Ventilation Survey: Assessing IAQ, Trickle Vents and MVHR in Existing Homes

Quick Answer: A ventilation survey assesses indoor air quality, mechanical and natural ventilation systems, and compliance with Approved Document F. UK survey cost £180–£500 typical for residential. Includes airflow measurement at extracts and supply, trickle vent area calculation, CO₂ and humidity logging, and assessment of MVHR balance and filter condition. Common findings: undersized trickle vents, blocked extract ducts, MVHR systems out of balance, and condensation risk in poorly ventilated rooms.

Summary

Indoor air quality (IAQ) and ventilation have become major topics following both the Approved Document F 2022 reforms and the broader public-health focus on respiratory and viral risk. A typical 1990s UK home was designed around natural ventilation (open windows, trickle vents, extract fans in kitchens and bathrooms); modern airtight new builds rely on mechanical ventilation (MVHR, MEV, dMEV); retrofitted period properties often have a hybrid combination. Surveying is the diagnostic process that identifies whether the system is working as designed.

The relevant regulations and standards:

• Approved Document F (Ventilation, 2022 update) — ventilation requirements in new and retrofit work
• BS EN 16798 — energy performance of buildings — ventilation
• BS EN 13141 — ventilation system component testing
• PAS 2035:2023 — retrofitting domestic buildings (requires ventilation assessment)

In 2026, ventilation surveying has become more sophisticated. Affordable monitoring kit (CO₂, PM2.5, RH loggers) gives 7–28 day data on actual conditions. Anemometers measure airflow at extracts and supplies. Smoke pencils trace airflow patterns. The survey market has grown alongside retrofit, with PAS 2035 retrofit assessors routinely including ventilation in their fabric assessments.

Key Facts

• Survey cost (2026) — £180–£500 for typical 3-bed residential
• Survey duration — 1.5–3 hours on site for typical residential
• Trickle vent requirement — 8,000 mm² equivalent area in habitable rooms (Approved Document F 2022)
• Trickle vent in wet rooms — 4,000 mm² for intermittent extract; not required for continuous extract
• Kitchen extract — 30 l/s if cooker hood ducted to outside; 60 l/s if intermittent at hob
• Bathroom extract — 15 l/s intermittent; 8 l/s continuous
• MVHR designed flow — typically 0.4 l/s/m² supply and extract balanced
• Acceptable CO₂ — under 1,000 ppm in habitable rooms; under 1,500 ppm short-term peaks
• Acceptable RH — 40–60% in habitable rooms; up to 70% in bathrooms briefly
• Acceptable PM2.5 — under 25 µg/m³ as 24-hour average
• Filter replacement — MVHR filters every 6–12 months
• System balance — MVHR systems require commissioning balance to ±10% of design

Quick Reference Table — Survey Findings & Causes

Detailed Guidance

Three ventilation strategies

UK homes use one of three ventilation approaches:

1. Natural ventilation with intermittent extract — trickle vents in habitable rooms, extract fans in kitchen and bathroom. Most pre-2010 housing.
2. Continuous mechanical extract (MEV/dMEV) — small fans run continuously at low speed, boost on demand. Common in mid-2010s new builds.
3. MVHR (mechanical ventilation with heat recovery) — supply and extract balanced, heat exchanger recovers warmth from extract to supply. Common in 2020s new builds and Passivhaus.

Survey approach varies by strategy:

• Natural ventilation — measure trickle vent areas, extract airflow, internal door clearances
• MEV/dMEV — measure individual extract flows, system balance, control settings
• MVHR — measure supply and extract balance, filter condition, ducting integrity, commissioning records

Trickle vent assessment

Approved Document F (2022) requires:

• 8,000 mm² equivalent area in habitable rooms (living, bedroom, study)
• 4,000 mm² in wet rooms (kitchen, bathroom, utility) where intermittent extract present
• Background ventilation provided by trickle vents OR window-night-vent OR mechanical supply

Common defects:

• Trickle vents installed but blocked by curtain or net curtain
• Trickle vents painted over during decoration
• Original 1990s trickle vents under-sized for current standards (typical 4,000 mm² historic vs 8,000 mm² required)
• Trickle vents in wrong location (e.g. above radiator — convection draws warmed air out)

Remediation: replace or upgrade trickle vents. Most window manufacturers offer retrofit upgrade ventilator units for £40–£100 per window.

Extract performance

Bathroom and kitchen extracts must achieve specific airflow rates:

• Kitchen — 60 l/s intermittent at the cooker (peak); or 30 l/s if hood ducted to outside; or 13 l/s continuous (MEV)
• Bathroom — 15 l/s intermittent peak; 8 l/s continuous (MEV)
• Utility — 30 l/s intermittent peak; 8 l/s continuous

Measurement: anemometer at extract grille gives air velocity; multiply by area to get volumetric flow. Typical fans achieve 50–70% of advertised performance after 5+ years of use. Many installations achieve <50% of designed flow due to:

• Long, twisty duct routing reducing flow
• Crushed flexible ducting in lofts
• Excessive bends and elbows
• Filter and grille obstruction

Remediation: re-route duct in straight, smooth path; replace flexible with rigid duct; replace fan if motor degraded.

MVHR balance and commissioning

MVHR systems must be commissioned to balance supply and extract within ±10% of design. Common findings:

• Commissioning never done — system installed and running but never balanced
• Balance lost — filter blockage on one side has moved system out of balance
• Wrong commissioning — set up to design wrongly (e.g. design errors in spec)
• Boost not working — boost button or humidity sensor failed

A poorly balanced MVHR system can cause:

• Negative pressure pulling air through structure (causing damp, sound transmission)
• Insufficient supply air to bedrooms (high CO₂ at night)
• Excessive energy use

Re-commissioning: airflow measurement at every supply and extract grille, then dampers adjusted to balance system. Typical cost £450–£900 for a 3-bed MVHR system.

CO₂, RH and PM2.5 monitoring

For comprehensive surveys, install monitoring loggers in occupied rooms for 7–28 days:

• CO₂ — peaks at night in bedrooms; spikes during cooking and group occupancy. Target: <1,000 ppm sustained.
• Relative humidity (RH) — peaks after showers, cooking, drying laundry. Target: 40–60% in habitable rooms.
• PM2.5 — peaks during cooking (especially frying), candle burning, woodstove use. Target: <25 µg/m³ daily.

Monitoring identifies real-world performance, not just design intent. Common findings:

• Bedroom CO₂ regularly exceeding 2,000 ppm at night (under-ventilated)
• Bathroom RH peaks above 80% with extract not turning on or under-running
• Living room PM2.5 peaks above 100 µg/m³ from cooking with poor cooker hood

Remediation: address specific findings — increase extract, improve trickle vents, balance MVHR, ducting changes.

When MVHR is appropriate

Three scenarios where MVHR is the right answer:

1. New build to current Building Regulations — usually included in spec; required where airtightness is below 5.0 m³/h.m²
2. Comprehensive deep retrofit — air-tight building requires controlled mechanical ventilation
3. Persistent damp / condensation — when other options exhausted

Three scenarios where MVHR is wrong:

1. Leaky building — MVHR is undermined by air leaks; install in airtight buildings only
2. Short-life use — MVHR has high install cost (£3,500–£8,000 for 3-bed) and 25+ year payback
3. Simple homes with adequate natural ventilation — over-engineered for the application

Frequently Asked Questions

What's the cost of a ventilation survey for a UK 3-bed semi (homeowner-friendly)?

Typical UK 3-bed semi ventilation survey: £180–£350 for a basic survey (visual inspection, airflow measurement at extracts, trickle vent assessment); £350–£500 for full survey with 7-day monitoring. Most surveys identify £500–£2,000 of useful remedial work. Especially valuable in older properties with damp/condensation problems, retrofitted properties where airtightness has been improved without ventilation upgrades, and properties with reported respiratory issues among occupants.

What does Part F require in retrofitted properties?

Approved Document F applies to new dwellings and to "material change of use" works. For typical retrofit (e.g. external wall insulation), Part F doesn't strictly apply but PAS 2035:2023 requires a ventilation assessment as part of retrofit design. Most retrofits add insulation and make buildings tighter without addressing ventilation, causing condensation and damp issues 12–24 months after retrofit completion.

Should I install MVHR in my Victorian terrace?

Usually not — Victorian terraces are typically too leaky for MVHR to work effectively. Air leaks bypass the heat exchanger, the system loses efficiency, and the cost is wasted. For Victorian properties, focus on:

• Adequate trickle vents (Approved Document F compliant)
• Good intermittent extracts in kitchen and bathroom
• Address damp and condensation at source
• If retrofitting heavily for energy: improve airtightness AND add MVHR together, not separately

How often should I service my MVHR system?

Filter change every 6–12 months (varies with environment); ductwork visual inspection annually; commissioning re-check every 3–5 years. Most homeowners forget about the system after install — many MVHR units run for 5+ years without filter changes, by which point efficiency has dropped 30–50% and air quality is being compromised.

Are bathroom extract fans mandatory?

In new builds, yes — under Part F. In existing buildings, ventilation must be "adequate" — defined in practice as either mechanical extract or operable window for opening. A bathroom without extract or window is non-compliant; remediation usually means installing an extract fan ducted to outside.

Regulations & Standards

• Approved Document F (Ventilation, 2022) — ventilation requirements in dwellings

• PAS 2035:2023 — retrofitting domestic buildings (ventilation assessment)

• BS EN 16798 — energy performance of buildings — ventilation

• BS EN 13141 — ventilation system components testing

• BS 5440 — installation and maintenance of flues and ventilation for gas appliances

• BS EN ISO 7730 — ergonomics of the thermal environment

• The Housing Health and Safety Rating System (HHSRS) — local authority enforcement framework

• WHO Indoor Air Quality Guidelines — PM2.5, CO₂ and RH targets

• Approved Document F — Ventilation, 2022

• PAS 2035:2023 — retrofitting domestic buildings

• BSI — BS EN 16798 building ventilation

• GOV.UK — Housing Health and Safety Rating System

• WHO — indoor air quality guidelines

• thermal imaging survey — combined with ventilation for full IAQ assessment

• specialist damp survey — adjacent diagnostic for condensation

• interstitial condensation — moisture failure mode in retrofitted walls

• heating controls and Boiler Plus — system control combined with ventilation