Knowledge/Systems/Sizing an evaporative cooler on air changes per hour
Sizing an evaporative cooler on air changes per hour
The range from the Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH) is typically at least 20 air changes an hour (ACH), up to about 40. The trade shortcut is 30. Divide cooler airflow in m³/h by the room volume. Then look up the cooler's airflow at 80 Pa. Your outlets together must deliver at least that.
- Engineering range
- 20 to 40 ACHAIRAH
- Trade shortcut
- 30 ACHused by many suppliers
- Check at
- 80 Paexternal static pressure
- Outlet rule
- Outlets at least matchthe cooler's airflow at 80 Pa
The air change method
AIRAH gives a range. The space you're cooling should typically get at least 20 air changes an hour, and no more than about 40. The Commonwealth research backs up the shortcut the trade actually uses. Many suppliers work on a rule of thumb that the cooler delivers the equivalent of 30 air changes an hour.
Both numbers are real and they're doing different jobs. 20 to 40 is the design range. 30 is the number you'd use on the phone.
The arithmetic
- ACH = cooler airflow in m³/h, divided by space volume in m³
- Or ACH = cooler airflow in L/s, times 3.6, divided by space volume in m³
| House | Ceiling | Airflow at 30 ACH |
|---|---|---|
| 130 m² | 2.4 m | 9,360 m³/h, or 2,600 L/s |
| 200 m² | 2.7 m | 16,200 m³/h, or 4,500 L/s |
Seeley's current dealer method
- Work out the cooling capacity you need, in kWNot airflow first. Capacity first.
- Pick a model that meets it or slightly beats itSlightly. Not double.
- Check that model's airflow at 80 PaThis is the number that matters, not the headline airflow on the brochure.
- Check the outlets can actually deliver itThe rule, word for word: 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 |
Put those two together and a 2,600 L/s cooler wants seven or eight 400 mm outlets. If you've inherited a system with four, you've just found why the back of the house doesn't cool.
Why 80 Pa is the most useful number here
Evaporative fans are specified at roughly 80 Pa external static - that's the resistance of the ductwork they're pushing against. A refrigerated ducted fan coil works against several hundred Pa. That one comparison explains most of what's different about evaporative ductwork.
It's why evaporative needs oversized ducts and large outlets. It's why the outlet count matters so much more than it does on a refrigerated system. And it's why evaporative ductwork can't simply be reused for reverse cycle, which is the question you'll get asked every time somebody converts.
Weighing up what to put in front of a customer?
Most of the hard calls aren't technical. They're about what you can stand behind when the customer rings you in February.
Kevin's been having that conversation for thirty years. Ask him.
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.
Related
- How an indirect gas ducted heater worksIndirect means the flue gases never mix with the air you breathe. How it holds outlet temperature, what shuts it down, and the gas pressures it needs.
- Can you reuse the ducts on a gas to reverse cycle swap?The honest version. No engineering document covers reusing the ducts, so here's what's checked, what's our best judgement, and the seven things that go wrong.
- Hydronic heating, and what the rental rule does to itSteady radiant warmth, slow to respond. And under the 2027 rental rule, a failed gas boiler points to reverse cycle rather than an air to water heat pump.
- Does a Victorian gas heater have to be replaced with electric?If the owner lives there, a dead gas ducted heater doesn't have to go electric. Space heating was dropped from the rules. Rentals are different from 2027.
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.