Feedback for a 2-zone minisplit plan in Zone 4C
Hi GBA – I’ve been working my way through a medium-depth remodel of a largely uninsulated 1955 house in Seattle, WA, and it’s time to tackle the heating for my main floor. Current heat is from a 60k Btu/hr natural gas furnace blowing through poorly-insulated ductwork in a vented crawlspace. In addition to being more than 3x oversized, the furnace is located in a utility closet that I’d like to convert to a master bathroom. I’ve attached the following:
1. Fuel use analysis from 2/5/2019 – 3/14/2019. This period includes possibly the coldest couple weeks we have had here in a decade. I read my gas meter once in the morning and once in the evening, converted to Btu/hr, and assumed that 5% of my gas was used by my water heater, my furnace was running at its rated 94% efficiency, and 5% of the heat was lost in the ductwork. Conservative assumptions, but enough to show an ‘upper bound’ on my heating needs of around 18k Btu/hr, 15k Btu/hr if I go with a 95% design temperature which is was 33deg F for my location (West Point Lighthouse).
2. Coolcalc results, which show a heating load of 33k Btu/hr. Their number is bananas to start with when compared to my fuel use, but even stranger when considering that the tool didn’t even have options for me to choose “no” insulation in the wall cavities, so it’s using R-13. Not sure it properly captured my uninsulated floor over my vented crawl either, but at this point given the fuel use I think we can disregard coolcalc.
3. Hacky sketchup plans for the post-remodel floorplan, with proposed locations for the high wall unit in the front living/dining room, and a horizontal ducted unit in the crawlspace serving the master bedroom and bathroom, music room, and office.
Summary – I preliminarily am leaning toward a Fujitsu AOU18RLXFZ outdoor unit paired to a ASU9RLF1 wall unit in the great (more like a good) room, and a ARU9RLF hitting the four back rooms.
A couple thoughts:
1. I expect that the community here will have some comments on the wisdom of having the ARU9RLF pushing air into four ducts given that air will flow from the master bedroom to the master bathroom, and from the music room to the office, but until I can get some insulation into these walls I think the heat lost through them by rooms without supply will exceed the heat that comes in from circulation.
2. I’ve got a local guy who does commercial HVAC work during the week and moonlights doing minisplits on the weekends. He can make $500 doing a single zone high wall unit so he turns up his nose at running ductwork for a horizontal-ducted unit, but if I run all the ductwork and the refrigerant lines he’s ok just coming out to charge them and getting everything running properly. Seems like the stuff I’m on the hook for should be within my abilities but I welcome any feedback.
3. I get that part of a Manual J is the room-by-room loads, and I haven’t done that. If I had even relative room-by-room loads I could scale my fuel use to that and get something accurate for sizing the wall unit vs. the horizontal ducted unit, and getting the right airflow to each room on the ducted zone. Given my struggles with coolcalc though I don’t think that such an analysis is within my abilities or my time, and I doubt that hiring someone is within my budget. Maybe I’m putting too much credence in the fuel use, and the reports from this site that most not-egregiously-missized minisplits will modulate pretty well…
4. In addition to insulating the walls, I need to seal up the crawlspace. I know, I’m working on it.
5. The ‘great’ room has a cathedral ceiling that slopes from 18 feet to just under 14 feet in the middle. From what I have seen – looking into an existing range hood exhaust vented through the ceiling, but without taking down drywall – the ceiling is vented and uninsulated cathedral; literally no insulation above the drywall. I’m also starting to see what looks like the effects of condensation on that drywall, on the paint on that drywall, and the hardware that holds that drywall to the rafters. I am working on a plan to fix this too. At this point though it’s all in the budgeting (raising 4 kids in this town is killing me) and sequencing for when drywall comes down and what needs to happen before it goes back up, hopefully to stay. Also, my wife really wants a new kitchen and bathroom, and a lot relies on getting rid of that massive noisy gas unit and getting some minisplits going.
Thoughts? I’m sure I’m leaving some stuff out and that my drawings, fuel use, etc are not industry standard ways of communicating, so thanks in advance to anyone who digs through them. Do I get a Dana stamp of approval, or better yet, advice that saves me from making any major mistakes??
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