Return-Air & Vent Myths: What to Open, What to Close

A practical airflow guide for Durham Region homes.

First Principles: How Supply and Return Actually Work

Your forced-air system needs a loop. Warm or cool air leaves the furnace/air handler via supply ducts, circulates through the rooms, then returns through return grilles to be reconditioned. When supplies are closed or returns are blocked, the loop breaks: static pressure rises, airflow drops, comfort suffers, and equipment may run longer for the same result. Well-sealed, balanced ducts improve comfort and efficiency; many homes lose significant airflow through leaky or poorly connected ducts.

Takeaway: think unimpeded loop, open path out and open path back.

Myth #1: “Close Supply Vents to Save Energy”

Closing multiple supply registers rarely saves energy. It raises duct static pressure, which can increase duct leakage and reduce system airflow and efficiency.

Better approach:

• Keep most supplies open; use only small reductions where needed to trim comfort.
• Use minor, incremental register adjustments (a notch or two) only to trim comfort, never to “turn off” a room.
• If a room is truly unused for long periods, reduce the supply slightly and ensure there is an open return path (see below).

Note: Actual comfort and energy outcomes vary by home, duct design, and equipment condition.

Return-Air Rules That Prevent Hot/Cold Rooms

• Keep returns clear. Don’t block them with furniture, storage, or drapery.
• Avoid restricting return paths; pressure imbalances can cause depressurization and combustion spillage in atmospherically vented appliances.
• Leave the grille on. Removing a return grille can create noise and dust issues; the grille spreads the intake across the opening.
• Check for filters behind returns. If your home uses filter-back returns, inspect monthly during heating season and replace/clean as needed.
• Basement returns matter. Unfinished basements with few or no returns often feel chilly; keeping returns clear upstairs and supply/return pathways open downstairs can help the whole house balance.

Doors: When Closed, Doors Close for Comfort

Shutting a bedroom door with no return grille (or no undercut/transfer grille) traps supply air and pressurizes the room. That air has to escape somewhere, often under the sill or through wall gaps, while the rest of the home goes slightly negative. The result can be temperature swings, drafts, and furnace cycles that feel “off.” Closed interior doors without a return/transfer path create room-to-room pressure differences that can disturb airflow and comfort.

Simple fixes:

• Keep doors partly open when conditioning rooms without returns.
• If privacy requires a closed door, ensure there’s a decent undercut or a through-wall transfer grille so air can get back to a return.

What to Adjust (Modestly) vs What to Leave Alone

Okay for homeowners

• Registers: Nudge supply registers slightly to trim comfort.
• Balancing dampers (only if clearly labelled at branches): Minor tweaks are okay, record positions and move in small increments; avoid trunk dampers near the furnace.
• Filters: Replace on schedule; low airflow from a dirty filter can cause hot/cold complaints.
• Grilles: Vacuum dust from supply and return grilles.

Pro-only

• Blower speed, temperature rise, static pressure, and staged controls.
• Duct modifications (adding/relocating returns, sealing major leaks, resizing).
• Heat-pump and furnace diagnostics (defrost/condensate, safety limits, control boards).
• Combustion safety/spillage diagnostics and any work affecting venting or combustion air.

Safety note: Do not tape over registers, defeat safety switches, or open sealed equipment panels. Power down the system before removing any grille for cleaning.

A Quick, Room-by-Room Airflow Tune-Up (10 minutes)

1. Open loop check: Confirm each room has a supply path and an open return path (return grille, undercut, or transfer).
2. Clear returns: Move furniture/rugs away; clean the grille.
3. Set registers to neutral: Fully open, then reduce only the rooms that run too warm by one notch.
4. Doors: Keep problem-room doors cracked during heating/cooling cycles.
5. Filter + fan: Replace the filter if dirty; try Fan “Circulate” or short hourly fan windows to even temperatures. (Benefits vary with duct design; start with short windows and adjust.)

1. Re-evaluate after a day. Small changes often stabilize comfort after several complete cycles.

When DIY Tweaks Aren’t Enough

If rooms stay stubbornly uneven after the basics, there may be duct leakage, undersized returns, improper damper positions, or equipment settings to address. A Limcan technician can measure static pressure, verify temperature rise, confirm blower setup, and recommend duct or return improvements. Prefer set-and-forget? Enrol in a Maintenance Plan so we can catch airflow issues early and keep the system dialled in across seasons.

Heat Pumps & Humidifiers (Durham Region Context)

• Heat pumps: Airflow is even more critical in heat-pump mode; blocked returns and closed supplies can reduce comfort and extend run times. Keep the loop open and filters clean.
• Humidifiers: Proper humidity (within safe limits) may make the home feel comfortable at slightly lower winter setpoints, reducing the temptation to over-close registers.

FAQs

Is it okay to close vents in unused rooms?

A slight reduction may be fine if there is a clear return path. Avoid shutting multiple supplies completely; pressure rises and airflow drops.

My bedroom is always stuffy. Where do I start?

Open the supply fully, clear the return, and keep the door slightly open. If there’s no return or undercut, consider a transfer grille, and book a Limcan assessment if it persists.

Will cleaning vents alone fix uneven rooms?

Cleaning grilles and accessible openings can help, but persistent imbalances usually trace to airflow/duct design, leaks, or settings. Focus first on sealing major leaks, ensuring proper airflow and balancing, and booking a service visit if the problem persists.