# LDD-05 · HVAC System (Overview)

> **Status:** 🟢 LOCKED v1.1.

## One-line intent

Each zone uses the simplest correct system; no single system serves the entire building.

## Why this matters

This is the right design principle for a building with this much program variety. Trying to push a single ducted system across 7,200 sqft of mixed-use space (basketball gym + ILS + living + workshop + LOW) creates duct chaos, balancing problems, and the soffit-everywhere fail mode the rest of the LDD set explicitly avoids.

## Locked decisions

**System breakdown**

| Zone | System | Notes |
|---|---|---|
| Living wing | Ducted heat pump | Trunk + 5 branches; details in LDD-06 |
| Gym | 2 mini-splits + destratification fans | Athletic / large-volume strategy |
| Bedrooms | Ducted heat pump (separate zone) | Upper level |
| UCR | Radiant heat + passive cooling, fan-assisted | Lowest-velocity comfort |
| ILS | Mini-splits | Independent operation |
| Workshop | Mini-split | Independent |
| LOW | Rough-in Phase 1 | Active equipment deferred |

**Gym air strategy**

- Large destratification fans required
- Reduce vertical temperature gradient
- Improve perceived comfort across large air volume

## Constraints

- **No soffits** — the exposed ceiling philosophy depends on it.
- **No overlapping systems** — no two systems competing in the same zone.
- **No duct chaos** — all routing aligned to structural grid.
- **No diagonal duct runs** — layout clarity.

## Open items / requires engineer review

- **Equipment selection** for each subsystem — make/model TBD, but capacity targets need Manual J per zone.
- **Heat pump heat source** — air-to-air vs ground-source? The doc doesn't say. Ground-source is a major budget hit (~$25–50K extra) but performs much better in cold climate and integrates well with hydronic radiant via a single buffer tank. Air-to-air is simpler and cheaper.
- **Make-up air for kitchen island system** (LDD-07) — not specified here, must be coordinated with whichever ducted system serves the living wing. A high-CFM exhaust without make-up air will pull combustion gases backward through any other exhaust path.
- **Ventilation strategy** — there's no explicit ERV/HRV in the LDDs. For a tight IMP-clad airtight building, a balanced ventilation system is **essential** (not optional) — code in most jurisdictions and required for IAQ. Add an LDD entry.
- **Humidity control** — neither dehumidification nor humidification is specified. With radiant heating and tight envelope, both can be issues.

## Cross-references

- ← [LDD-02 radiant](02-radiant-slab.md) — radiant + air systems must not fight; radiant provides baseline, ducted/mini-split handles response.
- → [LDD-06 living wing HVAC](06-living-wing-hvac.md) — distribution details.
- → [LDD-07 cooking ventilation](07-cooking-ventilation.md) — interlock with main HVAC for make-up air.
- → [LDD-12 ceilings](12-exposed-ceilings.md) — HVAC is layer 2/3 of the hierarchy.

## Cost drivers

- **Living wing ducted heat pump** (~3-ton): unit $6–10K, install + ducting $25–45K = **$30–55K**.
- **Bedroom ducted heat pump** (~2-ton): unit $4–7K, install $18–28K = **$22–35K**.
- **Gym mini-splits** (2 × ~3-ton ductless heads from single condenser, or 2 separate 2-ton systems): **$15–25K** including line sets.
- **ILS mini-split** (~2-ton): **$7–12K**.
- **Workshop mini-split** (~1.5-ton): **$5–8K**.
- **Destratification fans** (4 high-volume low-speed, 12'–20' dia): **$20–35K** installed.
- **ERV/HRV** (currently missing) — add **$8–18K**.
- **LOW rough-in** (line set, electrical, condensate stub): **$3–6K**.

**Likely-case rollup: $115–195K total HVAC.** Mid-point ≈ $155K (matches budget waterfall's $175K which includes some destrat and ventilation contingency).

## Air-gap concerns

1. **Missing ventilation system in the LDDs.** Tight envelopes need balanced ventilation (ERV in cold climate, HRV in mixed). It's not in the doc, it's expensive, and it can't be added cleanly after walls close. Add it now.
2. **Gym cooling load is the load not accounted for.** West-facing clerestory (LDD-04) + ~3,600 sqft of gym slab + 20'+ ceiling = a big summer cooling load. Two mini-splits may be undersized depending on tonnage chosen and climate.
3. **Make-up air strategy missing.** The island vent system (LDD-07) pulls a lot of CFM; without designed make-up air, you back-draft something else.
4. **No mention of zoning controls or thermostats.** Multi-system house with radiant + 4 forced-air systems = at least 5 thermostats and probably a zone-coordination strategy. Budget for it.
5. **Bedroom ducted system trunk path?** LDD-06 describes the living wing trunk explicitly. The bedroom system is named but never sited. Where does its trunk run, and how does it relate to the exposed ceiling hierarchy?

## Diagram

![Exposed ceiling hierarchy: structure, HVAC trunk, branches, electrical, lighting](../../diagrams/10-ceiling-hierarchy.svg)

## Status

🟢 **Green — strategy is correct.** Add an explicit ventilation LDD entry and resolve heat-source decision (air-to-air vs ground-source) before bidding.