My furnace and evaporator coil are in a closet in my house. I have a 5 ton system. An A/C contractor measured my external static pressure (between the entry to the blower and the top of the furnace) as .99 in. w.c. which is over the maximum static pressure on the furnace info plate of .8. From what I was told, I have the following static pressure readings:

.07 return air
.25 furnace mounted media filter (almost new)
.29 evaporator coil
.38 supply air

From what I understand, the .38 in. w.c. for supply air is high. It was measured just above the evaporator coil, near the start of the supply air plenum. I can take out the media filter at the furnace and put some filters at my return air grills. I have 1224 sq. in. of return air grill. However, I would still be pretty close to the maximum.

After the supply air plenum, the ductwork ‘T’s to the 2 sides of my house. I have tried to make a fairly accurate drawing of the ductwork with lengths, fittings, locations of turning vanes, etc. I want to find someone who can troubleshoot where the source of the high pressure is and fix it. I have a 40+ year old house with hard metal ductwork (with turning vanes) that is wrapped in foil covered fiberglass. Most of the lines are rectangular with the ‘Y’s being round metal pipe. I don’t see any place where the ductwork is collapsed. The contractor who took the measurements wants to replace all of the ductwork. I would like to see if there is a less drastic option since I’ve been told by others that it’s hard to get better than hard metal ductwork.

Is there any way to isolate where the problems might be?
Can the static pressure be measured at the beginning of the 2 lines after the ‘T’s?
Can someone determine if the problem is in the supply air plenum or in one of the 2 lines going off of the ‘T’?
When I am interviewing contractors, what kind of approach should I be looking for?
I have read about using Manual D on this forum. Would or should that be used to look at possible problems since I have the basic layout info?

Thanks.