Why would a vent blow cold air when the heating is on, and how can I fix it?

Quick answer

A vent can blow cold (or cool) air with the heat on due to a few common issues: the thermostat fan is set to ON instead of AUTO, the system is still warming up, a dirty filter or blocked return is restricting airflow, a heat pump is in defrost or struggling in low temps, or there’s a duct problem (leak, disconnected run, closed damper). Less commonly, a furnace ignition fault or safety switch is preventing heat while the blower keeps running. Most of these can be checked and fixed with basic steps.

Why this happens

• Fan set to ON: The blower runs constantly, pushing room-temperature air between heat cycles.
• Warm-up/short cycles: Furnaces and heat pumps need a minute or two to heat the coil/heat exchanger. Frequent starts/stops can make it feel cold.
• Heat pump behavior: Supply air may be 85–95°F (feels cool to skin). Defrost cycles can also blow cooler air briefly.
• Dirty filter/high restriction: Low airflow causes the furnace to overheat and shut the burner off; the blower then cools the unit with unheated air.
• Duct issues: Disconnected, crushed, or leaking ducts (especially in attics/crawlspaces) pull in cold air. Closed or stuck dampers misdirect air.
• Thermostat or zone problems: Wrong mode, bad wiring, stuck zone damper.
• Ignition/fuel fault: No flame, tripped limit switch, closed gas valve, or condensate safety switch will stop heating while the blower still runs.
• You’re feeling a return grille: Returns pull in room air and can feel cool. Verify if the “cold vent” is supply or return.

Typical supply-air temperature (at a nearby supply register)
- Gas furnace: ~90–120°F
- Heat pump: ~85–100°F (varies with outdoor temp)
Temperature rise across furnace nameplate: usually 30–60°F

Step-by-step checks and fixes

1. Verify supply vs. return (1–2 min)

   • Hold a tissue at the grille. If it pulls in, it’s a return (normal to feel cool). If it blows out, it’s a supply.

2. Check thermostat settings (2–3 min)

   • Mode: HEAT. Fan: AUTO, not ON.
   • If heat pump, avoid running COOL recently; wait 5 minutes after switching modes to protect the compressor.

3. Let the system run 5–10 minutes

   • Initial air may be cool. If no heat after 10 minutes, continue below.

4. Replace/inspect the air filter (5–10 min)

   • Turn system OFF. Slide out the filter at the return grille or furnace/air handler.
   • Replace if dirty or older than 1–3 months. Use the same size and an appropriate MERV (8–11 for most homes). Overly restrictive filters can cause problems.

5. Look for duct issues at the furnace/air handler (10–20 min)

   • Turn power OFF at the service switch/breaker. Use a flashlight to inspect accessible supply and return ducts.
   • Re-seat any slipped flex duct collars and tighten bands. Seal gaps with UL 181 foil tape or mastic (not cloth “duct tape”).
   • In attics/crawlspaces, look for disconnected or crushed runs to the affected room.

6. Check supply registers and dampers (5–10 min)

   • Make sure room registers are open and not blocked by furniture/rugs.
   • If your system has manual balancing dampers near the plenum, verify the affected branch isn’t closed.

7. Observe the heating cycle (5–10 min)

   • For a furnace: With the access panel on, call for heat. You should hear inducer fan, then ignition, then main flame, followed by blower start. A small window often shows a steady light (normal) or blinking codes (fault).
   • For heat pumps: Outdoor unit should run; light frost is normal. Steam and a short period of cooler air during defrost are expected.

8. Measure supply temperature (optional, 5 min)

   • Use an IR or probe thermometer at a nearby supply. If it’s under ~90°F for a furnace or under ~85°F for a heat pump (after warm-up), there’s a heating or airflow issue.

9. Condensate and safety switches (5–10 min)

   • High-efficiency furnaces and air handlers have condensate drains. A clogged drain can trip a safety switch and stop heat. Clear the trap/drain with a wet/dry vac and reinstall caps firmly.

Tools and materials

• Flashlight or headlamp
• Replacement air filter (correct size, MERV 8–11 typical)
• UL 181 foil tape or duct mastic
• Nut driver/screwdriver for duct collars or access panels
• IR thermometer (helpful) and a small step ladder
• Wet/dry vac for condensate cleaning

Safety considerations

• Turn power OFF before opening panels or touching wiring/ducts.
• If you smell gas or see signs of flame rollout, stop and call a pro immediately.
• Never run the furnace with the blower door off (some units need the door switch closed).
• Use only foil tape or mastic on ducts; cloth “duct tape” can fail and create leaks.

Tips for best results

• Keep at least 60–75% of registers open to avoid high static pressure.
• Insulate long attic duct runs (R-6 or higher) to reduce heat loss.
• For heat pumps in cold weather, consider a programmable thermostat that coordinates auxiliary heat properly.
• Mark manual dampers with a sharpie (summer/winter positions) to avoid accidental closures.

Common mistakes

• Setting the fan to ON and assuming the system isn’t heating.
• Using very high MERV filters without verifying the blower can handle the restriction.
• Closing too many registers in “unused” rooms, causing overheating and limit trips.
• Sealing ducts with standard duct tape instead of UL 181 foil tape or mastic.

When to call a professional

• Repeated ignition failures, fault codes, or short cycling.
• Ice buildup on the outdoor heat pump beyond normal light frost.
• Suspected duct disconnections you can’t access (inside walls/ceilings).
• CO detector alarms or any gas odor.
• You’ve completed the steps above and supply air remains below expected temperatures.

Rough costs: filter ($5–$25), foil tape/mastic ($10–$30), basic tune-up ($100–$200), duct repair varies ($150–$600 per run), major heat pump or furnace repairs can run higher depending on parts.