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Choosing an HVAC System

Jonny_H | Posted in Mechanicals on

Hey y’all, looking to crowdsource some opinions on HVAC for my renovation.  Just got Manual J load calcs done by Energy Vanguard, but had them hold off on equipment selection / duct design because I have a few options that I want to consider — but now I can’t make up my mind!

The house is 2-story with full basement, and I’m doing extensive work on the first and second floors; the basement sucks but fixing it is a later phase of this project (read: probably a couple years down the road).  2nd floor has 2 bedrooms, laundry room, bathroom, and an open hallway / study / staircase area — heating load 10k BTU/h, cooling load 5k BTU/h; interior completely gutted.  1st floor has 2 bedrooms, bathroom, and an open-ish living / dining / kitchen/ traffic circulation area, also open to the 2nd floor staircase — heating load 16k BTU/h, cooling load 10k BTU/h.  Basement adds another 10k BTU/h heating load and no cooling load.

Plan 1: My initial idea was a ducted mini-split serving the second floor; based on these calcs it could be probably be the smallest (9k) Fujitsu Halcyon unit.  A second ducted unit located in the basement could serve the basement and first floor — though I’m not sure of the implcations of either (a) sizing it for the full current load, but then insulating the basement later, or (b) undersizing it and just letting the basement be cold.

Plan 2: The 1st floor bedrooms, bathroom, and hallway sum to ~3.5k BTU/h heating load.  The remainder of the first floor would then be close to 12k BTU/h heating load.  That tempts me to (a) make the 2nd floor ducted unit a Fujitsu 12k unit and also serve the two first floor bedrooms (BUT the ductwork would be quite challenging if even possible) and (b) use a 12k ductless unit for the remainder of the first floor, centrally located in the basically open area.  The basement gets neglected in this plan, and the ductwork for the 1st floor bedrooms might not be possible, but it is semi-tempting.

Plan 3: The wildcard here is that I have a 2.5-ton-ish water-source heat pump unit in my garage.  It’s a Climatemaster Tranquility single-stage unit that I got new but super cheap at a surplus place — BUT it’s a commercial 480V 3-phase unit that I got running with a transformer and a VFD from the same industrial surplus place 😉 (the VFD *might* let me fake multi-stage or continuously variable capacity — but I’m not sure if its specific compressor would be OK with that).  Anyway, unlike most geothermal installs, I already have the unit and would DIY most of it, but I’d still need to have the outdoor part installed — due to not wanting to kill all my trees, I’d either need to go vertical wells or directional boring.

So, I have three (or more?) potential plans, all of which will cost several thousand dollars (either buying air source heat pumps, or paying for the ground-source wells / directional boring install), and all of which are complex in varying ways.  I think my original plan has overall the most straightforward ducting, but plan 2 is tantalizingly elegant.  Plan 3 mostly justifies my impulse buy of a commercial surplus heat pump 😉

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Replies

  1. DCContrarian | | #1

    The drilling is typically the lion's share of the cost of a ground-source system.

  2. Walter Ahlgrim | | #2

    Designing drilling and installing the pumps is lion's share of the cost of a ground-source system.
    You may find this forum helpful it is dedicated to ground source HPs.

    https://www.geoexchange.org/forum/

    The skeptic in me suspects when people say how little it cost to run a ground source system they conveniently forget to include the cost for running necessary the water pumps.

    When I see them post loop temps at the end of the heating season they always seem to be below 32° far from the 55°in the sales pitch.

    Walta

  3. Kyle R | | #3

    I would vote plan 1. With these units I would try to keep the ductwork as simple as possible. What is your design temp? Do you need a base pan heater?

    1. Jonny_H | | #4

      Sorry, forgot to mention Zone 5 / near Cleveland, OH. Design temp used for the Manual J was 10F. Would need a cold climate unit with a base pan heater from my understanding.

  4. Kyle R | | #5

    In my opinion you are on the edge of whether or not you need a base pan heater. The only small 1:1 ducted mini split that I know with a base pan heater is Carrier (Midea). I live in Zone 5 (Michigan, design temp 5F). I have a ducted Fujitsu for the upstairs (no pan heater, not an option for 1:1), but I have a ductless (with pan heater) on the first floor which actually heats the whole house. The upstairs unit actually cools the whole house in the summer.

    1. Jonny_H | | #6

      Hm, your setup sounds like my plan 2 -- maybe your first floor is more open than mine is? what do your loads & room layout look like? Do you have a basement?

      I only just realized the differences in outdoor:indoor unit pairing -- I had naively thought there would just be one set of outdoor units that would pair with any style of indoor units -- so I'd be able to get the cold-climate versions with ducted indoor units. I was specifically looking at the Fujitsu units because their heating capacity is substantially larger than the cooling capacity, which seems like a better match for my load conditions.

      While my design temp is 10F, we have on occasion had several-day polar vortexes of single-digit temperatures. Record low is around -20F, but below-zero temperatures are pretty rare.

      I wonder if a modified Plan 2, with a ductless (ie 12LZAH1) on the first floor and ducted (9RLFCD?) on the second floor, leaving the first floor bedrooms (actually more likely to be used as guest room / office / misc kids stuff type rooms) to fend for themselves, possibly with a bit of supplemental resistance heat -- or incandescent bulbs, the heat load for the larger of the rooms is only 500W. Not sure of the implications of leaving the basement chilly... A third mini-split seems excessive though.

  5. Kyle R | | #7

    My house is a 30x40 2 story rectangle with full basement. Basement is insulated but unheated (never gets below 60 F). My first floor is completely open except for a half bath and laundry room. I have a 15k Fujitsu cold climate floor mount. Open stair way to upstairs which has three bedrooms and 2 bathrooms. I have a 12 Rlfcd with one register in each bedroom. BTW there is very little difference between the 9 and 12 rlfcd.

    I would give the Carrier units a hard look. They are rebranded Mideas which is one of the largest HVAC equipment companies n the world. At least where I live there are way more contractors that carry Carrier than Fujitsu. I am also not thrilled with Fujitsu’s “thermostats”. The units work great, the controls leave something to be desired. I imagine the Carrier units would be more similar to a conventional thermostat at minimum I bet they are better than Fujitsu.

    Given a chance to try again, I would have gone with a ducted Carrier on each floor.

    1. Jonny_H | | #8

      Hmm, if your first floor is basically open, what would be your motivation for going ducted rather than ductless?

      I'll take a closer look at the Carrier offerings -- they hadn't really been on my radar as everyone seems to talk about Fujitsu, Mitsubishi, and Daikin most often.

      1. Kyle R | | #9

        Two reasons really other than the controls issue I already mentioned. 1) With a ducted unit you have a standard disposable air filter. Ductless heads will require being taken a part and cleaned at some interval. 2) When I do put the house up for sale a ducted system would be easier for most buyers to understand and accept.

  6. Jonny_H | | #10

    Update: Just had a call with EnergyVanguard. Forgot to ask their opinion on the base pan heater thing, but it sounds like the direction that makes the most sense is the Plan 1 -- all ducted systems. Even given my mostly-open first floor plan and above-average envelope, they were concerned that it was spread out enough that a single ductless unit wouldn't distribute air adequately enough for comfort in that space -- let alone in the first floor bedrooms. My main attraction to ductless had been ease of install / the elegance of not having to deal with ducts, but it sounds like there'd be too many compromises. Will update again for posterity when I have a final plan, but that seems to be the direction I'm headed.

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