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Ducted minisplits for an older house

Phillip_Bullard | Posted in General Questions on
I want to install AC in my 1925 stone twin in Philadelphia (zone 4a, I think). It has a hot water radiator system that we plan to keep, but no ductwork. It has replacement windows in good shape, but little to no insulation or air sealing.  
I have a quote for a conventional system and for a high velocity system, each of which require installing an air handler in the vented attic, which I want to avoid. We are removing the back staircase and picking up some additional room on the second floor, currently planned as closets but which could house an air handler.
I am leaning toward a combination ductless minisplits, supplemented with wall units or window AC where necessary.  I have a quote for minisplits on the second and third floor. I like the firm but lack confidence they are up to speed on the ducted units; it is also around $4k/unit installed, which I understand may be reasonable but more than I’d like to spend, if possible. I have a contractor who is an HVAC tech and am thinking if I can get the design firmed up then he and I can handle the installation. I have a whole house Manual J as well as a  room-by-room calculation for each of the five bedrooms and the downstairs spaces I want to cool. I do not object to purchasing a Manual S and/or D, but wanted to get a reality check here before proceeding, and possibly some direction. 
I understand that the system(s) will perform best if slightly undersized, and that connecting indoor units 1-1 with outdoor units may have a slight efficiency gain. I don’t mind building soffits to hide ducts, but would like to limit duct size to ~8″ round, if possible.
I am trying to evaluate the following options
Option 1: A conventional system for the first floor, with ductwork in the unconditioned basement; and
a ducted unit in the closet of bedroom 3 connected to its own outdoor unit, serving that bedroom and Room 18 (also a bedroom);  combined cooling load of 11,155 BTU/hour; and
a second ducted unit in one of the rear closets on the second floor, also connected to its own outdoor unit, servicing either (Option 1-a) bedrooms 1 and 2 (combined cooling load 8,894) or (Option 1-b) bedrooms 1 and 2 and room 14 (combined cooling load of 14,430). With Option 1-a, room 14 (currently planned as an office, but may eventually become a bedroom) would require either a window unit or a separate wall mounted head; if the latter, it would need to be connected to the same condenser as the ducted unit. It’s not clear though if I can get a duct up.
Option 2 (all minisplits, no conventional system): a larger ducted unit in the closet of bedroom 3, still connected to its own outdoor unit, servicing bedrooms 3 and 18 and also the living room (combined cooling load 17,438); 
a larger ducted unit in one of the closets on the rear of the second floor, connected to its own outdoor unit, servicing bedrooms 1 and 2 (and possibly top rear bedroom), and also servicing the kitchen and dining room (combined cooling load 18,421 or 23,957). 
Option 3: same as above, but with kitchen and dining room on their own ducted unit with separate condenser (for a total of 3 outdoor units); combined cooling load 9527. If we peel off the LR from the upstairs system and give its own wall unit, I could have the entire downstairs on a separate system (combined cooling load 15,810). 
Option 2 requires only a single air handler for the rear of the house (maybe an LG LV241HV4?). The ductwork required to reach the two bedrooms on the second floor is minimal. I would like to run a duct along the wall between the kitchen and dining room, so each room could have two registers, but I don’t know if a ducted unit can support that much ductwork. Option 3 has the advantage of allowing us to turn off most or all of the downstairs overnight. 
So glad I found this website, thanks for any help you can provide. 

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  1. Expert Member
    Akos | | #1

    It would be good to post the full ManJ for your place for the individual rooms. The cooling load comes out suspiciously close to 500sqft/ton, which feels like somebody had their finger on the calculation scale.

    Taking a quick look at the layout, I would tend towards option 2 but with some modification. I think you can put a slim ducted unit into the ceiling above the landing or the bathroom on the second floor and have a pretty simple run to all the rooms there and the 3rd floor.

    For the main floor, I would suggest first and foremost insulate your basement. This will probably cut your winter heating bill by 15% to 2o%. Installing the main floor unit there and running the ducts in the now conditioned basement is not a problem.

    I would not go for more than 2 outdoor units, it starts looking messy for not much gained. There are pretty simple ways to do a bit of zoning with ducted units if needed.

  2. user-1072251 | | #2

    Start by conditioning your basement and air sealing the rest of the house! Basements drive the comfort of the house much more than is understood. Having converted a few old homes and built nearly twenty new ones over ten years, I believe strongly in minisplits, but heating an old house (or a new one) with an unconditioned basement is like trying to stay warm on a cold winter day while in bare feet on cold concrete.

    1. Expert Member
      Dana Dorsett | | #3

      +1 (What Bob & Akos said!)

      It's often too difficult & expensive to retrofit insulate the above grade walls in finished rooms of an old stone house, but air sealing and insulating stone foundation basements is fairly straightforward, and well worth it.

      I also agree that both the heating and cooling load numbers in that Manual-J are suspiciously high, especially since we're not dealing with leaky 95 year old single-pane windows. Hopefully those replacement windows have a low-E coating of some sort(?).

      If you're not sure, hold a candle, lighter, or single-LED flashlight up to them at night and look at the reflections. There should be two pairs of reflections (4 total). If they're all about the same color it's unlikely to have any low-E- the reflection off a low-E coating will usually have a significantly different color. It'll be either on the exterior side of the interior pane (called "surface #3" in window-nerd speak) or the interior side of the exterior pane (surface #2.) Which surface the low E coating is on makes a slight difference, but that affect is small compared to low-E vs. clear glass.

      The Manual-J summary posted doesn't break down the contribution from glazing, but in high-mass construction the peak cooling loads are usually heavily driven by the glass. ANY type low-E coating (with a few notable exceptions) is going to cut the glazing portion of the cooling load by about half compared to clear glass double-panes, sometimes more. High-gain low-E double-panes are available, but the exception rather than the rule.

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