This tech tip is based on a presentation by Joey Henderson, a veteran symposium speaker and excellent HVAC educator. You can learn more about Joey and the work he does at joejoehvac.com and his YouTube channel HERE.

We recently published a tech tip about economizers as a means of bringing fresh air into commercial buildings. Economizers are built into rooftop units (RTUs) and bring in fresh air based on different temperature or enthalpy sensors. 

However, economizers aren’t the only way to bring fresh air into commercial buildings through a controlled pathway. We can also use a common technology that we use in residential buildings: HRVs and ERVs—just much bigger versions.

The Benefits (and Challenges) of Fresh-Air Ventilation

I’ve said it before, and I’ll say it again. Indoor air is (often) disgusting, especially when you’re in public and hear a bunch of strangers around you coughing and sneezing (or just giving off carbon dioxide when they exhale). The HVAC system may have filtration, but it’s ultimately recirculating the same stale air—unless we bring in fresh air.

ASHRAE Standard 62.1 sets minimum ventilation rates for a variety of commercial buildings. The outdoor-air fraction (OAF) is the ratio of outdoor air intake to total supply air, and it varies depending on the nature of the structure. For example, retail and office spaces have an OAF of 15%, whereas auditoriums and gymnasiums can have an OAF between 20 and 40%. 

While fresh air can come in through doors and gaps in the building envelope, that’s a drop in the ocean for a building like a superstore. It’s also completely uncontrolled, so we can’t match it to conditions we’re trying to maintain inside the building—sensible and latent.

That’s where HRVs and ERVs come in. 

Basics of HRVs and ERVs

Like economizers, HRVs and ERVs are mechanically driven devices that bring in fresh air and exhaust stale return air. They rely on a blower assembly to pull air in and exhaust it. They also have filters to remove contaminants from the airstream before that air enters the occupied zone, so they address the main concern of dumping unfiltered, untreated outdoor air into the conditioned space. 

Whenever we exhaust air, we put the building under negative pressure and lose some energy. HRVs and ERVs balance the air intake and exhaust by bringing in outdoor air and recovering some of the “lost” energy from the outgoing exhaust air by letting it temper the incoming air. The stream of outdoor air crosses over the stream of indoor air within a heat exchanger, essentially “pre-conditioning” the incoming air without the streams making direct contact.

However, HRVs and ERVs have their appropriate uses, and they’re not the right fit for all situations and climates.

HRVs (Heat Recovery Ventilators)

HRVs allow for sensible heat exchange only; they have a heat-exchanging surface—often a fixed-plate crossflow surface—with filters. 

Remember the high-to-low principle? In many cases, the outdoor air will be warmer or cooler than the indoor air being exhausted, and the hotter airstream will give off its heat to the cooler one over the heat exchange medium. 

They’re called “heat recovery” ventilators because they save energy by recycling the heat generated by the HVAC system. The HVAC system doesn’t have to spend as much energy to heat the incoming air, and the work that has been done by the HVAC to heat the exhausted air isn’t for naught.

For that reason, HRVs are common in cold, dry climates. During the winter, the indoor air is warmer than the incoming outdoor air, so it gives off some of its heat to the incoming airstream. When that happens, we’re not just dumping frigid air into the space and making everyone uncomfortable. 

ERVs (Energy Recovery Ventilators)

Unlike HRVs, ERVs allow for sensible AND latent heat exchange. The heat-exchanging medium contains a desiccant and may be fixed-plate crossflow or rotary. The rotary form looks like a pie and is also called an enthalpy wheel. We’ve talked about residential ERVs before, but look at the sheer size of a commercial ERV’s enthalpy wheel!

Heat exchange takes place over the medium—the same as in an HRV. The main difference between the two methods is that the desiccant in the exchange medium removes moisture from the incoming air to control the latent heat content, not just sensible, of the air coming in. This method of moisture removal is not as potent as something like a ventilating dehumidifier, so ERVs may not be effective enough at moisture removal for a climate like ours (in Florida). Some ERVs may also have an optional sensible-only mode.

While ERVs may come with all sorts of bells and whistles, they’ll have the same general parts:

• Blower
• Exchange medium (often an enthalpy wheel)
• Filters
• Digital control box
• Access panel

ERVs are pretty versatile, and they’re often a good choice for what we call “green grass” markets. These markets can have high dew points in the summer, but the climate is temperate, and winters are cold but not frigid. They’re not constantly damp year-round (looking at you, Florida and Louisiana). While ERVs can work in cold, dry climates, an HRV might make more sense, especially since the desiccant isn’t going to be very effective when the air is already dry.

Advanced IAQ Controls

While temperature control is one of the major benefits of HRVs and ERVs in their respective markets, some commercial clients may primarily use HRVs and ERVs to control contaminant concentrations in buildings. Churches commonly use ERVs to bring in fresh air, even if temperature or humidity control isn’t the main objective.

Some ERV manufacturers allow IAQ sensors to connect to their controls. Then, the BAS system can bring in fresh air via the ERV as necessary; it’s essentially a form of demand control ventilation. For example, the RenewAire Premium Controls can tie into CO2 and VOC sensors with alarm setpoints (like 1200 PPM) and a time delay (often 60–90 seconds). This control combination will allow the building automation system (BAS) to respond to elevated CO2 and VOC levels.

Source: RenewAire Premium Controls Manual, pg 87

Like economizers, ERVs and HRVs are energy-efficient ways to bring in fresh air for the benefit of the space and its occupants. HRVs and ERVs pre-condition the air so that we’re not haphazardly dumping in outdoor air and making the HVAC go back to square one to condition it. 

The combination of filtration and heat exchange—using the stale air from indoors—makes fresh air intake a lot more manageable while helping facilities meet ventilation standards. ERVs can even dilute contaminants on demand with some modern control setups, which is pretty nifty. 

Provided that you use HRVs and ERVs in their appropriate climates and understand what they do (e.g., keep HRVs as far away from Florida as possible), they can be worthwhile ventilation strategies for commercial clients, not just residential ones.