How People Normally Execute the Ducting in a Retrofit Situation

Understanding the Problem in Tight Homes

New, airtight homes are great for energy efficiency, but they often trap CO₂ and VOCs. Opening windows fixes air quality quickly, but it also brings in unwanted humidity, heat, or cold—forcing your HVAC and dehumidifier to work harder and increasing energy costs.

That’s why many homeowners turn to ERVs (Energy Recovery Ventilators) or HRVs (Heat Recovery Ventilators). These systems provide controlled fresh air while recovering heat (and moisture, in the case of ERVs), making them ideal for modern construction.

ERV vs. HRV in Dry Climates

In dry regions like Colorado, ERVs are usually preferred. They help retain some indoor moisture during winter, reducing how hard a steam humidifier needs to work. HRVs exhaust moisture more aggressively, which can worsen dryness in winter.

For homes that already require a humidifier, an ERV strikes the best balance between fresh air and comfort.

How People Normally Execute the Ducting in a Retrofit Situation

In existing homes, ERV/HRV ducting almost always follows one of three patterns:

1. Fully Dedicated Ducting

This approach uses separate ducts for:

• a. Stale air pickup (bathrooms, laundry, hallways)

• b. Fresh air supply (bedrooms, living areas)

Pros

• a. Best room-by-room control

• b. Excellent CO₂ reduction in bedrooms

Cons

• a. Most expensive

• b. Requires attic or wall access

• c. Harder to retrofit cleanly

This is common in new construction but less common in retrofits unless attic access is easy.

2. Hybrid Installation (Most Common Retrofit Solution)

This is the most popular retrofit method.

• a. Stale air is ducted directly to the ERV from bathrooms, laundry, or a central hallway.

• b. Fresh air is supplied into the furnace return duct.

• c. The furnace blower runs periodically to distribute fresh air.

Pros

• a. Much less ductwork

• b. Excellent whole-house air quality

• c. Works well with single-zone HVAC systems

Cons

• a. Slightly less precise bedroom control than fully dedicated

• b. Requires proper balancing

For most existing homes, this method delivers 80–90% of the benefit with far less disruption.

3. “Into the Return Only”

Both supply and exhaust connect to the furnace return.

Pros

• a. Simplest install

Cons

• a. Poor air distribution

• b. Relies heavily on furnace fan

• c. Least effective for bedrooms

How Effective Are ERVs That Use Existing HVAC Ducts?

When installed correctly, hybrid ERV systems are very effective at reducing CO₂ and VOCs.

Real-world results commonly show:

• a. Bedroom CO₂ dropping from 1,800–2,000 ppm to 600–800 ppm

• b. Faster recovery after cooking, showers, or gatherings

The key is:

• a. Enough CFM

• b. Strategic stale-air pickups

• c. Periodic furnace fan circulation (e.g., 20 minutes per hour)

Sizing the ERV: How Much CFM Do You Need?

For a 4,300–4,500 ft² home with ~9 ft ceilings, total air volume is about 44,000 cubic feet.

Using ASHRAE 62.2 and ~4 occupants:

• a. Required ventilation ≈ 160–170 CFM continuous

Best Practice

Choose an ERV rated for 200–250 CFM, then run it at:

• a. 150–180 CFM continuously

• b. Higher “boost” mode when needed

ERVs are quieter, more efficient, and last longer when operated below max capacity.

One Large ERV vs. Two Smaller ERVs

One Large ERV (Recommended First Step)

• a. Installed in the basement

• b. Hybrid connection to HVAC return

• c. Serves entire house

Why this works

• a. Simplest

• b. Least cost

• c. Often solves the problem completely

Two ERVs (One Per Floor)

• a. Better bedroom control

• b. More complexity and cost

• c. Best when doors stay closed at night and CO₂ remains high

Best strategy:
Install one large ERV first, measure CO₂, and add a second unit upstairs only if needed.

Bathroom Exhausts and ERVs

ERVs do not replace bathroom exhaust fans.

Reasons:

• a. Bathroom fan ducts are usually too small (4")

• b. They include backdraft flaps

• c. Roof caps are exhaust-only

Correct approach

• a. Keep existing bath fans

• b. Add separate ERV pickup grilles in bathrooms or nearby hallways

• c. Use ERV boost switches for odor and moisture control

Duct Sizing Basics (Keep It Simple)

Typical quiet airflow targets:

• a. 30–50 CFM per room

Common duct sizes:

• a. 4" → 20–30 CFM (often too small)

• b. 5" → 30–50 CFM

• c. 6" → 50–80 CFM

Even if you have more supply than return branches, proper trunk sizing and balancing dampers solve the issue. Many modern ERVs are self-balancing, but good duct design still matters.

Recommended Layout for Your Home

Best practical solution:

• 1. One 200–250 CFM ERV in the basement

• 2. Stale air pickups:

  • a. Upstairs hallway

  • b. Laundry room

  • c. Main-floor living area

• 3. Fresh air supply:

  • a. Tied into furnace return

• 4. Thermostat:

  • a. Fan circulate enabled (15–20 min/hr)

This setup minimizes work while delivering excellent air quality.