Knowledge/Install and commissioning/Relief air, and the two tests that prove you have enough
Relief air, and the two tests that prove you have enough
Give every room an exhaust opening about twice the vent size, using the windows or doors furthest from the vents. Then prove it two ways. At high fan speed, a door should stay slightly ajar without slamming. And a piece of tissue paper shouldn't stick to the fly screen while the unit runs.
- Relief opening
- 2 × vent areaper room
- Door test
- Stays ajarat high fan speed
- Tissue test
- Doesn't stickto the fly screen
- Published area ratio
- None foundm² per litre per second, don't trust one
Why does relief air matter so much?
An evaporative cooler is a one-way air machine. It pushes outside air in, and that air has to leave somewhere. The Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH) puts it this way. "The evaporative air cooler will not be able to supply air if the air has no relief path, and the effectiveness of the relief air path will, to a large extent, determine the effectiveness of the overall system."
In other words, if the air can't get out, the cooler can't push it in. How well the air gets out decides how well the whole system cools. So a system that measures perfectly at the cooler and cools nothing in the back bedroom usually isn't undersized. The air can't get out of that room.
Seeley says it in two different documents. Provide an exhaust opening "about 2 times the vent size of the room", and make sure the exhaust area, an open window or door, is "at least twice the size of the vent". Open the ones furthest from the vents so the air crosses the room on its way out.
What are the two tests?
- The door testRun the unit at high fan speed. A door should stay slightly ajar without slamming shut. If it slams, air is building up in the room with nowhere to go, and you haven't got enough relief.
- The tissue paper testA piece of tissue paper shouldn't stick to the fly screen while the unit is running. If it sticks, air is trying to get out through the window screen faster than the opening allows. Again, not enough relief.
Both take about ten seconds and need nothing from your kit. They're also the easiest thing in the world to show a customer, which helps when you're explaining why they need to leave a window open at the far end of the house.
What are the three ways to provide it?
| Method | What it is | Watch for |
|---|---|---|
| Natural | Open windows and doors | Security, noise and prevailing wind |
| Ducted relief | Dedicated relief duct to outside | Needs openings that close when the system is off |
| Mechanical exhaust | Powered extraction | Running cost, and it has to be interlocked with the cooler so they work together |
Checked,
not
found
There's no published square metres per litre per second figure
We went looking for a relief air figure given as square metres of opening per litre per second of airflow, because that's how an engineer would want it. No published Australian figure of that kind appears to exist.
The industry standard is the per-room rule, twice the vent area, plus the two field tests. That's it. If somebody quotes you a neat area per litre per second ratio, ask them where it comes from, because we couldn't find a source for one.
We'd rather tell you the figure doesn't exist than invent a tidy one. A made-up ratio that looks authoritative is worse than no ratio at all.
Chasing a figure the manual doesn't give you?
Some of them genuinely aren't published, and some of them are published somewhere nobody thinks to look. Either way you shouldn't be guessing.
Tell Kevin what you're after and he'll point you at it or tell you it isn't there.
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
- Commissioning the water side of an evaporative coolerSupply pressure, water level, prewet and the pad flush. Plus two plumbing traps, one that stops the water shutting off and one that locks up the solenoid.
- Add-on cooling needs a second thermistor, and here's whyA cold coil in the supply duct soaks up the heat rise the heater looks for to confirm it has lit. That's why V1.10 boards onwards want two thermistors.
- The outlet thermistor goes 3 metres down the supply ductAbout 3 metres down the supply air duct, never in the return, and never in a zone that closes. Get it wrong and the heater shuts down after 90 seconds.
- Duct sizing, bends and return air on gas ducted heatingAirflow per outlet, when to upsize the duct, minimum bend radii, and the grille table that shows exactly what a filter costs you in return air size.
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.