Select VRF equipment from the operating envelope

A large nameplate capacity can still fail the actual job. Test operating mode and each delivered duty before recommending equipment.

Original case · Suggested 18 minutes · Edition 2026-10-12.1

By the end of this workshop

Choose a method that respects the supplied conditions.
Explain how a changed condition affects the decision.
Identify a physical or procedural cross-check and an applicability limit.
Record the assumption you would verify next time.

Start with the complete model

Givens and assumptions

Two zones simultaneously need 60,000 Btu/h cooling and 30,000 Btu/h heating.
The table supplies corrected delivered capacities at the stated design condition, including the required piping and defrost corrections. They are hypothetical selection data, not a real product catalog.
Simultaneous operation is mandatory. Evaluate only the listed duty constraints; remaining selection checks are separate.

Original selection data at the design condition
Option	Simultaneous heating/cooling?	Corrected cooling / heating, Btu/h
A · common-mode heat pump	No	84,000 / 66,000
B · heat recovery	Yes	82,000 / 42,000
C · heat recovery	Yes	78,000 / 24,000

Study calculator, scratch notepad and whiteboard

Expression Trigonometry in degrees

Supports +, −, *, /, ^, parentheses, sqrt, abs, log (base 10), ln, sin, cos, tan, pi and e. Confirm units independently.

Scratch notepad

Scratch text and drawings stay in this tab and clear when you leave or reload. Save important reasoning in a response or reflection below before leaving.

Scratch whiteboard

Sketch a control volume or air path with a mouse, pen or touch. Ink stays in this page session and clears on reload. Use the response and reflection fields as a keyboard alternative. Sign in before starting and save a reflection to retain your reasoning.

1 · Choose the method

Which listed option satisfies simultaneous 60,000 Btu/h cooling and 30,000 Btu/h heating at the supplied design condition?

Choose one responseA, because its heating capacity exceeds every other option.C, because its cooling capacity exceeds the cooling demand.B satisfies both delivered duties and the simultaneous-mode requirement.Any option, because subtracting the two zone loads leaves only 30,000 Btu/h.

ConfidenceWhy did you choose this method? Optional personal reasoning, up to 1,000 characters.

2 · Change the condition

Predict before you inspect the calculation.

The coincident cooling duty rises to 88,000 Btu/h; heating remains 30,000 Btu/h.

Simultaneous cooling/heating remains mandatory. Corrected capacities: A 84,000/66,000 without simultaneous mode; B 82,000/42,000 with simultaneous mode; C 78,000/24,000 with simultaneous mode.
Required delivered duties are now 88,000 Btu/h cooling and 30,000 Btu/h heating.

Check the first decision to open the changed-condition response.

Personal notes are not automatically graded. Workshops are linked learning experiments, separate from timed diagnostics and mocks. They do not predict a pass or award instructional hours.

Reference navigation drill

Search concept: refrigeration, heat pumps, capacity

Distinguish a cycle energy balance from an equipment-selection envelope. Identify which performance data the original problem supplies.

Use an authorized reference. A search term does not guarantee that a current examination reference contains every relationship used in this teaching case.

Where this model stops

The capacity table is original hypothetical data. Real selection requires manufacturer simultaneous-operation ratings, capacity corrections, refrigerant and piping checks, controls and applicable safety requirements.

The PE Mechanical: HVAC and Refrigeration examination uses the NCEES PE Mechanical Reference Handbook. These source links provide study context; they are not additional exam-day references.

Found an unclear condition or calculation? Include workshop vrf-operating-envelope and the step in your comment.

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