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Jonathon Pullen

Technical Services Manager Gulf Coast District

No “Os” in Commercial Heat Pumps

Have you ever installed or started up a commercial heat pump?  Did you notice there is not an “O” terminal on the units?
 
Commercial heat pumps do not operate like the residential heat pumps in regard to controls.  I have taken several calls in the last few weeks where the heat pump was not working correctly and doing strange stuff. I always ask the tech how the thermostat is configured and I always get the same answer “Iits setup for heat pump.” This is the wrong way to do this.  There is no “O” terminal anywhere on the units because we do not use an”O” terminal, we are not energizing the reversing valve with “O.” 
 
All the logic is done through the defrost board. When you energize 24v to Y1 it knows to run in cooling, and when you energize 24v to W1 it knows to run in heat pump heating. 
 
When you install and set up the thermostat you configure it for conventional heat, basically telling it that it is a straight cool unit with heat using Y1, Y2 for cooling and W1 for heating. Do this and you wont have any problems. 
 
I do recommend the Pro1 T855 for the commercial heat pumps so you can set up the minimum run time for 5 minutes to prevent a nuisance fault code 3 on the CADM. Please see my article on fault code 3 HERE. 
 
All of this info is found in the install manuals that come with the unit. Below are some helpful reference images.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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