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Old 06-07-2010, 01:14 PM   #1
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2 story HVAC design questions


New poster here. Hoping I can get some input from you fine folks.

I purchased a house last fall, built in 1997. Its a 1 1/2 story house, walkout, with a homeowner finished basement. ~2000sqft in the down/upstairs, with a ~1200 sqft drop ceiling basement. Bryant natural gas furnace with a Goodman 3ton AC, both original.

Once summer came, I noticed the AC was having trouble keeping up (couldn't cool below 81F) and the basement was very humid (up as high as 77% RH.). A serviceman came and saw the condensor coils were clogged and it was overcharged with R22. The unit is cooling better now (measured 19 deg drop between ambient and supply and it can cool to about 75 deg without running constantly) and removed some of the moisture (now 66% RH), but flow isn't very good to middle levels of the house. While looking at the system, I noticed that the *only* returns to the system are in the upstairs. The downstairs has supplys in each room, but not a single return. The basement has no returns either. The return plenum goes from the furnace in the basement up to the attic, where its connected via a 14" insulated flex duct to a common return vent at the top of the stairs/hallway that then breaks off to each of the bedrooms upstairs.

I noticed a couple things that definately need addressed:
- The flexible ducts in the attic aren't sealed to their mating metal ducts. Some have nylon ties holding them on, others (like the main 14" return connection) are just slipped on. This seems like a good thing to address with some metal tape and/or mastic.
- The return plenum where it meets the filter rack at the furnace is not even remotely sealed along the top edge of the rack. This is sucking in air near the furnace, which i understand is not good for flue backdrafting, never mind a big giant leak in pulling hot air from the returns.
- I need to look to see how the basement vents are tapped into the downstairs floor venting. If those are poorly made or leaking, that would be a source of my low flow I'm seeing in the downstairs vents.
- I looked at my furnace blower and while some of the vanes were a little dirty, it seems to be working fine. Its a 1/2 HP unit.

Some other questions:
- Is the "returns only upstairs" design common? Compeletely wrong? Reworkable?
- With all my returns being in the attic, it seems it might help to reduce the attic temperature with some radiant barrier (like attic foil).
- How to get some of the moisture out of the basement? With no returns, that air is just stagnant (especially when the basement door is closed). I could install a dehumidifier but I already have one in my A/C unit!
- The A/C may be reaching the end of its service life, but its cooling within spec, it seems like more of my issues are one of flow and getting filtered air to the right areas.
- How aggressive should I be with sealing up the duct work? The metal work downstairs seems well done and tight, but its the crimped style slip fit with screws. No tape or mastic/caulk is over any of the joints.

Any thoughts or suggestions? I'm trying to make my new home comfortable for the summer!

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