the down seems a little high but that may just be the way it is with that co. as far as the duct it needs to be insulated. the coil should be in front of the furnace not in the return. if the furnace is farely new the warrenti on the heatexchanger will be voided if old then it may be time to replace the furnace anyway. call the contractor and find out what he plans to do and let him know what you want for your parents. make sure they do the job right
but be careful not to ofend your parents at the same time
GET THE MONEY BACK I will say it again in case you didn't hear me...GET THE MONEY BACK.
Even if it is impossible, or you have to take the guy to court, don't use him. It sounds like he is proposing a hack install. Better to lose a little now, than a lot later, as you will have bought a piece of cr**, and will have to spend thousands more to have it torn out, and the damaged its going to cause repaired, and a new one put in.
How old is the furnace? 35 years if its original to the house, may want to seriously consider changing it.
Uninsulated duct work in unconditoned space, you are losing a lot of heat, and will lose a lot of cool to that crawl space.
The guy wants to try and band aid the duct sweating by putting in a dehumidifier? Man I would have my Nikes on and you would be seeing my dust as a got away from him.
You need to get some reputable people in to look at what you are going to need. Its not going to be cheap, but then again you will not have mold growing in your crawl space.
The old ductwork will be undersized for Air Conditioning. You'll need approximatly 400CFM airflow per ton of AC. Thus the 4 AC Ton unit will need 1600CFM of airflow, a lot more than the old furnace only system. You may be able to shove that much air through the old ducts but they'll be noisy.
The unisulated ducts in unconditioned space will sweat, and while sweating lose cooling capacity from the system. Also make sure they size the controls transformer for both loads. They'll either need to put in a larger 24V transformer or put another one in the system ( and make sure the 2nd one is in phase with the new one)
Properly designed it's not a big deal, Just retrofitting in an A-coil and condenser to a previous furnace only system is a trainwreck waiting to happen.
Allan in Austin.
Still looking for work.
I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.
― Benjamin Franklin