Air conditioning Question from a client.

Do the registers need to be all open for optimum performance?
If some registers are closed…Does that make a difference.



For optimum performance, all registers should be open.

Think of it this way: if ALL the registers were closed, the system would not be putting any conditioned air into the building. And the system would be pushing against a wall, so to speak. The system would be stressed trying to push air that wasn’t moving. Performance would be zero.

If you open one vent, then a small amount of relief would be present and the system would have to work a little less hard. Open 2, a little more relief. 5, some more, etc. etc.

So, it’s sort of like a scale from zero (all closed) to optimum (all open). Can you measure the effect of closing one vent? I doubt it. But obviously it’s a scale…the more you close, the closer you get to zero efficiency. Trying to ‘pump’ air that cannot move is more likely to damage the system.

Sounds like a crock to me.

That’s why there are return vents and adjustable supply vents. You don’t need to have all of them all open for the system to perform properly.

Course you definitely don’t want all of them all closed either.

For optimum performance the system needs to be balanced.
I did some certifications with Enviromental Engineering Consultants, Ernie was a good guy, he is missed.

When you start closing off supplies you affect the operation. One or maybe two may not hurt but eventually you will get to the number that where it seriously affects operation.

Example: 2 tons of cooling requires 800 cfm of air flow for proper operation. A 6 inch round pipe is rated for close to 120 cfm airflow. Start closing off enough and you will have reduced airflow.

Evaporator coils come in two models. Those with thermostatic expansion valves which maintain a fixed superheat. That is no liquid refrigerant going back to the compressor. The other style is a flow accurator system. This system has a varying superheat. This system with reduced air flow will result in zero superheat condition and liquid refrigerant going back to the compressor. Compressors do not like to try to compress liquid. The liquid refrigerant returning to the compressor will also result in oil foaming.

The system should have been designed and sized to cool the entire house.