Introduction
Hypothetical case study — illustrative example, not a real project. The numbers below are worked through a realistic scenario to show how the full ventilation workflow fits together: surveying a house, sizing each space, selecting equipment, budgeting, and verifying the result. No real property, client, or contractor is involved; the scenario exists to demonstrate the method.
The scenario: a hypothetical 2,000 ft² (186 m²) single-story home with 3 bedrooms, 2 bathrooms, a kitchen with a 40,000 BTU gas range, and 8 ft (2.44 m) ceilings throughout. The owners’ complaints are familiar — a steamy bathroom mirror that never clears, cooking odors that linger for hours, and a stuffy primary bedroom. The Ventilation Calculator provides the per-room math used at each step below.
The Starting Point: Survey Findings
The walkthrough records each room’s dimensions and the existing ventilation:
| Room | Dimensions | Volume | Existing ventilation |
|---|---|---|---|
| Primary bedroom | 14 × 12 × 8 ft | 1,344 ft³ (38.1 m³) | None dedicated |
| Bedroom 2 | 12 × 10 × 8 ft | 960 ft³ (27.2 m³) | None dedicated |
| Bedroom 3 / office | 11 × 10 × 8 ft | 880 ft³ (24.9 m³) | None dedicated |
| Living room | 18 × 14 × 8 ft | 2,016 ft³ (57.1 m³) | None dedicated |
| Kitchen | 12 × 10 × 8 ft | 960 ft³ (27.2 m³) | Recirculating microwave hood |
| Hall bath | 8 × 6 × 8 ft | 384 ft³ (10.9 m³) | 50 CFM fan, 4 sones, vented to attic |
| Primary bath | 10 × 8 × 8 ft | 640 ft³ (18.1 m³) | 50 CFM fan, 3.5 sones, vented to attic |
Three problems are visible before any math: both bath fans vent into the attic (a moisture and code problem), the kitchen has no real exhaust (recirculating only), and the fans are loud enough that the owners admit they rarely run them. The common mistakes guide covers each of these failure modes.
Step 1: Room-by-Room Sizing
Applying the standard planning rates via the calculator:
| Room | Target | Required airflow |
|---|---|---|
| Primary bedroom | 5 ACH | 5 × 1,344 ÷ 60 = 112 CFM (190 m³/h) |
| Bedroom 2 | 5 ACH | 5 × 960 ÷ 60 = 80 CFM (136 m³/h) |
| Bedroom 3 / office | 5 ACH | 5 × 880 ÷ 60 = 73 CFM (124 m³/h) |
| Living room | 5 ACH | 5 × 2,016 ÷ 60 = 168 CFM (285 m³/h) |
| Kitchen hood | 100 CFM / 10k BTU | 400 CFM (680 m³/h) |
| Hall bath | 50 CFM intermittent | 50 CFM (85 m³/h) |
| Primary bath | 50 CFM intermittent | 50 CFM (85 m³/h) |
The bedroom and living-room figures describe desired air movement in those spaces — delivered by the whole-house system, not by dedicated exhaust fans in each room. That’s the reconciliation step next.
Step 2: Whole-House Baseline
ASHRAE 62.2 for 2,000 ft² and 3 bedrooms: 0.03 × 2,000 + 7.5 × 4 = 90 CFM (≈ 153 m³/h) continuous. The design decision: a 120 CFM ERV (one size up for duct losses, with boost mode) supplies the bedrooms and living areas continuously, satisfying both the 62.2 baseline and the per-room ACH targets through distribution. The estimating guide details this reconciliation logic.
Step 3: Equipment Selection
| Location | Selected (hypothetical) | Rationale |
|---|---|---|
| Hall bath | 80 CFM, 1.0 sone, humidity-sensing | One size up; auto-run guarantees moisture control |
| Primary bath | 80 CFM, 1.0 sone, humidity-sensing | Same; larger bath gets same class |
| Kitchen | 600 CFM ducted chimney hood | 400 CFM target + duct-loss margin; ducted, not recirculating |
| Whole house | 120 CFM ERV with boost | Covers 90 CFM baseline + distribution margin |
Critical corrections bundled into the scope: both bath ducts re-routed from the attic to proper roof terminations, and a make-up air damper interlocked with the 600 CFM hood (above the typical 400 CFM trigger). Equipment reading guidance is in the equipment guide.
Step 4: Budget (Hypothetical Planning Figures)
| Item | Planning range (USD) |
|---|---|
| 2 × quiet humidity-sensing bath fans | $300–$500 |
| Bath fan install + duct re-route to roof | $700–$1,400 |
| 600 CFM ducted hood (unit) | $700–$1,200 |
| Hood install + new duct + make-up air | $1,200–$2,200 |
| 120 CFM ERV (unit) | $1,000–$1,800 |
| ERV install with dedicated ducts | $2,000–$4,000 |
| Planning total | $5,900–$12,100 |
These are typical US planning brackets with regional variation — the cost guide breaks down each line. The owners in this scenario phase the work: bath corrections first (moisture damage prevention), hood second, ERV third.
Step 5: Verification
The hypothetical commissioning: delivered airflow measured at each grille with a flow hood — bath fans delivering ~65–70 CFM against their 80 CFM rating (duct losses as expected, still above the 50 CFM target), hood pulling ~480 CFM at the capture zone, ERV balanced to 90 CFM continuous with boost verified at 150 CFM. Make-up air damper operation confirmed interlocked with the hood. The mirror-fog test: the primary bath mirror clears within 10 minutes of a shower with the humidity-sensing fan running automatically — the original complaint, resolved.
Lessons from the Scenario
- The survey found the real problems. Attic venting and a recirculating hood were visible before any calculation — the math sized the fix, but observation found it.
- Noise drove behavior. The old fans were technically 50 CFM but effectively 0 CFM, because nobody ran them. The sone rating was as important as the CFM.
- One big load dominated design. The 600 CFM hood drove the make-up air requirement, which drove a meaningful cost line — a reminder to check interactions, not just rooms.
- Phasing made it affordable. Moisture corrections first, comfort and efficiency later — the estimate supported a phased plan because each line was sized independently.
- Measurement closed the loop. Commissioning readings turned the design from a paper exercise into a verified system.
To run the same workflow on a real house, follow the estimating guide end to end, verify each step with the checklist, and confirm code requirements with the local AHJ per the code-compliance guide.
Related Guides
- Ventilation Estimating Guide — the workflow this scenario follows.
- Ventilation Cost Guide — budget each line of the equipment list.
- Ventilation Equipment Guide — select the hardware.
- Pro Tips — installation practices used in the scenario.
- Ventilation Calculator FAQ — quick answers to the most asked questions.
Frequently Asked Questions
Is this case study based on a real project?
No. It is explicitly a hypothetical, illustrative example — realistic numbers worked through a fictional scenario to demonstrate the method. Your house will differ; run your own measurements through the calculator.
Why replace working 50 CFM bath fans?
In the scenario they had three compounding problems: attic venting (moisture/code violation), high noise (so they were never run), and no automatic control. The CFM rating was the least of their issues — a pattern the common mistakes guide documents.
Could the ERV have been skipped to save money?
In the scenario, the owners phased it last — but the 90 CFM continuous baseline it provides is a code requirement where ASHRAE 62.2 is adopted, not an optional upgrade. Skipping it permanently would leave the design non-compliant in such jurisdictions.
Why a 600 CFM hood for a 400 CFM target?
The 400 CFM is the cooktop-output minimum; the selection steps up for duct losses (a long run with elbows) and capture effectiveness over a 30-inch pro-style range. The make-up air damper handles the above-threshold consequence.
How were the results verified?
By measuring delivered airflow at each grille with a flow hood and comparing against targets — the commissioning practice described in the pro tips guide. Paper calculations become verified performance only when measured.
Apply the Method to Your Home
The scenario’s value isn’t its specific numbers — it’s the sequence: survey, size each room, set the whole-house baseline, reconcile, select equipment, budget, verify. Run your own rooms through the Ventilation Calculator, follow the estimating workflow, and confirm requirements with your local building authority before committing to equipment.
Disclaimer: This is a hypothetical illustrative example, not a real project — figures are for demonstration only. Actual ventilation requirements, costs, and code provisions vary by house, region, and jurisdiction. Verify targets with your local building authority (AHJ) and consult a licensed HVAC professional for system design and installation decisions.
