Context
Two-story garage (main radiant slab heat; loft apartment). Goal: cool main garage, heat/cool loft; consider future-proofing for capacity and cold weather.
Note: See Temporary Heating for the construction-phase heating solution using VEVOR diesel heaters during interior completion.
Proposed System
Mini-Split Configuration (3-Zone)
| Zone | Location | Size | Coverage | Purpose |
|---|---|---|---|---|
| 1 | Garage main floor | 18k BTU | ~960 sq ft open space | Cooling primary |
| 2 | Loft common/kitchen | 12k BTU | ~400-550 sq ft | Cooling + supplemental heat |
| 3 | Loft bedroom | 9k BTU | ~350 sq ft | Cooling + supplemental heat |
Total system: 36,000 BTU, 3-zone
DIY vs Professional Installation
| System | Equipment | Installation | Total |
|---|---|---|---|
| MrCool DIY 5th Gen | ~$4,000-5,500 | $0 (DIY) | $4,000-5,500 |
| Mitsubishi Hyper-Heat | ~$4,000-6,000 | $3,000-5,500 | $7,000-11,500 |
Decision: DIY installation with MrCool DIY 5th Gen system.
Why DIY makes sense for this project:
- Primary purpose is AC (radiant handles main floor heating)
- Supplemental heat only needed upstairs
- Installing insulation ourselves = can plan lineset routes before drywall
- Pre-charged quick-connect lines eliminate brazing/vacuum/EPA certification
- Potential savings: $3,000-6,000+
Cold Climate Performance
| Model | Min Operating Temp | Best For |
|---|---|---|
| MrCool 5th Gen Standard | -5°F efficient | This project (radiant backup) |
| MrCool 5th Gen Hyper Heat | -22°F | Sole heat source applications |
| Mitsubishi Hyper-Heat | -13°F | Professional install required |
Recommendation: Standard 5th Gen is sufficient since radiant floor handles primary heating. Hyper Heat models cost ~$500-800 more per zone.
Recommended Product
MrCool DIY 5th Gen 36,000 BTU 3-Zone (9k + 12k + 18k configuration)
- 23 SEER2 efficiency
- Pre-charged R454B quick-connect linesets
- 7-year compressor + 5-year parts warranty
- No special tools or HVAC license required
Finalize sizing after: Manual J load calculation (post-insulation)
Other Systems
- Radiant slab: gas boiler; coordinate thermostats/zoning with mini-split.
- Slab sensor: ½” PEX conduit installed for future slab temperature sensor (see Decisions - Slab Sensor Conduit); enables advanced radiant control and integration with smart thermostats.
Outdoor Unit Placement
Two location options under consideration:
Option A: Side Wall (Original Plan)
- Located on 24’ side wall near mechanical room
- Pros: Shorter electrical run from panel; close to mechanical room
- Cons: Longer refrigerant lineset runs to far side of garage (40+ ft potential)
Option B: Center-Back Wall (Preferred)
- Located on ground, centered on 40’ rear wall
- Pros:
- Shorter, more balanced lineset runs to all indoor units (~20-25 ft max)
- Copper lineset is expensive ($3-8/ft) vs. electrical wire
- Ground-level placement simplifies condensate drainage and service access
- Better efficiency with shorter refrigerant lines
- Cons:
- Longer electrical run from panel
- Must verify rear property line setback (garage is 6’ from rear property line)
Current Preference: Option B - the lineset savings outweigh the longer electrical run. Refrigerant line length affects efficiency, installation complexity (brazing, vacuum, refrigerant charge), and cost more significantly than electrical wire length.
Action Required: Confirm setback requirements with Clare County Community Development before finalizing. See Clare County Setback Research for details.
Outdoor Unit Weather Protection
Idea (2026-02-09): Build a small angled roof/shelter over the outdoor heat pump unit to protect it from direct rain, snow, and ice accumulation. Potential to use leftover metal roofing scraps from the Hershberger build (owner sorting through scrap pile for usable materials once snow/ice melts).
Design considerations:
- Must maintain manufacturer-required clearances on all sides (typically 12-24” sides, 48”+ top — check MrCool specs)
- Open sides required for airflow — this is a rain/snow deflector, not an enclosure
- Angled roof pitched to shed snow away from unit and away from building
- Mount to wall or freestanding posts — avoid attaching to the unit itself
- Material: Burnished Slate metal roofing scraps would match garage roof aesthetically
Loft Bathroom Conditioning
The loft bathroom will not have a dedicated mini-split head. Standard wall-mount units are not recommended for bathrooms due to high humidity causing mold growth, frozen evaporator coils, and premature system failure.
Solution: Transfer Fan with Timer Control
Use a through-wall transfer fan to move conditioned air from the common area into the bathroom when needed.
Components:
| Item | Product | Est. Cost |
|---|---|---|
| Transfer fan (supply) | Suncourt ThruWall TW108 | ~$100 |
| Passive return grille | Standard 10”x6” grille | ~$20 |
| Timer switch | Leviton DT160-1LW Countdown Timer (10/20/30/60 min) | ~$25 |
| Total | ~$145 |
Installation:
- Transfer fan mounted HIGH on shared wall (near ceiling) - supply side
- Passive grille mounted LOW on shared wall (near floor) - return side
- Transfer fan wired to countdown timer switch
- Creates circulation loop with common area
Operating Logic
| Bathroom State | Exhaust Fan | Transfer Fan | Result |
|---|---|---|---|
| Unoccupied, needs cooling | OFF | ON (timer) | Cool air flows in from common area |
| Occupied / in use | ON (timer) | OFF | Odors/humidity exhausted outside |
| Sleeping / away | OFF | OFF | Zero power consumption |
Why timer control:
- Prevents transfer fan from pumping odors/humidity into living space
- Manual activation = no sensors to fail
- Auto-shutoff prevents “left on” scenarios
- Matches existing bathroom fan timer workflow
- Zero standby power when not in use
Usage pattern:
- Hot summer day, bathroom stuffy → hit transfer fan timer (10-30 min)
- Using bathroom → hit exhaust fan timer (transfer fan stays off)
- After shower → let exhaust run 15-20 min to clear humidity before using transfer fan
- Away/sleeping → both fans off
Performance Expectation
Based on similar installations: bathroom stays within 1-2°F of common area temperature when transfer fan is running at ~80 CFM.
Alternative Options Considered
| Option | Pros | Cons | Verdict |
|---|---|---|---|
| Dedicated mini-split | Direct cooling | Humidity damage, oversized for space | ❌ Not recommended |
| Slim-duct unit serving multiple rooms | Professional solution | Complex, MrCool doesn’t offer DIY version | ❌ Overkill |
| Passive grilles only | Cheapest, silent | Less effective | ⚠️ Backup option |
| Exhaust fan pulls conditioned air | No extra equipment | Less efficient | ⚠️ Supplement only |
| Transfer fan + timer | Simple, effective, DIY | Manual operation | ✅ Selected |
Makeup Air for Fume Extraction
The Problem: Negative Pressure
The garage uses sealed HVAC systems — in-floor radiant heat (no air exchange) and mini-split heat pumps (recirculating, no air exchange). Neither system introduces or removes air from the building envelope. This means any exhaust system that vents outside will create negative pressure inside the garage.
The planned fume extraction system (for welding, grinding, solvent work) must vent outside. When running, it actively removes air from the space, pulling the interior to negative pressure relative to outdoors.
Why this matters:
| Concern | Risk | Severity |
|---|---|---|
| Gas boiler backdraft | Negative pressure can pull combustion gases (CO) back down the flue instead of venting outside | Critical (if boiler is atmospheric/power-vented) |
| Exhaust inefficiency | Starved airflow reduces fume extractor performance — it struggles to move air when there’s no replacement air entering | Moderate |
| Door seal stress | Garage doors and entry doors become harder to open under negative pressure | Minor |
Gas Boiler Safety
The radiant slab gas boiler’s combustion type determines urgency:
- Sealed combustion / direct vent (two-pipe system — dedicated intake + exhaust both going outside): Combustion air loop is isolated from room pressure. Backdraft risk is low.
- Atmospheric / natural draft (draws combustion air from the room, open flue): Negative pressure pulls exhaust back down the flue. Backdraft risk is high — makeup air is a safety requirement, not optional.
- Power-vented (draws from room air, fan-assisted exhaust): Fan helps, but strong negative pressure can overwhelm it. Backdraft risk is moderate.
Action: Confirm boiler combustion type before finalizing makeup air design.
What Does NOT Create Negative Pressure
- Central vacuum (shop vac based): Exhausts filtered air back into the garage interior. Air enters the hose and exits the vac’s filter — net zero air exchange with outside. No pressure differential.
- Mini-splits: Recirculate indoor air across a heat exchanger. No air enters or leaves the building.
- Radiant floor heat: Heats the slab with hot water. No airflow component at all.
Solution: Passive Makeup Air Inlet
Install a through-wall makeup air duct with a gravity backdraft damper on the garage main floor. This is the same concept as the fresh air intake duct found in homes with furnaces — a passive opening that allows exterior air in when a pressure differential exists.
How it works:
- Fume extractor turns on → removes air → interior pressure drops
- Pressure differential pushes the gravity damper open → outside air enters
- Fume extractor turns off → pressure equalizes → damper closes under its own weight
- No power, no controls, no maintenance
Components:
| Item | Specification | Est. Cost |
|---|---|---|
| Wall sleeve | 6” or 8” round galvanized duct (length = wall thickness) | ~$15-25 |
| Exterior wall cap | Louvered or hooded rain cap with screen (keeps insects/rain out) | ~$15-30 |
| Backdraft damper | Gravity/spring-loaded backdraft damper (mounts inside sleeve) | ~$15-25 |
| Interior grille | Round vent grille (optional, for finished appearance) | ~$10 |
| Total | ~$55-90 |
Sizing guideline: The makeup air opening should flow at least 75-80% of the fume extractor’s CFM rating. A 6” duct provides ~100-150 CFM passively; an 8” duct provides ~200-250 CFM. Size based on the extractor specs once selected.
Placement
- Location: On an exterior wall on the opposite side of the garage from the fume extractor exhaust point. This creates cross-ventilation and ensures replacement air sweeps across the work area rather than short-circuiting directly to the exhaust.
- Height: Low on the wall (12”-24” above floor). Cool makeup air entering low is less disruptive to the heated air volume above, and rising shop fumes are captured by the extractor above the work area.
- Rough-in now: Install the wall sleeve during framing/before drywall. Cap both ends until the fume extractor is installed. A capped sleeve costs almost nothing and avoids a painful retrofit later.
Future Upgrade Path
If the fume extractor CFM is high enough that the passive damper can’t keep up (noticeable difficulty opening doors, poor extractor performance), upgrade to a motorized makeup air damper interlocked with the fume extractor switch:
- Motorized damper wired in parallel with fume extractor — opens when extractor turns on, closes when it turns off
- Can add a filter (MERV 8-13) to keep shop air cleaner
- Estimated upgrade cost: $100-200 additional over passive setup
Electrical & Routing
- Provide exterior electrical box for outdoor unit; dedicated breakers per nameplate.
- Plan lineset + condensate routes before drywall; provide sleeve through wall.
- Consider noise placement for outdoor unit; maintain clearances.
- If using center-back placement, plan for longer electrical run from panel in mechanical room.
Actions
Planning Phase
- Call Clare County (989-539-2761) to confirm rear property line setback for outdoor unit. — stage:: 3
- Finalize outdoor unit placement (center-back vs. side wall) based on setback confirmation. — stage:: 3
- Complete insulation installation. — stage:: 3
- Perform Manual J load calc for garage and upstairs zones (post-insulation). — stage:: 3
- Confirm MrCool DIY 5th Gen 36k BTU 3-zone sizing based on Manual J. — stage:: 3
- Plan lineset routes and wall penetrations before drywall. — stage:: 3
- Plan bathroom transfer fan location (shared wall with common area). — stage:: 3
- Confirm gas boiler combustion type (sealed vs. atmospheric vs. power-vented) for makeup air urgency. — stage:: 3
- Select fume extractor and confirm CFM rating for makeup air duct sizing. — stage:: 3
- Plan makeup air inlet location (opposite wall from fume extractor exhaust). — stage:: 3
- Rough-in makeup air wall sleeve (6”-8” round) before drywall. — stage:: 3
Procurement
- Order MrCool DIY 5th Gen 3-zone system (9k + 12k + 18k). — stage:: 5
- Order bathroom transfer fan (Suncourt TW108). — stage:: 5
- Order countdown timer switch (Leviton DT160-1LW). — stage:: 5
- Order passive return grille (10”x6”). — stage:: 5
- Order makeup air components (wall sleeve, exterior cap, backdraft damper, interior grille). — stage:: 5
Installation
- Install lineset sleeves during framing/before drywall. — stage:: 3
- Install bathroom transfer fan (high) and return grille (low) on shared wall. — stage:: 6
- Wire transfer fan to timer switch. — stage:: 6
- Install outdoor unit on concrete pad. — stage:: 6
- Mount indoor heads (garage, common area, bedroom). — stage:: 6
- Connect linesets using quick-connect fittings. — stage:: 6
- Commission system per MrCool instructions. — stage:: 6
- Install makeup air inlet (exterior cap, backdraft damper, interior grille). — stage:: 6
Procurement
Estimated Costs
| Item | Est. Cost |
|---|---|
| MrCool DIY 5th Gen 36k BTU 3-zone (9k+12k+18k) | $4,500-5,500 |
| Bathroom transfer fan + timer + grille | ~$145 |
| Concrete pad for outdoor unit | ~$200-400 |
| Electrical materials (wire, breaker, disconnect) | ~$200-300 |
| Makeup air inlet (sleeve, cap, damper, grille) | ~$55-90 |
| Total estimated | $5,100-6,500 |
Comparison: Professional Mitsubishi install would be $8,000-12,000+
Order Tracking
- Order: Mini-Split HVAC Order
- Index: Orders Index
Product Links
- MrCool DIY 5th Gen 3-Zone Systems
- Chill Mini Splits - 36k BTU Options
- Suncourt ThruWall Transfer Fan TW108
- Leviton DT160 Countdown Timer
Notes
- High wall R-values have limited ROI if large garage doors remain R-9; address biggest losses first (doors, air-sealing).
- GFCI where required; follow NEC.
- Cellular connectivity: Metal roof will significantly attenuate cell signals in loft area. Consider cellular signal booster installation. See homelab project
Cellular Connectivity.mdfor detailed analysis and implementation plan. - DIY Installation: MrCool pre-charged quick-connect linesets require no brazing, vacuum pump, or EPA 608 certification. 7-year compressor + 5-year parts warranty.
- Bathroom humidity: Never install wall-mount mini-split directly in bathroom. High humidity causes mold, frozen coils, and system damage.
- Installation timing: Plan lineset routes during framing phase, before drywall. Much easier than retrofit.
- Makeup air: Fume extraction creates negative pressure in the sealed envelope (radiant + mini-splits = no air exchange). Passive makeup air inlet required. Rough-in wall sleeve before drywall even if fume extractor is installed later.
References
- Outdoor unit requirements: Mini-Split Outdoor Unit Installation Requirements
- Chat summary: Chat Reference
- Vent planning: Utilities & Conduits
- Slab sensor conduit: Decisions - Slab Sensor Conduit
- Product specifications: Products Used
External Resources
- MrCool DIY Direct - Official retailer
- MrCool Barndominium Install Review - Similar project experience
- Garage Journal - MrCool Reviews
- GreenBuildingAdvisor - Bathroom Mini-Split Discussion