Whole house, or particular rooms?
Walk round with your hand at each vent. That five-minute check splits the diagnosis.
Everywhere is weak. The system is not moving the air it should: the filter, the blower, or something restricting the main trunk.
Some rooms are fine and others are not. The system is moving air but not delivering it: dampers, duct runs, leaks, or a room that was never served well.
The whole-house version
- A blocked filter. Always check first.
- A dirty blower wheel. The blades load up with dust and stop scooping air, while the motor sounds fine.
- A failing blower motor or capacitor, turning slower than it should. See blower not working.
- A crushed or disconnected duct, particularly flexible duct under a house that has been leaned on or come apart at a collar.
A furnace that cannot move enough air also overheats and shuts itself off on its limit switch, so weak airflow and a furnace that keeps shutting off are often one fault.
The room-by-room version
- Closed or blocked registers, including ones closed on purpose. Closing vents does not usefully redirect air; it raises the pressure in the system and usually makes things worse overall.
- Dampers in the ductwork left in a position somebody set years ago.
- Long or badly routed runs to the far end of the house.
- Leaking ducts in a crawl space or attic, heating somewhere nobody lives.
- No return path. A bedroom with a supply vent and no return, with the door shut, has nowhere to send its air, and the room pressurizes. Undercutting the door is sometimes the whole fix.
When nothing is broken
In some houses a system was fitted to heat the ground floor well and the far rooms were an afterthought. No repair fixes that, because nothing is broken: it is a distribution problem that needs duct changes, zoning or a separate heater for those rooms. Replacing a working furnace will not fix it, which is the main reason it is worth knowing.
What a technician measures
Static pressure across the system, which says whether the blower is fighting a restriction and roughly where; the temperature rise across the furnace; and airflow at individual registers where the complaint is one room.
No heat? Tell us what the furnace is doing.
Gas, electric and high-efficiency furnaces across Caldwell, Nampa, Middleton and Parma. Start with the symptom and your ZIP code.
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