Cooling Capacity Isn’t Always What it Seems
We all learned how to read the tonnage off of a model number within a few weeks of beginning in the trade. What you may (or may not) have learned is that just because something has an 036 in the model number does not mean it actually produces 36,000 btu/hr even during RATED conditions let alone real world conditions.
Some of you may be used to pulling up an AHRI rating to find the true capacity of a system match. This is a good start and often you will find out that the the system produces slightly less to up 4,000 btu/hr less than the nominal rating. Here is the AHRI ratings for the system I have on my home.
You will notice that the 2-ton matches actually produces 24,000 btu/hr at the rated conditions, which are REALLY WARM temps inside and out by the way. However the 4-ton match produces 46,000 btu/hr at the same conditions.
Here is an example of some real world capacity readings I took on my Carrier VNA8 4-ton system with the Testo Smart Probes app and two 605i thermo-hygrometers.
This is a 4 ton unit with a proper charge (right at 11.6 subcool like the Infinity stat calls for) a 0.45 TESP and it’s been running for 30 minutes at high stage. You might be tempted to think something is wrong with the measurement or the unit, but we need to look closer.
Also, the outdoor temperature is only 72 degrees DB. In order to tell if 41,000 btu/hr is within range or not we will need to look in detail at the manufacturers expanded performance data located in the product data.
Here is the expanded data for this particular match and we lucked out. My air handler, condenser and suction line size are the match that the rating is based on. In some cases you will need to use a multiplier based on an alternate match or smaller copper sizes which can further reduce the rated capacity and possibly the efficiency as well like in the case of the FE4ANF003 or 002 below.
Now let’s zoom in on the performance data that applies to our actual conditions and see how we did.
The highlighted figure is the closest this chart comes to our actual conditions, though our indoor dry bulb is actually significantly lower than the 75 degree DB on the chart. So now the real world 41,223 btu/hr actually stacks up pretty well with the 42,870 btu/hr on the chart.
All of this to say that when sizing equipment and when testing capacity there is a LOT more to it than just the nominal tonnage in the model #. The only real way to know is to dig into the manufacturer product data and really understand that piece of equipment of equipment.
Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade