In a heat-pump study diagram, a line that carries liquid one way in cooling may carry it the other way in heating. A standard directional filter drier and a bi-flow filter drier are therefore not interchangeable just because the pipe size matches. This is a component-selection reasoning exercise, not a field installation instruction.
What changes when the heat pump reverses?
Danfoss explains that its standard liquid-line driers are designed for one direction of flow, marked by an arrow on the housing. In a reversing heat-pump circuit, liquid flow through the relevant section can reverse. Trace the line through both modes on the equipment diagram before deciding what the drier must handle. A reversing valve's presence alone does not prove that every line reverses flow.
This question differs from locating a plugged drier. The undercharge versus restriction article deals with symptoms after a flow restriction; here the choice is whether the component is designed for the stated flow path.
Why use a bi-flow design in a reversing path?
Danfoss describes a bi-flow drier that accepts liquid flow in either direction without releasing previously captured foreign matter. It also describes a more complex arrangement using two standard driers in parallel with opposing flow directions and check valves. In an exam-style paper sketch, label cooling and heating arrows first; then check whether the proposed one-way drier would see reverse flow.
Selection also depends on the actual manufacturer's refrigerant, capacity, pressure-drop and installation specifications. The diagram exercise is not permission to substitute a drier in a real system.
How do you solve a two-mode diagram question?
Draw two arrows beside the same liquid-line section: rightward in cooling and leftward in heating. If a proposed standard drier has one arrow pointing right, it matches only the cooling path in this simplified example. A rated bi-flow design is the direct fit for the section that reverses. If both arrows point right at the actual drier location, the diagram alone does not require a bi-flow component. State the location and mode before choosing.
Use the Ontario 313A exam guide and current-standard note for the applicable study outline. Red Seal still labels the newer occupational standard as awaiting new exams on 2026-10-04. The free sample is original, unofficial draft practice awaiting certified-mechanic review, not a recalled exam question or pass prediction.
Sources
- Danfoss: Filter driers and sight glassesChapter 4 on directional and bi-flow driers; checked 2026-10-04
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent-exam standard status; checked 2026-10-04
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.