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Ducts and airflow

Airflow problems rarely announce themselves directly. Instead they show up as a cold room, a system that mysteriously stops running, or ice where there shouldn't be any.

Two separate systems, one shared cause: restricted flow

This section covers two different kinds of flow: the air moving through supply ducts and registers to heat or cool rooms. And the condensate water draining away from the furnace and the AC coil. Both fail in similar ways. Something gets blocked, and the system either works poorly or shuts itself down as a safety response.

Ducts and airflow guides

Condensate Clogged condensate drain line: the safety switch that stops the system

A full drain line trips a float switch on purpose. What causes the clog, and what's reasonable to clear yourself.

Airflow Uneven heating: why some rooms never get warm

Duct length, blocked registers, leaky runs, and a single thermostat that can only see one room.

Condensate Condensate pump not working: why it's leaking or won't shut off

A pump has a motor, a float switch, and a check valve, so it fails in ways a plain drain line never does.

Duct sizing Air duct sizing: what actually decides the size of a duct

Duct size comes from the airflow each room needs, not the size of the furnace. And the signs the ducts are wrong.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

US EPA. Improving Indoor Air Quality
US EPA. Introduction to Indoor Air Quality