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Community and Q&A

How to replace inefficient AC in an old house?

pghbob | Posted in Plans Review on

I’m planning a major renovation of my 90+ year old house in western Pennsylvania (zone 5). The existing central air system was installed by a previous owner at least 10 years ago. It boosts my electricity cost about $200 a month in the summer time and does an adequate job of cooling & drying the indoor. As it is close to EOL and I’m going to make a lot of other changes, I’m looking for suggestions on how to replace/upgrade this system, reusing existing pieces to the extent that makes sense.

The house is about 2100 sf: basement, 3 floors, small unconditioned attic, brick exterior, slate roof. It has a gas boiler (20 yrs old) and big old cast iron radiators (original), which I plan to replace with Runtal or similar units. That’s a separate discussion…

The AC uses a Goodman condenser in the back yard, AC lines piped up to the air handler and heat exchanger crammed into the attic. This unit is about 3’x2’x7′, with the top side about 1′ under the ridge beam. The attic is accessed via scuttle with a plywood cover.

Six inch round supply ducting was retrofitted into ceilings on floors 1, 2, and 3 (ceilings are in good condition.) Framed-in and dry-walled chases were added to the stairwell and at a corner. Ceilings were opened to run the ducts on each floor. Each room has 9″ round diffusers. The single return register, about 18×18″, is cut into the ceiling between the 3rd floor and attic, at the top of the stairwell, just under the end of the air handler.

The unconditioned attic space has about 6′ of floor which is about 5′ under the bottom of the ridge beam. The attic runs from gable to gable, about 30′. The rafters are 2×6, supporting 3/4×9 roofing planks, gapped about 3/8″, covered with tar paper, original slate on top. The AC ducts snake throughout the attic to the chases and are mostly insulated with about 1″ of a material with a black plastic surface. Fiberglass insulation was randomly stuffed in to a few places.

With the AC off, 3rd floor rooms will get to over 95 deg on a hot day (I’d guess the attic gets to at least 140.) The intake fan is very noisy.

After educating myself about all this with GBA articles, sealing and insulating the unconditioned space seemed the obvious approach. But after checking the existing attic space, I don’t see how to add enough material to achieve R-38, clear the air handler, and allow service access. I’d like to relocate the air handler/exchanger to the basement, but don’t see how to do that without abandoning or ripping out most of the existing duct work and adding new ducting – a major cost/hassle I’d like to avoid. Any suggestions for an overall system approach would be appreciated.

Thanks in advance

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  1. user-2310254 | | #1

    Exterior rigid foam would be one solution, but I can understand why you wouldn't want to use that approach if your slate is in good condition.

    R-49 is code minimum for Zone 5. Do you have room to install gussets to the rafters? If there is enough space, you could probably get close to code using a combination of closed cell foam and air permeable insulation. (Perhaps Dana will chime in with his thoughts.)

    As for replacing your AC, I would recommend hiring a third-party HVAC engineer. He or she can work with you to do an accurate Manual J and spec a system that delivers optimal comfort and efficiency.

  2. pghbob | | #2

    Half of the original slate roof removed when the previous owner had it replaced with asphalt shingles to repair a small leak in the roof. I plan to put in a dormer on that side and restore with new slate over all. The other half is in good condition, but at 90 years, is questionable. So, exterior rigid foam is not out of the question.

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