What is the physical location of this 750 sq ft? Upstairs? Is it downstairs with concrete slab? There are a lot of factors to consider.A 1-1/2 ton unit will typically cool between 750 and 900 sq ft,but a heat load calculation needs to be performed.

Single story house on concrete slab. 6" ext. walls with R-20 insulation. Attic has R 30. Double pane Low E vinyl windows ( 2 windows -48 X 60" ea. ) with avg. sun exposure thru these windows. Also do you think utility costs would be comparable between the larger 2 ton 16 SEER vs. the 1 1/2 ton 14 SEER? I believe the variable speed air handlers run all the time,but mostly at very low speeds. Given a choice, which one would you use?

I would go with the 1-1/2 ton unit.The concrete slab helps you tremendously in the summer time.It is always cool to the touch.

I would most definitely go with the 1 1/2 ton 14 SEER unit with variable speed blower. The variable speed blower will help with humidity removal.

Even the dual stage 16 SEER unit will have a net capacity of around 75-80% capacity on low stage, which would be 1.5-1.6 tons--possibly a bit much, but only a load calculation can tell.

Is this a new install?

Yes it is a new install. Since it is master bedroom/bathroom space and will be primary night usage, it should rarely go into high stage. Both units would probably do a decent job. I'm just trying to use efficient equip. without going overboard. My main question is if the 14 SEER 1.5 ton single stage would cost about the same to run as the 2 ton 16 SEER? I'm going to use a variable speed blower in either choice. Thanks again for all advice.

The Allegiance 16 will have higher efficiency ratings, but those don't mean much if it's oversized (which, in low size, I guess it would roughly be the same capacity of the 1.5 ton Allegiance 14) since it will cycle off and on more. Since it's for a relatively small space and will be used at night primarily, I wouldn't spend big bucks on the high end two-stage equipment (but I would keep the variable speed air handler). Have whoever is installing the unit do a load calculation to ensure sizing.

Do you know how many supply vents/returns they are going to install?

There are 2 ea. 6" supplies to the bedroom, 1 ea. 6" supply to the bathroom & 2 ea. 4" supplies to closets. 24 X 24 return air to the return plenum. The variable speed air handler has a 5" Perfect Fit media filter. My contractor did a load calc. and said the 16 SEER 2 ton would not be oversized because the fan runs all the time and would remove humidity (which was my concern) He has already installed the air handler & evaporator coil. Everything is roughed in except the outside condenser coil. I expressed my concern to him re this subject and he was going to research it with Am. Std. He did say that if we end up using the 14 SEER 1.5 unit, he would have to reduce the size of the evaporator coil as well to match the smaller condenser coil. My gut feeling as a layman is that I would save inital $$ with the smaller 14 SEER and that utility consumption would be about the same. What are your thoughts?

5 supply registers at about 100 cfm each = 500 cfm. 400-450 cfm/ton would put you at around 600-675. You may want to find another spot for a supply or two. I don't know though--I didn't design the system.

Ask him what the calculated BTU's were (exactly) to see if he really did a load calculation and to see if the 2-ton, 2-stage unit would be undersized. The low stage won't be of much use if it's the maximum capacity the place needs (1.5 tons), but I don't know how many BTU's were calculated.

I just hope he doesn't want you to go with the more expensive unit. Ask him if he truly thinks the high stage will be needed. Low stage on that unit will be around 1.5-1.6 tons already. 2-tons is a lot for an add-on, for new construction. I could see a 2-ton unit serving 1000-1400 square feet.

There is one more 4" supply to the potty room which I forgot to mention. Don't the 6" supplies put out more cfm than the 4" ? He doesn't split the supplies. Each supply comes off of the trunk plenum. All of the runs are fairly short. Do you know the approx. cfm for 4 and 6" supplies? The reason I'm asking is that if I go with the 1.5 unit, I would like to be able to add maybe one more supply in the bedroom if needed later. Thanks again.

I don't know how much CFM each vent will get--I was going with a ballpark of 100 cfm each. The size of the duct, the design layout and the dampers will determine this. The airflow can be adjusted by the blower speed and balancing with dampers. I trust that he would know to design the ductwork system to the system's needs. Let your contractor know of your possible desire to add another supply or two down the road.

Have you talked to your contractor about the load calculation? I think the 1.5 ton 14 SEER unit is the way to go--smaller place, occasional usage, etc. You need to make sure a load calculation was done to determine what size unit is needed--very, very important. Good luck.