Balanced Ventilation System
A mechanical system where supply and exhaust airflows match to maintain neutral pressure, preventing uncontrolled infiltration or exfiltration.
What is a balanced ventilation system, and how does it differ from heat recovery ventilation?
A balanced ventilation system supplies fresh air while exhausting stale air in equal volumes, maintaining neutral pressure; no infiltration through leaks, no exfiltration through cracks. Balanced ventilation is NOT the same as heat recovery ventilation (MVHR). Heat recovery is a function;an exchanger that transfers warmth from exhaust air to incoming air. Balance is a regime;the airflow condition that makes the building operate predictably. Many MVHR systems are promoted as 'heat recovery' units, but their foundation is the balanced airflow. You can install a simple balanced system with two fans of equal capacity and no heat recovery; conversely, a heat recovery exchanger only works effectively if airflows are balanced. The two concepts are separable but interdependent in practice.
Why does neutral pressure matter in ventilation design?
Building pressure imbalance is the root cause of ventilation failure in existing homes. Most older Slovak flats rely on extract-only ventilation: bathroom fans and kitchen hoods pull air out, while 'makeup air' enters through envelope leaks, gaps around windows, and under doors. This extract-only regime depressurises the building, drawing unfiltered, untreated outdoor air through gaps and pulling humid air into cold cavities, causing condensation and mould. Modern airtight construction (airtightness) seals these leaks, which is why extract-only fans fail once a house is tightened: there is nowhere for makeup air to enter, so pressure crashes and humidity rises.
In contrast, a balanced system maintains neutral pressure. Supply and exhaust volumes are equal, so no net pressure differential drives infiltration or exfiltration. Air enters only through designed, filtered supply inlets. Stale air exits only through designated exhaust points. Humidity follows the exhaust path, not into walls. The building becomes decoupled from outdoor pressure variations (wind, stack effect), and indoor air quality depends entirely on filter performance and maintenance, not on envelope leakiness.
How is a balanced ventilation system commissioned and tuned?
Commissioning is the process of verifying that the system actually achieves balance in practice. On paper, fans rated for equal airflow may differ 10-30% in reality due to duct friction and damper settings. Commissioning requires measuring actual velocity at outlets and adjusting dampers until flows match within 5-10%. Only after commissioning can the design intent be verified. A system installed without commissioning is an untested pressure imbalance waiting to be discovered.
In passive houses and strict energy-efficiency standards, commissioning is mandatory and documented. The building regulation STN 73 0540 PASÍVNY DOM requires commissioning for all passive-house projects. For renovations and general building work, commissioning is often omitted to save cost, leading to disappointed occupants who open windows because they feel the 'ventilation isn't working' (when in fact it never balanced). Recommissioning after 1-2 years is also prudent to account for duct blockage or filter loading that shifts the balance.
What factors disturb balance after commissioning?
Balance is a design condition that drifts with operation. Four factors are the primary culprits:
- Filter loading: As supply and exhaust filters accumulate dust, airflow resistance increases. If filters load at different rates (kitchen exhaust collects more lint than bedroom supply), pressure drop becomes asymmetrical and balance shifts toward the cleaner filter.
- Competing exhausts: An open fireplace or an extract cooker hood running simultaneously with the MVHR can unbalance the entire system. The fireplace or hood creates a localized depression that overshadows the MVHR exhaust, pulling air from unwanted sources.
- Duct blockages: Dust accumulation or pipe collapse in poorly routed ducts can block one side (usually exhaust, which carries lint and hair) more than the other.
- Damper drift: Over time, dampers can stick or creep due to vibration, dust, or cable stretch, shifting design flow distributions.
Maintaining balance requires regular inspection: visual check of filters monthly, replacement on schedule (typically annually for central systems, monthly per unit for decentralised systems), and anemometer re-check every 2-3 years. Many occupants skip this because they cannot see the balance or feel the result. This is why central systems have better satisfaction: annual filter changes at one location keep the whole system visible and maintained.
What are typical ventilation rates, filter classes, and design parameters?
The table below sets out typical balanced-ventilation airflows and filter specifications for residential buildings in Slovakia:
| Building Type | Typical Supply Airflow | Standard Filter Class | Filter Replacement Interval | Pressure Drop (clean filter) |
|---|---|---|---|---|
| Passive house, 150 m² | 150–250 m³/h (0.4–0.5 air changes/hour) | ISO ePM10 55% (G4 coarse) | 12 months | 15–25 Pa |
| Energy-efficient renovation, 100 m² | 100–150 m³/h (0.5–0.7 air changes/hour) | ISO ePM10 55% | 12 months | 15–25 Pa |
| Allergy-prone household (same area, upgraded filter) | 100–150 m³/h | ISO ePM2.5 85% (F7 fine) | 8–10 months | 30–40 Pa |
| Extract-only (non-balanced, for contrast) | 50–100 m³/h bathroom exhaust only | None (or simple screen) | N/A | N/A (uncontrolled infiltration) |
How do supply and exhaust imbalances reveal themselves in practice?
The table below describes the symptoms of imbalance and their causes:
| Imbalance Condition | Cause | Observable Symptom | Consequence |
|---|---|---|---|
| Exhaust > Supply (Depressurised) | Extract fans larger than supply; undersized supply ducts; supply filter loading faster than exhaust | Door slams shut; difficult to open external doors; cold air leaks around window frames | Unfiltered outdoor air infiltrates through gaps; humidity cannot be expelled; cold bridging increases condensation risk |
| Supply > Exhaust (Pressurised) | Supply fans larger than exhaust; undersized exhaust ducts; extract filter loading faster than supply | Doors blow open; air leaks from building toward outdoors; visible drafts around electrical outlets | Moist indoor air forced into wall cavities; potential for hidden condensation; occupants feel 'stuffed up' and open windows |
| Balanced (Neutral Pressure) | Supply and exhaust fans matched; ducts sized equally; filters maintained on schedule | Doors operate normally; no drafts; even temperature and humidity throughout | Controlled ventilation works as designed; filters capture all infiltration; indoor environment stable and healthy |
What is the role of filter selection and maintenance in sustaining balance?
Filter class affects both balance stability and air quality. A G4-class (ISO ePM10 55%) filter is standard for residential balanced systems because it removes dust, pollen, and particles down to 10 micrometres with moderate pressure drop. A fine F7-class (ISO ePM2.5 85%) filter captures ultrafine particulates and allergens but creates higher pressure drop (25-40 Pa versus 15-25 Pa for G4), increasing fan power and noise. Fine filters also load faster, so replacement becomes a burden if the household is not disciplined. Choosing F7 means committing to monthly or bi-monthly checks instead of annual replacement; most passive-house projects specify G4 initially and recommend F7 only if occupants have allergies or the building is in a high-pollution location.
Maintenance discipline is the unglamorous truth about balanced ventilation. The system's fundamental purpose;neutral pressure and controlled air quality;depends entirely on keeping filters clean and airflows tuned. Unlike heat recovery efficiency, which appears in marketing and compliance documents, balance is invisible and often forgotten until something goes wrong. The cost of filter replacement and annual commissioning check-ups (EUR 50-100 per year) is trivial compared to the health and building-durability benefit, but many occupants perceive ventilation as 'set and forget' infrastructure. This is why education at handover is critical: the ventilation system is not an accessory but a core building function that must be actively maintained.
Frequently asked questions
- What is the difference between balanced ventilation and heat recovery ventilation?
- Balanced ventilation is the airflow regime: supply and exhaust flows matched so the building runs at neutral pressure. Heat recovery ventilation is a function that recovers heat energy from exhaust air. You can have balanced ventilation without heat recovery (simple supply and exhaust fans of equal capacity), but you cannot have functional heat recovery without first achieving balanced airflows. Many MVHR systems are marketed as 'heat recovery' when their foundation is actually the balance.
- Why does it matter if my ventilation system is balanced?
- Without balance, the building develops pressure imbalances. If exhaust exceeds supply (extract-only, or underpowered supply fans), the building is depressurised and unconditioned, unfiltered air leaks in through envelope gaps. If supply exceeds exhaust, the building pressurises and moisture-laden air is forced outward through walls, risking condensation and rot. Only neutral pressure allows clean, controlled air to flow predictably through the system and filters.
- What happens if an MVHR unit is not commissioned or balanced?
- An uncommissioned unit is a pressure imbalance disguised as a system. The fans may run, but if the ductwork is undersized, dampers are not tuned, or supply and exhaust fans have mismatched capacity, the building will still be imbalanced. Occupants then open windows to compensate, defeating the purpose. Commissioning means physically measuring airflows in each room, adjusting dampers, and verifying neutral pressure; this is not optional.
- How do fireplaces and cooking hoods affect balanced ventilation?
- An open fireplace or an unvented cooking hood can instantly unbalance a ventilation system. A fireplace draws air up the chimney, creating local depressurisation that forces the exhaust fan to extract from the fireplace rather than from the kitchen and bedrooms. A recirculating (non-vented) cooker hood adds moisture without extracting it. If an extract hood vents directly outside, it can exceed the MVHR's exhaust capacity. The system remains balanced only if fireplaces are sealed, cooker hoods vent to outdoors with a backdraft damper, or they run only when the MVHR supply is increased.
- Why do commissioning and balancing cost extra?
- Because they require a technician to use a flow measurement hood or anemometer to measure air velocity at each outlet, adjust dampers, and verify that the building achieves neutral pressure under operation. This takes 4-8 hours for a typical house. Without commissioning, the system may look correct on paper but fail in practice. It is not a design luxury; it is verification that the balance actually works.
- How does filter loading affect balance over time?
- As filters accumulate dust, they create increasing airflow resistance. If supply and exhaust filters load evenly, balance is maintained but total airflow drops. If one side loads faster (e.g., the exhaust filter collects more lint from kitchens and bedrooms), that side's flow decreases, and the system begins to drift toward imbalance. Regular filter inspection and replacement on schedule prevents this drift. The 'balance is a design intent that drifts as filters load' means balance must be actively maintained through maintenance discipline.