The issue may not be confined to airflow alone. An air conditioner makes us more comfortable through two basic rules of physics.
It removes sensible heat (the temperature you feel) and removes moisture from the air. Dry air enhances the bodies own cooling system allowing perspiration
to evaporate more quickly which speeds the removal of heat from the body, this is why 85 and dry feels more comfortable than 72 moist degrees.
to ensure the ducts are balanced properly one must use an anemometer which
is a tool used to calculate airflow by the pro's. Being that most homeowners do not have access to one there are some good observations to make using a pocket thermometer and your own senses:
- Ensure the guest room vent is the same size as the master bedroom.
- Ensure the supply duct for the suspect vent is the same diameter and approximate distance from the main plenum (where the ducts originate) as the master bedroom.
- Is the air emminating from the suspect vent approximately the same in volume as the master bedroom (does it feel and sound the same).
- The issue exists even if the guest bedroom door is left open.
- The air temperature measured at the vents is within 3-5 degrees of each other.
If 1 - 4 are true check for #5, if # 5 is false make sure the suspect ducting is insulated or covered well by attic insulation.
If 1, 2, 3 or 4 are false: Ensure the following:
- The guest room ducting does not have a damper hidden under insulation or duct wrap.
- The duct has not been crushed or restricted.
NOTE: internaly insulated ducts can be restricted on the inside by fallen insulation while appearing normal on the outside.
If #5 is false (issue goes away if the door is left open) the door may need to be cut off a bit more at the bottom or a grill installed to allow the air coming into the room a place to go.
If none of the above reveal the culprit concider the room itelf, it may have a higher heat load than the others.
A higher heat load may be due to dislodged,
compacted and or wet insulation in the walls or attic, feel the walls and ceilings with your hands, how does it compare with cooler rooms?
If the walls and ceiling feel about the same and the tests above did not expose an issue its time to call the contractor back in, make sure the static pressure is checked in each duct a foot or so downstream of the the plenum and again just upstream of the vent. A duct with air pressure higher or lower when compared with the others could mean a variety of issues with the duct or where it connects to the plenum.