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HVAC School - For Techs, By Techs

May 21, 2024

In this short Q&A podcast episode, Bryan answers Jeremy's question about checking the charge on rooftop units, and he gives some RTU tips.

Rooftop units often come in sets, so you can use a thermal camera to look at all of the RTUs and compare them. A thermal imaging camera may even show you where subcooling begins in the condenser, which can be a useful diagnostic hack. Try to avoid hooking up gauges each time you check the charge on a rooftop; RTUs have a fixed factory charge, and losses from your gauges can add up over time. Your senses will also come in handy; check for odd sights, sounds, and smells.

Checking suction pressure and superheat is less risky and imperfect, but it'll give you some diagnostic data. You can also check discharge temperatures to check for signs of compressor overheating (remember: 225 stay alive). 

Here are some tips for RTUs:

  • Get familiar with the service manual and wiring diagrams; these items will help you with the job and help you build on your knowledge of RTUs in general.
  • Make sure you have all of your materials at the ready, including extra screws.
  • Make sure you have access to the BAS system, if necessary.
  • Safety is critical on rooftop units, especially when it comes to ladders, lockout/tagout, and ensuring that you have safe access to the unit (this burden is often on the client). 
  • Pay attention to specialized equipment, including ERVs and economizers.
  • Be mindful of curb installation, transformer taps, and phase monitoring, as those items may not be set up properly.
  • Build up a basic working knowledge of variable frequency drives or VFDs (or know where to find information about them; THIS livestream is a good start),
  • Keep systems clean to prevent overheating.
  • Prevent oil logging whenever you have to replace a compressor.


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