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Advice on getting a new A/C system

Gary__B | Posted in Mechanicals on

Greetings, and thanks in advance for help on my first post.

I own a rental house here in Portland OR. It’s a simple, small ranch-style home, about 1300 sf with 3 bedrooms and 1 bath. Built 1960, unconditioned attic with a low-pitch roof, dirt crawl space foundation, very limited insulation in walls and ceiling. Currently the only HVAC is wall resistance heaters in each room and a window a/c in the kitchen/dining area. I’m looking to give the tenants some immediate relief from the impending summer heat, and also more comfortable heating in the winter.

So my going-in assumption is get a minisplit system installed. While I spend far too much time reading all the helpful articles and posts on GBA about such things, I still need a general “lay of the land” to get me going. So a few questions/confirmations:

1) Ductless minisplit? With the unconditioned attic this seems the obvious choice. 
2) I’m thinking 4 zones inside. Each bedroom would be a small unit and a larger one for the combined kitchen, dining, and living room. I know I could cool multiple rooms with a zone, but I prefer to keep this simple (avoid ducting between rooms) and give the tenants control over each room. 
3) Is there a preferred manufacturer(s) to look into? I see a lot of talk here about Fujitsu. 
4) Is DIY out of the question? I do just about everything myself, and it doesn’t seem particularly hard (except of course charging the system), but is it a bad idea?
5) Lastly, and obviously, I know I need to do air sealing and more insulation. That will come in due time, but unfortunately I’ll need to get this system in first then worry about improving performance of the house. So any suggestions with an eye toward the future are helpful.

Thank you,
Gary B.

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Replies

  1. Jon_R | | #1

    Consider converting the crawlspace to conditioned and putting ducted mini-split ducts there.

    1. Gary__B | | #3

      I'd love to, but I don't think it's feasible. The crawl is 18" high or less for much of the house. I've done some plumbing down there, and it's an awful place to work, army-crawling around with tools and materials. That, and very uneven terrain and posts that sit on pier blocks that are resting on top of the dirt, which I think would make installing a vapor barrier futile.

  2. Expert Member
    Dana Dorsett | | #2

    "1) Ductless minisplit? With the unconditioned attic this seems the obvious choice"

    Not really. See Jon R's suggestion.

    But really just air sealing and insulating the place makes a huge improvement in overall comfort, and it could come in less than the cost of a single 3/4 ton name brand ductless.

    >"2) I’m thinking 4 zones inside."

    The smallest 4 zone multi-splits are 3 tonners, which would be ridiculously oversized for the loads of a 1300' house with R13 cellulose in the walls and R30 or more of fluff in the attic, and clear-glass storms over wood sash double-hungs. Even without addressing the envelope deficiencies probably looking at ~1.5 tons of cooling load even with the house as-is, and a design heating load of less than 20,000 BTU/hr. If you tighten that up with air sealing & insulation you'll have a cooling load of 1-ton or less, and a cooling load than 13,000 BTU/hr. A 3/4 ton or 1-ton ducted Fujitsu with ducts in the crawlspace would probably do it, and a 1.5 tonner would do it even without converting it to a conditioned crawl.

    How deep are the ceiling & floor joists?

    Do pipes in the crawlspace ever freeze?

    If the floor plan is amenable it may be possible to right-size a mini-ducted zone to cover the low load rooms, with a single right-sized ductless head in the more open space, either as a pair of mini-splits or a 2-zone multi. A cold climate 2-zone multi- is worth more than 20,000 BTU/hr @ +17F (8F colder than your 99% design temp), and more appropriately sized for your actual loads:

    https://nonul.mylinkdrive.com/files/MXZ-2C20NAHZ2_ProductDataSheet.pdf

    https://ashp.neep.org/#!/product/29024

    http://portal.fujitsugeneral.com/files/catalog/files/24RLXFZH3.pdf

    https://ashp.neep.org/#!/product/25329

    I'm not sure if there is any subsidy money in Oregon or locally in Portland for doing the insulation or converting to heat pumps, but retrofit air sealing and insulation is pretty cheap. In a crawl space it can be cheaper (and definitely better) to insulate the foundation and put down a ground vapor barrier than insulating between the joists. In WA there is usually subsidy rebate money for heat pumps, depending on the utility.

    1. Gary__B | | #4

      1) See above. Unfortunately converting the crawl space is probably out. Running (sealed and insulated) ducts down there might be possible, although very difficult. Air sealing and insulating would go a long way, but unfortunately Portland as of late tends to get pretty miserable for extended stretches in the summer. We just cant count on cooling down at night and keeping it through the day like we used to.

      2) Thanks for the thoughts. I'll definitely factor some insulation/air improvements into the sizing and do that concurrently. Does your advice still hold if I ran ducts in the attic instead? As I note, it's next to impossible to work in the crawlspace.

      It's been awhile since I've been in there, but floor joists are only 2x6 I believe, I think just 2x4 ceiling joists (trusses). I've had no problems with pipes freezing. The floor is uninsulated and pretty leaky, and since the crawlspace is so short and Portland mild, I think it keeps warm enough.

      1. Expert Member
        Dana Dorsett | | #5

        >" Does your advice still hold if I ran ducts in the attic instead? As I note, it's next to impossible to work in the crawlspace."

        Yes- insulate the place, make the duct system balanced & air-tight, they BURY the ducts in the insulation when you insulate the place. Portland has low outdoor dew points in summer, with effectively zero latent loads, and would not have condensation issues with ducts buried in fiber insulation, especially with high SEER systems such as mini-splits.

        Calculate the loads- don't guess, and don't just take the WAG of some guy on the internet. Even a freebie online tool like loadcalc.net is better than a hunch, a rule of thumb or a WAG. When using those tools assume the house and ducts are a lot tighter than you think they might be right now or you risk oversizing. (Loadcalc comes up with numbers 25-35% higher than a professional aggressive Manual-J on my house, even more if I assume anything but a very tight house with very tight or no ducts.)

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