How to Calculate Your Refrigerant Charge Guide – Part 2 of 3

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How to Calculate Your Refrigerant Charge Guide – Part 2 of 3

I’ve found that many in the HVAC/R field are calculating refrigerant charges wrong, particularly when it comes to field-assembled equipment.

So, in this post, which is Part 2 of our 3 Common HVAC Mistakes series, I’d like to explain how to calculate your refrigerant charge properly and plus provide a free quick guide on it. 

Because you, too, could be calculating refrigerant charge incorrectly


Field-Assembled Equipment Prone to Refrigerant Charge Calculation Woes

All HVAC/R systems are comprised of several key components:

  1. Compressor
  2. Condenser
  3. Evaporator
  4. Controls
  5. Piping to connect everything

Importantly, there are more than 100 million appliances installed in the US.

For the purpose of today’s discussion, we are going to designate two types of systems:

(1) those that are field assembled, charged in the field, and have parts and pieces both in/outside; and

(2) those that are factory charged.

The factory charged systems are easy to identify—They are referred to as “packaged,” or stand-alone, and can include systems like vending machines (a few ounces) as well as all the way up to 150 ton packaged roof top units (hundreds of pounds). 

Other than the packaged equipment, many systems are field-assembled, and, in this post, this is our primary concern.

Let me explain further.

First, field-assembled equipment has 3 components that contain refrigerant:

  1. The Compressor/Condenser section
  2. The Evaporator
  3. The Piping

No complicated math required, you simply add together the gas in the pipe, the compressor/condenser section, and the evaporator = total charge

As simple as this process is, most commonly techs and staff only read the “nameplate” capacity on the label of the compressor/condenser section and then they leave off the evaporator and the piping… (we show you how to properly calculate refrigerant charge in this quick guide here)

Get My Free Guide on Refrigerant Charge


As a result, they only capture between 30-40% of the system charge and therefore under report capacity charge.

This underreporting ultimately causes more problems than it solves (as we will discuss later).


Total System Charge Isn’t Necessarily What’s On the Nameplate

Back in the day, the system charge was always determined by the tonnage of the unit and then, depending on where you were located (cold vs warm climate), you would either choose 3 lbs per ton or 4 lbs per ton.

Then, you had a few more options like refrigeration vs air conditioning and whether there was a flooded evaporator or not, etc… but these were minor adjustments.

Basically, if someone had a 20 ton (non-packaged) unit with multiple air handling units (AHU’s), regardless of the piping, it would be assumed that the unit had somewhere between 60-80 pounds of refrigerant. 

However, starting 10 or more so years ago, regulations started to focus on leaks and indeed agencies began to write requirements that targeted equipment 50 lbs and over…

And somewhere along the way, instead of reporting normal system charges, we as an industry began to report only the nameplate.

Because if the nameplate was less than 50, then the tech didn’t have to record as much information

(This is where you can get your refrigerant charge calculations wrong…)


Miscalculating Your Refrigerant Charge Can Lead to Operational Problems

There are many problems with this process, but the most significant are the following:

1. Underreporting of weight means that as systems leak – the leak rate looks larger. For a

✔ 20-ton unit with 80 Pounds and leaks 10 pounds = leak rate of 12.5%
✔ 20-ton unit with 49 pounds and leaks 10 pounds = leak rate of 20%

In Scenario A, the leak rate is high, but not as high as it is with fewer pounds reported (i.e., Scenario B). 

2. If you report fewer pounds in a system, then your maintenance needs look lower than they actually are. So, when developing budgets, you are always underestimating the need you have to keep these systems operational. Additionally, in the event of a system failure, your new gas charge will be larger than what you reported—We call this “the can’t get 10 lbs of flour into a 5 LB sack” rule.

3. Plus, underreporting can lead to fines and violations if exposed during an audit or investigation 

Inaccurate system reporting is compounded when you consider that:

1) very few technicians use a scale when charging a system; and,

2) that, although R-410A has 25 lbs in each cylinder, it is well-documented that at least 10% of that charge remains in the cylinder at all times, meaning that you only can get 22-23 lbs. out of each cylinder.  


So Make Sure You Get Your Refrigerant Charge Calculations Right

Think of it this way: Refrigerant is to an HVAC/R system what antifreeze is to a car.  If you under or over fill, then the car won’t operate as needed. 

Keep this in mind going forward when you calculate your refrigerant charge.

And, feel free to grab a copy of our guide, “How to determine how much refrigerant gas is in your HVAC/R system.”

Get My Free Guide on Refrigerant Charge


(Calculating total refrigerant charge—It’s an essential thing for you to know!)

It’s what you need to know about refrigerant charge calculations in one quick guide.

Thanks for reading, and be sure to join us next week for Part 3, the third and final part, of our new series.


If you enjoyed this article, you might want to check out more in this series:

3 Common HVAC Mistakes Your Facility Should Avoid – New Series

The Impact of Missing Equipment From Your Installed Inventory – Part 1


1 Comment
  • avatar
    Muthusamy Gounder
    April 6, 2020 at 5:27 am

    What is the formula to calculate refrigerant charge

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