the ROI is rarely there on spray foams in walls, if ever. Marketing BS has people thinking spray foam is the next step up from batts to ultimate energy savings. The truth is, unless you are going with closed cell (most do not) your wall r value increases 1 or 2 points, however you do get a boost in air sealing within the stud bays. However you still have a thermal bridge in either system, and spray foams can not address that, as well as air leaks at your top and bottom plates of your walls. I would be curious to see how you are getting r21 in your exterior walls. if they are using fiberglass batts, you arent hitting r21. For zone 6, it would be money in your pocket to increase that value (especially after thermal bridge and batt insulation short falls your whole wall r value is closer to r16) by adding exterior foam sheathing. The extra upfront cost of adding a couple inches of foam on the exterior will be cheaper then the spray foam upgrade, as well as decreases the thermal bridge shortfalls (reduced "cold drafting" on your walls) and provide a better air seal assuming seams are caulked/foamed/taped. Get your HVAC guys to do a load simulation and adding foam sheathing should reduce your tonnage for the equipment, thus reducing the upfront cost of it and offsetting some of the cost of the sheathing. After that, depending on your energy usage, rates, size of home, etc your payback on your sheathing will typically be less then 5 years. I would also recommend switching from batts (again, assume you are using them) and going with a blown product such as fiberglass or cellulose. It will do a better job of fully insulating the wall cavity. Finally not everything needs to be looked at as ROI. Some things will provide added comfort but not always come with a payback. Full foam below slab could be one of those items. I have had clients that do it and are happy they have, and I have clients that save the couple k and dont, and wish they have. One even took up their entire wood floor (floating) and added foam on the slab after the fact since it was so cold. These are people in your same climate zone 6. People never like to spend money on things they dont see, and insulation is typically the first area people do not always care about. Most builders do not understand air infiltration, thermal bridging, and how to do energy simulations and payback so most do not understand the advantages to increasing the shell insulation and tightness of the home. Most stick by code minimums (like in your case). Energy rates may be low at the moment. But do keep in mind...energy rates will always keep going up.

So my NJ house actually has something GOOD about the hvac? Each of the 4 upstairs bedrooms has its own return. They are not cut through to the hall. The mbr one goes up onto the attic as does the hall one (I think) and they must join up together to go down to the basement. I think 2 of the br are back to back and go down to join up with one from the dining room, which seems to line up with one br. Not sure where the other one br return goes. Now you have me wondering. I have pretty high door undercuts. I never knew there was a reason for that. I thought it was poor workmanship with measuring! I though poor air return was leading to humidity in my house. I have been Having high humidity in the house, might be because it's not hot enough for the ac to run for long. I've been running it to try to lower the humidity. I think my air sealing is part of the problem (though it helped with humidity in the winter!). He checked the return flow this week by drilling a hole and sticking some measurement tube in. I think he said it was a 2 (?) but that it could be higher. Anyway, I have never noticed the return vents the op showed. I will have to pay attention to homes I visit because now I am curious.

2 separate units will do the job, but cost of maintenance and break downs of 2 units as opposed to one with zone controls will in the long run be more costly. You have 2 units to maintain, you have 2 units to repair when they break. Tag team event. One system breaks down one month. The other system breaks down the next month. Doesn't always happen this way... but it could. zoning has to be done right. So what does that cost? I don't know I am not there. 'Design, Design, Design' other wise 'Trouble, Trouble, Trouble'.

WHAT IS A HIGH LIMIT SWITCH? https://www.newcentralairconditioner.com/ The high limit switch in your gas furnace is a small part with two very important jobs. This switch plays a role in the normal operation of your furnace. Turning the fan motor on and off is the high limit switch’s main job. This is the role it plays in every cycle of your furnace’s normal operation. But that’s not the only thing that the high limit switch does. It’s also safety device that helps prevent your furnace from overheating. The high limit switch consists of a long temperature-sensing probe attached to a metal mounting plate. From the mounting plate, the switch is wired to the blower fan motor and to the gas valve. The switch is generally located just past the heat exchanger. This is so that it can detect the temperature of the supply air. In other words, the air that’s about to be blown into your home.