MVHR and ERV Grille Placement: Where to Put Supply and Extract in an Airtight Home

Why Grille Placement Is a Retrofit Issue

As a retrofit makes a home more airtight, leaky building fabric stops providing accidental ventilation. Ventilation then has to be planned. That’s why PAS 2035 treats ventilation as part of the whole-house approach and doesn’t leave it as an afterthought. Retrofit Assessors record what is already there. Retrofit Coordinators make sure the improvement plan includes a ventilation strategy that suits the fabric work. Installers under PAS 2030 have to install and commission it so it actually performs.

A recent North American training video took one floor plan and walked through where to put the supply and exhaust points for a balanced energy recovery ventilator (ERV). The units and standards are American: CFM rather than litres per second, and ASHRAE 62.2 rather than Approved Document F. The design thinking carries over directly to UK MVHR work. This post converts the figures and pulls out the lessons that matter for retrofit exams and site practice.

Tip: In UK terms, an ERV is a balanced mechanical ventilation unit with heat and moisture recovery. Most UK specifications call it MVHR, and some units have enthalpy cores that behave like an ERV.

Step One: Set a Target Rate

The presenter’s main point is simple: start with a recognised standard, calculate a whole-house ventilation rate, and work from that number. In the UK, the reference is Approved Document F, alongside the ventilation guidance used within PAS 2035. Whichever standard applies, you want one continuous rate of fresh outdoor air that dilutes the pollution people produce just by living in a house.

His worked example used a continuous target of 105 CFM (about 50 l/s) for a large house of roughly 850 m³. That doesn’t mean replacing all the air every hour. It works out at roughly one full air change every five hours, which is reasonable for steady background ventilation.

Figure in the video	Approx. metric	What it represents
105 CFM	50 l/s	Calculated whole-house continuous rate
25 CFM	12 l/s	Each main extract point (shower, store, laundry)
15 CFM	7 l/s	Smaller extract points (WC, internal closet)
130 CFM	61 l/s	Total of all extract points
160 CFM	75 l/s	Unit capacity chosen to allow boost

The extract figures above are the presenter’s own design choices. They aren’t regulatory minimums. For UK work, take the extract rates for each type of wet room from Approved Document F and the system designer’s calculations.

Step Two: Put Extracts Where the Pollution Is

This is the most useful part of the video. You have two ways to make extract work well: fan power or geometry. Getting the geometry right is cheaper to run, quieter, and works better.

The Bathroom Rule

In the example bathroom, the door is on one side, the WC is in the middle and the shower is in the far corner. The extract goes inside the shower zone, in the far corner. Replacement air comes in under the door, crosses the whole room and leaves the room at the point where the steam is made.

Most builders put the extract in the middle of the ceiling. The moving air then takes a short route from the door to the grille and leaves the shower corner untouched. The only way to fix that is to turn up the fan. The presenter says that with the extract in the shower, the mirror over the basin never fogs up.

Did you know? Air takes clear, predictable routes through a building. Anyone who has run a blower door test has seen draughts rush towards the fan from rooms away. Your extract grille is the end of that route, so place it on purpose.

The Diagonal Rule for Every Room

The general rule is: put the extract diagonally opposite the door. The aim is to draw air through the whole room before it leaves. An extract just inside the door lets air in and straight back out, so the rest of the room never gets fresh air.

Apply it to:

1. Bathrooms and shower rooms: in or beside the shower, away from the door.
2. WCs: a smaller extract on the far side from the door.
3. Utility and laundry rooms: especially rooms with no window, where moisture has nowhere else to go.
4. Internal stores and closets: rooms with no external walls, so little heat loss or gain, and often no heating or cooling supply at all.

That last point is easy to miss. A room with no heat loss may get no conditioned air delivered to it, so it can become stale or damp. Putting a small extract there pulls air through it. This is directly relevant to the damp and mould risks covered in Damp, Mould and Condensation: A Complete Guide for Retrofit Professionals.

Step Three: Add Up the Extracts and Recheck the Rate

Once every extract point had a rate, the total came to 130 CFM, which is higher than the 105 CFM whole-house figure. The presenter then used the higher number as the continuous design rate. A whole-house calculation is a minimum. If the extract points you actually need add up to more, the design has to match that higher total.

Remember: Approved Document F sets minimum whole-dwelling ventilation rates and minimum extract rates for each type of wet room. A good design meets both, and in practice the extract total often sets the figure.

Step Four: Size the Unit for Boost

The presenter is clear on this: don’t buy a unit rated at exactly your design rate. With a 130 CFM continuous rate, he would look for a unit of about 160 CFM. The spare capacity allows a boost mode for parties, cooking, a household illness or several showers in a row.

He also warns about the opposite mistake. Some manufacturers push oversized units, so know your numbers before you ask a supplier for advice. A unit that is far too big costs more, can be noisy at low speeds, and may be hard to commission accurately at the bottom of its range.

Sizing mistake	Consequence
Unit rated exactly at design rate	No boost, so the fan runs flat out to cope with peak moisture
Unit grossly oversized	Higher cost, possible poor control at low flow
Rate taken from floor area only	May miss the total of the wet-room extracts

Supply Side: Keep It Simple

On the supply side, the presenter admits he built a fully separate supply and extract duct network and wouldn’t recommend it again. For homes with a ducted air handler (common in North America, less so in UK retrofit), he now prefers to feed fresh air from the ERV into the air handler’s return, just before its filter. The fresh air then gets filtered and mixed before it is sent round the house.

Most UK retrofits don’t have ducted warm-air systems, so supply normally goes to habitable rooms (bedrooms and living rooms), with extract from wet rooms. The principle still holds: don’t add duct runs you don’t need. The transfer paths between supply and extract rooms (door undercuts or transfer grilles) must be in place, or the balanced system can’t move air through the house.

For lower-disruption options in existing homes, see Retrofit Ventilation Systems That Fit Existing Homes, which covers cascade supply and room-by-room extract.

ERV or HRV in a Damp Climate?

The presenter strongly prefers ERVs to HRVs for most homes, especially in damp climates. An ERV recovers some moisture as well as heat, so it can limit very dry indoor air in winter and reduce the moisture load on the house in humid weather. In the UK, the choice between a standard heat-recovery core and an enthalpy core depends on occupancy, moisture risk and the manufacturer’s guidance. The Coordinator should justify it in the design and not leave it to whatever the supplier recommends by default.

What This Means for Each Retrofit Role

• Retrofit Assessors should record where existing extract fans are, whether they work, door undercuts, rooms with no windows, and signs of condensation. That record is what the ventilation strategy is built on.
• Retrofit Coordinators have to check that the improvement option evaluation and medium-term improvement plan include ventilation that suits the airtightness gained. Under PAS 2035, ventilation can’t be left until after the insulation goes in.
• Retrofit Installers working to PAS 2030 need to install grilles where the design puts them, keep transfer paths open, and commission flow rates accurately.

For a wider look at the building physics behind this, see Building Physics Revision for Retrofit Exams.

How RetrofitReady Fits Into This

Ventilation questions come up in every retrofit route because they link fabric, moisture and occupant health. Practising them by topic is the quickest way to get ideas like extract placement, transfer paths and boost capacity firmly fixed.

• RetrofitReady: Coordinator: 600 PAS 2035 questions across domestic retrofit, risk, fabric, services, ventilation, MTIP, and monitoring. Ideal for working through ventilation strategy decisions.
• Retrofit Assessor: 480 questions across eight syllabus areas, including multi-answer and diagram-based practice on recording existing ventilation and condition.
• RetrofitReady: Installer: 480 PAS 2030 questions across the two core units and six fabric measures, covering the installation and commissioning steps that make ventilation work.

Across the full suite, RetrofitReady offers 1900+ questions across 4 apps. Start with the free quizzes or browse the complete RetrofitReady app suite.