When measuring the performance of an HVAC system, the term 'delta' is often used to express the difference between two measurements. Let’s take a look at the family of HVAC industry “deltas” that we include in our day-to-day HVAC slang terms. Delta T, Delta P, Delta V and Delta H and let’s add Delta C.

Besides the “difference between two related measurements” that we use, the word delta has an interesting history. Going way back, Delta or “Δ” just happens to be the fourth letter of the Greek alphabet. The name came from the appearance of a three-sided island that forms where two rivers meet. From there, any symbol that looks like a triangle also became known as a delta.

Delta T

“Delta T” is the most common use of the word delta in the HVAC industry, meaning temperature difference. If the temperature before a cooling coil is 75F and the temperature after the cooling coil is 55F, subtract 55F from a 75F to find a delta t of 20F.

Since the first time a service technician tried to explain delta to me, I’ve wrestled with a way to make the definition a little easier to swallow. It took him 20 minutes and I went away scratching my head. Remember that when talking to new technicians or the office staff. Take time to explain delta to those who need to become familiar with this valuable term.

Delta P

In our industry, Delta P means the difference in two related pressures. For example, when you measure the pressure before a filter and it’s .12-in. WC, then, measure the pressure after the filter and it's .22-in. WC. Now, subtract the .12-in. WC from the.22-in. WC to find the Delta P or pressure difference of .10-in. WC.

Once you find a Delta P over a component in an air stream, compare the number to an industry standard. As you learn the industry standards, and compare your test results to them, test results suddenly start making sense.  

The typical filter pressure budget for a constant speed fan is 20% of fan rated pressure. For example, a rated system fan of .50-in. WC multiplied by 20% reveals an acceptable filter pressure drop of .10-in. WC. So our example above is ideal.

Measure the pressure drop over the typical high efficiency filter and you’ll find Delta Ps around .30-in. WC. Now if you match that filter with a .50-in. fan, and a coil with a Delta P of .30-in. WC, you have already exceeded the capacity of the fan. The Delta P of the duct system hasn’t been added to these pressures yet. This system is in trouble.

Delta V

You guessed it. This one is the slang for voltage difference between two legs on feeding a blower motor or a compressor. Voltage differences are found by measuring the voltage of two power wires entering equipment and then by comparing the difference. If the entering voltage varies more than 10% between the wires, serious damage to the equipment may follow.