GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

AC design questions

rhl_ | Posted in Mechanicals on

I’m guessing this is a Dana question.

I ran a Manual-J calculation for my house. I did it in the state “before I purchased the house” and after I am done with all my insulation work. I feel the models are decent, (as an aside, the only trouble I have is the house is currently quite leaky, and i dont know how to estimate what the results of future air sealing will look like).

For cooling loads, I see a breakdown like this (in Btuh):
Post Insulation:
17K Downstairs
9k Upstairs
~2k Basement

Pre insulation:
25k Downstairs
19k Upstairs
~2k Basement

Our living room is large ~400 sqft, with a 16ft at it’s highest point “frustrum” ceiling, and a large 32 sqft window. That room has a post-insulation load of about 10k (pre ~14k). All the remaining rooms of which there are 3 rooms downstairs and 3 rooms upstairs have loads in the 1-4k range. This is excluding closets and bathrooms and hallways.

The question becomes what is a reasonable + cost effective way to add air conditioning?  I’m trying to get something decently efficient, ideally, i can keep operating costs low, but i obviously also want to keep a cap on install costs.

My house has 2×4 walls, which rules out most AC ducts, except the high velocity ones.

That is, it seems like I should make one 10k-14k ductless unit with its only matching outdoor unit for the large living room, but for the remaining rooms, it seems crazy to buy 6k BTU ductless heads. What are my options? I guess its that or ducts, or bust, right? How should I approach this?

EDIT: My current system is in wall air conditioners, most operate at EER like 8 or worse. If I turn them all on, my electricity usage is like 5KW. So I definitely want to improve on that benchmark. That doesn’t seem hard, even at EER 11 i’m beating that benchmark by about a factor of 2, but, if it’s possible to do better, than I would like that.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    You’re right, this is a question Dana can probably give you the best answers to. I haven’t seen any posts from him today so perhaps he has taken some much deserved time off :-)

    Anyway, he’s going to need some extra info from you to get you the best info such as what climate zone are you in, and what type of heating system do you currently have? The better the info you have, the better the results you can get.

    I can tell you that you’ll probably have most of your air sealing done at the same time as you finish your insulating since it makes sense to do those two things together while you have things open, and while you’re squished into all those tricky little confined spaces. Trust me that you don’t want to make two trips into nasty attic spaces! If you have lots of air sealing to do, it’s worth buying the great stuf gun and using the “profession” cans instead of the cans with the disposable straws.

    There are regular ducts that will fit in 2x4 walls. Those ducts are 3.5” x (other dimensions). They’re made to fit in the cavities inside 2x4 stud walls. I recommend you not put any ducts in exterior walls though since the ducts will displace insulation. You probably won’t need the high velocity systems.

    Mini splits also avoid the need for ductwork, and it’s much easier to run the refrigerant lines in limited space.


  2. rhl_ | | #2

    We are in Climate Zone 4A, and we have hydronic heat. I’ve looked into air source to water heat pumps. I’m definitely interested, because bundling mechanicals makes a lot of sense to me, but, not sure it’s mainstream enough yet.

    I understand that air sealing will take place and all of the air sealing and insulating work would be carried out by my passive house builder friends contractor, but, I don’t think anyone wants to try to provide expectations of the results..

    1. charlie_sullivan | | #5

      The Chiltrix system would be great for you, but it's also true that finding the right contractors do it it well is not easy. It kind of depends whether you want to help blaze that trail or wait until others have done that for you.

  3. Expert Member
    Akos | | #3


    The cheap way is one wall mount in the living room up high and one wall mount in hallway upstairs. This has comfort issues, need to keep doors open and some of the rooms furthest from the head will be hot.

    For better comfort wall mount in the living room plus a ductected unit somewhere. Location depends on layout of the rooms upstairs and downstairs and where you want bulkheads. The basement I find typically needs some dehumidification only.

    If you want to keep them inside your wall you can go with either oval pipe or square ducts. You have to be careful with both as some of the fittings are extremely restrictive. Good design here will pay off on having a comfortable house. The key with ducted units is that you don't want lots of bends/transitions. Something on the order of 2 90deg bends.

    If you don't mind bulkheads, I find working with semi rigid aluminum duct the easiest. Its a bit more restrictive than straight pipe (%15) but you can do bends in them that actually flow unlike with flex duct. Not the cheapest, but you can get it long length and do the entire run without fittings.

  4. rhl_ | | #4

    If there is a way to have a “manifold” where ducts can exit any direction then I can do all the runs with just straight paths in the attic. My guess is that the most cost effective thing is ducts for the small load rooms.

    Can you use like a plastix product or something? Something like the zehnders use?

    1. Expert Member
      Akos | | #6


      Ducts in the attic are swear words around here.

      Some of the manufacturers make plenums for their ducted units, that would be your easiest solution. You can also get an HVAC sheet metal shop to make a plenum and use standard takeoff fittings for your runs. I've done DIY plenum and it is a pain in the &^%.

      As for the pipe, you can use whatever your local inspector is OK with. The important thing is to watch for losses.

    2. Expert Member
      BILL WICHERS | | #7

      Yes, you can — a “manifold” in ductwork is known as a “plenum”. It’s basically a big main duct and individual lines branch off of it, often with a damper in each branch run. I agree with the other poster, don’t try to make your own plenum. Call local HVAC contractors and find one with a sheet metal shop for making ductwork (this is common for commerical contractors). Have the contractor fab the plenum for you with all the cutouts for branch runs, then just do the install part yourself. Sheet metal fabrication requires special tools and metalworking skill.

      I’d use rigid metal duct as much as possible for best flowrate. Try to avoid flex as much as possible, and don’t put ducts in the attic! Attic ductwork is notorious for air leaks and is an energy loser. Maintenance is a pain too since attic work of any kind just plain sucks. Try to put your plenum in the basement/crawl space and branch off and go up. You’ll still want to seal all the seams with tape or mastic.


      1. rhl_ | | #8

        Hm. This approach is challenging indeed. We don’t have many interior walls to play with. Also the ducts would clearly need to make many turns to achieve getting from basement to 2nd floor.

        1. Expert Member
          BILL WICHERS | | #9

          One trick I’ve used in the past is to run ducts up columns, if you have any. You can either squeeze the ducts into existing columns, or enlarge the columns slightly to get the space you need. Obviously this only works if you actually have any columns :-)

          You do want to keep duct runs as straight as possible. Excess bends can be somewhat compensated for by dampering down other ducts, but it’s not ideal.

          Putting utilities in renovated structures is something of a 3 dimensional puzzle to challenge your mind. Sometimes you need to be creative to find a solution. What kinds of things do you have in your house that you can use to hide things like ducts? I’ve used walls, columns, the inside part of vertical steel girders, sometimes built-in cabinets can be shortened a bit in the back. See what you have that might work. Every site is different.


        2. charlie_sullivan | | #10

          In my house, there was a chase for the oil-boiler chimney that we could use for HRV ducts once the oil boiler was removed. Any opportunities like that?

          I keep thinking of how well Chiltrix would work for a situation like yours. And then I think of the fact that in 12 years of living with a hydronic heat-pump system, I've never found a contractor who really understood it...

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |