Why is my furnace blowing cold air?

Why cold air is actually useful information

This Old House describes how a furnace works in two moving parts:

They operate by a blower pulling air into the furnace through the return side and pushing it out the supply side.

And the heating half:

A heat exchanger is a burner inside a heating system's airway in a sealed chamber. Gas enters the heat exchanger and ignites, heating the metal that makes up the c[hamber]... The conditioned air blows across the chamber, which the burner has heated, pulling heat energy from the heat exchanger. As it does, the temperature of the air rises

Our reading, marked as ours. If air is coming out of the vents at all, the blower works. The fault is on the combustion side, or it is the thermostat telling the blower to run when no heat is called for. That eliminates the most expensive part on most repair lists, the blower motor, before anyone has looked at anything.

Start with the two free checks

The thermostat fan setting. If the fan is set to run continuously rather than only when heating, it will push unheated air between burner cycles. This Old House's winter preparation guide recommends replacing an old thermostat with a programmable or smart one, and warns never to mount a thermostat near a fireplace, even an inoperable one, because the reading will be wrong.

The filter. The same guide starts there: plenty of dust collects on the filter during use, and replacing it is one of the easiest ways to keep the system performing. A badly blocked filter can make a furnace overheat and shut its burner down on the safety switch while the fan keeps running, which is exactly the symptom on this page. That last sentence is our reading, not a quotation, and it is the reason the cheapest part in Fixr's table matters so much.

What it costs, part by part

Fixr's full table, cheapest first, including the service call:

The average furnace repair is $485, with most jobs between $349 and $650. Fixr's low end for the whole guide is $66 and its high end is $2,673, which are the first and last rows of that table.

Our reading, marked as ours. Five of the eight parts cost less than $180 fitted, and four of those five sit on the combustion side: limit switch, flame sensor, burner and ignitor. On the symptom this page is about, the odds favour the cheap end of the table by a wide margin.

Labor is $87 to $162 per hour, so on the sub-$180 rows you are largely buying the visit.

The row that is not a repair

The heat exchanger at $1,346 to $2,673 is the exception, and This Old House explains why it matters beyond the price:

A furnace that hasn't been maintained properly is susceptible to exhaust issues or cracked heat exchangers. A cracked heat exchanger will allow carbon monoxide-lad[en gases into the duct system]

Our arithmetic, marked as ours. Fixr's furnace guide puts a new furnace at a national average of $4,800. A heat exchanger at the top of its range, $2,673, is more than half of that, which is the point at which our furnace page becomes the page you want instead of this one, with the fuel-specific figures on our gas, electric, propane and oil furnace pages.

This is also the one fault on the list that is a safety matter rather than a comfort one. This Old House notes that a technician tests the supply air and the exhaust gases during regular maintenance precisely to catch it, and Fixr lists a carbon monoxide alarm among the signs needing immediate attention. If your detector has sounded, this is not a page about cold air any more.

The other signs that come with it

Fixr lists what else to look for alongside the cold air, and each one narrows the diagnosis:

• The pilot light is yellow instead of blue. A combustion problem.
• The furnace turns on and off constantly. Fixr says this has several causes and needs a technician.
• Unusual noises, squealing, whistling, groaning or clunking, which our noise page covers.
• Leaking water.
• Fuel or burning smells. This Old House distinguishes these from the normal smell of a furnace first fired for the season: abnormal is burning plastic or electrical wiring.
• Higher bills than the same month last year.

Before you call

1. Check the thermostat is set to heat and the fan to auto, not on. It is the one cause that is not a fault at all.
2. Change the filter. It is the cheapest thing in this whole section and it appears first in every professional sequence we have read.
3. Look at the pilot light if you have one. Yellow rather than blue tells the technician something before they arrive.
4. Describe which half is working. "The fan runs but the air is cold" rules out more than any other sentence you can offer.
5. If the carbon monoxide alarm has sounded, stop and get out. That is not this page.
6. Ask for the part to be named before the price. The table above turns a named part into a range, and five of the eight are under $180.