R-134a refrigerant has recommended pressure levels to keep a system properly charged:

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My question is if you're trying to make an ac system as cold as possible, and you're willing to ignore potential side effects, should you slightly over charge, or under charge the system?

The reason I ask is when an ac system is low on freeon, one of the dangers is the evap coil gets so cold it literally freezes over. So it seems to me like you'd want to "under charge" the system as much as possible to the point it's just about to freeze but does not.

However I've also heard if your ac is slightly undercharged the best way to make it blow colder is to bring it up to 'spec' even if it's not currently freezing.

These 2 things sound contradicting to me, so I'm just wondering what is the right way to interpret this?

  • $\begingroup$ You might be better off asking about nominal pressure to handle a range of external temperatures. $\endgroup$ May 16, 2022 at 14:21
  • $\begingroup$ If you're asking how to get your room the coldest, then you need to look at the ability of the heat exchanger to pull energy from the airflow, which is a function of contact area as well as cold-side temperature. $\endgroup$ May 16, 2022 at 14:24

2 Answers 2


Firstly, lets agree on what you mean by blow colder. You want more cooling. This is not the same as having the air come out the vents colder.

The setup that works best is different for every out door temp, indoor temp, and indoor humidity combination. So it isn't a trivial problem at all.

Also, you probably care about this when it is hotter than standard conditions outside. The California Energy rating is more applicable to these conditions, compared to the Federal SEER.

When it is really hot out, the problem often is getting the heat out of the condenser and into the outside air. To improve this, add refrigerant to boost the highside pressure and increase the condensation temp. It is pretty common to have to adjust the refrigerant load in large systems seasonally. The system check that tells you if you have it right is to calculate the supercooling at the condenser exit.

The evap pressure and temp may increase also, but the added density should improve cooling. If you lose air delta T across the evap coil, it isn't working.

Modern efficient systems don't have the adjustability that the older systems with their higher compression ratios did. I used to run 175 psi condenser pressures in South Florida in the summer with R22. Nothing less would dehumidify the hotels.

If you run into obstinate permitting people who simply won't permit the property for an adequately sized unit, look for the brands with the highest compression ratios and highest water extraction numbers.


If you want a colder output, then change the working fluid.

A refrigerant has a vaporisation temperature that is fixed and a condensation temperature that is also fixed.

If you need more cooling power, more fluid moved and larger working area for condenser and evaporator will be needed.

Running low on refrigerant just refuces the efficiency.


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