# Air System Specification

## 1. Design Philosophy

In a well-sealed Baufritz home, 100% of incoming air passes through the ventilation system. The goal is to engineer indoor air that is **measurably superior to outdoor air** across every parameter — not merely acceptable, but actively health-positive.

**Target: Whole-house H13 HEPA filtration (ISO Class 7–8 equivalent), with full control over particulates, gases, humidity, CO₂, and pressure.**

---

## 2. Filter Cascade — Corrected Sequence

The filter stages are ordered so that **every upstream stage's potential contaminants are caught by the next downstream stage**. The H13 HEPA is always the final particle barrier before air enters the living space.

```mermaid
flowchart TD
    A["Outdoor Air Intake\n weather-protected, north-facing preferred"] --> B
    B["Stage 1: G4/M5 Pre-filter\nCoarse particles, pollen, insects"] --> C
    C["Stage 2: F7 Intermediate Filter\nFine dust, mold spores"] --> D
    D["Stage 3: Activated Carbon\nVOC, NO2, ozone removal"] --> E
    E["Stage 4: H13 HEPA - FINAL BARRIER\n99.95% efficiency at 0.3um\nCatches everything incl. carbon fines"] --> F
    F["MVHR Unit\nCounterflow heat recovery 80-95%"] --> G
    G["Steam Humidifier\nelectrode or resistive, inherently sterile"] --> H
    H["Motorized Zone Dampers"] --> I
    I["Room Distribution"]

    style E fill:#1a5e1a,color:#fff,stroke:#0d3d0d
    style D fill:#4a4a00,color:#fff,stroke:#333
    style F fill:#1a3d5e,color:#fff,stroke:#0d2840
```

### 2.1 Stage Details

| Stage | Filter Class | Function | Face Velocity | Initial ΔP | Change Interval |
|-------|-------------|----------|---------------|------------|-----------------|
| 1 | G4 / M5 | Coarse protection — pollen, insects, large dust | < 2.5 m/s | 30–50 Pa | Every 3 months |
| 2 | F7 (ePM2.5 ≥ 65%) | Fine particulates, mold spores, extends H13 life | < 2.0 m/s | 40–60 Pa | Every 6 months |
| 3 | Activated Carbon | VOC, NO₂, ozone adsorption | < 1.5 m/s | 30–50 Pa | Every 12 months |
| 4 | H13 HEPA | Final barrier — 99.95% @ MPPS (0.3 µm) | **< 1.0 m/s** | 80–150 Pa | Every 12–18 months |

### 2.2 Critical Design Rules

- **Carbon BEFORE HEPA**: Carbon filters shed microscopic activated carbon fines. The H13 HEPA downstream catches these. Never reverse this order.
- **Face velocity across H13 panels must stay below 1.0–1.5 m/s**: This requires filter housings 2–4× larger than the duct cross-section. At low face velocity, pressure drop drops to 80–100 Pa initial, noise disappears, and filter life doubles.
- **Filter housing must accommodate 600×600 mm panel filters** (or larger) — not inline cylindrical filters.
- **Differential pressure sensors across each stage**: Electronic, connected to Home Assistant. Change filters based on actual loading, not calendar time. Spring pollen will load pre-filters in weeks.

---

## 3. Exhaust Side Filtration

The exhaust path also requires filtration to protect the MVHR heat exchanger:

```mermaid
flowchart LR
    R["All Rooms\n(except kitchen)"] --> EF["F9 Exhaust Filter\nProtects heat exchanger"]
    EF --> MVHR["MVHR Exhaust Side"]
    MVHR --> OUT["Outdoor Exhaust"]

    K["Kitchen"] --> KE["Dedicated Kitchen Exhaust\nNEVER recirculated into MVHR"]
    KE --> KOUT["Separate Outdoor Exhaust"]

    style K fill:#8b0000,color:#fff
    style KE fill:#8b0000,color:#fff
```

- **Minimum F9 on exhaust** before the MVHR heat exchanger
- **Kitchen exhaust is entirely separate** — cooking aerosols, grease particles, and combustion byproducts must never enter the MVHR system
- Kitchen hood: ducted directly outside through a dedicated penetration with backdraft damper

---

## 4. MVHR Unit Requirements

For whole-house H13, the MVHR must handle significantly higher system pressure than standard residential units.

### 4.1 Minimum Specifications

| Parameter | Requirement |
|-----------|-------------|
| Available static pressure | ≥ 450 Pa (preferably ≥ 500 Pa) |
| Airflow capacity | ≥ 400 m³/h (for ~150–200 m² at 0.8 ACH) |
| Heat recovery efficiency | ≥ 85% (counterflow plate, no enthalpy wheel) |
| Fan type | EC (electronically commutated), variable speed |
| Filter bay | External filter box connection, or large internal bay |
| Controls | KNX or Modbus interface for building automation |
| Noise at rated flow | ≤ 35 dB(A) at 1 m |

### 4.2 Recommended Units

| Unit | Max Static Pressure | H13 Support | Notes |
|------|---------------------|-------------|-------|
| Drexel & Weiss aerosilent | ~600 Pa | Native options | Austrian, Passivhaus standard |
| Swegon CASA R5 | ~600 Pa+ | Via external box | Semi-commercial, bridges residential/commercial |
| Zehnder ComfoAir Q600 | ~500 Pa | Via external box | Largest residential Zehnder, KNX native |
| Paul Novus 450 | ~450 Pa | Via external box | Very quiet, excellent German engineering |
| Hoval HomeVent FR | ~500 Pa | Via external box | Commercial-grade residential |

**Recommendation**: Drexel & Weiss aerosilent or Swegon CASA — both designed for the pressure demands of H13 whole-house filtration.

---

## 5. Humidification

### 5.1 Requirements

| Parameter | Target |
|-----------|--------|
| Relative humidity | 40–60% year-round |
| Humidifier type | **Steam ONLY** (electrode or resistive) |
| Position | Downstream of MVHR, in supply duct |
| Control | Hygrostat per zone, integrated with central controller |

### 5.2 Why Steam Only

Since the H13 HEPA is the final particle barrier upstream of the MVHR, anything added to the airstream after the HEPA must be inherently sterile:

- **Steam humidifiers**: Water boiled to 100°C — output is sterile vapor, no mineral dust, no biofilm
- **Ultrasonic**: Creates aerosols containing dissolved minerals and bacteria from reservoir — **PROHIBITED**
- **Evaporative (unsterilized)**: Biofilm grows on wetted media within days — **PROHIBITED**

**Recommended brands**: Condair, Hygromatik — industrial-grade steam humidifiers designed for duct integration, commonly used in German clean rooms and hospitals.

### 5.3 Dehumidification

In summer, dehumidification is handled by:
- The MVHR cooling bypass (partial)
- Optional: small split heat pump unit on the supply duct, or dedicated dehumidifier in the technical room
- Target: never exceed 60% RH at any surface

---

## 6. Duct Design

### 6.1 Velocity Requirements

| Duct Section | Max Velocity | Typical Diameter |
|-------------|-------------|-----------------|
| Main supply/extract trunk | ≤ 2.0 m/s | 200–250 mm round |
| Branch ducts to rooms | ≤ 1.5 m/s | 125–160 mm round |
| Final connection to diffuser | ≤ 1.0 m/s | 100–125 mm round |

At 0.8 ACH for 150 m² with 3 m ceilings = **~360 m³/h airflow**. Main duct at 2.0 m/s → ~250 cm² cross section → **200 mm round duct minimum**.

### 6.2 Material and Sealing

- **Rigid steel or aluminum ducts throughout** — no semi-rigid flex (adds resistance, collects contamination)
- **Exception**: final 0.5 m connection to each diffuser may use flex for vibration isolation
- **All joints sealed with mastic** (not just tape) — Passivhaus duct leakage class
- **Pressure-tested after installation** before walls are closed

### 6.3 Duct Routing

Must be coordinated with Baufritz at structural design stage:
- Duct runs in ceilings and walls need to be planned and boxed in
- Dedicated vertical risers for multi-story distribution
- No ducts in exterior walls (condensation risk, thermal bridge)
- Acoustic silencers at MVHR outlets and before each room diffuser
- **Ground floor alternative**: branch ducts may route through the Lindner NORTEC Doppelboden cavity (see 04-flooring-ceiling-spec Section 6). Supply air may be distributed via Plafotherm AirHybrid ceiling panels (see 04-flooring-ceiling-spec Section 5), potentially replacing conventional wall/ceiling diffusers

---

## 7. Pressure Management

| Parameter | Target |
|-----------|--------|
| House pressure vs. outside | **+3 to +5 Pa** (slight positive) |
| Supply vs. exhaust airflow | Supply 5–10% greater than exhaust |
| Kitchen exhaust compensation | Dedicated make-up air or interlock with MVHR boost |

Positive pressure ensures:
- All infiltration passes through the filter stack
- Radon and soil gases cannot infiltrate from below
- Garage pollutants cannot enter (garage must be entirely pressure-separated)

---

## 8. Air Quality Targets

| Parameter | Target | Danger Threshold |
|-----------|--------|-----------------|
| PM2.5 | < 5 µg/m³ | > 15 µg/m³ |
| PM10 | < 10 µg/m³ | > 25 µg/m³ |
| CO₂ | < 800 ppm | > 1000 ppm |
| TVOC | < 200 µg/m³ | > 500 µg/m³ |
| Relative Humidity | 40–60% | < 30% or > 70% |
| Radon | < 100 Bq/m³ | > 300 Bq/m³ |
| Temperature uniformity | ± 1.5°C between rooms | > 3°C delta |
| Formaldehyde | < 30 µg/m³ | > 100 µg/m³ |

---

## 9. Estimated Filter Costs (Annual)

| Stage | Interval | Cost per Change | Annual Cost |
|-------|----------|----------------|-------------|
| G4/M5 pre-filter | 3 months | 10–20 EUR | 40–80 EUR |
| F7 intermediate | 6 months | 20–40 EUR | 40–80 EUR |
| Activated carbon | 12 months | 50–100 EUR | 50–100 EUR |
| H13 HEPA panels | 12–18 months | 80–200 EUR per panel | 80–200 EUR |
| **Total** | | | **210–460 EUR/year** |

This is the cost of breathing clean air. For context, this is less than a monthly gym membership.