Furnace Blowing Cold Air? What to Check Before You Call

When a furnace runs but the air coming out of the vents is cold, it usually means the blower is moving air but the burners aren't heating it. Start with the thermostat fan setting and the air filter. If the air is still cold after that, JC Mechanical can find the cause. Call (720) 779-7263.

Cold air from the vents right now? Call (720) 779-7263.

Start here: four checks that take five minutes

1. Thermostat settings

Set the thermostat to Heat, not Cool or Off. Set the fan to Auto, not On. Raise the setpoint a few degrees above the room temperature, and replace the batteries if the screen is blank or dim. If the thermostat itself seems to be the problem, see our thermostat and zoning services.

2. Furnace power switch and breaker

Most furnaces have a light-style switch on or near the unit. Make sure it's on. Check the furnace breaker once. If it trips again, leave it off and call.

3. Air filter

Pull the filter and hold it up to a light. If you can't see light through it, replace it. Denver's dry, dusty air can load a filter faster than you'd expect.

4. Registers and return grilles

Make sure supply vents are open and return grilles aren't blocked by furniture, rugs, or boxes.

If the air turns warm within a few minutes, you're probably done. If it's still cold, read on.

Is a little cold air at startup normal?

Yes, briefly. When a call for heat starts, the burners light first and the blower waits until the heat exchanger warms up. Some furnaces push a short puff of cool air from the ducts before the warm air arrives. If the air stays cold for the whole cycle, something else is going on.

Why the air is cold: the likely causes

Fan set to "On"

With the fan on On, the blower runs all the time, even when the burners are off. Between heating cycles the air feels cold. Switching to Auto fixes it.

Dirty filter and the high-limit switch

A clogged filter chokes airflow, the furnace overheats, and a safety switch called the high-limit shuts the burners off. The blower keeps running to cool things down, so you feel cold air. A new filter often solves it. If it keeps happening with a clean filter, have the furnace checked.

Dirty flame sensor

The flame sensor confirms the burners are lit. When it's coated, the furnace thinks the flame went out and shuts the gas off. If your furnace lights and then quits over and over, that points to the flame sensor or a safety switch, and a technician should check it.

Igniter or ignition problem

If the furnace hums, tries to start, and never lights, the igniter or the ignition controls may have failed. That's a repair for a technician. If the furnace won't start at all, see our no-heat emergency guide.

Condensate drain on high-efficiency furnaces

High-efficiency furnaces with white PVC vent pipes make water. If the drain line or trap clogs or freezes, a safety switch stops the burners. After a Denver snowstorm, check that the outdoor intake and exhaust pipes aren't buried in snow or ice.

Gas supply

A closed gas valve or a utility interruption means no flame. Don't try to work on gas parts yourself. If you smell gas, follow the safety steps below.

Leaky or disconnected ducts

If only one room or one floor gets cold air, a duct may have come apart in the attic, crawlspace, or basement, or the system's airflow may be out of balance. Our ductwork services cover finding and fixing duct problems.

Cracked heat exchanger

This one is rare, but it matters. Cold air together with soot around the furnace, a yellow or flickering flame, or a carbon monoxide alarm is a reason to turn the furnace off and call. More detail is on our furnace carbon monoxide safety page.

Still cold after these checks? Call JC Mechanical at (720) 779-7263.

What's DIY and what isn't

Fine to do yourself: thermostat settings and batteries, the furnace power switch, one breaker reset, the filter, opening registers, and clearing snow from outdoor PVC pipes.

Leave to a pro: anything with the gas valve, igniter, flame sensor, control board, wiring, heat exchanger, or condensate safety switch. Repeated resets can hide a real problem, so if the furnace keeps shutting down, stop and call.

Denver factors worth knowing

Denver has a long heating season and big temperature swings, so furnaces run hard and cycle often. Dry, dusty air fills filters faster. Snow drifts can block the pipes on high-efficiency furnaces. Altitude also affects how gas furnaces are set up, which is one reason combustion work belongs with a technician.

Safety box: gas smell or CO alarm

• Smell gas? Get everyone outside. Don't flip switches or use phones inside. Call 911 from outside, then Xcel Energy.
• CO alarm going off? Get everyone outside and call 911.

Call an HVAC company after the home is safe. JC Mechanical sells CO alarms, and every tune-up includes a furnace safety inspection.

What happens when you call JC Mechanical

Call (720) 779-7263 and tell us what the furnace is doing. A technician diagnoses the problem first (furnace repair service), then explains what they found and your options in plain language. If an older furnace is failing again and again, we'll talk honestly about repair or replace.

• Service call: $89 during normal hours (Monday to Saturday, 7 AM to 7 PM)
• 24/7 service is available. After hours there is a larger fee, which we tell you before we come out
• Maintenance plan: $240 per year. A tune-up is $129 and includes a furnace safety inspection. See maintenance plans

Frequently asked questions

Why is my furnace blowing cold air?

Usually the blower is running but the burners aren't heating. The most common simple causes are the thermostat fan set to On and a dirty filter that trips the high-limit switch. Flame sensor, igniter, condensate drain, gas supply, and duct problems need a technician.

Why does my furnace blow cold air when the thermostat is set to heat?

Check that the fan is on Auto, the setpoint is above room temperature, and the batteries are good. If those are right, the burners are likely shutting down from a safety switch, a dirty flame sensor, or an ignition problem. Call (720) 779-7263.

Is it normal for a furnace to blow cold air at first?

A brief puff of cool air at startup can be normal, because the blower waits for the heat exchanger to warm up. If the air stays cold through the whole cycle, something is wrong.

Can a dirty filter make a furnace blow cold air?

Yes. A clogged filter restricts airflow, the furnace overheats, and the high-limit switch shuts off the burners while the blower keeps running. Replace the filter and call if it keeps happening.

Why is only one room getting cold air?

Closed or blocked registers, a leaky or disconnected duct, or airflow that's out of balance are the usual reasons. A technician can check the ductwork.

Can a clogged condensate drain stop a high-efficiency furnace from heating?

Yes. High-efficiency furnaces with white PVC vents make water, and a clogged or frozen drain trips a safety switch that stops the burners. Clear snow from outdoor pipes yourself and call for drain or switch work.

Is a furnace blowing cold air dangerous?

Usually it's a comfort problem. Cold air together with a CO alarm, soot, or a yellow or flickering flame is a safety problem. Get everyone outside and call 911 if the CO alarm sounds.

What does it cost to have JC Mechanical look at it?

The service call is $89 during normal hours, Monday to Saturday, 7 AM to 7 PM. 24/7 service is available for a larger after-hours fee, which we tell you before we come out. Call (720) 779-7263.

Furnace still blowing cold air? Call JC Mechanical

Call (720) 779-7263. We'll help you figure out whether it's a quick fix or a repair, and we can send a technician across south Denver, including Denver, Centennial, Highlands Ranch, Littleton, and the other cities on our service areas page. You can also request service online.

JC Mechanical Heating & Air Conditioning LLC · Owner Jimmie Ciccone · In business since 2019 · 7600 S Grant St, Unit B4, Littleton, CO 80122 · (720) 779-7263