Whisper quiet, ductless, heating and cooling from one slim wall unit: the mini-split is the tiny house climate hero. But the classic DIY mistake is buying too much of a good thing. An oversized unit short-cycles, leaving the house cold and clammy while the compressor bangs on and off like a screen door.
Sizing, placement, line sets, and commissioning: the complete DIY sequence is laid out here. You will learn the BTU-per-square-foot rule, where to mount the indoor and outdoor units, how to route and flare the line set, and the install errors that void warranties. One careful afternoon of work buys years of silent comfort.
The Tiny House HVAC of Choice
The ductless mini-split is the default heating and cooling system for tiny houses: efficient (20+ SEER), no ductwork, quiet, and it both heats and cools. A 9,000–12,000 BTU unit conditions most tiny houses. This is the complete DIY installation guide — what you can do yourself and where the pros come in.
1. Sizing: Right-Size or Regret
Rule of thumb: 20–30 BTU per sq ft for well-insulated tiny houses. A 300 sq ft house: 6,000–9,000 BTU; 400 sq ft: 9,000–12,000 BTU. Oversizing is the classic mistake (an oversized unit short-cycles — cools too fast to dehumidify, leaving the house cold and clammy; and short-cycling kills efficiency and the compressor). Undersizing means the unit runs flat-out and never catches up on the hottest/coldest days. The pick: 9,000 BTU for most tiny houses (the sweet spot — handles 250–400 sq ft); 12,000 BTU for hot climates, poor insulation, or big glass. Cold climate: verify the low-ambient heating rating (quality units heat to -13°F or lower; cheap ones quit at 30°F — the spec that matters).
2. Placement: Indoor and Outdoor Units
Indoor head: high on an interior wall (heat rises, cool air falls — high mounting serves both modes), centered on the longest wall for even distribution, away from the loft sleeping area’s direct blast (nobody wants 55°F air on their face at 2 AM), and NOT above the bed (drips happen — condensate lines clog; don’t sleep under one). Outdoor condenser: on the ground on a pad or wall-mounted on brackets (off the ground = no snow burial, less vibration), within the line-set length limit (typically 50–80 ft max — shorter is better), shaded if possible (a shaded condenser works 5–10% more efficiently), and with 2+ feet clearance all around (airflow is the condenser’s lifeblood).
3. The Line Set: Connecting the Halves
The line set (insulated copper pair + condensate drain + communication wire) runs through the wall: drill the penetration (2.5–3″ hole, sloped slightly downward to outside — water drains out, not in), route the lines (no kinks — a kinked line set is a ruined line set; minimum bend radius 4–6″), insulate both lines (the factory insulation is often thin — add armaflex sleeves, especially the suction line), condensate drain (sloped continuously down and out — no traps, no uphill sections; in heating mode the OUTDOOR unit makes condensate — plan its drainage too), and seal the penetration (foam + silicone + wall cap — the hole is a water, air, and insect entry without sealing).
4. Electrical: The Dedicated Circuit
Circuit: dedicated 15–20A 230V (most mini-splits are 230V — verify; some 9k units are 115V), 12 AWG wire, GFCI/breaker per code. Disconnect: a service disconnect at the outdoor unit (code requires it — the $30 box that lets a tech kill power safely). Surge protection: mini-split boards are lightning-sensitive ($200 boards fry in storms — a whole-house or dedicated surge protector is cheap insurance). Off-grid note: the startup surge (locked-rotor amps) can trip small inverters — verify your inverter handles the surge (most 3,000W+ pure sine inverters do; soft-start kits exist for marginal cases).
5. The Refrigerant Question: DIY vs Pro
The honest answer: mounting units, running line sets, wiring, and drilling are solid DIY. The refrigerant work — vacuuming the lines and releasing the charge — legally requires EPA Section 608 certification in the US. Options: (a) DIY everything, hire a tech for the 1-hour vacuum-and-commission ($150–300 — the standard approach), (b) pre-charged quick-connect line sets (Mr. Cool DIY series — genuinely DIY-legal, no vacuum needed — $1,200–2,000 for the kit; slightly less efficient, but truly DIY), (c) full pro install ($2,500–4,500 all-in). Never: skip the vacuum (moisture in the lines destroys the compressor — the vacuum boil-off is what removes it) or release refrigerant to atmosphere (illegal and environmentally destructive).
6. Commissioning: First Start-Up
With the tech (or your pre-charged kit): pressure test (nitrogen to 300+ psi — holds 24 hours = no leaks), vacuum (500 microns or lower — the deep vacuum that boils off moisture), release the charge (open the service valves — the factory charge fills the system), test both modes (cooling AND heating — verify temperature split: 15–20°F across the indoor coil in cooling), check condensate drainage (pour water in the indoor pan — it must flow freely outside), and verify electrical (amp draw within spec, no tripped breakers). Document: the install date, refrigerant type, and charge amount (on the unit — the next tech needs it).
7. Maintenance: The 30-Minute Annual Ritual
Monthly: clean the indoor filters (rinse — 5 minutes; dirty filters are the #1 efficiency killer). Annually: hose the outdoor coil (gentle — bent fins strangle airflow; fin comb for straightening), clear the condensate drain (algae tablets or vinegar flush — clogs cause indoor waterfalls), check line set insulation (UV degrades it — replace cracked sections), and verify mounting (vibration loosens brackets — the house moves, the unit must not). Every 3–5 years: pro check (refrigerant pressures, electrical connections, deep coil clean). Total cost of ownership is minimal — the mini-split is the lowest-maintenance HVAC you can own.
Final Considerations
Size it right (9k BTU for most), mount the head high and the condenser shaded, run the line set without kinks, wire the dedicated circuit, let a certified tech handle the refrigerant (or buy the DIY pre-charged kit), and clean the filters monthly. The mini-split is the rare appliance that heats, cools, dehumidifies, and sips power — install it well and it disappears into the background of comfortable tiny living.