Furnace Blowing Cold Air in Philadelphia? 7 Causes and What to Do
If your furnace is blowing cold air in your Philadelphia home, the usual causes are a thermostat fan set to On, a clogged filter that makes the furnace overheat and shut the burner off, a dirty flame sensor, an ignition problem, a gas valve that's off, or a blocked drain line on a high-efficiency unit. A few of those you can fix in five minutes. The rest need a tech. Here's how to tell which one you've got.
I've been working on commercial and residential heating and cooling since 2005. The first real cold week of the year, this is the call I get more than any other: "The furnace is running, but the air coming out is cold." With temps dropping into the 30s this week, you don't want to find out at 2 in the morning.
First: is it cold the whole time, or just at the start?
When the heat kicks on, a lot of furnaces light the burner first and start the blower a bit later. You might feel a short puff of cool air that was sitting in the ducts. That's normal. Give it two or three minutes.
If the air is still cold after that, or it starts warm and turns cold, the furnace is telling you something. Pay attention to which one it is, because it points to different problems.
7 reasons your furnace is blowing cold air
1. The thermostat fan is set to On
This is the easy one, and it gets people every year. With the fan on On, the blower runs all the time, even when the burner is off between cycles. So you get warm air, then cool air, then warm again. Set the fan to Auto. On a Honeywell or most other thermostats, it's a separate Fan setting from Heat.
2. A clogged filter
A dirty filter chokes the airflow. The heat has nowhere to go, the furnace gets too hot, and a safety switch shuts the burner off while the blower keeps running. That's cold air out of the vents. If your filter is gray and caked, that's probably your answer.
3. A dirty flame sensor
The burner lights, runs a few seconds, then shuts off. Then it tries again. That pattern is often a flame sensor coated with buildup, so the furnace can't confirm the flame is there and cuts the gas for safety. It's a small part, and it's one of the most common no-heat fixes I do.
4. The pilot or igniter isn't working
Older furnaces have a standing pilot that can go out. Newer ones use a hot surface igniter or spark igniter that eventually cracks or wears out. No ignition, no heat, and the blower may still push room-temperature air.
5. The gas is off or not getting through
Check that the gas valve near the furnace is open (handle in line with the pipe). If your other gas appliances aren't working either, the issue may be the supply, and that's a call to your gas utility.
6. A blocked condensate drain
High-efficiency furnaces make water and drain it out through a small line or pump. If that line clogs, a safety switch keeps the burner from lighting. If you see water around the furnace or a full condensate pump, this could be it.
7. Leaky or disconnected ducts
If the furnace is making heat but some rooms still get cool air, the warm air may be escaping through a loose or broken duct in the basement, crawl space or attic before it reaches you. Common in older Philly rowhomes and in homes around Havertown, Springfield and Broomall where ductwork has been added onto over the years.
What you can safely check yourself
- Thermostat: set to Heat, fan on Auto, a few degrees above room temp. Swap the batteries if it has them.
- Filter: pull it out and look. If you can't see light through it, replace it.
- Power: check the furnace service switch (it looks like a light switch near the unit) and the breaker.
- Furnace door: make sure the front panel is on tight. Many furnaces won't run with it loose.
- Gas valve: handle in line with the pipe means open.
- Vents and returns: make sure furniture or rugs aren't blocking them.
If you ever smell gas, get out of the house and call your gas utility first. Don't flip switches on the way out.
When it's time to call a tech
Call when the burner lights and drops out, the furnace keeps shutting off, you see water around it, you hear clicking with no flame, or a new filter didn't change anything. Don't keep hitting reset. That can hide the real problem or make it worse.
When we come out, we find the cause first, show you what we found, and tell you straight what it needs. We work on what's actually in Philly-area basements, including Goodman, York and Airtemp furnaces and Honeywell thermostats. Every job comes with our 90-day guarantee on all work.
Keep it from happening again
Most cold-air calls trace back to a furnace nobody looked at since last winter: a dirty sensor, a clogged filter, a weak igniter. A yearly tune-up catches those before the first cold night. Ours is $76: we clean it, vacuum it, wipe it down, take readings and tell you what's going bad.
And if your furnace is past saving, you'll hear that straight too. Free in-home estimates for qualified replacement projects.
Quick answers
Why is my furnace blowing cold air when the heat is on?
Most often the thermostat fan is set to On instead of Auto, the filter is clogged and the furnace is overheating and shutting the burner off, or the burner lights and then drops out because of a dirty flame sensor or an ignition problem.
Is it normal for a furnace to blow cold air at first?
For a few seconds, yes. Many furnaces start the blower a little after the burner lights, but you may feel a short puff of cool air from the ducts. If it stays cold after a couple of minutes, something is wrong.
Can a dirty filter make my furnace blow cold air?
Yes. A clogged filter chokes the airflow, the furnace gets too hot, and a safety switch shuts the burner off while the blower keeps running. That's cold air from the vents. Swap the filter and see if the heat comes back.
Why does my furnace blow cold air at night?
If it happens when the system cycles more on a cold night, look at the filter and the flame sensor first. A burner that lights and then shuts off after a few seconds is a common cause, and it gets worse when the furnace runs more.
Should I keep running a furnace that's blowing cold air?
Don't keep resetting it over and over. Shut it off at the thermostat and call. If you smell gas, leave the house and call your gas utility first.
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