Work out the outdoor air a space needs, then check what it actually delivers from measured CO2, one step at a time. The tool does the arithmetic. Deciding whether the space is adequately ventilated, and what you would change, is yours.
The fields open pre-filled with a worked example (a real teaching room) so you can see each step working. Type over them on tabs 1 and 2 with your own space, and every tab follows.
Activities 1 and 4 use no tab. Activity 1 is the sensor measurement that produces your numbers, and Activity 4 is the share-back where you set the required airflow against the actual.
Numbers on screen: the worked example (a real teaching room).
How much outdoor air the standard says this space needs.
ASHRAE 62.1 sets a floor for outdoor air. Two things pollute the air indoors: the people, who exhale CO2 and other bioeffluents, and the building itself, whose finishes and furnishings off-gas whether or not anyone is there. So the standard adds a per-person allowance to a per-area allowance. That is why a packed teaching room is asked for far more air than a corridor of the same floor area. Pick your space type to load the standard's values, then read off the required outdoor airflow, Voz.
From ASHRAE 62.1 Table 6.2.2.1, Rp is the outdoor airflow per person and Ra the outdoor airflow per unit floor area. The breathing-zone airflow is Vbz = (Rp × Pz) + (Ra × Az), and the outdoor airflow the system must supply is Voz = Vbz / Ez. The "breathing zone" is the region 75 mm to 1.8 m above the floor and at least 0.6 m from walls, where people actually breathe. Ez = 1.0 assumes fully mixed air; real systems are often 0.5–0.8, and lower effectiveness means more airflow is needed for the same result.
What the space actually delivers, back-calculated from your CO2 reading.
People add CO2 at a fairly steady rate and ventilation carries it away. When the concentration stops rising, those two rates are in balance. That is what lets you work backwards. Read the flattened value off your measured curve and the tool returns outdoor airflow per person, the room total Qtotal, and air changes per hour, then checks the result against the requirement from step 1.
The per-person outdoor airflow is Qperson = G / (Ci − Co), with G the CO2 generation rate per person (default 0.005 L/s for an adult at rest), Ci the indoor steady-state CO2, and Co the outdoor value. CO2 in ppm is converted to a fraction (ppm ÷ 1,000,000) internally, so you enter values in ppm. The room total and air-change rate follow: Qtotal = Qperson × N and ACH = (Qtotal × 3.6) / Volume, with Volume the floor area × ceiling height. One subtlety worth discussing: a ventilation rate derived from measured CO2 already reflects how well the air reaches the occupants, so when Ez is below 1.0 the like-for-like target arguably sits between Vbz and Voz.
The whole curve: how fast CO2 builds up, and how fast it clears.
A plateau tells you the steady state. How quickly the room reaches it, and how fast it clears once a window opens, is a ventilation measurement in its own right. This runs the full transient mass balance, so you can watch CO2 rise as a room fills and decay after people leave. It is the same curve your sensor draws. Room volume, generation G and outdoor CO2 carry over from step 2. You set who is in the room, and how it is ventilated, over time.
Occupancy schedule. Each row sets the people count N and ventilation Q that apply from that time onward. Add a row to open a window (raise Q) or have the class leave (drop N to 0) partway through.
| From (min) | People, N | Ventilation Q (L/s) | Remove |
|---|
The transient single-zone mass balance V·dC/dt = N·G + Q·(Co − C) solves to C(t) = Css + (C₀ − Css)·e−(Q/V)t, with time constant τ = V/Q. Css is the steady-state level the curve heads toward; the steady-state shortcut in step 2 is just this equation once the exponential has decayed to near zero. The time constant τ is how long the room takes to cover 63% of any change. The same τ governs both rise and decay. After about 3τ the curve has effectively flattened, which is when the step-2 estimate becomes valid.
How many people this room can hold before the air quality slips.
This holds the ventilation you measured in step 2 fixed and sweeps occupancy, showing where steady-state CO2 would land against the 1,000 ppm guideline (the dashed line). The question shifts from whether the room is acceptable now to how many people it can take before it stops being acceptable. Find the occupancy at which your space would cross the guideline.
Rearranging the steady-state balance for indoor CO2 under a given occupancy gives Ci = Co + ((N × G) / Qtotal) × 10⁶. Raising N (or lowering Qtotal) lifts the predicted steady-state CO2; the explorer plots this line for you against the 1,000 ppm guideline.