DavidAndersen's blog

Thermal Infrared HVAC Analysis

Excerpts from: http://www.nachi.org/forum/f58/helping-people-understand-laymans-terms-40440/ 

How would you describe these A/C units to a novice or homeowner that has no building science knowledge? I am trying to put together some general terms or statements for people so they understand things like this when they see them. Any help would be appreciated.

helping-people-understand-laymans-terms-june-3rd-2009-068.jpg   helping-people-understand-laymans-terms-june-3rd-2009-069.jpg 

If you are addressing the stripes on the coil, you need to describe how an HVAC condensing unit works.

Hot gas in the vapor state is being discharged from the compressor into the condensing coil.

The hot stripes that you see is this hot gas being discharged into several circuits that make up the condensing coil.

Air passes through the coil, induced by the outdoor fan. The cooler air temperature cools the hot gas to a point where it will begin to condense at the current head pressure. This pressure changes with outdoor ambient conditions.

The greatest heat transfer occurs during the "change of state" of the refrigerant from a vapor to a liquid.

Once the refrigerant returns to the liquid state it gives up heat at a different rate.

The darker stripes as you see in the scan are saturated vapor to refrigerant in a liquid state. The refrigerant exits the condensing unit through multiple distribution tubes into a single liquid line to be sent back to the evaporator metering device to gather more heat.

The cooler the outdoor air temperature, the faster this process will occur and the larger the dark lines will appear. Refrigerant is utilized at a lower rate under a lower heat load and head pressure drops lowering the overall temperature within the coil. So things look different as load and outdoor temperatures change.

My interpretation of the infrared scan data (which is uncorrected for emissivity) is that the outdoor ambient temperature is around 75°F. The highest temperature in the scan of one picture is 95°F and 105°F in the other. This is a temperature rise of 20° in one in 30° F. in the other.

A 30° rise in discharge air temperature which is closely resembling a highest temperature in the scan above the fan is the expected temperature rise of the air passing through a condensing coil.

Further, a pressure rise at a 75° outdoor ambient could acceptably be 212# psig. The temperature-pressure conversion of this 212# psi is equivalent to 105°F saturation temperature.

This is exactly the numbers that we have derived previously.

If we were to rely on the spot measurement temperature of the infrared scan, 34°C is equivalent to 180# psig. Which is probably also an acceptable range for an outdoor ambient temperature of 75°F (depending on the indoor cooling load).

So without further information (which is always required) it appears that these units are operating within acceptable parameters.

This information may or may not be correct because we have insufficient information provided. But this is statistical information that can be derived from a thermal infrared scan. Accuracy is totally dependent upon accurate data collection and camera operation.

 David A. Andersen & Associates 
Clarksville - Nashville Home Inspector Lic#40
ITC Certified Level II Thermographer Cert#1958
Building Science Thermographer Cert#33784
HVAC Certification EPA Cert#2046620

Submitted by DavidAndersen on Thu, 06/11/2009 - 10:03.
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