MVHR Installation: Mechanical Ventilation with Heat Recovery — Ducting, Commissioning and Part F

Quick Answer: Mechanical Ventilation with Heat Recovery (MVHR) is a balanced whole-house ventilation system that extracts stale air from wet rooms, transfers heat to incoming filtered air via a counterflow heat exchanger, and supplies tempered fresh air to living spaces. Building Regulations Part F 2021 sets minimum extract rates by room type. Heat recovery efficiency must be ≥85% (System 4) for new build. Ducting design, balancing and commissioning by SAP Appendix Q listed products is critical — a poorly commissioned MVHR underperforms a passive system.

Summary

MVHR (Mechanical Ventilation with Heat Recovery) is now the standard ventilation strategy for airtight new builds and increasingly for deep retrofits. The principle is simple: instead of losing warm, stale indoor air through trickle vents and extractor fans (and replacing it with cold air through gaps in the fabric), an MVHR system captures the heat from outgoing air and transfers up to 90% of it to incoming filtered fresh air via a counterflow heat exchanger. The result is continuous mechanical ventilation with very low energy penalty.

Part F of the Building Regulations (2021 edition) categorises ventilation systems as System 1 (background ventilators + intermittent extract), System 2 (passive stack), System 3 (continuous mechanical extract — MEV), and System 4 (continuous mechanical supply and extract with heat recovery — MVHR). At the new Part L 2021 fabric energy standards, System 1 is largely incompatible with airtight construction. MVHR has become the dominant system for new build to 0.5 ACH50 air tightness or better.

MVHR works only when designed, ducted and commissioned correctly. The most common failures are: undersized ducting causing fan strain and noise, unbalanced supply and extract causing pressure imbalance and moisture problems, filters not changed (clogged filter destroys SCOP), and bypass dampers not commissioned (summer overheating). A poorly installed MVHR system is worse than no MVHR — it consumes electricity and delivers no fresh air benefit. See ventilation strategy for system selection and heat pump sizing for whole-house energy strategy.

Key Facts

Quick Reference Table

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Room Type Function Part F Min Flow Typical Duct
Living room Supply 0.3 L/s per m² 75mm radial × 2
Master bedroom Supply 0.3 L/s per m² 75mm radial × 1
Single bedroom Supply 0.3 L/s per m² 75mm radial × 1
Kitchen Extract 13 L/s boost 100mm rigid or 2 × 75mm
Bathroom Extract 8 L/s boost 75mm radial × 1
En-suite Extract 8 L/s boost 75mm radial × 1
Utility Extract 8 L/s boost 75mm radial × 1
WC Extract 6 L/s boost 75mm radial × 1
Hallway Air path — 10mm door undercut
Plant room (MVHR location) — — 200mm OD intake/exhaust

Detailed Guidance

Step 1: Design heat exchanger size

The MVHR unit is sized by the design flow rate — sum of supply rates across all habitable rooms or sum of extract rates across all wet rooms (whichever is higher). For a 3-bed house with 100m² floor area, the background flow is 30 L/s. Add boost capacity by selecting a unit rated 30–60 L/s normal / 80+ L/s boost.

Step 2: Unit location

Locate the unit in a heated space (cupboard, utility, plant room) — not in an unheated loft. Heat losses from the unit casing reduce heat recovery efficiency, and the intake/exhaust ducts in cold space risk condensation. Where loft location is unavoidable, the unit must be fully insulated and a condensate drain installed.

Provide:

Step 3: Duct routing

Plan duct routes to minimise length and bends. Each 90° bend adds approximately 1.5–3.0 m of equivalent straight pipe loss. Rigid spiral duct is preferred for trunk lines (200–125mm). Semi-rigid (Polypipe Domus / Ubbink) 75mm radial is preferred for room connections — runs to a central manifold, with one duct per room.

Avoid:

Step 4: Insulation and vapour seal

The intake and exhaust ducts between the MVHR unit and the external terminals carry cold air and must be insulated and vapour-sealed. Use closed-cell insulation (25mm minimum) or insulated semi-rigid pipe. The supply distribution ducts in heated space need not be insulated, but should be acoustically separated where they pass walls.

Step 5: Terminal selection

Supply terminals are diffuser grilles or jet nozzles, ceiling-mounted, with directional vanes. Avoid blowing cold supply air directly onto occupants — aim toward a wall or upward. Extract terminals are simple grilles, ideally close to the moisture source (cooker hood, shower). Cooker hood may be either MVHR-connected (full extract through MVHR with grease filter) or separate recirculating (preferred — MVHR cannot handle grease loading well).

Step 6: External terminals

Intake and exhaust grilles on the external wall must be separated by ≥1.5m to prevent short-circuiting. Avoid placing the intake near pollution sources (driveways, bin stores, boiler flues). Both terminals are insect/bird-screened.

Step 7: Commissioning

Commissioning is the single most important step. Every terminal must be measured at design flow rate and adjusted via the manifold dampers until the readings are within ±10% of design. Use an anemometer or vane hood. Record:

COMMISSIONING SHEET (example)
Room        | Design L/s | Measured L/s | Damper position
Kitchen     |     13     |     12.8     |     65%
Bath 1      |      8     |      8.1     |     80%
Bath 2      |      8     |      7.9     |     78%
Bed 1       |      6     |      6.0     |     90%
Bed 2       |      5     |      4.9     |     85%
Living      |      8     |      8.2     |     70%
TOTAL SUP   |     30     |     30.0
TOTAL EXT   |     30     |     30.0
Balance     |      0     |      0.0
SFP at boost|    0.6 W/(L/s)

The supply and extract totals must balance (±5%). Unbalanced systems create pressure differentials that drive uncontrolled infiltration or interstitial condensation.

Step 8: Boost control

Boost mode triggers from humidity sensors (in bathrooms) or kitchen switches. Set humidity boost threshold at 65% RH (winter) — too low causes constant boost, too high allows mould risk. Cooker hood boost should be manual via switch on the hood or wall plate.

Step 9: Summer bypass

Summer bypass diverts incoming air around the heat exchanger when external air is cooler than extract air, preventing summer overheating. Commission the bypass setpoint at the unit (typically 24°C extract trigger, 18°C external lockout to avoid frost cycle). Test by simulating the temperature differential.

Step 10: Handover

Provide the homeowner with: filter change schedule, MVHR operating instructions, commissioning record, and warranty paperwork. Demonstrate boost switches and filter access. Building Control requires the commissioning sheet for completion.

Frequently Asked Questions

Can I retrofit MVHR to an existing house?

Yes, but air tightness is the gating factor. At 10 m³/h.m² @ 50 Pa (typical existing house), the MVHR's recovered heat is overwhelmed by uncontrolled infiltration — the system delivers little benefit. Retrofit MVHR is cost-effective from approximately 5 m³/h.m² @ 50 Pa downward, which usually requires deep retrofit including airtightness layer, IWI/EWI, and triple glazing. For uninsulated existing housing, dMEV (continuous extract) is more pragmatic.

Where should the intake be located?

On the windward elevation (typically S or W in UK), at high level (≥2.5m above ground), away from pollution sources: boiler flue (≥3m horizontal), driveway, bin store, septic vent. The exhaust should be on the leeward elevation if possible, but minimum 1.5m from the intake. Avoid both terminals on the same façade unless separated vertically by at least 1.5m.

Do I still need extractor fans?

No — wet rooms (kitchen, bathroom, en-suite, utility, WC) are served by the MVHR extract only. The cooker hood is the one exception: a recirculating hood is preferred so grease does not enter MVHR ducts. If a vented cooker hood is fitted, it must be a separate ducted fan, not connected to MVHR, to avoid grease loading.

How often must filters be changed?

Manufacturer-specified, but typically: F7 supply filter every 3 months in heating season (longer in summer), G4 extract filter every 6 months. In high pollen areas or near a busy road, supply filters may need monthly attention. Set a calendar reminder. Filter replacement is a homeowner job — typically £15–£30 per filter set.

What's a normal SFP for a well-designed MVHR?

0.4–0.7 W/(L/s) at design flow for a Passivhaus-certified or top-tier MVHR. Part F caps at 0.8 W/(L/s) for new build. SFP >1.0 indicates undersized ducting (too much pressure drop) and forces the fan to overwork. Always check the in-situ SFP at commissioning, not just the catalogue claim.

Regulations & Standards