Two things to check first
Before anything else, look at the thermostat. If the fan is set to On, the blower runs all the time, including between heating cycles, and the air out of the vents feels cold most of the hour. Switch it to Auto. That single setting accounts for a surprising share of "cold air" calls in Arvada, especially on new smart thermostats.
Second, pull the filter. A furnace that cannot pull enough air overheats, the high-limit switch shuts the burners off, and the blower keeps running to cool the heat exchanger. Warm for two minutes, cold for ten, repeat. A packed filter is the most common cause of that pattern, and a new one costs less than the diagnostic visit.
Carbon monoxide alarm going off? Get everyone outside and call 911. Then call us from the driveway. A cracked heat exchanger or blocked flue can put CO into the house with no smell at all, which is why every Arvada home with a gas furnace needs a working CO alarm on each floor.
What it is when those are fine
Burners lighting and dropping out (flame sensor)
You hear the burners whoosh on, then click off after a few seconds. The blower starts on schedule and moves air that never got warm. A dirty flame sensor cannot confirm the flame to the control board, so the board shuts the gas for safety. Cleaning or replacing the sensor runs $150 to $300 in Arvada and is one of the quickest repairs on the truck.
No ignition at all (ignitor or pilot)
Older furnaces with a standing pilot: if the pilot is out, the furnace goes through its motions and blows room-temperature air. You can relight it if the instructions are on the panel and you are comfortable doing so. Newer furnaces use a hot surface ignitor, and when it cracks, the same thing happens without the option to relight. Ignitor replacement is $150 to $300.
Condensate blocked (high-efficiency furnaces)
If your furnace has white PVC pipes instead of a metal flue, it makes water while it runs. That water drains through a trap and a hose. When the trap clogs or the hose freezes in an Arvada garage or crawlspace, a float switch shuts the burners off and the blower keeps going. Clearing the line is a small job, typically inside the diagnostic fee plus a little labor.
Ductwork and the house itself
Some cold-air complaints are not the furnace. A disconnected duct in the crawlspace dumps heat under the house. A closed damper on a basement run starves a bedroom. On Arvada's 1960s and 70s split-levels, the run to the upstairs bedrooms is the usual suspect. A tech can find a disconnected duct in minutes; fixing it is usually a few hundred dollars.
Cracked heat exchanger
This is the one that matters. A cracked heat exchanger can trip the rollout or limit switches, so the furnace runs cold, and it can push combustion gases into the air stream. A tech will inspect it with a camera or a combustion analyzer. Replacement runs $1,000 to $3,700 in parts and labor, and on any furnace over fifteen years old that number almost always points to a new furnace instead.
Cold air repairs, Arvada, this quarter
Prices verified September 2026| Cause | Tell-tale sign | Parts and labor |
|---|---|---|
| Thermostat fan set to On | Air is cool between cycles, warm during them | $0 |
| Clogged filter, limit tripping | Warm briefly, then cold, over and over | $10 - $40 (filter) |
| Flame sensor | Burners light, drop out in seconds | $150 - $300 |
| Ignitor | No whoosh, blower runs anyway | $150 - $300 |
| Condensate trap or line | Water near the furnace, PVC vents | $100 - $250 |
| Disconnected duct | One room cold, crawlspace warm | $150 - $500 |
| Heat exchanger | Rollout trips, soot, CO alarm | $1,000 - $3,700 |
| Compiled from published Denver-area contractor pricing and national cost guides, Q3 2026. Diagnostic visit $49 to $180, credited toward the repair. Written price before any work starts. | ||
Why it happens more at 5,300 feet
Gas furnaces lose roughly four percent of rated output per 1,000 feet of elevation. Many Arvada furnaces were sized from a sea-level chart and never derated, so they run hotter than designed, trip the limit more often, and produce exactly this warm-then-cold pattern once the filter starts to load. A tech who works this side of Denver checks the derate before blaming the part.
Send a photo of the sticker and the burner compartment through the photo quote and the range comes back narrowed to your furnace, usually within minutes.