Faith and Jultzya,
Originally posted by faith
“They will install only one return” mlbussert, I assume that “they” is a qualified HVAC Contractor, if so “they” would have designed the air distribution system to meet the required manufacture’s tolerances for pressure drop and velocity limits. Both of these factors depend upon the duct geometry and the type duct of material. Selecting the correct blower for the system weighs into the mix, and there are many factors to be considered there also. Driving the equation is the fact that your residence is located in Arizona thus it’s ACTUAL capacity needs to be carefully analyzed. Cooling is made of two components, sensible and latent. The former is what you see on a thermometer (delta in degrees) and later is what you see on a humidity gauge (delta in relative humidity). Arizona is a decidedly “sensible” only environment therefore all latent cooling capacity should by subtracted from the aggregate total capacity stated (3-Tons).
In your region the qualified Contractors realize all the above and hopefully use sound professional judgment to design a system that will meet the cooling loads as calculated by ACCA Manual J procedure (or similar methodologies).
I have to smile at your focus on the thermostat, which while obviously a necessary component in the mix, albeit such a minor one in the total scheme of things. So many times I’ve ask the caller “Who is the manufacture of your unit?” only to be told WHITE-RODGERS. Thus the highly visible thermostat gets top billings and the remainder of the components remains a mystery. I agree that the user interface is omnipresent in the owners mind, however if you are installing a completely new system please try to come up the curve on all the elements in the equations giving the most credence to the most critical of the aforementioned.
How much thought have you placed on the length of warranty being offered, and does it cover compressor only, parts and compressor (if so are they both covered for the same delta time) and labor. I would be far more concerned about getting a 10-year compressor, 10-year coil; 5-year other parts and 1-year labor (minimum). For a very reasonable price you should be able to get 10-years on everything. Mlbussert, these are extremely important considerations that YOU control, does this make sense?
I would also want to know if the unit size was determined by a cooling load calculation and if so I would want to review the input data used and see if room-by-room loadings were generated. If the contractor is able to accommodate these request, then assuming you’ve checked references you can relax a bit knowing that you’re indeed in competent hands.
The other factors in the equation would require a substantial time commitment on your part to study the necessary methodologies to become competent enough to make meaningful decisions on this design issue. For your immediate questions start with ACCA Manual D, ISBN 1-892765-00-4 (2002 with 2004 reprint available also) available free of charge through your local library via inter-library loan. If your not in a hurry and have lots of free time then camp out on this HVAC Forum and drink in the knowledge made available by a wonderful multitude of talent, enough said.