Can I add a register?

I have two upstairs bedrooms that are pretty large but also with very high ceilings. I estimate that they are about 1,800- 2,000 cubic feet each due to the high ceilings. They each have only one register and are uncomfortable. I once heard (from an AC guy) that AC installations are designed for precisely the number of registers that they have. But can I add a register to each of these rooms? Is there anything to consider?
The other large room that is comfortable up there (though it is larger) has two registers.
The other bedrooms on that floor are smaller with one register each. The unit serves a total of 11 registers including halls and bathroom areas and it is a 3-ton heat pump unit by Lennox HP26-036.
Thanks.

 

Ok.... it's a heat pump system but where is the ductwork..... in the attic ?

You should be able to add an additional register but remember that will take from the others.
You may not need it to be wide open.

 

I added a vent/register to my bed rm. I was able to access the duct in the crawlspace and installed a Tee on the duct going to the laundry rm. Both vents have reduced air flow but since the laundry rm is small it's not noticeable.

If you just add an extra vent off of the existing supply duct you may not have any extra air/heat coming into the room although it's possible it will be better distributed. Is it feasible to come off of the trunk that services the attic?

 

As said, if you add a register it will take away from others.

The "AC guy" was correct.
The registers are sized to distribute the air that the duct work is able to supply.
Unless your duct work is oversized, adding a register to the far end of the air stream will not cause more air to come down the duct.

Provided there is sufficient capacity at the air handler you should try to add a duct to serve an additional outlet.

 

Yes, the ductwork is in the attic and I can easily access it. As for the trunk - I'll have to get back up there and see exactly what the lay of the land is regarding that versus the room location.

The "AC guy" had also said not to close registers in unused rooms. But if I do this, might it actually be a good idea to do so? Or would it help at all?

 

The ductwork should be sized to carry all the air the fan can deliver.
Any potential gains you would see by not cooling specific areas would likely be offset by reducing the total airflow through the cooling coil.
The lower the coil tempetature the less the overall system efficiency becomes.

Any dampers there may be on the ducts are there for small changes to the airflpw to balance the system, not to shut off airflow to an area.

 

A slight help- would be to add ceiling paddle fans, set to the winter position. They would take the warm air at the ceiling and push it back down to your comfort zone.