The Difference Between Suction Line and Vapor Line

The suction line is to return gas to the compressor. It is between the evaporator and compressor on a standard cooling system and between the reversing valve and the compressor on a heat pump.

On a split system heat pump, the connecting lines are the vapor line and the liquid line. The vapor line can be cool suction vapor or hot gas depending on the mode of operation (heat/cool).

You map an additional clamp to the vapor line on a heat pump. You would have one on the true suction, one on the liquid and one on the vapor.

See the full probe placement guide

For a deeper dive into how measureQuick uses these measurements, see How measureQuick Calculates Superheat & Subcooling.