Ducted Minisplits vs. ceiling registers vs. standard central air conditioning
We have a 100 year old Washington DC row house with cast iron radiators. The house is 1520 square feet. It is a corner unit with the front of the house facing west and the outside wall facing south. New windows were put in on the first and second floor. There is a “sleeping porch” at the back of the house. The 1st floor framing was constructed of rough-sawn 2×8 floor joists at 16” on center, and the roof framing was built of rough-sawn 2×6 roof rafters, running from front to back. I’m attaching rough schematics.
We want to add central air conditioning (but keep the radiators). We don’t like boxed in ducts, but we also hate ductless mini splits. We think that ducted mini-splits might be the best solution because of less ductwork.
I am having trouble reconciling the bids that we have received with what I am reading here. Basically, no one is giving me anything like the detailed info that I am seeing here. Normally someone comes out (usually a sales guy), walks around the house and takes a few photos. No measurements or notes are taken. No one sends detailed plans of what the system will be like/where the ducts will be. I mostly get “we’ll put in an X ton system, that has some ducts, and it will cost you $X.” No one has mentioned a Manual J, so I’m guessing they are just sizing the system using a rule of thumb? The contractors I have contacted have come from a variety of sources (including a couple from Fujitsu’s list of contractors).
This is the most detailed proposal I received:
“The preferred option for the most control with the least disruption is a 2-zone Fujitsu mini-split system using 2 slim-ducted air handlers (ARU18s) and one AOU36 outdoor unit. One air handler will be mounted on the wall in the basement with a simple duct system feeding floor supplies and one return on the first floor with a little dehumidification help in the basement. The other air handler can be installed in the attic area or in a closet near the hallway with one main return and filter grille. This will serve the entire second floor. There will be some duct exposure to get to the rear old sleeping porch.”
These are my questions:
1. Am I correct in understanding that the company above is proposing a 3.6 ton unit for 1520 square feet? Does that sizing seem reasonable?
2. I’ve read that ducts should not be in uninsulated attics. So if we put in a ducted minisplit system, how does the air get into the 4 upstairs bedrooms if the ducts are not in the attic? If they’re inside the house, how is that different from a standard system?
3. How do I find a company here in DC that will size and install the system properly, and let me see in advance exactly how they propose to install it? Or is sketching upfront not normally done? Can anyone recommend a company in this area?
4. What is the difference between ducted mini splits and ceiling registers? Are ceiling registers conceptually the same as the ductless mini splits, except that they are recessed into the ceiling (as opposed to being on the wall)? If so, approximately how many ceiling registers would we likely need on each floor? Our ceilings are reasonably high (almost 9’ on the first floor). Would we have to lower the ceiling to accommodate ceiling registers?
I would deeply appreciate any guidance anyone can offer.
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