Tag: capacity

Enthalpy is easy… it’s just a state function that depends only on the prevailing equilibrium state identified by the system’s internal energy, pressure, and volume. It is an extensive quantity. Simple.

Like most things, the scientific definition is as clear as mud. In HVAC/R we use enthalpy measurement to come up with the total heat change in a fluid, whether it’s refrigerant, water or air.

That total change in heat content or enthalpy change is called Delta H (ΔH) which is just another way of saying “total heat split” and it is generally measured in BTU/lb in the US.

In air, we need to use probes that measure humidity and temperature like the HUB2 probes shown above or the Testo 605i probes in order to calculate the enthalpy of the air. Air has both the energy associated with the temperature of the air as well as the latent heat stored in the water vapor.

UEI HUB Screenshot

If you want to use the Δto calculate the total heat added or removed from the air you would then use this formula to calculate BTUs of heat added or removed from the air.

Total Heat = (H1-H2) x 4.5 x CFM

In the case above it would be

Total Heat = (29.68 – 22.77) x 4.5 x 730 (CFM we measured)

so

29.68 – 22.77 = 6.91 Δ

6.91 x 4.5 x 730 = 22,699.35 BTU/hr

This total air enthalpy change is a required part of calculating total system capacity and is a pretty simple thing to understand.

Don’t confuse ΔH (Total Heat Change) with ΔT (Temperature Difference). ΔH includes both latent and sensible heat and is a measure of heat quantity in BTU/lb while ΔT only calculates temperature difference and isn’t converted to BTUs at all.

— Bryan


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.


You will notice pretty quick that my indoor temperature is low (68.3db)with a low indoor RH (54%) which equates to a 57 degree wet bulb indoor return.

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

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