVs dump the humidity, especially during the colder months when it’s pretty dry and we benefit from the humidity. The entire discussion around whether you should have an HRV or an ERV is changing. The variables historically included the size of the home, average humidity, and whether it is a heating or a cooling climate which is defined by heating/cooling degree days. It is our understanding that ERVs are the current direction in general, especially with high-performance, airtight homes.
With ERV systems, the ducting is a vital piece of the process. While some units do a lot of sophisticated work, the design of the ducting is crucial for success with all types of ERV and HRV systems.
Here are a few key guiding principles for duct design:
- If possible, keep ducting as short, fat, and straight as possible to minimize static pressure and air turbulence. Static pressure makes it harder for the blowers to do their job and the system wears out sooner as a result. Go deeper into the math on duct design!
- Set up dedicated ducts for an ERV in new construction. In retrofits, existing ducts can be leveraged as well. There are some important considerations on where to tap into those ducts.
- Insulate Fresh and Stale air ducts as much as possible. R-8 or better is the general recommendation. These are the ducts that go to the outside wall and bring in the fresh air and exhaust the stale air. Also, these ducts shouldn’t terminate right next to each other on the wall. They should be at least 10 feet apart.
- Return ducts are generally for polluted and humid areas – kitchens, baths, or even laundry spaces.
- Supply ducts generally go to the living areas and bedrooms.
- Consider a sound dampening option with insulated flex or a silencer, especially in high-performance homes. Blowers and airflow generate some sound; generally, high-performance homes are super quiet. Our feedback from clients has been the “ERV is making noise” and silencing ducts help reduce the noise.
ERVs We Love
Our team is a big fan of Build Equinox’s CERV 2, which uses a high-efficiency heat pump to exchange energy(heat) between incoming supply and outgoing exhaust air. The CERV 2 measures and manages indoor air pollutants, CO2, and humidity. It brings in fresh air only when you actually need it. It also has an efﬁcient inverter drive heat pump, which performs the heat exchange. The heat pump assists the primary HVAC system with heating and cooling. Built with the user experience in mind, their color touchscreen controller is easy to read and simple to use, and you can even manage the system remotely!
Your ventilation system is likely the most important system in a Passivehaus design. We care about air tightness for a home, and we care about good air quality as well, especially for the Passivehaus. As a result, you can’t afford to just phone in this part of the process. That’s why we encourage our customers to talk to their HVAC expert, like BrightSense, to help you create the optimal design with product selection and ducting design. Please note that your HVAC expert should also be able to help you confirm your flow rates with a duct blaster.
While we aren’t HVAC design experts ourselves, the AE Building Systems team knows a lot about the CERV2 system and many of the nuances you should have on your radar. We’re always here to help point you in the right direction and think through the various angles on your own project!