Delta What?

“Delta t” is the most common use of the word delta in the HVAC industry. Delta t means temperature difference. If the temperature in a supply duct is 100F and the temperature in the attic around the duct is 60F; subtract 60F from a 100F to find a delta t of 40F. Let’s take a look a few other “deltas” that we include in our day-to-day HVAC industry terms.

Delta Definitions

Besides being defined as “the difference between two related measurements,” the word delta has an interesting history. Delta or “Δ” happens to be the forth letter of the Greek alphabet. The name came from the appearance of a three sided island that forms when two rivers meet. From there, any symbol that looks like a triangle also became known as a delta.

When a well seasoned dispatcher uses the letter D, they might replace the letter “D” with the word “delta”, so we don’t confuse “D” for “E” or “C”.

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 to explain it 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 air enters the filter, and say its .12-in.wc, and you measure the pressure after the air leaves the filter and it’s .22-in. wc. Simply subtract the .12-in.wc from the .22-in.wc to find the delta p or pressure difference of .10-in.wc.

The next step once you find a delta p over a component in an air stream is to compare the number to an industry standard. As you learn the industry standards, and compare your test results to them, the test results really start to make sense.

The typical pressure drop industry standard for a constant speed fan is 20% of fan rated pressure. For example, if the system fan is rated at .50-in.wc, times 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 p’s 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, and the delta p of the duct system hasn’t been added to the pressure profile yet. This is an example of a system in trouble.

Delta V

This one is the industry term for the voltage difference between two legs on feeding a blower motor of 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.

Delta H

Sounds like it should be delta humidity, but it isn’t. Although most HVAC deltas use the first initial for the related word, sometimes that rule just doesn’t apply. Scientists rarely use the first initial of the value being measured in their formulas. Q often stands for airflow in their language. But that’s not the language most of us use in the field.

Delta h stands for enthalpy change. Delta h is the difference in measured enthalpy as air enters and exits a cooling system. Simply measure the wet bulb temperature before and after the equipment in the appropriate locations. Convert wet bulb temperatures to enthalpy and subtract to find the delta h or enthalpy change through cooling system.

The list of deltas goes on and on. Listen for and use the term around the shop and see how many you can weave it into your tech-talk in the coming week.

One final note, remember our deltas are our HVAC industry terms and are totally unfamiliar to those we serve. So when talking with clients in the field and explaining the problems with their systems that they need fixed, try to speak consumer English and keep it simple for those that are new to our language.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, a training company with technical and business level membership organizations. If you're a contractor, technician or energy rater interested in a procedure and chart to help you covert wet bulb readings to enthalpy in an instant, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.

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