# Ventilation Calculator: Fresh Air by ASHRAE 62.1 and by Air Changes

> A ventilation calculation sets how much air a space needs. ASHRAE 62.1's ventilation rate procedure gives the outdoor air: a rate per person plus a rate per square metre of floor, divided by how effectively the air reaches the occupants. The air-change method multiplies room volume by a target number of air changes per hour. Offices are normally designed on the first.

## What it calculates

The calculator works out the **airflow a space needs for ventilation** in two ways, and can
show both side by side:

- **Outdoor air by the ASHRAE 62.1 ventilation rate procedure**: the fresh air that has to
  be brought in from outside for the people in the zone and for the zone itself, in L/s,
  m³/h and cfm, with the figure for each person.
- **Airflow by air changes per hour**: the room volume multiplied by a target number of air
  changes, in the same three units.

Seeing the two together is the point. An office specification that asks for "six air
changes" and one that asks for outdoor air to ASHRAE 62.1 are asking for very different
things, and the comparison shows how different.

## Inputs

**Method** chooses which results to show: both, the ASHRAE 62.1 outdoor air only, or the
air changes only.

**Floor area of the zone** is the occupiable floor the air serves, entered in square feet or
square metres. Square feet are converted to square metres at 0.092903.

**Occupants at the design count** is the number of people the zone is designed for: the
largest number expected in it, not the average.

**Room height** is floor to false ceiling, used only to work out the volume for the
air-change method.

The **assumptions** panel holds the four figures that drive the result: the outdoor air rate
for each person (Rp), the outdoor air rate for each square metre of floor (Ra), the zone air
distribution effectiveness (Ez) and the number of air changes per hour.

## Method

The ventilation rate procedure in ASHRAE 62.1 builds the outdoor air for a zone from two
parts and then corrects for distribution:

- **Breathing zone outdoor airflow**: Vbz = Rp × Pz + Ra × Az, where Pz is the number of
  people and Az the floor area in m².
- **Zone outdoor airflow**: Voz = Vbz / Ez.

The people part covers what occupants release; the area part covers what the building
releases whether anyone is there or not, from finishes, furniture and equipment.

The air-change method is simpler: **volume = area × height**, and **airflow = volume × ACH**,
in m³/h.

Units are converted at 1 L/s = 3.6 m³/h = 2.119 cfm.

## Assumptions

| Assumption | Default | Basis |
| --- | --- | --- |
| Outdoor air for each person (Rp) | 2.5 L/s | Office space value in ASHRAE 62.1 |
| Outdoor air for each m² of floor (Ra) | 0.3 L/s | Office space value in ASHRAE 62.1 |
| Zone air distribution effectiveness (Ez) | 1.0 | Cool air supplied at ceiling level |
| Air changes per hour | 6 | A placeholder only; the target comes from the code that applies to the space or from the designer |
| Room height | 3 m | Floor to false ceiling |

Rp and Ra are the office values. A meeting room, training room, reception or pantry has its
own values in the standard, and a meeting room's design occupancy is far denser than the
open plan beside it, so it needs to be calculated as its own zone. Use the values for your
space type, from the edition of the standard your design basis adopts.

The air-change figure has no default that is right for every room. Toilets, kitchens, car
parks, plant rooms and stores each have their own requirements under the applicable code,
NBC 2016 Part 8 among them, and the designer sets the target.

## Limitations

- **It treats one zone.** Where a single air handler serves several zones, ASHRAE 62.1
  adjusts the outdoor air drawn in at the unit for occupant diversity and for system
  ventilation efficiency, so it is not simply the sum of the zone figures.
- **It does not check occupancy.** The result is only as good as the headcount entered. A
  floor that is later re-planned with more desks needs more outdoor air.
- **It ignores exhaust and pressurisation.** Toilet, pantry and kitchen exhaust, and the
  make-up air they draw, are designed separately.
- **It says nothing about air quality outdoors or filtration.** Where outdoor air is
  polluted, filtration and treatment are separate design decisions.
- **It does not size equipment.** The cooling and dehumidifying load of the outdoor air is
  worked out in the heat-load calculation.

## Worked example

A 5,000 sq ft office with 50 people and a 3 m ceiling, using the default assumptions:

| Step | Calculation | Result |
| --- | --- | --- |
| Floor area | 5,000 × 0.092903 | 464.5 m² |
| People part | 2.5 × 50 | 125 L/s |
| Area part | 0.3 × 464.5 | 139.4 L/s |
| Breathing zone outdoor air, Vbz | 125 + 139.4 | 264.4 L/s |
| Zone outdoor air, Voz | 264.4 / 1.0 | 264.4 L/s |
| In other units | × 3.6 and × 2.119 | 952 m³/h, 560 cfm |

The office needs about **264 L/s of outdoor air**: 952 m³/h or 560 cfm, which is 5.3 L/s
(11.2 cfm) for each person once the area part is shared out. As air changes, that outdoor
air is only 0.68 changes per hour of the room's volume.

By the air-change method, the room volume is 464.5 × 3 = 1,394 m³, and at 6 air changes per
hour the airflow is **8,361 m³/h**, or 2,323 L/s and 4,922 cfm. That is almost nine times the
outdoor air. The difference is the air the system recirculates and cools; it is not fresh
air, and meeting the air-change figure says nothing about whether the occupants are getting
enough outdoor air.

Double the occupancy to 100 people and the outdoor air rises to about 389 L/s (1,402 m³/h),
while the air-change figure does not move at all. That is the reason offices are ventilated
on the occupancy rather than on air changes.

## How engineers use it

An HVAC engineer applies the same procedure zone by zone, using the space types and
occupancy from the layout. Open office, meeting rooms, training rooms and reception each get
their own Rp and Ra, the distribution effectiveness comes from the supply and return
arrangement, and the zone figures are combined into the outdoor air the air handler or the
treated fresh-air unit must draw in. Exhaust from toilets and pantries is then balanced
against that supply.

The outdoor air quantity also feeds straight into the heat-load calculation, because in most
Indian climates it has to be cooled and dried before it is supplied. Demand-controlled
ventilation, which varies outdoor air with occupancy in rooms like meeting rooms, and heat
recovery from exhaust air are both ways an engineer may reduce that load; whether they suit
a project is a design decision.

## When a professional design must replace it

Use the calculator to understand the order of fresh air a floor needs, to compare it with
what a landlord's treated fresh-air supply provides, and to check a specification written in
air changes. Replace it with a ventilation design by an HVAC engineer before any equipment
or ductwork is sized, wherever one system serves several zones, for any space with exhaust,
process or contamination requirements, and whenever the occupancy changes. The engineer
sets the rates from the edition of ASHRAE 62.1 or the other basis adopted for the project,
alongside NBC 2016 Part 8.

## Frequently asked questions

### How much fresh air does an office need per person?

Under the ASHRAE 62.1 ventilation rate procedure there is no single per-person figure, because the requirement has a people part and a floor-area part. With the office values of 2.5 L/s per person and 0.3 L/s per m², a floor with one person for every 100 sq ft works out at about 5.3 L/s, or 11 cfm, per person. A denser floor needs less per person but more in total.

### What is the difference between fresh air and air changes per hour?

Fresh air, or outdoor air, is the air brought in from outside to dilute what people and the building release. Air changes per hour counts all the air moved through a room against its volume, and in an air-conditioned office most of that air is recirculated. A room can meet a high air change rate and still be short of outdoor air.

### Where do the Rp and Ra values come from?

ASHRAE 62.1 tabulates a people rate (Rp) and an area rate (Ra) for each type of space. The calculator's defaults are the office space values. Meeting rooms, classrooms, retail and other spaces have different values, and the rates can change between editions, so take them from the edition named in your project's design basis.

### What is zone air distribution effectiveness?

Ez allows for how much of the supplied outdoor air actually reaches the breathing zone. Cool air supplied at the ceiling is taken as 1.0. ASHRAE 62.1 gives lower values for layouts that short-circuit, such as warm air supplied and returned at the ceiling, and a lower Ez means more outdoor air has to be supplied.

### Does more fresh air mean a bigger air conditioner?

Usually, yes. In most Indian climates outdoor air is hot, humid or both, and it must be cooled and dehumidified before it reaches the room. That is a significant share of an office's cooling load, which is why the AC tonnage calculator includes a fresh-air load for each person.

## Sources

- [ASHRAE 62.1](https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2) — Ventilation rate procedure: breathing zone and zone outdoor airflow
- [NBC 2016, Part 8](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Building services, Section 3: air conditioning, heating and mechanical ventilation

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Source: https://hagerstone.com/calculators/ventilation-calculator