Retrofitting a Furnace Duct System for Minisplits
fourforhome
| Posted in General Questions on
I’m looking at retrofitting my house with slim ducted units
I currently have a 3.5-ton central ducted with the air handler in the conditioned attic. (3.5 ton cycles most of the time and then shuts off at 30°F) The house is two stories (like a Colonial) plus a daylight basement. All of the ducts for supply and return go down through wall cavities and those line up with floor joist bays, where the supplies are located near the center of the house. (All the ducts in the walls are hard pipe and are well sealed with mastic. All the ducts in the attic are fl** duct and have a bit more than 4% slack. I’d install mostly hard pipe in the attic myself.)
All that to say, I can’t change where the air handler/slim duct unit can go. The attic is the only option.
I’m considering one 12 and one 18 Fujitsu LUAS1 and putting the outside units either behind the garage or above the garage roof, and bringing the line sets through the garage attic and a bedroom closet, leading to the attic. How practical is this approach? Is there a better way?
Climate zone 4c/near Portland, OR. 3,700 ft², 1.25 ACH50.
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Replies
The larger lineset is not going to be a problem if lineset is not too long as then you will run into problems with oil return. If shorter lineset (I would say less then 75 feet) you will just need more refrigerant which can be calculated as Fujitsu has the formulas for linesetups up to 1" for the gas line.
Thx. I'll reconsider going that route.
What is wrong with putting the outdoor unit where the existing one is? These are so quiet that even bellow a window you'll barely hear it except maybe when it goes into defrost.
Mini splits have become very common around me with older homes that had radiant. There are a lot of line sets run all over the place and most of it looks quite shabby. Besides shabby, the routing can make for a great ladder for critters. This is not only annoying but they destroy the insulation on the lines.
You also never want to mount outdoor units directly onto wood framing. These units are very quite but they still produce low frequency vibration which carries very well through framing. Outdoor units should only be mounted onto masonry or on the ground. I would also avoid stacking them in snow country.
There is nothing wrong with flex especially if you are downsizing the air handlers. Oversized flex works just fine and is generally better for noise transfer between rooms. Even if you do all hard pipe, I would still include a length of flex just for this reason.
P.S. Good job on getting the house down to 1.25 ACH.