AC capacity: Large overcapacity or just a little?

I am considering purchasing an AC in the future.
Now I know approximately how much BTU or kW I will need.

But I have a question:
Is it better to buy a device that is just right, plus 10 or 20% to cope with the big heat spikes? (In extremely hot summer for example).
Or is it better to immediately buy a device with 50% or more overcapacity?

I can imagine that an AC with a lot of overcapacity will switch on all the time and then switch off again.
Or does an AC work like an exhaust fan that just runs slower when there is less heat to get out of your room?
A device with a bit of overcapacity may stay perhaps run more constantly.
I also have no idea what is better (or less bad) for an AC: Continuous running, or occasionally on and then off again?

If you get a minisplit with the inverter tech (most are) they can run at different levels as required to handle the load, they are not just on and off as with a standard AC unit. So oversizing isn't a big issue. If you have plans to expand and add heat loads later then oversize it all you want.

In the winter the compressor on my 4 ton units is running very slow and the condenser fans don't even turn. lol

Think of the normal AC unit as either 0 or 100 and the minisplit with inverter technology can run anywhere in between.

Renfro said:

In the winter the compressor on my 4 ton units is running very slow and the condenser fans don't even turn. lol

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Do you also use your AC as a heater during the lights-off period in the winter?

Keesje said:

Do you also use your AC as a heater during the lights-off period in the winter?

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Well short answer is no.

Longer answer is:
No, the flowering AC units are on a flip with the lights. I only have a 150 AMP main and I pulled a 125 amp sub panel to my basement, changed the range and dryer to gas. I have two flowering rooms, each about 10kW of HPS. Each has a 4 ton AC. The AC's are powered from a flip box I built with 40 AMP double pole contactors fed from a 40 AMP double pole breaker. In the winter there is enough heat that I never need to worry about lights off temps bottoming out. I actually have an intake blower (backup cooling) that brings in fresh filtered air when the lights go out to drop the temps down to about 75f (nighttime setting on the autopilot). That daily air exchange prevents any buildup of ethylene gas. Even with that cooling I don't ever see lights out temps below 72 - 73 unless I were to set the nighttime temp on my autopilot to a lower temp causing the intake blower to run more.

Even with my flowering rooms on a flip, when at peak load (one flower room and the veg room on and everything crack-a-lackin lol) I am pulling around 90 amps on both legs of the 125 amp feed for my subpanel. Not a lot left for the rest of my house, (150 x 0. - 90 = 30 amps per leg remaining power. I can still change the hot water to gas if I need to but I may not have a enough gas with only a 1 inch feed off my meter (I upsized it from 3/4 by eliminating a reducer at the meter). Id have to do some math but I really never use the furnace so it should be fine. I have the two CO2 generators (only one can run at once with the flip), dryer and range on natural gas along with the furnace that never runs. Having the grow downstairs makes for radiant floor heating. I often run my bedroom AC unit in the winter lol.



If someone does wish to use their minisplit as a heater when the lights are off that can be easily done with a wifi AC controller like the Pebble, Sensibo and there is another one I can't remember. But they can be configured to schedule a change to heat mode and a change back to cooling mode. You can check the room temp and make changes to the AC settings from your internet connected android.

Renfro said:

If someone does wish to use their minisplit as a heater when the lights are off that can be easily done with a wifi AC controller like the Pebble, Sensibo and there is another one I can't remember. But they can be configured to schedule a change to heat mode and a change back to cooling mode. You can check the room temp and make changes to the AC settings from your internet connected android.

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Let's say the outside temperature where you live is 66 in the morning, 80 in the afternoon and then going down again.
Is it possible to have your AC set at 74... then the AC would have to heat in the morning and cool in the afternoon to achieve that temperature.
Would it switch automatically from heating to cooling at some point?
Is the set temperature leading?

Keesje said:

Let's say the outside temperature where you live is 66 in the morning, 80 in the afternoon and then going down again.
Is it possible to have your AC set at 74... then the AC would have to heat in the morning and cool in the afternoon to achieve that temperature.
Would it switch automatically from heating to cooling at some point?
Is the set temperature leading?

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Some units will do that if you leave them on "Auto" mode and set a desired temp. Other units require a user intervention to change the mode. The owners manual would be the place to check on that. Ask the sales rep if you are selecting a unit. They can point you towards the unit you want.

I learned more from you in the last few posts then checking out websites of AC manufacturers.
I also asked a supplier. But he replied "why would you do that?"

Some units also have a setting that will disable the LED lights on the unit so they won't mess with your dark cycle. It's sometimes an internal dip switch setting. Only some units have it, I have seen it on an LG unit a friend has. If you have to cover the LED's with electrical tape then be sure not to cover any IR sensor for the remote, saw that in a thread today.

Also be sure to spec a unit that will run low ambient should you need to use it when the outdoor temperature is cool / cold. Some units will not go below say 32 degrees F and others have a low ambient rating like -5F or -11F, I have even seen a mitsubishi unit that went down to -28 lol. They use them in server rooms. Mitsubishi makes the best but they are expensive. Spec a unit that has "Auto Restart after Power Outage".

Keesje said:

Would it switch automatically from heating to cooling at some point?

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Also know that multizone units can't run a heating mode on one zone while running a cool mode in another zone. Some units will allow you to run a dehumidify mode in one zone while running cooling in another.