Poor airflow, a faint breath where there used to be a steady stream, is one of the most common complaints and one of the easiest to misread. The blower pulls air in through the returns and the filter, pushes it across the indoor coil or heat exchanger, and sends it out through the supply ducts. A restriction anywhere in that loop weakens it, and the system then runs longer, heats or cools less, and in summer can freeze its coil. The first clue is simple: is the air weak at every register, or only at some?
Weak at every register: look at the equipment
When the whole house is affected, the restriction is usually at or near the indoor unit.
- The filter. A clogged filter is the most common cause. So is a filter that is too dense for the system.
- A dirty indoor coil. Dust that slips past the filter mats onto the coil, and air has to squeeze through it.
- A frozen coil in cooling season. Ice can close the coil almost completely; see frozen AC coils.
- Blower problems. A failing motor or capacitor turns slowly; a blower wheel caked with dust moves far less air; a wheel loose on its shaft slips.
- Starved returns. A system can only push out as much air as it pulls in. One small return for a large house, or a sofa against the return grille, chokes it.
Weak at some registers: look at the ducts
When most rooms are fine and a few are not, the problem is usually the path to those rooms.
- Disconnected or crushed ducts in the attic or crawlspace. Flexible duct sags, kinks and gets stepped on during other work.
- Closed dampers or registers. A balancing damper bumped shut, or a register closed and forgotten behind a bed.
- Leaking joints and seams. Air escapes before it reaches the end of a long run.
- Long runs off one trunk. Rooms at the far end of the house or on another level often get the least.
If those same rooms are also the hardest to keep comfortable, read uneven temperatures too; the two problems often travel together.
Checks you can do safely
- Replace the filter with the size and type printed on the old one’s frame.
- Walk the house, open every register and make sure nothing sits over a return grille.
- Stand near the indoor unit while it runs and listen. A steady rush of air is normal; a hum with little air, a squeal or silence is not.
- From a garage, basement or crawlspace opening, look at the ducts you can see for a fallen or separated run. Don’t crawl into tight spaces or step on ducts.
- Think back: did the airflow drop suddenly (something broke or came apart) or slowly over months (dirt building up)?
Turn it off and call when
- You smell burning from the furnace or air handler. An overheating blower motor or a slipping belt can smell hot.
- The blower hums but does not turn.
- There is ice on the indoor coil or the copper lines.
- The furnace keeps shutting off while the air it does push out feels very hot. It may be overheating for lack of airflow.
What a technician measures
Airflow is measured, not guessed. A technician reads static pressure at several points, which works like a blood pressure reading for the duct system and shows whether the restriction is the filter, the coil or the ducts. Then the blower: motor amps, capacitor, speed setting and the state of the wheel. On the duct side, a look through the attic or crawlspace for disconnections and crushed runs, and a check of the size and number of returns. The fix may be a coil and blower cleaning, resealing and reconnecting runs with duct repair, adding a return, or, where a duct really is clogged, duct cleaning. You see an itemized price after the diagnosis, before any work starts.
Better filtration and cleaner air are worth having, but they have to be matched to the blower; indoor air quality covers the options that do not cost you airflow.
Ducts in older and tract homes
Novato has many 1950s to 1970s tract homes whose ducts run through attics or crawlspaces and have been cut into and extended during remodels over the decades. Older homes elsewhere in Marin often have ducts squeezed into tight crawlspaces where nobody looks for years. In both, a joint that has slipped apart or a run flattened by a later project is a common finding, and fixing it often does more for airflow than any change to the equipment. A yearly maintenance visit keeps the filter, coil and blower from becoming the bottleneck.