MVHR interaction with heat pump heating

Mechanical ventilation with heat recovery extracts stale air and supplies filtered outdoor air through a heat exchanger. It can reduce the heat lost through planned ventilation, which can reduce room and whole-building heat loads. It does not make the home airtight or remove the need for a complete heat-loss calculation.

The heat pump and MVHR designs need the same account of the building: which rooms are supplied or extracted, how air moves between them, what flow rates are commissioned and how much unplanned leakage remains.

Planned ventilation and infiltration are different

MVHR recovers heat from the extract air before that air leaves the building. The recovered heat raises the temperature of the incoming supply air, so the heating system has less ventilation loss to replace.

Air still leaks through gaps and openings under wind and stack pressure. This infiltration bypasses the heat exchanger. Treating the quoted heat-recovery efficiency as a reduction to every air loss in the home would therefore understate the design load.

A heat-load calculation should represent separately:

• mechanical supply and extract airflow
• heat recovered between extract and supply air
• infiltration through the envelope
• deliberate openings, flues and other ventilation devices
• transfer air moving between rooms

Use design and commissioned data

The MVHR unit’s headline efficiency is not enough. The heat-loss calculation needs performance relevant to the installed duty, including the designed airflow and suitable heat-recovery data. The ventilation design also needs to consider duct resistance, imbalance, leakage and frost or bypass behaviour.

Useful evidence includes:

• unit make and model
• design supply and extract rates by room
• whole-system design airflow
• heat-recovery performance at the relevant operating point
• duct layout and insulation where ducts pass through cold spaces
• intended frost-protection and summer-bypass behaviour
• commissioning airflows and balance
• fan electrical demand for the energy estimate

The heating calculation should not assume the commissioned value will match a catalogue best case regardless of airflow or installation.

Airtightness matters

MVHR is most effective when most ventilation air follows the designed duct route. In a leaky building, a greater share of air bypasses the heat exchanger and the fans may not control the air path as intended.

Measured air permeability is the strongest input when it is available and compatible with the calculation method. Otherwise, the permitted default should be recorded. A visual description such as “modern airtight house” is not a test result.

As at 22 July 2026, the MCS Heat Load Calculator applies a warning when MVHR is selected without measured air-permeability data. Its current method determines room ventilation loss from the relevant room air-change requirement, intentional mechanical ventilation or the apportioned building infiltration rather than blindly adding all three.

Heat recovery does not equal heat production

MVHR preserves part of the heat already in the extract air. It is not normally the home’s space-heating system. The supply air is warmer than untreated outdoor air, but that does not mean it can meet every room’s design heat loss.

A separate air-heating coil or exhaust-air heat pump changes the system and must be designed as heating equipment. It needs its own capacity, temperature, airflow, control and room-distribution checks. It should not be inferred from the presence of ordinary MVHR.

Room-by-room effects

Supply air normally enters habitable rooms and moves towards extract rooms through transfer paths such as door undercuts. The room calculation should match that arrangement.

An extract room can receive air and heat from adjoining rooms. A supplied bedroom receives tempered outdoor air but still loses heat through its envelope and infiltration. Internal doors, transfer routes and flow imbalance influence the actual distribution.

Every heated room still needs a design load and suitable emitter. Reducing the whole-building ventilation loss by one percentage and applying it equally to all rooms can misallocate the benefit.

Sizing the heat pump and emitters

The compliant whole-building result, including the modelled MVHR effect, is used to select the heat pump. The room results select the emitters. The MVHR should be part of the same design case as the heat pump, not added as an informal deduction afterwards.

If MVHR is planned but not yet installed, show separate cases. Selecting a smaller heat pump on the assumption that future ventilation work will happen creates a shortfall if that work is delayed, altered or omitted.

The same caution applies when airtightness improvements are proposed. Their effect belongs in a post-improvement case supported by a defined scope and, where appropriate, testing.

Frost, bypass and extreme conditions

Heat-recovery performance can change when the unit protects its heat exchanger from frost. Strategies include changing airflow, bypassing part of the exchanger or using preheat. The consequences vary by product and climate.

The designer should check the manufacturer’s cold-weather operation rather than assume the annual or declared efficiency applies unchanged at the heat-pump design condition. Summer bypass is mainly a warm-weather function and should not be treated as winter heat recovery.

Controls and operation

The ventilation system should provide the airflow needed for indoor air quality even when the heating demand is low. Turning it down simply to save heat can increase moisture and pollutant levels.

Heating and ventilation controls should be coordinated without making either service dependent on an inappropriate signal. Boost airflow for cooking or bathing increases the short-term ventilation load; the heating system should remain stable without being sized from an arbitrary permanent boost condition unless the formal method requires it.

Filters, terminals and heat exchangers also need maintenance. Restricted or dirty components can change airflow and balance, which affects both ventilation performance and the assumed heat recovery.

Commissioning and handover

The finished installation should be balanced and measured at its terminals. The handover record should include:

• commissioned room supply and extract rates
• unit settings and boost controls
• filter type and replacement or cleaning instructions
• heat-recovery and frost-protection basis used in the heating design
• airtightness result or default used in the calculation
• any differences between the design and installed system

If the commissioned airflow or unit differs materially from the design, the ventilation heat-loss calculation should be reviewed before the heat-pump design is treated as final.

Related entries

• Heat loss survey
• Room-by-room heat loss
• Heat pump sizing margin
• Design flow temperature
• Commissioning flow rates