Should an ERV Be Tied Into the HVAC System? What Massachusetts Homeowners and Builders Need to Know
A HERS-Based Perspective for Today’s High-Performance Homes
As Massachusetts homes get tighter and more energy efficient, mechanical ventilation is no longer optional—it’s essential. Energy Recovery Ventilators (ERVs) play a critical role in maintaining healthy indoor air quality (IAQ) while minimizing energy loss.
One of the most common questions we hear during design reviews and site visits is:
“Should the ERV be tied into the HVAC ductwork, or should it be a dedicated system?”
From a HERS Rating and building-science perspective, the answer matters more than many people realize.
What an ERV Is (and What It’s Not)
An ERV is designed to provide continuous, controlled ventilation by exchanging stale indoor air with fresh outdoor air while transferring heat and moisture between the two air streams.
Its job is simple but critical:
Deliver fresh air to living spaces
Exhaust stale air from bathrooms and kitchens
Recover energy to reduce heating and cooling penalties
Maintain healthy IAQ in airtight homes
What an ERV is not meant to do is replace your heating or cooling system—or rely on it to function properly.
The Temptation to Tie an ERV Into HVAC Ductwork
At first glance, tying an ERV into existing HVAC ductwork can seem appealing:
Fewer ducts
Lower upfront cost
Perceived “air mixing” benefit
However, from a performance, control, and HERS modeling standpoint, this approach often introduces more problems than it solves—especially in Massachusetts homes built to Stretch Code or Specialized Stretch Code.
Why Tying an ERV Into HVAC Ducts Is Often a Bad Idea
1. Ventilation Should Not Depend on the HVAC Running
ERVs are designed to operate continuously or intermittently, independent of heating or cooling demand. When an ERV is tied into HVAC supply ducts:
Fresh air distribution depends on the air handler running
During shoulder seasons, ventilation can be inconsistent
Bedrooms and low-use spaces may not receive adequate fresh air
From a HERS perspective, inconsistent ventilation does not improve performance—and can work against comfort and IAQ goals.
2. Low ERV Airflows Don’t “Mix” the House
Most ERVs operate in the 50–100 CFM range. That airflow is intentionally low and steady.
When tied into HVAC supply ducts:
The airflow is too small to meaningfully distribute air throughout the home
Fresh air often short-circuits to nearby returns
The idea of “whole-house mixing” is largely theoretical unless the air handler runs constantly
In practice, this setup provides less effective ventilation, not more.
3. Duct Leakage and HERS Penalties
Connecting an ERV to HVAC ductwork increases:
Total duct length
Number of joints and transitions
Risk of duct leakage
For HERS ratings, this matters. Increased duct leakage—especially leakage to outdoors—can negatively impact the score and make compliance harder, not easier.
4. Conditioning Fresh Air Through the HVAC Is Often Counterproductive
Another misconception is that introducing fresh air into HVAC supply ducts “pre-conditions” it efficiently. In reality:
ERVs already temper incoming air
Injecting air into supply ducts during heating or cooling cycles can disrupt system balance
It may actually increase runtime and energy use
From a modeling standpoint, this rarely improves performance and often complicates commissioning.
Best Practice: Dedicated ERV Ducting
From both a building science and HERS rating perspective, the preferred approach is clear:
Use a Dedicated ERV Duct System
A properly designed ERV system should:
Exhaust air from bathrooms, kitchens, and utility areas
Supply fresh air to bedrooms and main living spaces
Operate independently of heating and cooling equipment
This approach delivers:
Predictable ventilation rates
Better IAQ
Cleaner HERS modeling inputs
Fewer duct leakage risks
When Can Partial Integration Make Sense?
There are limited scenarios where partial integration can be acceptable—such as using a shared return pathway only if:
The system is carefully commissioned
Airflows are verified
Controls ensure ventilation runs regardless of HVAC operation
Even then, these setups require careful review and are not the default recommendation for high-performance homes.
How This Impacts a HERS Rating
A well-designed ERV system can lower a HERS score by:
Allowing tighter building envelopes
Reducing uncontrolled infiltration
Providing verified, code-compliant ventilation
Minimizing duct leakage penalties
Conversely, a poorly integrated ERV can:
Increase duct leakage
Complicate modeling assumptions
Deliver no meaningful performance benefit
The Bigger Picture: Build Tight, Ventilate Right
One of the most important principles in modern construction is this:
It is far better to build a tight home and manage IAQ with a properly designed ERV than to rely on a leaky envelope for “fresh air.”
Uncontrolled air leakage is inefficient, uncomfortable, and unpredictable. Controlled ventilation is intentional, verifiable, and healthier.
Conclusion
While tying an ERV into HVAC ductwork may seem convenient, it often undermines the very benefits ERVs are meant to provide—especially in Massachusetts homes pursuing Stretch Code compliance and low HERS scores.
A dedicated ERV system, designed with proper airflow, insulation, and commissioning, delivers better IAQ, more reliable performance, and cleaner HERS results.
If you’re designing or building a high-performance home in Massachusetts and want to ensure your ventilation strategy supports both comfort and HERS compliance, Spectrum Energy can help.
