For an air-conditioned basement garage whose AHU runs on the minimum outdoor air: when cars start and idle, how many seconds until carbon monoxide (CO) and nitrogen dioxide (NO₂) pass the acceptable limit, and how much more fresh air keeps them within it. It adds the idling cars' heat to the garage load and sizes the AHU for both cases: the supply airflow and supply air condition, the outdoor air and the cooling coil, for cars or SUVs, new or old, with the emission data and its sources shown.
Garage Ventilation Calculator
jerinsantosh.com
Units
Dry bulb (°C)
Humidity
Garage
%
Outdoor
°C
- Sensible and total load: the garage's base load only (lights, walls, roof, people and the rest), excluding the heat from the cars, the fresh air load and infiltration; the tool adds those itself.
- The supply air leaves the AHU ΔT below the garage.
- Extract above 100% keeps the garage at a negative pressure (ASHRAE 62.1 keeps a garage at or below the pressure of the building next to it): the difference is drawn in from outdoors, down the ramp, and its heat goes into the garage.
- Extract below 100%: the rest of the outdoor air is pushed out through the ramp and leaks. Either way all the air leaving the garage carries CO and NO₂ out.
- Altitude and pressure set each other.
- Cars idling: started in the garage and left running (warming up, air conditioning on).
- Idling time: how long they run before driving out.
- Ventilation effectiveness Ez: 1.0 for cool air supplied at the ceiling (ASHRAE 62.1).
With the minimum outdoor air, the air passes the limit after
– s
Outdoor air, cars idling
L/s
- Limit reached first
- –
- Level at the end of idling
- –
- Minimum outdoor air, cars off
- –
- Outdoor air needed, cars idling
- –
- Outdoor air used, cars idling
- –
- Extract air, cars idling
- –
- Additional outdoor air
- –
- If the cars keep idling
- –
- If every car idles
- –
AHU with the cars idling
- Heat from the idling cars
- –
- Garage load, sensible / total
- –
- Supply air
- –
- Cooling coil
- –
Minimum outdoor air Outdoor air needed Solid CO, dashed NO₂
Inputs Results
| Cars off minimum outdoor air |
Cars idling outdoor air raised |
Change | |
|---|---|---|---|
| Garage sensible load | – | – | – |
| Garage latent load | – | – | – |
| Garage total load | – | – | – |
| Outdoor air | – | – | – |
| Extract air | – | – | – |
| Air drawn in (+) / pushed out (−) | – | – | – |
| Load from air drawn in | – | – | – |
| Supply air | – | – | – |
| Outdoor air share | – | – | – |
| Supply dry bulb / wet bulb | – | – | – |
| Supply RH | – | – | – |
| Coil entering, dry bulb / wet bulb | – | – | – |
| Cooling coil, total | – | – | – |
| Coil sensible / latent | – | – | – |
| Cooling coil, tons | – | – | – |
| Condensate | – | – | – |
| Vehicle | CO g/min |
NOx (as NO₂) g/min |
NO₂ share of NOx, % |
Fuel at idle L/h |
Heat, sensible / latent kW |
|---|---|---|---|---|---|
| Car, new (Euro 6 / Tier 3) | – | ||||
| Car, old (1990s) | – | ||||
| SUV, new (Euro 6 / Tier 3) | – | ||||
| SUV, old (1990s) | – |
- Model: the garage is one well-mixed space starting at the outdoor air's level; while cars idle the level rises as C = Coutdoor + G ÷ (Q·Ez) × (1 − e−Q·t/V), G the cars' emission, Q the air leaving and V the garage's volume.
- Dilution: the AHU's recirculated air takes out no CO or NO₂; only the air leaving the garage dilutes them: the outdoor air, or the extract when it is more (the difference drawn in from outdoors).
- Override: the AHU starts on the minimum outdoor air; when CO or NO₂ would pass the limit while the cars idle, Override raises the outdoor air to the least that keeps them within it (or to your own value, at least the minimum), and the extract, the air drawn in, the garage load, the supply air and the coil follow it.
- Limit as a ceiling: the level is held under the limit at all times (as CO/NO₂ sensors do), which is safe for limits set as 8-hour averages.
- Emissions, old cars (first minutes after a start, before the catalyst warms up): CO 4.7 g/min, the cold-start rate in the ASHRAE parking garage design method (Krarti and Ayari, ASHRAE Applications Handbook, ch. Enclosed Vehicular Facilities); NOx from US EPA idle factors (EPA420-F-08-025: cars 71.2 g/h CO and 3.5 g/h NOx, light trucks and SUVs 72.7 and 4.07 g/h) × 1.5 for a cold start.
- Emissions, new cars: Euro 6b cold-start measurements (Atmosphere, 2019: 1.5 g CO and 83 mg NOx a km over the first 5 minutes), taken lower at idle.
- SUVs are cars × EPA's light-truck to car ratio.
- NO₂ is 10–30% of a petrol car's NOx, lowest at idle; diesel cars emit far more NOx and NO₂, so raise both for a diesel fleet.
- Fuel and heat: Argonne National Laboratory (2.0 L engine about 0.6 L/h at idle with no load, 4.6 L a little over twice that), with air conditioning on; all the fuel's energy heats the garage: sensible its heating value (petrol 43.4 MJ/kg, 0.745 kg/L), latent the water its burning makes (1.2 kg/kg).
- Minimum outdoor air: ASHRAE 62.1 Table 6-2 and IMC 403.3 exhaust 3.7 L/s·m² (0.75 cfm/ft²) for parking garages; IMC 404.2 lets CO and NO₂ detection drop it to 0.25 L/s·m² (0.05 cfm/ft²) with the full rate available; BS 7346-7: 6 air changes an hour.
- Limits (room ceiling used): IMC 404.1 fans on at CO 25 ppm, with NO₂ 0.5 ppm (New York City Mechanical Code 404.1; ASHRAE's garage method also designs to 25 ppm CO); Dubai Municipality CO 25 ppm (1 h) and UAE car-park practice; UK BS 7346-7 and Approved Document F CO 30 ppm (8 h; 90 ppm over 15 min), HSE EH40 NO₂ 0.5 ppm; EU Directive 2017/164 CO 20 ppm, NO₂ 0.5 ppm (8 h); Australia AS 1668.2 / NCC CO 30 ppm (8 h; 60 ppm 1 h, 100 ppm peak); ACGIH CO 25 ppm, NO₂ 0.2 ppm; NIOSH CO 35 ppm, NO₂ 1 ppm (15 min); OSHA CO 50 ppm, NO₂ 5 ppm ceiling; WHO CO 35 mg/m³ (1 h), NO₂ 200 µg/m³ (1 h). Where a standard sets no NO₂ limit, only CO is checked.