MVHR · Whole house & basement
Heat recovery ventilation (MVHR)
Ventilation is the half of a habitable basement that gets designed last and causes most of the complaints. A room below ground often has no window anybody can open, so the air has to be moved mechanically — and once it is being moved mechanically, there is no longer any reason to throw the heat away with it.
The system in section
What is actually behind the wall.
- The unit, in the basement plant space. Two air paths through one box that never mix: stale air out of the wet rooms on one side, fresh air in from outside on the other, and most of the heat stepping across between them. It is the only moving part in the system and the only thing that needs a filter change.
- Fresh air intake, above external ground level. Deliberately the lower of the two, and deliberately not beside a flue, a boiler terminal or a bin store — everything this system delivers to the house for the next twenty years comes through this one opening.
- Exhaust, separated from the intake. Put them close together and the unit spends its life drawing back the air it has just thrown out, which quietly costs you both the ventilation and the heat. The separation is a design decision, not an accident of where the holes were easiest to cut.
- Extract from the wet rooms. Shower room, kitchen, utility, WC — air is drawn from where the moisture is actually produced, which is what keeps it out of the rest of the basement rather than merely diluting it once it has spread.
- Supply to the habitable rooms. Filtered, and arriving at close to room temperature rather than at whatever it is outside. The branch rising through the slab is the same system serving the floors above: below ground is usually where the plant goes, not the limit of what it serves.
- Lagging where a duct crosses a cold zone. A duct carrying cold outside air through a warm house sweats, exactly like a cold pipe, and an unlagged run above a ceiling will drip into it. Insulating these two is not a refinement — it is the difference between a ventilation system and a slow leak.
- Condensate, trapped, to foul. The exchanger produces water and it has to go somewhere. It goes to a trapped foul connection — not into the waterproofing sump, which discharges to surface water and already has the one job the whole basement depends on.
- A sealed collar wherever a duct crosses the membrane. This is the detail nobody else is looking for. Every one of these is a hole through the waterproofing, and a hole through the waterproofing is how a drained system fails two winters later. We form and seal them as designed penetrations while the membrane is going on, rather than coring through a finished wall and hoping.
Why a basement cannot be ventilated by opening a window
Approved Document F sets out how a home is meant to breathe, and its default assumptions are a window somebody can open and a trickle vent in the frame. A basement frequently has neither. A light well is not a window anyone opens in January, and a room with no external wall at all has nothing to put a vent in.
So below ground, mechanical ventilation is not an upgrade on the natural sort — it is the only route to a compliant, comfortable Grade 3 room. That is worth stating plainly, because it is usually sold as an extra.
And once the air is being moved by a fan rather than by the weather, throwing the heat out with it becomes a choice instead of a fact of life. That is the whole proposition: the ventilation a basement has to have anyway, without the heating bill it would otherwise carry.
What heat recovery actually recovers
The unit is a box with two air paths through it that never mix. Stale, warm air leaves the wet rooms; cold, fresh air comes in from outside; the two pass either side of an exchanger and most of the heat steps across from the outgoing stream to the incoming one. What reaches the room is outside air at something close to room temperature.
Below ground that is worth more than it is upstairs. A basement is thermally stable and slow to warm — the same property that makes it pleasant in August makes it expensive in February — so heat you have already paid for is heat worth keeping hold of.
What we will not do is quote you an efficiency figure off a brochure. Manufacturers test in a laboratory and the number is true there. What you actually get is decided by the duct layout, the tightness of the joints and whether anybody commissioned the thing — which is why the rest of this page is about the installation rather than the box.
The ducts go through the waterproofing, which is the whole reason we do this
A Type C system works because it is continuous. Every duct that crosses it is a penetration, and penetrations are where drained systems fail — the same sentence appears on our cavity drain membrane page, and it is not a coincidence.
A ventilation contractor arriving at a finished basement will core through a wall to get a duct where the design needs it. Behind the plasterboard is the membrane; behind the membrane is the reason you had the work done in the first place. They will seal it with whatever is in the van, in complete good faith, having never seen the system they have just gone through.
We set the duct routes while the membrane is going on, and detail every crossing as a designed element alongside the rest of them. It is the same argument as running a conversion with one team instead of two: the join between trades is where responsibility gets lost, and this particular join puts a hole in your waterproofing.
Dew point, and the one case where more ventilation makes it worse
Condensation is not decided by humidity. It is decided by the gap between the dew point of the air and the temperature of the surface it touches. A wall at 13 °C in a room whose air has a dew point of 12.3 °C is seven tenths of a degree from wetting, and no humidity reading on its own will tell you that.
In winter, ventilation moves that gap the right way. Cold outside air carries very little moisture in absolute terms; bring it in, warm it through the exchanger, and the humidity in the room falls. The dew point drops away from the wall surface and the risk goes with it.
In summer it can run the other way. A basement stays cool, and warm humid August air brought into it can arrive with a dew point above the temperature of the wall — at which point ventilating harder makes condensation more likely rather than less. That is what a summer bypass is for, and occasionally the honest answer is a dehumidifier alongside rather than more air. Anybody who tells you ventilation is always the cure for damp has not thought about July.
If you would rather have the numbers than the argument, this is precisely what basement monitoring measures: the air, the wall surface, and the margin between the two.
Filters, noise, and whether you are still using it in three years
An open window lets in whatever is outside, which in a light well onto a London road is not nothing. A mechanical system is the only kind that filters what it brings in, and for a basement bedroom near a main road that is often a bigger benefit than the heat.
Filters are consumables. A neglected one strangles the system slowly enough that nobody notices — the room simply gets stuffier over a couple of years. We set the replacement interval at handover and tell you what the filters cost, because a running cost discovered afterwards feels like a fault.
Then noise, which is what actually kills these systems. You may well have put the cinema or the gym below ground because it is quiet down there. So: a unit sized to run slowly rather than flat out, ducts sized so the air is not forced through them, attenuators where the design calls for them, and cross-talk between rooms designed out rather than discovered. A system you can hear is a system somebody switches off, and a ventilation system that is off protects nothing at all.
Commissioned, not just installed
Rigid or semi-rigid duct rather than the flexible sort — a crushed or sagging flexible run can cost a system a large part of its performance, and it is invisible the moment the ceiling goes up. Joints sealed. Insulation wherever a duct crosses a cold zone, because otherwise the duct sweats and you have built a condensation problem into the cure for a condensation problem.
Then it is commissioned: the flow rates measured room by room, adjusted until they meet the design, and the results written down and handed over. That record is part of the Building Regulations paperwork, and it is also the only evidence that the system does what it was sold as doing. An installer who gives you no figures has given you an untested system.
And where MVHR is the wrong answer, we will say so. An unheated cellar used for storage does not need it. Nor does a house so draughty that the recovery has little to recover — in a genuinely leaky building the air is leaving through the fabric regardless, and the money goes further on the fabric first. Sometimes the right answer for a single basement room is a single-room unit rather than a whole-house system, at a fraction of the cost.
Common questions
What people ask us about heat recovery ventilation.
Is MVHR the same as air conditioning?
No, and it is the most expensive misunderstanding in this area. MVHR ventilates — it replaces the air in the building and recovers the heat while doing it. It does not cool a room on a hot day. The supply air arrives at roughly the temperature of the air it is exchanging with, and in summer the bypass stops the system warming the house further, but that is not cooling. If the requirement is a basement cinema that is cold in August, that is a separate conversation about comfort cooling and we would rather have it at survey than at handover.
Will it dry out a damp basement?
No — and be careful with anyone who says it will. Ventilation manages the moisture the room itself produces: people, showers, cooking, drying clothes. It does nothing whatever about water arriving through the structure, which is a waterproofing problem and needs a waterproofing answer. Fitting ventilation to a basement with water coming in gives you a well-ventilated wet basement.
Does it work in an old house that is not airtight?
Less well, and that is the honest limit of it. Heat recovery only pays back on air that actually goes through the unit; air leaking through gaps in the fabric bypasses it entirely. In a very leaky house you still get controlled ventilation and filtration, but the recovery is doing less than the brochure implies, and sealing the fabric is the better first pound. A basement is often the airtight part of an old building — it is buried on five sides — which is why basement-only and single-room systems can make sense in a house where a whole-house one would not.
How noisy is it?
A properly sized and commissioned system is quieter than a fridge, and the unit itself normally sits in a plant space rather than in a room. Noise complaints almost always trace back to one of three installation decisions: a unit sized too small and therefore running near its maximum, ducts too narrow for the flow they are carrying, or flexible ducting used where rigid was specified. None of them is a fact about the technology.
What maintenance does it need?
Filter changes, typically twice a year depending on the filter grade and how dirty the outside air is, plus an annual check and clean of the unit and its condensate drain. It is genuinely low but it is not zero, and a system with clogged filters is worse than no system at all, because you believe you are ventilated.
Can it be added after the basement is finished?
Yes, and it is a worse job for a worse price, for the reason set out above: the duct routes have to cross the waterproofing, and doing that through a finished, boarded and plastered room means coring through a membrane you cannot see and cannot inspect afterwards. If the basement is being done now, decide about ventilation before the boards go on rather than after.
Is it notifiable under Building Regulations?
Yes. Ventilation work in a dwelling falls under Approved Document F, and the installation has to be commissioned with the measured flow rates recorded and handed over. We do the commissioning as part of the job and the paperwork comes to you at handover. Where the ventilation forms part of a wider basement conversion it sits inside that project's Building Control application; for standalone work we confirm at survey how it will be notified, so you know before we start rather than afterwards.
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