What needs improving?

Upper floor cooling, especially the two west-side rooms, which are furthest from the air handler. And possibly, overall system efficiency. The former gas furnace died an early death in part because it was oversized. The new heat pump is is better matched to the house size, but might still be stressed if the duct system is undersized on supply or return side.

You ask if you need a pro to look at it then you say more than one pro has looked it over and said it's fine. You need somebody that can talk you down off the fence.

Were any sizing calculations done as the basis for declaring the current ductwork acceptable?

No, no calculations, just looking around and declaring "it seems about right. What you really need is a mini-split, which we can install for $15k." Or in the current proposal, which is to redo the entire crawl space supply ducting for $14k, but no increased return air.

To klem1's point, if someone measured duct sizes, ran some Manual J calculations, and told me things were correct or unfixable, I would believe them. They don't do that. They look around, tell me it's all exactly as it should be, and pitch a solution that's expensive but doesn't address the issues that basic internet searching flags as a problem. And we've been burned a couple of times now by pros. The guys who installed an 8 ton gas furnace in a 1,750 square foot house. Then the guys who "fixed" the overheating furnace by installing a larger AC unit and air handler, for a much larger cost than replacing the furnace would have been. Then the HVAC sub-contract who reconfigured some of our ducting during a kitchen remodel, in ways that made upstairs cooling worse. So for this round, I want to understand as much as a layperson reasonably can, and ask the right questions, and feel confident that anything I pay $10-$30k for is technically sound.

I had 3 "professionals" take a look at our house to add AC. Early 50s house. Only one told me I wouldn't be happy with the final result without extensive duct rework. She actually did calculations on the spot. The others just wanted to make a sale and just gave me a number after a quick look.

Exactly, Seabornman!

Well, first of all, your return grills are way undersized. 180sq in for 3 tons of cooling? The general rule is 200 sq in per ton of cooling. Also, 3 tons is kinda big for a 1750 sqft home - oversizing will impair dehumidification. You definitely need a heating/cooling load performed before signing anything. Dump the people who said everything is fine, and the the ones who suggested a mini-split (at $14,000) and talk to the guy who suggested reworking the ductwork.

Edit: I see you're using a heat pump. Heat pumps are sized based on cooling requirements. Any addition heating requirement is supplied via aux/backup heat (usually electric heat strips).

Thanks @sktn77a. Thing is, the guy who suggested reworking the ductwork has also said the return grills are fine. I did send him a follow up email with the exact return dimensions. He was a tech who was running everything by a manager. So maybe central office will look at the numbers and come up with a sensible plan.

@Charles Ross Homes that makes sense, especially for a new build or a major remodel. Our bedrooms are on the second floor, with vaulted ceilings. Adding return ducting to the west-side rooms would be a big project. Fortunately both rooms can be left open almost all of the time.

Have you measured air temp and flow at the registers? Tools to do so are inexpensive, and I'll suggest, consistency is more important than accuracy.

You only describe the distribution system as a spyder in the crawlspace.

Where are the registers and returns located -- low or high?

How does the supply air make its way to the 2nd floor and to those west bedrooms?

Is this imbalance new?

You said, the cooling was better, before you had a remodel, and the runs were changed. How much changed, and that may well be part of your problem now.

PPF & cat_ky, a combined answer to both of your questions. A friend who was planning to do an HVAC startup did do some measuring at the registers. Flow to the SW bedroom was low. Flow to the NW bedroom was basically nothing. Can't say how new the imbalance is as we only measured after the duct changes. But cooling in the upstairs was always poor. The main change after the remodel was that cooling in the east rooms actually improved a bit, cooling in the west rooms is worse than before.

Registers are all in the floor, in first floor and second floor. First floor supply ducts go through the crawl space directly to each register. Second floor supply runs from the crawl space up east and north walls and then through the floor joist cavities to registers. First floor air return is in the central hall ceiling. Second floor air return is in the central hall near the ceiling, runs down the wall and then through the floor.

Something that might be diagnostic--we just hit real summer temperatures late last week, and we found a surprisingly effective solution. We've used a second-floor window unit for years. It keeps the second floor within 7-8 degrees of the first floor. This weekend I tried closing down the register dampers in the master bedroom, where we have the window unit, and in our first floor bathroom, a tiny room that gets massively overchilled. That is forcing enough air to the other bedrooms that the entire house is now evenly cool and comfortable. At 8 pm the second floor east bedrooms were only one degree warmer than the first floor, and the NW bedroom. That is a huge improvement over previous years. This could be a liveable solution, if closing down two registers won't harm the system.

If the ductwork is accessible, installation of balancing dampers would be a better solution than using supply registers to redirect air flow.

Thanks. That makes sense. The ducts are only accessible in the crawlspace. Can dampers be added that can be controlled remotely? We would want all supply registers fully "fed" during heat season.

Another company visited yesterday. Their rep thought return was our main problem. He said the same as @Charles Ross Homes, that ideally we would get a return in each bedroom. Since that's a major remodel-level project, he's writing an estimate to add a first-floor return, enlarge the return trunk line and plenum, and remove two first-floor ducts that supply a tiny bathroom and a garage(!) register. That might be enough to make a big difference. If not, we can look at more significant duct renovation or return air additions in a few years. Talking to one more company tomorrow though. Will be interesting to see what their take is.

Most residential HVAC systems are starved for air on the return air side. I'll bet from your description that yours is too--big time. You'll be well served to improve return air flow. Balancing dampers are relatively inexpensive and easy to adjust. I have two in my crawl space that I adjust each summer and each winter to alter the temperature in the primary bedroom. If you mark the ductwork at both settings, you can return to the same settings each time.

If your HVAC system has duct work supplying the garage, it is a code violation. Blank it off so it's isolated from the home and install drywall over the opening.

Yep! Plan is to remove and patch up that garage register and duct ASAP. I'm going to try to block it with foam this morning, to at least minimize that cooling loss. That register was supposed to come out during our remodel a few years ago. I lost track of that and forgot it was even there (it's in a weird spot that's easy to miss).

We will keep dampers in mind for future improvements. We're only closing registers right now to maximize the benefits of a window unit. If we're lucky, the return improvements would let us cool the primary bedroom with our main system and ditch the window unit.