I agree that you are undersized on return. How is your return air filtered? Furnace is attic installation? I would recommend a whole house filter media cabinet with easy access at the new furnace. You have been quoted a low end basic system. I would get an upgrade quote to the Performance series, both furnace and AC condenser. Is dealer installing new lineset or reusing old? New thermostat included? Just for info, Bryant is same as Carrier. Single speed vs two speed both furnace and AC is more about comfort than savings. Of course 2 speed condensers and two stage furnaces will cost more.

IMO

Excellent information, gentlemen.

Ionized, I am completely happy with current performance but realize there is little value in limping this older system along. Current sizing provides all the humidity control we've needed, etc.

Tigerdunes, due to the terrible in-attic layout' I'm using 1" filters on the two 20x20 returns, and the system is quoted with the rotation to make a 4" attic filter a much more usable option, in addition to improving duct layout/flow and overall access. I'll have to research exactly what you mean by a whole house media cabinet.

Seeing it was a Comfort System, I have asked about a quote for a Performance system. The quote is using current ducting (which they have checked/sealed in the recent past), adding some return ducting but reusing current lineset (they are familiar with it, is there a consideration there?) and didn't include a thermostat as I already have a Nest I'm happy with. I did advise on the additional quote request that I not married to the Nest, and would consider a Cor or such.

Mention has been made of high and low pressure switches to protect the outdoor unit, and I not sure if this is a concern...and if any of the Comfort Series Units have them.

If 2-speed would be overkill for the house, given it's current quite acceptable system performance, the cost savings would never pay us back given our current (efficient) electrical use.

I don't think the Comfort condense has high and low pressure switches.

A 2-stage system is more about comfort than energy savings. In a 5 ton installation having the condenser start up in the low stage means it as noticeable when it turns on in terms of noise and having a cold blast of air blow down the back of your neck. The same would be true in the winter. The furnace would probably rarely go to the second stage in your climate.

There are mixed opinions about the Nest thermostat. I think you are better off using the appropriate Carrier thermostat. It would have a 10 year parts warranty.

The current line set can be reused if it is flushed properly and it is the correct diameter.

Your AC company is correct about the return size, although I think you could get away with your present 800 sq in if it were't for the master bedroom issue. While adjusting dampers to provide more airflow to the master might do the trick, you may still need another return in the MBR to get the temperatures evened out. You should have 1000 sq in of filter for a 5ton AC so a 20x20 in the MBR would be more than adequate.

A two stage unit would be nice for a couple of reasons: the first stage airflows (supply and return) would likely be inaudible in the MBR and would dehumidify the air better as the run times on first stage tend to be longer. And on those rare occasions you kick into 2nd stage, the 1200 sq in of return with your existing supplies shouldn't be distracting while you sleep.

Prices are regional but your estimate doesn't appear too outrageous, especially seeing that they appear to have done their homework on sizing and ducting. A two stage system should add about $1000 on up (depending on the level of system you upgrade to).

Yea, that's why he spec'd a 10x10 IN the bedroom and a 20x20 outside...figured that outside return would reduce the draw noise on the MBR return. Our humidity control is fine, so I'm leaning single-stage get right now, no point in trying to fix things that ain't broke. Thanks.

Is it humid there?

Not terribly humid. The Nest tracking humidity has never shown much above 52% or below 38%. Generally hangs happily in the 42-48% range, and that is with "cool to dehumidify" disabled on the Nest. I have no complaints on our current systems control of the humidity.

If you can hold 74F on a 110F day, you're doing just fine.

Ok, now have a Comfort vs Performance comparison for your input, from the same vendor. Both include two additional returns, plenums and an inline 4" filter with rotation of the unit in the attic for superior access and function. Both will run off my current Nest (as quoted), but if there's a reason for the Cor unit, tell me. The Performance unit is 4 db quieter (consideration because of closenesss to master bedroom window, but both units are quieter than the current unit) and has high/low pressure protection. We have already decided it is under returned, single speed is fine, and 5-tones is properly sized.

Carrier Comfort: 16 Seer 5-ton 24ABC660A003, 80% 90,000 BTU gas furnace 58STA090-1-20 and a 5-ton CNPHP6124ALA evaporator coil. $7,480.

Carrier Performance: 16 SEER 5-ton 24ACC660A003, 80% 110,000 BTU gas furnace 58PBH110-1-20 and a 5-ton CNPHP6124A1 evaporator coil. $9,260.

Given the slight db reduction and the increased cost, which system is a better choice.

I don't care for the Performance configuration since it has a 110K BTU furnace. It is a 80% efficiency furnace so the output is 88K BTU. That seems over sized for the Texas climate with a house of your size. I would expect a single speed furnace to short cycle on most winter days.

I can't find the AHRI directory listing for the Comfort configuration. That means it may be a legal matching configuration. Ask the dealer to give you the AHRI directory number. The 90K BTU is more appropriate and probably the smallest furnace you can get to match a nominal 5 ton condenser.

Mike, I was wondering about that too...so I'm gonna get a requote on a Performance 90,000 btu unit.

The big questions for me now are:

1) Is the addition of the high/low pressure switches of sufficient value?

2) Will the Comfort outside unit stand up, given it is wire protected vs the louvered/covered Performance unit?

3) Do I need to utilize a Carrier thermostat?

After research I have concluded that 4 db is unlikely to even be noticeable to me, and that the Comfort series works fine, but lacks some things I like (but don't need?).

The high and low pressure switches are supposed to protect the equipment if some part fails. You can do a search for articles that describe their respective functions. What value they have is hard for me to judge. The Comfort series is intended to be the budget equipment. In your harsh summer climate of Texas I would recommend spending a little more money on better built equipment if you plan to stay in this house long term.

I don't see a major benefit of a Carrier thermostat for a single speed furnace and condenser.

I should have and could have mentioned that it is difficult to tell how efficiently systems based on variable speed compressors and blowers run compard to the the bang-bang type. As far as I know, they are rated in a way that locks them at a steady speed/frequency. That may not be the most efficient speed. They DO run more efficiently at relatively lower speed compared to flat-out. Figuring out how much coin that can save is a pretty complex engineering calculation compared to other HVAC calculatoins,