The calculation was done with an summer indoor temperature of 70F degrees. The normal practice is to use 75F degrees. Did you request 70F degrees? The lower the temperature the larger the required cooling.

I did. The "standard" 75F is below (3.50 tons / 4.19 tons).

Given the size of your home, windows, and your demands on HVAC, I would consider a Waterfurnace Series 7, 5-ton geothermal HP where all elements of the system are all variable speed.

I don’t know if adequate consideration has been given to your skylights - You might need a better Manual J calculation!

Waterfurnace Series 7 - Geothermal Heat Pump

YMMV

IMPO

SR

I doubt the ductwork from the original 3 ton system would support a 5 ton system. But I doubt the current system is delivering 3 tons of cooling. I suspect you could use a slightly larger system, maybe a 2 stage 4 ton cooling system. You don't want to oversize it as this will significantly compromise dehumidification.

I realized my information about skylights and windows may have been misleading and implied that we added 32 skylights for a new total of 48. I wish! We added two larger skylights, plus a wall of windows totaling roughly 125 sqft along the upper part of the cathedral ceiling. I have updated it in the OP and copied it below.

I did my own Man J using one of the free online calculators and it jibed pretty closely with what I attached.

The home originally had a fair amount of windows and two skylights of about 8 square feet each (16 sqft total). We've added two more skylights of 16 sq ft each (32 sqft added, 48 sqft total) plus added a large wall of windows (facing NNW) of perhaps 125 square feet of glass.

I would highly recommend a professionally done Manual - J and Manual - D particularly because of your windows, skylights and usage. Skylights are a big factor; you need to get that right.

Your ductwork will most likely have to be modified - at the very least!

You might consider future additions and zoning as well at this time.

IMPO

SR

When you say professional, what do you mean? The Manual J info I posted was done by a higher end local HVAC company. I think the duct info I posted might be the manual D you mention?

What duct changes would you expect? Since my first post, two local companies got back to me and indicated that the current ducting will support 1600 CFM while keeping static pressure within acceptable noise levels.

Good point on the future. Is there a way to make a system bigger than needed for potential future square footage Without causing over sizing issues now?

The original 3 ton AC was undersized for the house. Adding the skylights and additional windows increased the cooling load. The duct work appears large enough to support a 4 ton system. That is what I would recommend. The cost is dependent on what equipment you buy and the rates in your area.

Do you have shades on the windows and skylights to block the sun during the hottest part of the day? Have you considered adding ceiling fans?

mike_home, we have shades on some of the regular windows. The added wall of windows do not have shades and really couldn't from a practical or aesthetic standpoint. For the skylights, aesthetically we'd prefer not to have shades, and I also have had bad luck in the past with the motorized remote rail shades that would be needed. I think I'd rather spend a couple thousand more on the AC system than on motorized skylight blinds.

Then if you have the budget a 2-stage variable speed AC would provide good comfort. Have you gotten any quotes on equipment options?

I'm working on quotes. Got the first company's just now. They basically refused to quote a 2-stage AC. 3.5 ton 13 seer $4700 installed; 3.5 ton 16 seer $4800 installed. Indicated current furnace is "3-ton drive" so going above 3-ton AC requires new furnace. Quoted $4000 installed for 90K BTU 80% single-stage. "Comfortmaker-Tempstar".

Any thoughts on this pricing?

"Given the size of your home, windows, and your demands on HVAC, I would consider a Waterfurnace Series 7, 5-ton geothermal HP"

Yes, follow this suggestion and spend $50,000 instead of $12,000 or less with conventional equipment that will give you the same functionality.

Got another from an outfit I‘ve liked. Basically $8k for a 3.5t AC and 95% furnace. Additional $4k to go to a 4.0t two-stage stage AC and 97% two-stage furnace. Thoughts on prices, especially if it’s worth $4k to go to two-stage setup from a comfort stand point?

"I'm working on quotes. Got the first company's just now. They basically refused to quote a 2-stage AC."

Get a different company. Remember the golden rule: "he with the gold makes the rules!"

Are these installers choosing to ignore the load calculation and your desire for the temperature to be 72-74F degrees during the hottest part of the day? Do they think 3.5 tons will be sufficient or is it a matter of not spending the additional money on a furnace?

A 2-stage AC provides some additional comfort. It will be be quieter and you won't have the cold blasts of air that a 3.5 or 4 ton system normally provides. I am not sure why an installer would refuse to install a 2-stage system. Did he give a reason?

I am a little skeptical your current furnace does not have a blower capable of 1600 CFM of air flow. If you can provide the model number so this could be verified. Investing in a 95%+ efficiency furnace may be a good investment if you plan to live in this house for the next several years.

3.5 ton 13 seer $4700 installed; 3.5 ton 16 seer $4800 installed.

Can't comment on these prices without know what is being installed. You probably will not achieve a 16 SEER rating with your current furnace. The installer has no way of verifying the performance.

Quoted $4000 installed for 90K BTU 80% single-stage. "Comfortmaker-Tempstar".

I think you are being quoted a low end furnace. Avoid this if you have the funds.

Basically $8k for a 3.5t AC and 95% furnace. Additional $4k to go to a 4.0t two-stage stage AC and 97% two-stage furnace.

Another option is a single stage 4 ton AC and single stage furnace if the $12K quote is beyond what you want to spend. If you provide the model numbers of all equipment (coil, thermostat etc.) you will get some feedback

Thank you, Mike. I am not looking to waste money, but I have it if the value is there. Got three options from another company (Vendor L). I am impressed with them in terms of how long they spent looking and measuring the existing system and ducting and discussing options. They recommended I get several more quotes and, when I told them I was in that process, they asked who. My other top contender company (Vendor C) they not only didn't diss, but spoke highly of. I'm waiting on pricing and specs from Vendor C.

Vendor L (Trane based):

Option 1: XC95m 97% modulating furnace 100K BTU + XV20i 20 seer fully-variable 4-ton AC $12.5K

Option 2: S9V2 96% two-stage variable speed furnace 100K BTU + XL18i 18 seer two-stage 4-ton AC $10.1K

Option 3: S9V1 80% furnace 100K BTU + XR16 16 seer 3.5-ton AC $8.7K

+Trane ADP Matching Coil

+Options 1 & 2 require going to a 12x24" return drop $200

+Recommend new return in finished basement: $350

+This was their initial pricing -- got the impression there was a decent discount I could get especially in a cash offer

Does the above give you coil info? I will have to ask about thermostat although I believe they indicated Options 2 and 3 could/would use the existing Gen3 Nest while 1 requires new (assuming it's included in price but also need to check).

I'm interested in thoughts on these configurations, the pricing (esp value), and long-term ownership costs (esp repairs). Thank you everyone!

What was the heating load calculation? A 100K BTU furnace at 97% efficiency seems over sized. I think a 80K furnace with a 4 ton blower should work. I think he is proposing a third party ADP coil. I think you are better off with a Trane coil even if it costs a little more money.

The XV20i has two compressors has is likely overkill for your region. Option 2 or some variation of it is a better value. Hopefully @tigerdunes sees this post and gives his opinion on the Trane equipment.

I would be cautious about using the Nest thermostat. Make sure the installer is on board with using it. Most pros hate and avoid them.

Mike, I think I was able to attach an image of the Manual J summary to my OP. Regardless this is what I think is the info you're asking for:

Total Heating Required Including Ventilation Air: 83,330 Btuh 83.330 MBH

Vendor L showed their load info to me but I didn't request a print out because it was pretty close to the Manual J I already had. I believe his was 81 or 82K BTU. He was surprised and mentioned he expected to spec a smaller BTU furnace (current is 100K / 80%).

I have requested clarification from Vendor L regarding the coil. I will double check on the thermostat, but I know he knew Nest because he mentioned it "was ok as long as it's not the cheaper one with the white face".

I don't think there's any such thing as a "Trane ADP" coil. Trane makes coils, ADP makes coils, but they are not the same. Ask him to provide the matching AHRI number for his proposed system vs the all Trane system