house-spec/specs/01-air-system-spec.md

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.

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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.

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.

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3. Exhaust Side Filtration

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

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

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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.

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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

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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

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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)

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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

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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.