Effectively cool spaces large and small with the right unit.
When you need to cool an event space, office, school, retail business, manufacturing floor or computer server room and there’s no built-in HVAC system, or the one that’s there can’t deliver the necessary BTUs, it’s time to rent a portable air conditioner. But how big should you go, and what type of model do you need?
Here are six things to consider when choosing a portable air conditioner.
How much cooling power do you need?
Portable air conditioners are rated in one of two ways. The most common system is BTUs (British Thermal Units), which indicates BTU output per hour.
The smallest commercial portable AC units start at around 12 000 BTUs and go way up from there. While it’s smart to perform an energy audit to determine the cooling requirements for your space, you can use these numbers as a general rule of thumb:
- 12 000 BTUs cools roughly 400 square feet
- 14 000 BTUs cools roughly 600 square feet
- 18 000 BTUs cools roughly 1 000 square feet
- 30 000 BTUs cools roughly 1 500 square feet
- 60 000 BTUs cools roughly 2 000 square feet
- 120 000 BTUs cools roughly 4 000 square feet
You will also see AC units listed by tonnage. This isn’t a reference to the weight of the unit; one ton is the amount of heat required to melt a ton of ice in 24 hours. A one-ton AC unit is rated at 12 000 BTUs. To convert an AC unit listed in tonnage to BTUs, multiply by 12 000. A 5-ton unit, for example, delivers 60 000 BTUs.
A number of factors affect how efficiently you can cool a space. The shape of the room affects the AC you need, as well as its windows and insulation. Also, if you have tall ceilings, or the outdoor environment is very warm or humid, you should err on the side of renting a larger unit. The number of people working in the space is also a factor. And finally, server rooms need to be kept extra-chilly.
Should you choose air cooled or water cooled?
Air conditioners remove heat from the air, and in the process, they generate even more heat internally that needs to be eliminated. Many air conditioners are air cooled, which means that the hot air they generate must be vented to the outdoors. Water-cooled AC units, on the other hand, use water, which eliminates the need for venting hot air.
Water-cooled ACs tend to be a little more energy efficient. While you don’t need to rig any exhaust ducting, you do need to connect these units to a water source. So the ease of installation depends on the layout of the space.
You can find both water- and air-cooled AC units at the lower end of the size spectrum, but the largest portable units tend to be air cooled.
How much maintenance does it need?
As part of the cooling cycle, a portable AC removes moisture from the air. This water, called condensate, needs to be drained. The method of drainage varies with different types of AC units. A fully self-evaporative unit is the most convenient kind because it exhausts all the condensed water through evaporation back into the air; there’s no maintenance required.
A partially self-evaporative AC unit can evaporate most of the condensed water back into the environment, but some condensate will remain, and it needs to be drained, perhaps manually — meaning, you may need to empty the condensate tank. The accumulation rate is low enough that this kind of AC unit can be considered low-maintenance, and it can run unattended for long periods.
A gravity drain allows you to connect the air conditioner to a floor drain so you never need to deal with condensate. A portable AC unit might rely entirely on the drain, or you can use a gravity drain in conjunction with a partially self-evaporative unit so there’s never a need to empty the condensate.
What’s the venting path in your space?
An air-cooled portable air conditioner needs a way to vent the hot air that is created as part of the cooling cycle, so you’ll need to plan for a venting path, which may determine where the unit is placed. Most units come with kits that include the exhaust hose and hardware.
What are the electrical requirements?
Only the very smallest portable AC units, suitable for small spaces or spot cooling, use standard 120-volt power outlets. Larger AC units require special outlets and voltages, and you will need to wire it professionally as part of the set-up.
A 3-ton unit, for example, typically requires a 208-volt, single phase, 30-amp circuit, so you need to consider that when planning your cooling strategy. Another alternative: If practical, perhaps you could deploy two or three 1-ton units instead, each of which can be operated from a standard 120-volt electrical outlet.
Do you need heat, too?
Some portable air conditioners can do double duty as heaters. You’ll probably care about a unit like this only if you are using it through multiple seasons.
The heat in a portable AC unit generally comes from either an electric heater or a heat pump. Electric heaters can deliver better performance in colder climates. Heat pumps, which work by extracting ambient heat from outdoors, tend to be more energy efficient in more moderate environments, where they can move heat around without needing to generate it electrically.
Not sure which type of portable air conditioner will best meet your needs? Visit your local United Rentals branch for expert advice.
Dave Johnson is a Los Angeles-based freelance writer who has been writing about all aspects of business and technology since before there was an internet.