HVAC Questions & Answers

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Question: 

What can be done if your existing AC unit is oversized and is not able to dehumidify the space and has several start / stop events during the day. 

Answer: 

The reason is an oversized AC unit cannot remove the moisture from the air is it does not stay on long enough because it has already satisfied the room temperature. Several ways to help but not totally alleviate this problem are as follows: 

1. Set the fan to “automatic” and not to “on” all the time. If the fan is always on the water on the coil evaporates into the air flow when the compressor is off. 
2. Lower the unit total CFM to allow the unit to run longer, thus removing more moisture. Need to be carful here, lower it to much and you will damage the compressor. Do not lower the CFM to less than 320± CFM/Ton 
3. Some units have programmable fan controls in the unit that allow you to set a special mode that varies the indoor fan speed based on the unit load which allows the unit to remove the maximum amount of moisture. Normally this is a dip switch setting on the control board.   

Question:

What is the best setback temperature to set your AC unit to during the night, or when the building is not occupied? Some say turn it totally off and others say only 10⁰.

Answer:

Unfortunately, there is no one correct answer for this question because like many things in life, “it depends”. Here are some suggestions and the reasons for them: 

1. The “it depends” problem is because the mass of a structure changes how long it takes to get a space to the temperature setpoint. A high mass building, which is a more efficient building, takes a longer time to get to setpoint than a low mass, less efficient building which takes more load to change its temperature. The efficiency kicker is, if it takes more energy to change the temperature between occupied and unoccupied than it would to keep it at a constant temperature, then it is the wrong setback temperature.  
2. Normal suggested summer temperatures are 75⁰ occupied and 85⁰ unoccupied. Normal winter temperatures are 72⁰ occupied and 62⁰ unoccupied. If it takes longer than ±30 – 45 minutes to get to these morning occupied setpoints, then these unoccupied setpoints are too extreme and need to be reduced.