The other day, I was having a conversation with an experienced HVAC technician.  Let’s call him “Joe”.  Joe was telling me about a problem he was having on an air conditioning system that he had installed earlier in the year, explaining that everything was running, but the system would not maintain setpoint.

He told me that in the first part of the cooling season, the system ran great, but now that it was very hot, it wouldn’t keep up.  He had performed a load calculation before selecting the equipment and he was certain it was correct.  He even did it a second time when the customer called to complain.

“Is the coil clean?” I asked.

“Yup” Joe said.  “That’s the first thing I tried.  I put in a new filter, too, just to be sure.”

Before I had a chance to ask about any other details, Joe added “The Suction line is nice and cold, and it’s blowing cold air and everything.”

“What was your temperature drop?” I asked.

Joe responded with “I didn’t measure that” and then added that he felt cold air blowing from the supply registers.

“What do you think it could be?” Joe asked.

This conversation is typical, and is very similar to many other conversations I’ve had over the years with many technicians.

As HVAC techs, we are involved in the process of moving heat energy from one place to another.  Heat energy is a little hard to pin down.  I don’t know anyone who has held a handful of Btu.  You can’t put a bunch of Btu in a container and put it on the scale and weigh them.  You can’t pull out a tape measure and measure heat energy.

In fact, the transfer of heat energy from one place to another can’t really be measured directly.  The only thing we can do is measure the effect that heat energy has on something else as the heat is absorbed or rejected.  When a thing absorbs or rejects heat, either its temperature will change (sensible heat) or it will change phase (latent heat).

Test instruments are used so that you can visualize things you can’t see.  Things like electricity and heat energy.  When we’re talking about heat energy (which we usually are), we’re talking about measuring temperature.

By itself, a temperature measurement isn’t all that useful.  For example, it’s not very helpful to know that the supply air temperature is 60 degrees unless you also know that the return air temperature is 80 degrees.  That means something.  If the supply air temperature was 60 degrees when the return air temperature was 70 degrees, that would mean something very different.
Temperature measurements are comparative.  When we compare one temperature to another and calculate the difference between those temperatures, this is giving us an indication of the movement of heat energy into or out of the thing we are measuring.

I asked Joe to go back to the job and measure the return air temperature and the supply air temperature and report back.  The question he needed to answer was this: Is the unit operating properly and is undersized, or is the unit sized properly but having an operational problem.

“The weather report says we have a cold front coming through.  I don’t think we’re going to see the same temperatures we saw before.”  Joe said.

“That won’t make a difference.”  I told him.  “If you have a problem when it’s hot, you’ll have the same problem when it’s mild.”

Joe called back a couple days later.

“Supply air temp is 47 degrees!” Joe reported triumphantly!  “I knew it was blowing cold!”

“What about the return air temp?”  I asked.

“Looks like 74 degrees.”  Joe said.  “The thermostat was satisfied so I had to turn it down to get it to come on.”

“The difference between 74 and 47 is 27 degrees.” I said.  Under the conditions you had yesterday, you should be running a temperature difference between 20 and 22 degrees.  27 degrees is too much.  You may have an airflow problem.”

As air moves across the evaporator more slowly, it has more opportunity to fall in temperature.  Also, as air moves across the evaporator more slowly, fewer Btu are delivered to the evaporator and fewer Btu per hour get moved through the system.  Thus, you have nice cold supply air, but less refrigeration capacity.  This explains why it couldn’t keep up in the hotter weather.

“Calibrated hands” are not enough to measure temperature accurately.  Technicians must be using accurate calibrated digital thermometers, using proper measuring techniques, and comparing the results to calculated norms based on current operating conditions.

As I always say, “Btu and Temperature are not the same thing.”  Colder isn’t always better.

Joe went back in and looked for an airflow problem.  He called back a while later.

“I’m feeling a little silly.”  Joe said.  “I had the dip switches on the variable speed furnace set backwards.  As soon as I set them right, that fan took off and I started seeing a temperature difference of 22 degrees.”

“Don’t feel bad,” I said, “Everyone makes mistakes.”  It’s important to have a backup plan in order to catch your mistakes before you leave the job.  Check it, then correct it.

Word to the wise:  Failing to measure temperature appropriately is one of the main causes of missed diagnosis in air conditioning systems!

4 Responses

    1. On a new installation with a variable speed furnace, it is necessary to adjust the dip switches for proper airflow. At the end of a long hot day, it is an easy mistake to accidentally set the switches in reverse order. It happens all the time.

  1. This is a very common problem with many technicians. They seldom use their thermometers and temperature change and difference are of major importance in this field.

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