Our master bedroom is 14’x19’ with one HVAC vent. Keeps the room perfectly chilled or warm, as appropriate for the weather. The air return is not in the master bedroom.

We are in the process of building a new home. Our master bedroom is 16x13 and there is only one AC vent.

Ok?

1. How do I determine if that is sufficient?

You can not. The HVAC designer determines required number of CFMs using the overall load, that factors your insulation / windows / your climate, etc. Block loads for heating and cooling are different. After determining the load, Manual S performed to better select a system, followed by D / T for distribution. There are 3 different methods to design a distribution ductwork system. w/o knowing the load - even IF you buy equipment that measures CFMs - you have nothing to compare it to.

2. What should be the ideal distance between venting and return air? Right now they are almost next to each other, about a feet away.

You think that anyone here can see into your new house? We don't know if you are in the heating or cooling climate, or if the registers installed into a wall, floor, ceiling? I can tell you it is all about a register throw... https://www.continentalindustries.com/DD-SupplyRegisters

Independent of the air volume calculation, ideally a supply register should be under each window and the return grille should be near an inside wall. No one can tell you more without a floor plan.

The supply register should be located beneath a window. The best place for the return is on the opposite of the room.

Is there an attic above the bedroom? if so what is the insulation value in the attic and the exterior walls? Are there windows facing south or west? Where is the house located?

For a new construction a single supply register is probably sufficient. I would be more concerned about the return being too close to the supply.

How do I determine if that is sufficient?

Let me suggest two alternative methods to assess system performance: the first is to compare the conditions in the room against the design criteria for the system. You'll need to collect temperature and relative humidity data for the full range of climate conditions you experience, crunch some numbers, generate some graphs and compare design conditions with actual conditions. That sort of fun stuff. Or you could use the "Goldilocks" approach: is the room too hot, too cold or just right?

Ultimately, it's your comfort level that matters. If the conditions in the bedroom vary a lot from the thermostat's setpoint, then either the system--or something like insulation levels-- will need to be altered to correct that.

The best way to determine if the AC vent is sufficient is to set the thermostat to "cool" and wait a half hour to see if you are indeed cool. There is a way to calculate the performance of your AC system taking into account all the variables. To pay for such study will make the aforementioned technique attractive.

Express your concerns to your builder in writing and wait to see how the AC system performs. Doing something as simple as rearranging the furniture in a room can effect the AC system's performance.

Because bedroom doors can be expected to be closed, each bedroom should have a return or, at a minimum, a jump duct because air coming out of the supply needs a way to return to the circulating system. A jump duct can be short, it's only to provide a passageway as an air return from the room with a closed door to an open common area.

Grills are put into the ceiling (normally) of the bedroom and adjacent common area and are connected by flex duct.

Jump ducts are worth their weight in gold, especially in larger homes with many closed doors.

Dedicated returns are the best.

If you are cooling .. returns should be located high, heating climate... returns should be low. Ideally install both, but I never seen this happened. Jump ducts are pretty ugly IMHO.

Balancing return draw throughout with too many returns can be tricky, depending on the layout. Jump ducts are easier to deal with and cheaper, especially if used for bedrooms. I'm not sure what you've seen, but the ones I've seen look like smallish additional supply grills. They don't need to be large and they can be put in unobtrusive places.

As far as location of the returns high or low is concerned, aren't most homes heated in winter and cooled in summer? Do you suggest a switch between high and low returns depending on the season? You think putting in what looks like a few extra supply grills for jump ducts is ugly but you're ready to put in twice as many large return grills as the system needs? That's an unusual point of view.

There should be sufficient returns and you balance the supplies. No difference in designing returns vs supplies other than if supplies are designed for 0.1 TESP a return will designed for 0.05 [0.15 rule = 0.1 filter + 0.05 ductwork] .

Is the duct work for the bedroom returns installed in unconditioned space?

@Is the duct work for the bedroom returns installed in unconditioned space?

Not necessarily, but jump ducts [transfer ducts] almost always. Here OP asked about the distance between supply and return install presumably in a wall.

"There should be sufficient returns and you balance the supplies."

I have a long and narrow ranch house, with two returns (one at each end) and the HVAC equipment located at one end. The far end wasn't getting enough air flow and so was colder during heating season and warmer during cooling season. I had two HVAC contractors out to assess the problem- did the supply ducts need resizing or balancing or what else could help? Both said the problem was that the return ducts both went directly back to the equipment and because of how air flows, the shorter one would draw much more air than the longer one (at the far end of the house). Both said the ducts were properly sized but there wasn't enough return draw at the far end. With a forced air system, the air flow is affected not only by the blowing of the supply lines, it's also the negative pressure of the return ducts.

The solution was to run a trunk line to roughly the center of the house, and then do a Y to run two ducts of roughly equal length, one to each return grill. The result was amazing and the balancing of the return paths and size made the difference.

While rooms with closed doors (like bedrooms) do need to have the return air flow accomodated somehow, designing too much of an octopus for the return paths may affect the airflow throughout the house.

Yes, jump ducts can transmit sound but the passage can be baffled and if carefully placed, the problem is minimized.

Thanks everyone for taking time to clarify. Almost everything is new to me when it comes to home construction. I may not have used the right words / terms. But thanks for explaining these things to me. Learning everyday!!

I don't think anyone has said it but 1 foot is too close for a supply and return. The original post says "about a feet away" - so not sure if there is a number missing.

I am firmly in the camp of returns in every bedroom.

Returns in every bedroom works great so long as the folks doing it know how to do it in such a way to compliment and not conflict with the airflow intended by the supply duct layout and sizing calcs. And aren't doing their work trying to pinch out every penny possible.

I've run into more sad stories about poor HVAC contractor performance in new construction than with remodels and retrofits but I'm sure there are exceptions. HVAC jobs with new builds too often go to the lowest bidder the general contractor can find, not necessarily the most capable.