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

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

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:

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.