Indoor Air Quality - ERV and HRV Reference

Why this reference exists

Modern energy-efficient homes are tight. Tight homes don't leak air, which means they don't naturally exchange stale + polluted indoor air with fresh outdoor air. Without mechanical ventilation, CO2 climbs, humidity climbs, VOCs accumulate, and ultrafine particulates linger. ERVs (Energy Recovery Ventilators) and HRVs (Heat Recovery Ventilators) are the engineered fix. They've moved from "premium upgrade" to "code requirement" in many jurisdictions adopting IECC 2021 + ASHRAE 62.2.

ERV vs HRV - the difference

Both transfer heat between incoming + outgoing air streams using a heat-exchanger core. The difference is moisture:

HRV (Heat Recovery Ventilator):

• Transfers heat only
• Does NOT transfer moisture
• Best in cold + dry climates (Zone 6+)
• In winter, dumps outdoor cold humidity (dry air) into the house; recovers heat from exhaust

ERV (Energy Recovery Ventilator):

• Transfers heat AND moisture
• In summer, retains some indoor cooling + dries incoming humid air
• In winter, retains some indoor moisture (keeps humidity from dropping too low)
• Best in mixed + humid climates (Zone 3-5)

For most of the U.S. residential market (Zone 3-5): ERV. For cold + dry climates (Minneapolis, Montana, upstate NY): HRV. For Florida + Gulf Coast: ERV with dedicated dehumidification supplement.

Why ventilation matters

ASHRAE 62.2 recommends 7.5 CFM per person + 3 CFM per 100 sq ft as the minimum continuous ventilation rate. For a 2,000 sq ft home with 4 occupants: 30 CFM + 60 CFM = 90 CFM continuous.

Without that, accumulated indoor pollutants include:

• CO2 (3-5x outdoor levels by morning in tight homes)
• VOCs from carpets, paint, furniture, cleaners
• Cooking fumes (NO2, ultrafine particulates)
• Bathroom moisture (mold growth)
• Combustion fumes from gas appliances
• Radon (in radon-active regions)
• Pet dander, dust mites

Customer-facing benefit: better sleep, fewer headaches, less morning grogginess, reduced allergies.

When ERV/HRV is required

IECC 2021 (now adopted by many states) requires mechanical ventilation in new construction below a certain air leakage rate. ASHRAE 62.2 + IECC together push ERV/HRV from "optional upgrade" to "mandatory."

Existing homes: voluntary upgrade. Typical motivator is:

• Customer complaints (moisture, allergies, stale air)
• After heavy weatherization (tightening creates the problem)
• Post-COVID awareness of IAQ
• Customer's family member has asthma or chemical sensitivity

Sizing

Continuous ventilation rate (ASHRAE 62.2):

CFM = 7.5 × (occupants) + 0.03 × (floor area sq ft)

A 2,500 sq ft 4-person home: 30 + 75 = 105 CFM continuous.

Most residential ERV/HRV units handle 100-200 CFM continuous on low; higher CFM available on boost (when running bath fan or kitchen exhaust).

Installation options

Dedicated duct system: ERV runs separate ducts to each room + bath + kitchen. Quietest + best balanced. Highest cost. New construction or major retrofit.

Integrated with central HVAC: ERV ducts tie into return + supply trunks. Air-handler blower distributes. Most common retrofit. ERV operates on its own fan during HVAC-off periods; tied to HVAC fan during HVAC operation.

Through-the-wall single-room: a single ERV unit in a high-IAQ-need room (master bedroom). Limited utility; not a whole-house solution.

For most retrofit: integrated with central HVAC.

Standard install procedure (integrated retrofit)

Step 1: Site assessment

1. Verify air handler location + ductwork accessible
2. Identify exterior wall for fresh + exhaust air penetrations
3. Calculate continuous + boost CFM
4. Choose ERV vs HRV per climate
5. Sound + vibration considerations (avoid bedroom wall)

Step 2: Mount ERV unit

1. Hang from ceiling joists or wall-mount per manufacturer
2. Above the air handler is common
3. Allow 24-30" clearance for service access
4. Insulate housing (some units pre-insulated)

Step 3: Outdoor penetrations

Two penetrations needed: fresh air intake + stale air exhaust.

• 6"-8" round duct typical
• 6+ ft separation between intake + exhaust (no short-circuit)
• Intake away from: dryer vents, gas meter, vehicles, exhaust hoods, AC condenser
• Outdoor hoods with bird/insect screens

Step 4: Connect to ductwork

• Fresh air supply ties into return duct DOWNSTREAM of any filter (so fresh air gets filtered)
• Stale air exhaust pulls from kitchen + bathroom returns (or dedicated grilles in those rooms)
• Boost switches in bathrooms + kitchen
• Damper controls per design

Step 5: Drain (condensate)

ERV cores produce condensate in summer + winter. Drain to floor drain, condensate pump, or to existing furnace drain.

Step 6: Electrical

• 120V dedicated circuit (15A typical)
• Some smart units connect to home Wi-Fi

Step 7: Commission

• Set continuous CFM per ASHRAE calc
• Test boost mode (kitchen + bath grilles)
• Verify balanced supply + exhaust (manometer reading)
• Filter installed; replacement schedule briefed to customer
• Customer briefing on controls + maintenance

Common pitfalls

• Wrong product for climate: HRV in humid climate dries house excessively; ERV in cold climate keeps too much humidity
• No boost switches: customer can't override for cooking + showering
• Intake near dryer vent: pulls lint + warm wet air into house
• No condensate drain: water on floor
• Unbalanced flow: positive pressure pushes humid air into walls (sometimes intentional, sometimes problematic)
• Filter neglect: customer doesn't know about it; performance drops to zero after 6-12 months
• Sound complaints: bedroom-adjacent install with no isolators

Filter maintenance

ERV/HRV filters need regular service:

• Pre-filter (washable foam): clean every 1-3 months
• Main filter (MERV 8-13 pleated): replace every 6-12 months
• Core (housing the heat exchanger): wash annually
• HEPA upgrades: replace every 1-2 years

A customer-side checklist of "what to do when" sells maintenance contracts at.

ASHRAE + code

ASHRAE 62.2 is the residential reference; IECC 2018+ defers to it. Boost requirement: 50 CFM kitchen + 50 CFM bath. Balanced (ERV/HRV) is the gold-standard mechanical ventilation strategy.

Incentives

ERV/HRV does NOT qualify for 25C. Some state + utility programs (NYSERDA, MassSave) rebate when offered.

Customer talking points

The conversation that sells ERV/HRV:

1. "Your home is well-sealed - great for energy, but it traps stale air."
2. "CO2 in tight homes is often 2,000-3,000 ppm by morning. Outdoor air is 420 ppm. People complain of headaches + bad sleep at those levels."
3. "An ERV exchanges that stale air for fresh, while recovering 70-85% of the heat/cooling."

For asthma + allergy families: HEPA filtration on the ERV supply seals the deal.

References

• ASHRAE 62.2 Residential Ventilation
• IECC 2021 Section R403.6 Mechanical Ventilation
• ENERGY STAR ERV/HRV Product List
• DOE Indoor Air Quality Resources
• Manuall internal: Heat Pump Sizing for Cold Climate Reference