Heat Recovery Ventilation (MVHR)

What is MVHR and how does it work?

Heat Recovery Ventilation, or MVHR (Mechanical Ventilation with Heat Recovery), is a controlled ventilation system that continuously supplies fresh air to a building while recovering heat energy from the stale air being expelled. Unlike passive ventilation through open windows, MVHR maintains indoor air quality at a constant level by drawing outside air through filters, distributing it throughout the building, and simultaneously extracting humid or polluted air. The heat recovery happens in an exchanger where the two air streams pass near each other without mixing, transferring warmth from the warm exhaust air to the cold incoming fresh air.

Why is MVHR essential in passive houses and airtight buildings?

In a passive house, the thermal envelope is so tight that opening windows becomes impractical for ventilation. Windows create thermal bridges and lose heat rapidly when opened for fresh air. With an airtight envelope verified by a blower door test, MVHR becomes not just efficient but necessary. The system ensures continuous fresh air supply for occupant health and comfort while keeping heat loss minimal, which is why passive-house standards require a minimum heat recovery efficiency of 75%. Without MVHR, an airtight house would quickly become stuffy and damp.

How do heat recovery efficiency ratings work, and what numbers are realistic?

Heat recovery efficiency is expressed as a percentage describing how much heat energy from the outgoing air is transferred to the incoming air. An 80% efficient system means that if fresh air enters at 0 degrees Celsius and exhaust air leaves at 20 degrees, the fresh air will be preheated to approximately 16 degrees. Modern MVHR units typically achieve 75-95% efficiency depending on design and operating conditions. However, real-world efficiency is lower than laboratory test results because of thermal losses in ducts, imbalances between inlet and exhaust flow rates, and variations in seasonal temperature difference. For practical planning in Slovakia's temperate climate, assume 70-80% effective efficiency under actual conditions rather than peak-rated figures. The energy performance certificate calculations use standardized efficiency values, but field performance varies.

Scenario	Typical MVHR efficiency (lab)	Realistic field efficiency	Passive-house minimum
Air-to-air sensible heat exchanger, new	80-92%	70-80%	75% required
Enthalpy exchanger (heat + humidity), new	82-95%	72-82%	75% required
System after 3-5 years with standard maintenance	75-85%	65-75%	Meets standard initially
System with neglected filters (>12 months)	40-60%	30-50%	Does not comply

How do central ducted systems compare to decentralised room units?

The choice between central MVHR and decentralised (or point-source) heat recovery units fundamentally affects building design, running cost, and maintenance burden. Central systems draw air from all occupied rooms through a network of ducts, exchange heat in one large unit, and distribute fresh air back through a separate duct network. Decentralised systems install compact heat exchangers in the walls or windows of individual rooms, handling their own air balance locally. The table below compares the practical implications for residential projects in Slovakia.

Aspect	Central MVHR (ducted)	Decentralised units (room-mounted)
Capital cost	EUR 3,000-6,000 installed (whole house)	EUR 400-800 per unit; 3-5 units typical = EUR 1,500-4,000 total
Duct footprint	Requires 150-200 mm ducts in floor voids, roof space, or external walls; ties design early	No ducts; minimal building integration
Air balance across rooms	Can achieve pressurised supply and controlled exhaust; entire house balanced	Each unit balances only its own room; whole-house air balance difficult
Noise	Single unit noise source; good silencers available; ducts may transmit room-to-room noise	Individual units quieter in aggregate but multiple small fans; window-mounted units may rattle
Maintenance	Two filters (supply and exhaust) changed annually; duct cleaning every 3-5 years recommended; one control point	One filter per unit changed every 1-2 months; cleaning and filter replacement repeated 3-5 times; higher total labour
Commissioning and adjustment	Flow balance between rooms set once; future tweaks via dampers	Each unit adjusted independently; harder to diagnose imbalances across the house
Retrofit difficulty	Very difficult in existing buildings; ducts need routing through walls and structure	Easier retrofit; external wall units or window inserts avoid internal ducts

For new passive-house construction in Slovakia, central MVHR is standard because it integrates with the design from the start and achieves superior whole-house control. For retrofits or renovation of older buildings, decentralised units are pragmatic, though with higher lifetime maintenance and weaker heat recovery from rooms not equipped with units.

What is the real maintenance burden, and why do owners complain?

Maintenance is the biggest gap between MVHR theory and ownership reality. Central systems require annual filter changes (supply and exhaust, cost approximately EUR 30-50 per set) and professional duct cleaning every 3-5 years (EUR 300-500), plus occasional damper or control adjustments. Decentralised units demand individual filter changes every 1-2 months per unit, making households with 4-5 units responsible for 24-60 filter changes per year. This is why decentralised MVHR has poor owner satisfaction in Slovakia despite lower upfront cost. Filters clogged with dust reduce air flow, forcing the system to work harder and increasing running cost. Damp, dirty filters become breeding grounds for mould and odours. Central system owners rarely notice the maintenance requirement because annual filter changes are manageable; decentralised unit owners often abandon the system or patch it with open windows. For any MVHR project, filter class matters: ISO ePM10 55% class is standard for residential (captures most dust, pollen, and particles); upgraded ISO ePM2.5 85% (fine particulate) costs 20-30% more but is worth specifying in allergy-prone households or near highways.

What happens in summer, and how does the bypass valve work?

Summer is when passive-house ventilation strategy splits into two camps. In northern Europe, continuous MVHR cooling air through heat exchange is sometimes desirable; in Central European summers (and Slovakia's warming climate), the reverse is true. A summer bypass valve automatically switches the heat exchanger out of the circuit when outdoor air temperature rises above indoor temperature (typically triggered around 18-20 degrees Celsius). Without bypass, the exchanger would cool fresh incoming air, adding to the cooling load on hot days. With bypass, the incoming air passes straight through without heat exchange, allowing natural cooling. However, bypass does not solve summer overheating in poorly shaded passive houses; it only avoids making it worse. Overheating is prevented through solar shading design, thermal mass, night-time ventilation, or occasionally small reversible heat pumps. The bypass valve is a necessity in Slovakia's climate, not a luxury.

How does winter humidity and enthalpy exchange work?

Winter brings a different challenge: indoor humidity. Occupants cooking, showering, and breathing generate moisture that must be extracted. A standard air-to-air heat exchanger (sensible heat only) transfers temperature but not moisture. Cold outside air entering at 2 degrees Celsius and 80% relative humidity remains very dry after warming in the heat exchanger, potentially leaving interiors uncomfortably dry. An enthalpy exchanger (or desiccant exchanger) transfers both heat and moisture, pre-humidifying incoming air during winter and dehumidifying it during summer. This adds cost (30-50% premium) but is worth considering in sealed homes where no moisture escapes through leaks. Slovakia's building code and passive-house standards do not mandate enthalpy exchange, but it improves comfort and reduces drying-induced structural stresses.

What about duct noise and running cost?

Duct noise comes from two sources: the MVHR unit's fan vibration and air velocity noise inside ducts. Modern units with low-noise EC (electronically commutated) fans are much quieter than older AC models, producing 30-35 dB at normal flow. Proper duct design minimises velocity noise: too-small ducts cause whistling and hissing. Flexible ducts with acoustic lining absorb high-frequency noise but collect dust; rigid ductwork with separate silencers is cleaner long-term. For most occupied spaces, well-commissioned MVHR is inaudible. The utility room where the main unit sits should have acoustic treatment if the room is near bedrooms.

Running cost depends on system size and electricity price. A typical central MVHR for a 150 m² passive house draws 50-80 watts during normal operation, costing EUR 15-25 per year (assuming EUR 0.18/kWh in Slovakia). Variable-speed fans can reduce this by 30% when demand is lower. The electrical cost of MVHR is trivial compared to heating or cooling savings. In fact, any house that needs MVHR (passive-house or airtight retrofit) saves far more in heating than it spends on ventilation electricity.

What are common misconceptions about MVHR?

First misconception: MVHR forces you to live in a sealed box with no natural ventilation. False. MVHR and opening windows are compatible; if someone opens a window, MVHR continues operating alongside, though efficiency drops temporarily. Second: MVHR is always energy-positive. Partly true for passive houses (where it enables the envelope) but not for loose, leaky buildings where mechanical ventilation adds energy cost without benefit. Third: all MVHR systems are the same. False. The difference between a cheap unit with undersized ducts and a well-commissioned system is dramatic. Fourth: MVHR eliminates humidity problems. No; without proper shading and thermal mass, MVHR alone cannot prevent overheating or condensation on cold bridges. Fifth: decentralised units are cheaper lifetime. Wrong for careful owners; central systems are cheaper to maintain and less prone to abandonment. For Slovak residential projects, the single biggest misconception is that MVHR is a luxury accessory. In a passive house or any airtight building, it is the foundation of livability.