First, which do you have?
The answer differs completely. A gas furnace burns fuel to make heat. A heat pump moves heat from outside, and its supply air is cooler than furnace air even when working perfectly.
If you have a gas furnace
- It is not igniting. A failed igniter is common. The blower may still run, delivering unheated air.
- It lights then shuts down. Classic flame sensing problem: it ignites, fails to confirm the flame, shuts down for safety, and you feel the cool air left in the ducts.
- A safety has tripped. Limit switches open when something is wrong, usually airflow.
- The fan is set to on rather than auto. Genuinely worth checking. In that mode the blower runs continuously, including between heating cycles, so you feel unheated air.
If you have a heat pump
- It is in a defrost cycle. Normal. The system periodically reverses to clear frost from the outdoor coil, and cool air for a few minutes is expected.
- Heat pump supply air is cooler by nature. Noticeably cooler than furnace air, which surprises people who have switched.
- Low refrigerant charge. Reduces capacity, meaning a leak.
- Reversing valve problem. If it will cool but not heat, this is a prime suspect.
What to check
The thermostat is set to heat and the fan is on auto rather than on. The filter. Whether the outdoor unit is running. If you smell gas, leave and call your gas utility from outside.
Common questions
Why does my furnace blow cold air sometimes?
If the fan is set to on rather than auto, the blower runs continuously including between heating cycles. If it is on auto, it usually means the furnace is not igniting or not staying lit.
Is cool air from a heat pump normal?
Heat pump supply air is cooler than furnace air by nature, and during a defrost cycle it is cooler still for a few minutes. Persistent cool air is not normal.
What is a defrost cycle?
A heat pump periodically reverses to melt frost from the outdoor coil. It is designed behaviour and lasts a few minutes.
My furnace starts then stops after a few seconds. Why?
That pattern usually points at flame sensing. It lights, cannot confirm the flame, and shuts down as a safety response.
Reviewed and updated September 2026. Prices shown are owner supplied typical ranges, confirmed after an assessment. If anything here looks out of date, call (615) 400-4993 and tell us.