Installing new HP and ductwork

[LJT]

I am in the process of replacing an Oil Furnace (died) and straight 5 ton a/c, Trane outdoor unit, (14 years old). All 3 contractors are pushing Lennox systems. Biggest concern is IAQ, then cost. House is a block, ranch, 1966 vintage, 2600 sq. ft., new windows and wall insulation added. Located in Central FL. We are taking out the old sheet metal duct work, branches and returns. Have had a serious mold problem with significant health issues for myself and daughter. Dr. and remediation specialist recommend replacing everything. Mold was in the walls from very negligent window install. Have plaster walls and never saw any signs until too late... The HVAC estimates are for the XP19 4 ton HP (heat load calc done), variable or non-variable fan at 4, 5 and 5.5 ton, 7000 t-stat, new return plenums and Lennox PureAir. MYQUESTION: (1) Do we need the XP19 or would the XP14 save enough $$ to step down? (2) VS fan seems to be a no brainer, but what size and model? The indoor unit is in the garage, no a/c. (3) Is the PureAir worth the initial cost and annual maintenance? --VOC's are a concern as new material going into the house after remediation and 2 of us have asthma. Do we need an additional UV above the coil? (4) 2 companies want to do all FLEX Duct, 1 co. wants to do rigid Fiberglass ductboard with flex branches. What type of duct work is best? It is in the attic. (5) Should the return plenums be a certain material and what about filters here? (6) Fresh air ventilation/dehumidifier? Can not open windows year 'round due to severe indoor and outdoor allergies. (7) Cost is definitely a concern.
ANY HELP is greatly appreciated!

[tech45]

I can't comment on the latest Lennox products ( the last lennox system I installed was a Pulse furnace and an HS 19 series ac , lol ).

But if clean duct was my priority I would have nothing but clean sheet metal wrapped with the highest value fiberglass wrap ( attic ). Seal all seams on the metal before closing the duct wrap ...and no sound liner whatsoever. A little noise is the trade off.

The " clean houses " I worked on in the past had no flex either ( that is extreme I'll admit ). On one job we had to wash all internal surfaces of the metal with de-natured alchohol before we ran it through a machine to form a snaplock or pittsburg edge , lol.

[LJT]

Thanks for the reply. Metal sounds logical from a "clean" standpoint, but with the labor involved it might be cost prohibitive. I guess that is why everyone wants to use the Flex...

[tech45]

I hear you on that. I do commercial work now ( and have for many years ) ....the industry standard there is 6 ft on flex runs . I have adopted this policy for any residential job I may get involved with ...and it serves me well. But yeah...I do not compete with any " bid " for these jobs either ( and yeah , I ...like others here...speak from the standpoint of the " ideal " ).

Fiberglass ductboard....would I have it in my house ....depends who was cutting and fabbing it. There is a very specific set of tools required to do this " proper " ( and the knowlege to use them ). Done proper...the leakage and insulation factor is awesome.

Friction loss of insulated flexible duct ( as compared to metal pipe )...there are many types on the market and I'm sure there is someone who can discuss this more knowlegable than me.

I'm just a weathered old " metal dog " who's services were no longer needed in house work , lol ( many years ago )... so don't mind me. There are good ways to use all modern products for sure ( but many just make it so darned easy to " hack " away ). Good luck with your project.

[LJT]

I appreciate you sharing your knowledge. I too am worried about the quality of work...it is a refreshing thing to see when it happens! This house was so well built that it is a shame what has happened with it.... But it will recover and so will we! Thanks for wishing us luck! We are way overdue!