Feedback on DIY HVAC Plan
I’m building an 800 sq ft single story guest house in Phoenix AZ almost entirely DIY. It is the first house I’ve ever built, but my goals are to reach a high performance threshold like Matt Risinger, only with 1/10 the budget (if that) and zero experience. What could go wrong?
I worked up a tentative HVAC plan for the house and want feedback on it, if possible. One overriding goal is to have maximum performance for the absolute smallest amount of money and do it 100% DIY.
I’ve attached the floor plan of the house along with one possible layout of the HVAC components.
Some background: I did an online Manual J for the house and it claims about 11937 total BTU cooling. 5293 of that is the glass doors, since there is a lot of glass on all walls except the East — almost 40% of the North wall is glass! 2400 is in “appliances” which may or may not apply — this guest house is forbidden from having a stove, so the fridge, hot plate, and toaster oven are going to be it. Will they add 2400 BTU like a typical 220v or gas stove would? Don’t know.
I am almost surely going with a Mr Cool DIY option since I will be installing it myself, and they dominate in that space. The Manual J suggests 12k BTU but maybe oversizing to 18k makes sense? What if the Manual J isn’t good enough; the temp tops 120F; and the 12k can’t handle it? I flip-flop on that.
I’m proposing mounting the wall unit in the foyer, just above the bathroom door. This is mostly because it’s the high point in the house — the foyer has 9ft ceilings. The bedroom/bathroom/kitchen have 8ft ceilings and the great room has an 8ft soffit surrounding it, with only the center at 9ft. So it “fits” best above the bathroom door.
I have notable concerns about that. First, the instructions explicitly say to keep it away from doors and it’s literally by three of them. Second, is that one unit going to evenly cool the entire space? Am I going to need to put a transfer duct between the bedroom and great room? I was willing to pay an expert to design this part just to ensure that it works evenly, but couldn’t find one that would work so small… but yeah, very concerned that this is sub-optimal.
For ventilation, I’m shooting for around 20cfm and one pair of Lunos e2 HRVs should handle that. I’m thinking of putting one in the bedroom and one in the great room so that it pushes and pulls air back and forth through the house. Maybe that would even be enough to mix the conditioned air and negate the need for the transfer duct? Those locations are picked because that’s where I want the fresh air the most and, coincidentally, they are very shaded so hopefully they will have the coolest possible air to start with for their cycle.
Then there’s the bath fan in the shower. I’m planning on a bog standard Panasonic Whisper style fan that is either timer based or humidity based. No ERV/HRV, though – an ERV/HRV with enough cfms would easily double or triple my entire HVAC budget!
So hear me out: My thinking with the bath fan is that it will definitely be exhausting air faster than the house “normally” can provide it, so it will take the easiest input route… which is whichever of the two Lunos e2 tunnels is actively pulling air. That only does 20cfm and the bath fan does over 50cfm, so wouldn’t it just suck in air faster than the Lunos e2 can cool it, but it would still be fresh and filtered air? Does that make sense or is this just nonsense?
I did debate putting another exhaust fan through the wall behind the toilet, for odors. But I’m gambling that the one fan in the shower should be enough for that, too.
There will not be an exhaust fan in the kitchen. Like mentioned before, this cannot have a stove. There might be fumes and such from the hot plate and toaster/convection oven… but if it’s an issue, then I can either retrofit a recirculating hood or (more likely) construct a filter of some sort to handle the problem.
I’m open to literally any thoughts y’all might have on these ideas.
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