Upstairs too warm, trying to sort out causes and find solution
My 4 BR colonial was built in 1968, with single zone forced air and central AC. I've been in it for 8 years. The upstairs/downstairs temperature balance has fluctuated over the years, and now after our recent roof work (details below) we are finding the upstairs too hard too keep cool, while downstairs is fine. There seem to be a variety of factors at play here, and I'm trying to sort it all out (sorry for the long post). Any insight would be appreciated.
1) Our entire 4 BR 2 bath 2nd floor has a grand total of 2 returns, near the floor in the hallway. This seems to be a likely part of the problem, but the HVAC guys I've asked in the past have said that running new returns to the upstairs is very difficult (I'm going to ask again). There are plenty of supplies up there.
2) When I first moved in we discovered that the whole system had about 1/2 the necessary return air for the furnace size, causing the furnace to regularly overheat. This was fixed by adding a couple of very large returns downstairs (but doesn't help upstairs).
3) In the past, we added a bunch of additional insulation to our unfinished attic (blown cellulose). I think we're pretty well insulated up there, though I'm sure it could still be better.
4) With our recent roof job, we followed guidelines at green home advisor and removed the attic fan, while adding full soffit venting and a ridge vent. I do not know how the temperature in the attic compares to what it was before we got the job done, unfortunately (didn't measure attic temp last year, so I have nothing to compare to).
5) Typically, I'm finding the upstairs ceiling temperature tracks the indoor temperature, but on one hot day (a bit over 90F) it read about 5 degrees F above the room temp. I don't know if this is typical, very good, or very bad. I could always add the fan back if necessary, although it is not my first choice. More insulation is also a possibility. What should I expect here?
6) The problem is not isolated to very hot days, although I am not looking forward to seeing what happens when the mercury starts hitting the upper 90's in July and August.
7) We had our AC checked last year and it was working within spec. Haven't checked this year yet but I see no indications that it's working differently. I am told it is appropriately sized for our house.
Can anyone offer suggestions on how to diagnose the real source(s) of the problem and/or recommend ways to fix it?