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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"?

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  • Is your return network limited to a single return, or is it open elsewhere? The key is that you have enough airflow, so if you have open trunk lines it may be fine.
    – isherwood
    Jun 25, 2020 at 13:26
  • I currently have just the single 14" return installed (but i do plan to add more)
    – Tyler M
    Jun 25, 2020 at 13:30

2 Answers 2

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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.

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  • So the worry is that if I blow too little air over the a-coil, then not as much refrigerant will gassify as when I finish the ductwork? Thus, when I add in the remaining ductwork, too much refrigerant will gassify now due to the extra air flow, compromising internal pressures of the condensor system?
    – Tyler M
    Jun 25, 2020 at 13:29
  • Most new systems use thermal expansion valves to regulate the refrigerant flow the state of the refrigerant is regulated by that or a fixed orifice but there would be no difference now than running the unit on a cold day it may ice up being the problem.
    – Ed Beal
    Jun 25, 2020 at 13:44
  • It won't be a problem after all the ductwork is done because all the refrigerant should be converted to gas. They're concerned about just the running before the ducts are complete and that imbalance. It kind of like saying lets run the car engine to see if it works before installing the radiator
    – JACK
    Jun 25, 2020 at 13:46
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    Jack the conversion is temp based this is why we use super heat and sub cooling to charge a system the temp and pressure are taken because on a cool day liquid may be returning to the compressor this causes high pressure and shuts down the compressor normal operation on a fixed system. With a thermal expansion valve the valve closed when cold causing the high pressure and shutdown.
    – Ed Beal
    Jun 25, 2020 at 14:03
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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.

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  • Good point on the warranty. +
    – JACK
    Jun 25, 2020 at 13:51
  • Could incomplete ductwork result in the load on the air handler being too low so the motor overspeeds? Or is the required load supplied by the restriction in the metal cabinet of the air handler?
    – Jim Stewart
    Jun 25, 2020 at 23:17
  • Most of the motors for single phase are 2pole. 3600 rpm or 4 pole 1750 pole Some are 6 pole 1200 rpm , straight wound squirrel cage type with different configurations. Some motors that use series wound (DC) when the series motor looses the field it will continue to accelerate until it flys apart, two light of a load can also cause this (I think this is what you are thinking of) when the field is reduced the speed increases but the torque is reduced so if there is some load It slows and flying apart rarely happens, unless a big high torque motor.
    – Ed Beal
    Jun 25, 2020 at 23:47

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