Minisplit quantity and location
My name is Sam. This post may not come through with my name (as past posts have not) and I cannot find a way to add it!
I am looking for advice on heating systems for a 1000sf addition on a 1922 home in Seattle. The addition will share one wall with the existing structure, and I have attached a floor plan. Everything new is black, existing is gray.
The existing house is as insulated as one might expect. 2×4 walls and roof–not a lot there and far from airtight. New structure will be R-21 wall cavity insulation. Roof will be unvented with R-30 plus R-10 rigid above sheathing.
The existing house is heated with forced air and works reasonably well. I do not wish to add forced air to the addition. I would prefer a ductless minisplit, but I worry that the floorplan is less than ideal for this system.
I would probably add one or two minisplit heads to the addition. If just one, I would probably add it to the hallway. If two, I would add one to the master and one just outside the two kids bedrooms.
To complicate the issue, the addition will have an unvented crawlspace below. I will need to provide code-minimum ventilation to the crawl space (1CFM per 50 sf for a total of 22 CFM min). Per local code, the crawl space must exhaust to the outside.
My biggest concern is that locating one or two heads in/near the hallway will be thrown off by the proximity to the existing house heating system.
我的第一个选择是在走廊中只找到一个头。然后，我会在每个儿童室的地板上找到一个转移格栅，以与爬行空间交换。然后，我会在主卧室下方的边缘托梁中找到排气风扇。从理论上讲，这将为走廊提供空气，空气将流入儿童房，然后穿过转移格栅，然后穿过爬行空间，然后穿过另一端的外墙。
Is this just wishful thinking?
另外，在这种情况下，会有充分的理由考虑ERV或HRV吗？
Thoughts or requests for more info are greatly appreciated!
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Replies
Sam,
通常的建议适用：第一步是执行加热和冷却负载计算。
我的本能告诉我，您要求一个或两个无管的迷你底片做很多事情。您正在提出一个听起来像是勉强满足代码要求的添加（无论如何，就R值而言）。如果没有异常设计的信封，我猜您将有不可接受的房间对房间温度变化 - 这种变化需要管道溶液。
如果我正在计划这项工作，我将在房屋的旧部分中包括Blay-Ore定向的空气密封，我将包括一个全屋通风系统。有关通风选项的更多信息，请参阅以下文章：
Designing a Good Ventilation System
Revisiting Ventilation
The odds are pretty good that this rectangular shaped addition will come in within the output range of a single 1 ton mini-split if it's air tight and the foundation walls are insulated, possibly within range of a 3/4 tonner.
It's unlikely that it can meed code in the bedrooms without a direct heat source, unless the windows & walls are well above IRC code min. A half ton or 3/4 ton mini-duct cassette mounted in top of the closets separating the smaller bedrooms might work, with a separate head in the MBR. The hall would then be surrounded by mostly conditioned space, with very little exterior wall exposure.
The bathrooms may or may not need some resistance electric heat, or hydronic heat running off the water heater to get there.
I am also installing a Fujitsui 2 ton unit in the stairwell of our 1600 sq. ft. Townhouse condo. I am concerned about air flow to all rooms due to the location. We have 3 bedrooms up stairs and an open concept down. Has this been a topic you can comment on. Canada