Mini-split system design questions
Hi folks, a few questions about a nonstandard mini-split system design.
Background: A friend in western New York has a two-story single-family home. The basement has humidity issues. He doesn’t have air conditioning. He has a new, highly efficient natural gas furnace with a variable-speed distribution fan that serves the whole house. The distribution fan can run when the furnace is off, providing ventilation and mixing air throughout the house. The distribution fan no return ducts; it pulls air from the basement, which in turn pulls air from the first floor via the basement stairwell. The door to the basement is always open.
We have load estimates from multiple reliable sources that all place his whole-house design heating load in the range of 34 to 38 thousand BTU/h. (That’s at the local 99% design conditions of 70 F indoor, 0 F outdoor.)
Goals: My friend has two primary design goals. First, he’d like to get his basement humidity under control. Second, he’d like to add air conditioning to at least a portion of the house — he’d be happy with air conditioning on the second floor.
Beyond those primary goals, he’d also like to reduce his carbon footprint from heating. Electricity in his area is very clean, so displacing some natural gas heat in favor an electric heat pump makes sense. Because the heat pump would provide heating displacement, rather than replacing the (new, efficient) natural gas furnace, we’re thinking we’d size it to run efficiently during typical winter conditions: 65 F inside, 30 F outside. We estimate the whole-house heating load in those conditions at 17 to 19 thousand BTU/h.
Design options: We’re open to suggestions, but here are two designs we’re considering.
1. Mini-split indoor unit in the basement. It pumps heated or cooled air into the basement, which then gets circulated through the house by the furnace distribution fan. In summer, the heat pump also wrings moisture out of the basement air.
2. Mini-split indoor unit on the second floor. Run the furnace fan during the summer, exchanging some of the cool/dry air from the heat pump’s floor with the moist air in the basement. Get some basement dehumidification indirectly.
Questions:
A. Which design would you recommend? Or would you suggest a different design?
B. Suggestions for a good DIY mini-split brand? We’ve heard mixed things about Mr. Cool. We don’t know much of anything about the alternatives. Anyone have experience with DIY Pioneer, Senville or Daikin mini-splits?
C. Sizing suggestions? We’re currently thinking a model with either 9 or 12 thousand BTU/h of rated heating capacity should work, but we’re open to suggestions.
Thanks in advance for any help you folks can offer!
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Replies
>"The distribution fan can run when the furnace is off, providing ventilation and mixing air throughout the house. The distribution fan no return ducts; it pulls air from the basement, which in turn pulls air from the first floor via the basement stairwell. "
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>"My friend has two primary design goals. First, he’d like to get his basement humidity under control."
The first thing to do is to provide low-impedance return paths for all doored-off rooms to the furnace return. Without it air-handler driven pressure differences between rooms are going to use "the great outdoors" as part of the return path, dragging in humid outdoor air in summer, even if the basement door is open.
If it's just one common return located in the basement running the fan is actively de-pressurizing the basement relative to the outdoors, so air sealing the basement walls (and dryer vents, etc) is critical for getting this under control. Insulating the foundation walls to a code-min R15 continuous insulation from the slab all the way up to the subfloor would be in order, since the cool foundation walls and anything in contact with those walls will take up that moisture to levels that support mold growth, the main cause of "musty basement smell".
Any schemes for spreading AC-dried air with the duct system is bound to fail UNLESS the deficiencies of the duct system are addressed, along with the outdoor air leakage into the depressurized basement. Creating jump-duct paths for doored off rooms to where the room-to-common-area pressure differentials are under 0.02" water column would be a good start. Door-cuts would likely have to be huge, but using partition wall stud bays with grille near the floor on one side and another near the ceiling on the other side can work.
Getting the pressure difference that low to a commoned return located in the basement is going to be tough though. There may be locations in the house where floor grilles into panned joists can work, and it'll probably take more than one or two (and maybe even a louvered basement door).
An Energy Star duct system would have no more than 3 pascals (=0.012' w.c.) difference at any doored off room with the doors closed, under all air handler speeds.
>"Beyond those primary goals, he’d also like to reduce his carbon footprint from heating. Electricity in his area is very clean, so displacing some natural gas heat in favor an electric heat pump makes sense. Because the heat pump would provide heating displacement, rather than replacing the (new, efficient) natural gas furnace, we’re thinking we’d size it to run efficiently during typical winter conditions: 65 F inside, 30 F outside. "
Hot air rises- the first floor can't be heated with mini-splits on the second floor. With an open stairwell a decent amount of heat could be provided to the second floor via a first floor mini-split. For the same convective/ air-buoyancy reasons, an upper floor can't be cooled by a mini-split on the first floor. With the doors open an air conditioner on any floor will provide some amount of drying, but the basement would almost always need a room dehumidifier, since the temperature in the basement in summer would probably be on the order of 5F-15F cooler than the rest of the house. (Within the same volume of air a lower temperature = higher relative humidity.)
>" Sizing suggestions? We’re currently thinking a model with either 9 or 12 thousand BTU/h of rated heating capacity should work, but we’re open to suggestions."
Size it only for the COOLING load- forget about the heating load, and install one per floor. You're probably looking at a 3/4 tonner, or a pair of them. The one that has the lowest minimum-modulated output level is probably going to be the "right" one, independent of HSPF or SEER numbers.
None of the DIY mini-splits use cold-climate type compressors, and at 0F they won't really cut it., but most can still muster their rated cooling BTU/hr numbers for heat at +17F (pull their AHRI sheets.) But 1.5 tons of mini-split should be able to handle the whole-house load at the seasonal average heating season outdoor temperature on a house with a design heating load of 35-40KBTU/hr @ 0F, displacing more than half the total gas bill at reasonable efficiency.
Since there is a heating history on the place, use last winter's gas bills to run a fuel-use load calculation to verify the heating load estimate using these methods:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new