How To Add Basement To AC System

D107

We have a 1982 3-ton attic AC unit running fine but excludes the basement. I have been surviving in the summers down there with a dehumidifier that gets things quite hot --over 85º most of the time.

I've had HH offer some solutions just for the basement zone--minisplits, portable ACs etc but since we may have replace the whole house unit fairly soon and we may not have enough of a chase internally to run ductwork, I wondered if running the supply and return ducts--heavily insulated of course--from the attic on the outside of the building down into the basement is possible. Basement is about 500 sq ft + 150sq ft for small half bath at top of the stairs that's sealed from rest of the house. Of course the boiler room (150sq ft) in the basement doesn't really have to be cooled.

Jamie Hall

It's certainly possible, of course -- but they will need to be quite heavily insulated (I'd use rigid foam, not something soft and fluffy and water absorbing!) and weather protected.

EBEBRATT-Ed

Yes. I would TDC duct (it connects with flanges) and has butyl tape put on the joints so when assembled is water tight.

You could have them use rubber foam (like armaflex) on the inside for insulation. You going to need the right installer for this and then could also insulate the outside in addition if necessary

D107

Thanks @Jamie Hall and @EBEBRATT-Ed I just remembered that the current attic air handler is already connected to the outside compressor by copper tubing insulated with armaflex type black rubber insulation. (covered cosmetically overall by a three-sided bent metal column type covering.) So I guess if the refrigerant can work outside, so could the air supply and return. Duct size could be an issue. Current supply ducts to first floor are at least 6-7-8 inches respecively.

HVACNUT

No closets? 7 inch oval doesn't take much room.
Can the system handle the extra load? 

mattmia2

The min split might still be the best option cost, cosmetically and function wise.

ChrisJ

HVACNUT said:

No closets? 7 inch oval doesn't take much room.
Can the system handle the extra load? 

What's a single 7" oval going to cool a bathroom?

EBEBRATT-Ed

Due to the length of run needed, the cost of the wether proof duct installation and the size of the ducts needed that this approach will be impractical

pecmsg

Install a small mini split!

D107

Based on all that I'm hearing it seems like the easiest thing to do is install a minisplit to cover the 500sqft basement and the connected 150sqft half-bath above it = 650 sqft ––minus the boiler room––and then replace the central AC on its own. Main house is 1100+ sq ft, (600 main floor, 500 upstairs) currently works fine with old 3 ton unit. House walls are insulated with 4" sprayed cellulose and attic eaves and walls have 4" closed cell spray foam. (House is larger than 1100 sq ft but currently AC does not cover side porch or ground floor half-bath.)

I've seen great Minisplit jobs of family members where the outside pvc? piping and joints were beautifully done and combined into one large compressor in one location. I would consider that––come replacement time––maybe one wall unit upstairs and one on main floor could cover it if upstairs unit was in the main connecting hall. Problem is we use our bedroom with door closed at night and unless we put circulation opening in the door itself it wouldn't work. No headroom above the door. So adding a minisplit now would seem to solve the immediate problem.

pecmsg

A basement in most areas require heat and dehumidification while the rest of the building envelope requires cooling. ( The earth is great insulation )

A mini with a small dehumidifier is all that’s needed. 

HVACNUT

ChrisJ said:

HVACNUT said:

No closets? 7 inch oval doesn't take much room.
Can the system handle the extra load? 

What's a single 7" oval going to cool a bathroom?

He would obviously need multiple drops. The point being they don't take up much room, and just about anything is better than what the OP is proposing. 

D107

@hvacnut In the case of our small house--which already uses three closets as AC duct conduits (for 6,7 and 8" rounds for kitchen, dining and living room respectively) we are at our limit with closet intrusions. (The prior homeowners did not zone the Bryant R22 system they had installed in 1992, so the only return register is a relatively small one in the upper floor hall 12 x18.) So the only question remaining for us is are there savings and efficiency benefits to re-doing the entire house with mini-splits now and thereby having a single compressor, or, more likely perhaps, just installing the one mini-split now and replacing the 3 ton Bryant unit later.