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I'm in the middle of rebuilding my entire AC system. I've got one 14" return and two 8" supplies hooked up to the blower (on a 2 ton system). I plan to add much more, but for now I want to start the outside unit and make sure the system is functional before I proceed.

I've had 3 different licensed HVAC professionals tell me that I needed to finish the ductwork before they could start the condensor -- otherwise the "pressures would be off"

I'm very confused. In my mind, the condensor and A-Coil are a separate closed-loop system from the air being blown across the A-Coil. So why would more/less ductwork ever affect the condensor and its "pressures"?

Tyler M
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2 Answers2

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The air handler, blower, is designed to blow a certain amount of air over the cooling coils which in turn converts the refrigerant back to a gas from a liquid. It can only get the correct amount of air if all the returns and supplies are installed, otherwise the blower will not function as designed and can ice up the coils. This affects the compressor operation as the transformation of the refrigerant from gas to liquid back to gas is compromised. If the outside unit has to be charged or topped off if it's a precharged unit, everything needs to be running correctly to get the right refrigerant charge in the unit.

JACK
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They are correct the flow will not be correct but it will not hurt the system if fully functional , it may not cool as well because the lack of restriction across the evaporator.
In a way they are separate but the amount of heat removed at the evaporator or inside coil affects the condenser the outside coil, if the fan is not functional on the inside unit it could ice up and this is bad and can cause damage. So just for testing I would wait or add some restriction to the discharge side so you can feel cool air but as I mentioned it may not cool well without the proper amount of air flow over the evaporator. If the installer says don’t do it you could void your warranty on a new system by going ahead.

Ed Beal
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