Windowless rooms don’t get a “natural ventilation” shortcut—so the HVAC design has to do the heavy lifting. The goal is simple but non-negotiable: deliver verifiable outdoor air, remove pollutants through reliable exhaust, maintain intended pressure relationships, keep noise acceptable, and do all of it without wasting energy. This guide provides practical, code-aligned workflows for facility teams, MEP engineers, and contractors working on interior zones with limited or zero façade openings.

What “good” looks like: outdoor air and exhaust meet the adopted code path, pressure relationships are measurable (e.g., toilets negative to corridors), filtration captures fine particles on recirculated systems, sound levels support work, and controls modulate airflow based on occupancy—so performance is both compliant and economical.

1) Start with the baseline: what codes and standards typically require

Before selecting equipment, lock the compliance path and translate it into airflow targets. Most projects fall under one of these approaches:

• ASHRAE 62.1 (nonresidential): Use the Ventilation Rate Procedure (VRP), which combines a people component and a floor-area component to determine outdoor air. Apply system ventilation efficiency and confirm your jurisdiction’s adoption and amendments.
• EN 16798 (Europe): Method 1 uses person- and area-based flow rates with indoor air categories. Category II is commonly used for nonresidential applications (always verify the official table for your space type).
• Restrooms/toilets: Many jurisdictions specify minimum exhaust rates by fixture/space. Design exhaust to sustain negative pressure relative to adjacent zones, and ensure a defined makeup-air path.
• Filtration (recirculated air): A common baseline for nonresidential systems is MERV 13 or ISO 16890 ePM1 ≥50% where air is recirculated. Always verify fan static pressure headroom and plan for end-of-life filter pressure rise.
• Noise: Don’t treat acoustics as an afterthought. Duct velocity, terminal selection, and fan control strategy directly affect sound levels. Many offices/meeting rooms aim for NC/RC targets appropriate to the use.

2) Translate requirements into design: a step-by-step workflow

Step 1 — Define occupancy and compute design airflow

• Confirm peak occupancy and realistic schedules. Calculate outdoor air using your selected standard, then apply system ventilation efficiency and safety margins where appropriate. For high-variability spaces (e.g., meeting rooms), demand-controlled ventilation (DCV) can reduce energy while maintaining indoor air quality—provided sensors are correctly located, calibrated, and trended.

Step 2 — Choose the right system topology for an interior zone

• For windowless rooms, a DOAS paired with terminal units often provides better control of outdoor air delivery and humidity. Add an ERV/HRV to recover energy from exhaust air and reduce operating costs. Prioritize verified performance and ensure the selected unit can meet airflow at your required external static pressure.
• airwoods product references (for specification scoping): start with the Eco-Flex Energy Recovery Ventilator for compact applications, or review integrated solutions like the Airwoods Ceiling Mounted Heat Pump Energy Recovery Ventilator 350CMH. These pages can help you frame airflow range, available ESP, filtration configuration, and controls capability when building a basis of design.

Step 3 — Ductwork and terminals: prevent “short-circuit” airflow

• A common failure mode in interior rooms is supply air “dumping” directly into returns without properly mixing in the occupied zone. Use diffuser placement and throw characteristics to support mixing, keep return grilles out of the supply jet path, and avoid stagnant corners. Maintain separation between outdoor intakes and exhaust outlets, and plan access for filter replacement and maintenance.

Step 4 — Pressure relationships: make airflow go where it should

• Pressure is the traffic control system for air. Keep toilets negative to adjacent areas, maintain offices and meeting rooms neutral to slightly positive to corridors (when appropriate), and provide defined transfer paths (door undercuts or transfer grilles) consistent with fire/smoke strategy. Verify with differential pressure (Pa) and smoke visualization during TAB.

Step 5 — Controls and monitoring: don’t “set and forget”

• Use schedules and occupancy signals to reduce airflow when spaces are empty. Trend CO2 (where used), fan speed, outdoor air damper position, space pressure, and filter differential pressure. Add alarms for sensor failure, abnormal pressure, and filter DP thresholds—so IAQ problems are detected early instead of becoming comfort complaints.

3) Commissioning & verification: how to prove it works

For windowless spaces, commissioning is not optional—it’s how you confirm airflow targets, pressure relationships, and controls sequences. A practical checklist includes:

• Measure outdoor air, supply air, and exhaust at terminals; document the calculation path from breathing-zone airflow to system airflow.
• Verify restroom exhaust meets the required minimums and demonstrate negative pressure to adjacent spaces.
• Check diffuser performance and adjust balancing to eliminate short-circuiting.
• Validate DCV: confirm CO2 sensor accuracy, location, and control response under representative occupancy.
• Record filter baseline differential pressure and configure alarms for service thresholds.
• Confirm sound levels in representative rooms; add attenuation or isolation if required.
• Test ERV sequences (bypass, defrost/frost prevention) where climate conditions warrant.

Ongoing verification: Review monthly trends for CO2 peaks, pressure alarms, and filter DP. Recheck airflow after space reconfiguration, tenant changes, or major maintenance.

4) Example application (disclosed)

Disclosure: airwoods is our product.

Consider a windowless conference-room cluster served by a DOAS with energy recovery, paired with a nearby interior toilet block. A robust strategy is to size the DOAS to deliver the calculated outdoor airflow at peak meeting occupancy, while the ERV reduces heating/cooling load on the make-up air. A staged filtration setup (e.g., pre-filter + final filter) helps manage pressure rise and service intervals. Occupancy-based DCV reduces airflow between meetings, while the BMS trends CO2 and filter DP for continuous assurance.

For early-stage selection and product range review, see airwoods Commercial ERV options.

On the toilet side, exhaust is designed to meet local minimums, a clear makeup-air path is provided, and negative pressure is verified during TAB—so odors don’t migrate into adjacent office areas.

5) Common pitfalls in windowless rooms—and how to fix them

• Odors leaking from toilets: usually caused by insufficient exhaust, blocked makeup-air paths, or poor pressure control. Fix by verifying exhaust rates, adding transfer paths, and confirming negative pressure with DP readings and smoke tests.
• Short-circuit airflow in meeting rooms: happens when supply air flows directly to return. Fix by reworking diffuser/return layout, verifying throw/induction at design flow, and using CFD for complex spaces when needed.
• Fans can’t overcome filter pressure rise: occurs when end-of-life filter DP isn’t included in fan selection. Fix by adding static pressure margin, trending DP, and using staged filtration to extend service intervals.
• Cold-climate frosting on ERV cores: caused by missing or misconfigured defrost/bypass logic. Fix by verifying sequences and adding alarms so stuck modes don’t persist for weeks.

Next steps

• Select your compliance path, calculate airflow for real occupancy, and document the basis of design.
• Choose an ERV/DOAS solution with verified performance and appropriate filtration; confirm fan static pressure headroom for filters and duct losses.
• Design terminals and pressure relationships to prevent short-circuiting and odor migration.
• Commission the system and keep verification data in your routine operations review.

If you need specification-ready submittal packages or selection support for ERV/DOAS in windowless spaces, the airwoods team can help match design targets with certified equipment capabilities and control strategies.