When CAN we turn on the AC?

As the weather warms up, we’re caught in a bit of a pickle. Some days it’s 60 or 70 degrees outside, but other nights we can still see lows dipping to 26. Some days we look to our AC systems and desperately want to run them, but is it time? Is it good to run AC for just a few hours a day? Is it good to run them in the bitter colds nights? Are we going to regret running these?


Physics Hate Us

We’re going to run into a few issues running our air conditioning in less than roasting weather. We’ve built modern AC, from window units up to multi-ton rooftop monsters, to take a brutal 100 degree summer down to a cool 70 degree oasis. These machines can create temperature drops from twenty to forty or more degrees. This involves creating some intense coolness inside the air conditioner.

If the ambient air is 60 degrees and we drop that by 40 degrees inside the air conditioner, we’re going to make it 20 degrees inside. This extreme cold will create ice, the great nemesis of all things mechanical. We don’t start to get to ideal temperatures until it’s about 70 degrees outside. Around that point, we can actually run the AC without having to worry so much about the ice build up.

Why is ice a problem? It’s mother nature’s wedge. As water freezes it expands. As water expands, it pushes out in every direction. Imagine water freezing inside a coil (essentially a radiator for an air conditioner). It’ll break the fins open and there’ll be refrigerant everywhere.


Duty Cycle

There’s another problem too: short cycles are bad. Modern air conditioners are designed to run for longer and get the most out of it. Consider when the system first starts up, it draws a ton of amps to actually get working. From a dead-stop, the compressor has to get up to speed, it has to use the most torque to get all the refrigerant moving while its own lubricants are cool and stiff. Start-up is the harshest time for a compressor, once it’s running there’s momentum and less friction, it just has to maintain energy rather than build it up.

If we only need to lower the temperature by a few degrees, the system might run for all of five minutes and have to shut down because we’re already on course to overshoot our target temperature. You end up in this sticky spot of going full throttle for a few minutes, idling, and then full throttle again not much later to get close to your desired temperature.

This short-cycling is horrendous for air conditioners. You’re going to have a higher power bill, due to being in the start-up band more often than your performance band. The equipment is going to have that much more wear from having to spend so much time maximizing torque to operate. It’s a lose-lose scenario.

The problem changes a bit with scale. Industrial air conditioners are perhaps the most harmed by short cycling. With multi-ton units, replacement parts are more expensive, there’s more energy in play, and everything is amplified. Their efficiency is amplified with their scale and raw-output but the start up load to get it is just about as bad.

With smaller systems, window mounted air conditioners and the like, there’s less refrigerant, less power, and less of an impact. These things are small and cheap. They’re designed to be shut off arbitrarily by home owners, where an industrial system is meant to take big strides and run for weeks uninterrupted.


What about that big data center?

There are air conditioners that are run during the winter, but they’re designed for it. Factories, some office buildings, and certainly big data centers will produce more heat just from their operation than they actually need. These are facilities which may abandon heating systems entirely because their day to day operation naturally produces too much heat.

In these cases, there are hard-start capacitors installed on the compressors, there’s installation specific defrost equipment, and other mitigations to prevent icing from causing issues. At the same time, the facility’s native heat generation is enough to keep the system at a reasonable load and prevent it from short cycling or being underworked even in colder weather.


When is it time?

The first thing to do is consult with your maintenance crew that run the building’s HVAC equipment. They’ll know what it’s rated to run at and be able to tell you when they can run it without wasting power or causing unneeded wear and tear on the equipment. The second thing is to just consult your manufacturers’ documentation. The parts making up your system have already been tested so the manufacturer knows how long they’ll hold up under differing conditions.

If there is no documentation or you just cannot find out, our recommendation is to hold off until it’s unavoidable to run the system. If a window fan can cool you down, then use it. There’s no sense wasting power on the AC when it’s not even warm outside. When the lows start to grace the fifties and the highs are starting to reach the mid 70s, it starts to make sense.

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