Knowledge/Calculators and tables/Evaporative airflow and relief air, checked two ways
Evaporative airflow and relief air, checked two ways
Divide the cooler's airflow by the room volume to get air changes per hour (ACH). Aim for at least 20 and no more than about 40, with 30 as the trade shortcut. Then check the relief path. Each room needs an exhaust opening about twice its vent area, and the door test and the tissue test prove it.
- Air changes (ACH)
- airflow ÷ volumem³/h divided by m³
- From L/s
- × 3.6 ÷ volumevolume in m³
- Target range
- 20 to 40 ACH30 is the shortcut
- Relief opening
- 2 × vent areaper room
How do you work out air changes per hour?
- ACH = cooler airflow in m³/h, divided by the space volume in m³
- Or ACH = cooler airflow in L/s, times 3.6, divided by the space volume in m³
- Space volume = floor area in m², times ceiling height in m
So a 200 m² house with 2.7 m ceilings is 540 m³. A cooler delivering 4,500 L/s gives 16,200 m³/h, and 16,200 divided by 540 is 30 air changes an hour. The Commonwealth research uses this same house and cooler as its example. It's a handy check when a supplier's sizing looks wrong.
| House | Ceiling | Volume | Airflow for 30 ACH |
|---|---|---|---|
| 130 m² | 2.4 m | 312 m³ | 9,360 m³/h, or 2,600 L/s |
| 200 m² | 2.7 m | 540 m³ | 16,200 m³/h, or 4,500 L/s |
Which airflow figure should you size from?
Use the airflow figure at 80 Pa external static, the resistance the fan pushes against once it's ducted. That's roughly what an evaporative fan is specified against. The big airflow number on the brochure isn't that figure.
And the outlets have to be able to pass it. Seeley's rule, word for word, is "total outlet airflow must be greater than or equal to cooler airflow at 80 Pa."
| Outlet size | Capacity |
|---|---|
| 350 mm | 250 to 300 L/s |
| 400 mm | 350 to 400 L/s |
| 450 mm | 450 to 500 L/s |
How do you prove the relief air is enough?
Sizing the cooler and the outlets gets air into the house. Relief air is what lets it out, and without it the rest doesn't work.
Give every room an exhaust opening about twice the vent size. Open the windows or doors furthest from the vents. Then prove it with two tests that cost nothing.
- The door testAt high fan speed, a door should stay slightly ajar without slamming shut. If it slams, the room is pressurising.
- The tissue paper testA piece of tissue paper shouldn't stick to the fly screen while the unit runs. If it sticks, air is trying to leave faster than the opening allows.
There's no published Australian rule for relief air along the lines of "this much opening for this much airflow". We looked. If somebody quotes you a tidy square-metres-per-litre-per-second ratio, ask them for the source, because we couldn't find one.
Want a second opinion on a number?
Calculators are only as good as the assumptions under them, and the assumptions are where jobs go wrong.
If a figure looks off against what you see on site, that instinct is usually worth more than the calculator. Ask Kevin.
Who this is for: small and medium HVAC contractors in Victoria. If you're a homeowner after a rebate or a repair, you need a licensed contractor, not us, and yours can point you the right way.
Review date: 1 April 2027. Figures in this area change without much notice and this page carries some that will go stale. Every number above is dated and sourced so you can check it yourself. If you find something out of date here, tell us and we will fix it.