Mechanical ventilation with heat recovery
Fresh air in
every room.
The heat stays in.
A Scottish home built to today’s airtightness targets cannot breathe on its own. MVHR gives every room a continuous supply of filtered outdoor air, and keeps most of the warmth you have already paid for.
Fresh air, only where you live
Outdoor air is filtered and ducted to the bedrooms and living rooms. Nothing is recirculated: every breath of it came from outside.
Stale air, only where it’s made
Kitchen, bathroom and utility are extracted continuously. In between, air moves room to room under the doors, shown by the small grey arrows, so the whole house is swept in one direction.
and the heat stays behind
The two streams pass on opposite sides of a thin plate. Heat conducts straight through it; air cannot. That is the entire trick, and why the air leaving is nearly as cold as the air that arrived.
How it works
The plate is the whole trick
Incoming and outgoing air run past each other on opposite sides of a thin plate. Heat conducts straight through it. Air cannot.
Indoor temperature is held at 21 °C. Manufacturers typically quote 85–93% dry heat recovery under EN 13141-7 test conditions, measured in a laboratory on a new, clean unit.
In occupied homes real recovery more commonly lands between 60% and 85%, depending on how well the system was designed, installed, balanced and maintained. Passivhaus certification requires a minimum of 73%. We quote the number your system will actually hit, not the number on the box.
Outside, −1 °C. Inside, every room breathing at 18.
That gap is the entire product. The warmth you have already paid for stays in the building; only the stale air leaves, and it leaves cold.
Scottish building standards · Standard 3.14
How airtight you build decides what you are allowed to fit
Under the domestic technical handbook the ventilation strategy is not a free choice. It follows from the air infiltration rate the designer puts into the SAP calculation. Tighten the fabric to save heat, and you move down this ladder.
Trickle ventilators and extract fans
The handbook’s trickle ventilation guidance is written on the basis that infiltration of 5 to 10 m³/h·m² will be achieved. The fabric leaks enough to do part of the job for you.
Continuous mechanical extract (dMEV)
Individual fans running continuously, permitted where the infiltration rate is not less than 3 m³/h·m². Where they sit next to bedrooms, continuous noise should not exceed 30 dB LAeq,T. Replacement air still arrives cold, through vents.
Balanced supply and extract: MVHR
Where the designer intends an infiltration rate below 5 m³/h·m², the handbook says a continuously operating balanced supply and extract system should be used. Combine that with heat recovery and you have MVHR. Installation should follow BRE Digest 398.
MVHR, and nothing else
Below 3, decentralised extract is no longer offered as an option. There is not enough leakage left to supply the air an extract-only system depends on. This is the band a Passivhaus-standard house lives in.
Figures are the air infiltration rates given in Standard 3.14 of the Building Standards Technical Handbook (domestic), and they are set by the design, not measured afterwards.
Your building warrant application is where they get fixed. We would much rather be involved before that than be asked to solve it afterwards.
The three strategies, side by side
| Trickle vents + extract fans | dMEV continuous extract | MVHR | |
|---|---|---|---|
| Where fresh air comes from | Whatever blows through an open vent | Pulled in through vents and gaps | Ducted to each room, at a measured rate |
| Heat recovered | None | None | 60–85% typical in use |
| Filtration | None | None | Coarse on extract, ePM1 on supply |
| Works with everything shut | Poorly | Partly | Yes, that is the design case |
| Fabric it suits | 5–10 m³/h·m² | 3–5 m³/h·m² | below 5, the only route below 3 |
| Running cost | Heat lost to outside | Heat lost to outside | Fan electricity, heat retained |
What is coming, and when
Scotland is legislating a Passivhaus-equivalent energy standard. You cannot hit a space heating target that tight with a leaky building, which is why this is really a ventilation timeline.
-
In force now 2022
Standard 3.14, domestic technical handbook
Applies to building warrants submitted on or after 1 June 2022, with the ventilation methodology updated in 2023 to sit closer to the equivalent English standards.
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19 December 2025
The Scottish Government sets the direction
Full Passivhaus certification will not itself be mandated in the next revision of Section 6, but a certified Passivhaus will be accepted as an alternative way of demonstrating compliance with standard 6.1.
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Consultation closes 16 Oct 2026
Stage 2 consultation open
Detailed proposals for the performance targets and the compliance process, including a Target Space Heating Rate under standard 6.2. A voluntary version of the standard is proposed from 2026.
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31 March, mandatory 2028
Mandatory
The implementation date set out in the amendment regulations. Every new home will need to meet the space heating target, and the airtightness that implies puts ventilation squarely in the bottom two bands above.
What it costs
Honest numbers, and where they move
Two figures matter: what it costs to put in, and what it costs to leave running. We would rather show you the ranges and what pushes you up or down them than quote one number that suits us.
Installed
Covers unit, ducting, terminals, installation and commissioning for a whole house. What moves you within the range: house size and number of wet rooms, how far the unit sits from the terminals, whether ducting is rigid or semi‑rigid, and how much of the route is still open when we arrive.
Retrofit costs more than new build, not because the kit differs, but because the duct routes have to be found in a finished house. We survey before we quote, never after.
Running, per year
Most whole‑house systems cost roughly £30–£80 a year in electricity. Filters run £20–£120 a pair and are changed every 6–12 months, so all‑in maintenance plus electricity typically lands around £130–£250 a year. Set against that: the heat you are no longer throwing out of a trickle vent all winter.
What we do
Five stages. All of them, or any one of them.
Most MVHR that disappoints was not built badly. It was designed thinly, installed around other trades, and never properly commissioned. We work across the whole of Scotland, new build and retrofit alike.
Design
Room‑by‑room supply and extract rates, duct routes drawn before the joiners arrive, pressure drops and acoustics calculated, and the paperwork your building warrant needs.
New build install
First‑fix ducting sealed and supported properly, terminals set out with the ceiling grid, and the unit sited where it can actually be serviced in ten years’ time.
Retrofit
Into an occupied or refurbished home. We survey the real routes: joists, service voids, cupboards, bulkheads, and tell you honestly if the house suits a different strategy.
Commissioning
Every terminal measured and balanced to its design flow rate, unit settings set, and the results recorded. A system that was never balanced will not deliver the rates it was designed around, no matter whose unit is on the wall.
Servicing
Filter changes, core clean, condensate check and a re‑balance. Dirty filters make the fans work harder and can cost you 10–20% of your heat recovery inside a year.
Fixing other people’s
Noisy unit, cold draughts, condensation that never went away, a system nobody ever explained. Often it is commissioning, filters or a crushed duct rather than the unit itself.
- HandoverDay one
Commissioned and recorded, every terminal measured.
- Every 6 to 12 monthsFilters
Coarse on the extract side, ePM1 on the supply side.
- AnnuallyCore and condensate
Heat exchanger washed, drain checked.
- Every few yearsRe-balance
Flows measured back to their design rates.
What is in the air outside, and what gets past the filter
Supply-side filtration to ISO ePM1 50–80% catches pollen, fine dust and most bacteria before the air reaches a bedroom. It is a filter, not a wall: a little fine material still passes, which is why the filter class and the change interval both matter.
Straight answers
The questions we actually get asked
If the one you need is not here, ask. We would rather explain it properly once than have you find out after the ceilings are closed.
Can I still open my windows?
Yes. Open them whenever you like. The system is designed so that you never have to, which is a different thing from not being allowed to. In a Scottish January most people stop opening them because the house no longer feels stuffy, not because anyone told them to.
Do I still need trickle vents?
The handbook’s trickle ventilator guidance exists to give extract‑only systems somewhere to draw replacement air from when the doors are shut. A balanced MVHR system supplies its own replacement air through ducts, already filtered and already warmed.
Vents that let untreated outside air bypass the heat exchanger work against both the balance of the system and the airtightness figure the design depends on. It is a point to settle with your verifier at warrant stage, and one we will put in writing as part of the design.
Is it noisy?
A correctly designed system is quieter than the fridge. Noise comes from three places, all of them design decisions: air moving too fast because the ducts are undersized, the unit sited above a bedroom instead of over a hall or utility, and missing attenuators on the branches nearest sleeping rooms. For comparison, the handbook holds continuously running decentralised extract fans next to bedrooms to 30 dB LAeq,T.
Does it heat the house?
No, and be wary of anyone who says it does. MVHR recovers heat that is already in the air you have paid to warm; it does not create any. You still need a heat source. What changes is that your heating is no longer fighting a stream of cold air coming in through vents.
What about cooking smells and bathroom steam?
They go outside. Extract terminals sit in the kitchen, bathroom, ensuite and utility, and run continuously with a boost setting for heavy use. Because the two air streams are separated by the plates of the heat exchanger, only heat crosses over; nothing that smells travels with it. A recirculating cooker hood is a separate matter and should not be ducted into the MVHR system.
Can it go into an existing Scottish house?
Often, yes, but it earns its keep in proportion to how airtight the house is. In a draughty solid‑wall property the air you carefully warm can leak straight back out, and the recovery figures fall away. We will survey and tell you honestly whether to improve airtightness first, fit a different strategy, or do both in the right order.
What happens in summer?
Most units include a summer bypass that routes incoming air around the heat exchanger, so cool night air arrives cool instead of being warmed by the air leaving the house. It should be commissioned and set, not left on a factory default.
How is this different from a PIV unit in the loft?
Positive input ventilation pushes loft or outside air into the house through a single diffuser, usually on the landing, and pushes stale air out through the fabric. It is cheap and it can help a damp house. It is not balanced, it is not ducted to each room, and it recovers little or no heat, so it does not satisfy the strategies expected below 5 m³/h·m².
How do I know it was installed properly?
Ask for the commissioning record. It should list every terminal in the house with its design flow rate and its measured flow rate, plus the unit settings. If nobody can produce that document, the system was not commissioned. It was just switched on.
Get started
Talk to someone who surveys before quoting
Whether you are at warrant stage on a new build, part way through a retrofit, or living with a system that has never felt right, tell us where you are and we will tell you what we would do.
Details
- Phone
- 07841 674547
- Based in
- Aberdeenshire
- Covering
- Scotland
- Registered
- SC524725
Registered in Scotland, company number SC524725. We cover the whole country from Aberdeenshire.