HVAC Basics

Low subcooling may come from undercharge, insufficient liquid, flash gas, operating conditions or a bad reading. Learn why it does not prove a leak alone.

Learn how to record a furnace flashing-light code safely, match it to the exact model, preserve fault history, and avoid treating a code as a part diagnosis.

Overcharge and restricted airflow can create overlapping pressure, temperature and cycling symptoms. Learn how technicians separate charge from air-side faults.

Calculate heat-pump backup from the design-load shortfall, operating sequence and delivered output. Learn why default strip-kit sizing can mislead.

A furnace running constantly can reflect load, reduced output, duct loss, airflow, or thermostat bias. Learn what observations and measurements separate them.

When sensible cooling load is low, the thermostat may stop AC operation while latent moisture continues. Learn when independent dehumidification may be needed.

Sealed combustion describes isolation from indoor air; direct vent describes an approved outdoor intake and exhaust arrangement.

Learn how heat-pump COP changes with outdoor temperature, why it differs from capacity and HSPF2, and how to compare certified operating points.

Low superheat can indicate evaporator overfeeding, low load, restricted airflow or control behavior. Learn why it raises floodback concerns without proving overcharge.

Furnace output is measured at the appliance, while ducts can lose heat and airflow before rooms receive it. Distribution loss requires explicit analysis.