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More Than One Delta

Take time to explain delta to those who need to become familiar with this valuable term.

All good HVAC techs use the term “Delta T.” This is the most common use of the word delta in the HVAC industry. It literally means temperature difference. If the temperature in a supply duct is 100F degrees 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 at some other “deltas” that we include in our day-to-day HVAC terminology.

The Greek Definition

Besides the “difference between two related measurements” definition, the word delta has an interesting history. Going way back, Delta or “Δ” just 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 and I went away scratching my head. Remember that the next time you talk with 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 at .12” wc, and you measure the pressure after the filter to be .22” wc -- just subtract the .12” wc from the .22”wc to find the Delta P or pressure difference of .10” wc.

Once you find a Delta P over a component in an air stream, compare that number to an industry standard. As you learn the industry standards, and compare your test results to them, suddenly the test results really start making 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” wc, and you multiply it by 20%, you’ll see there is an acceptable filter pressure drop of .10”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” wc. Now if you match that filter with a .50” fan, and a coil with a Delta P of .30” 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 system is in trouble.

Delta V

You guessed it. This one is for the voltage difference between two legs on a compressor blower motor. Voltage differences are found by measuring the voltage of two power wires entering equipment and comparing the difference. If the entering voltage varies more than 10% between 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 into your tech-talk in the coming week.

One final note, remember deltas are our language and are totally unfamiliar to customers. So, when talking to clients about problems with their systems, try to speak consumer English and keep it simple.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free test procedure describing simple testing you can do, 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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