What the unit does and what it does not
I hear two contradictory sentences about heat recovery ventilation. The first: you cannot build without it today. The second: the neighbours switched theirs off after a year and are happier for it. Both are true, because each describes a different machine. The first describes a system with drawn duct routes, measured airflows and changed filters. The second describes a box someone hung in the plant room and never set up.
Mechanical ventilation with heat recovery does one thing, and does it continuously: it supplies fresh air to the living rooms, extracts it from the kitchen, bathrooms and WC, and on the way out takes the heat from that air and gives it to the air coming in. This is balanced ventilation, supply and extract match, so the house is neither pressurised nor depressurised. The heat is moved by an exchanger, usually a counter-flow core, whose sensible efficiency in good units sits in the eighty to ninety percent band. For the passive house standard the minimum is seventy-five percent.
Now the other half. It does not heat, although it does cut the ventilation heat loss. It will not fix mould caused by a thermal bridge or by water getting in, that has to be dealt with in the fabric. It does not fully replace the cooker hood or the steam load of cooking. And it will not rescue a badly built envelope: the leakier the house, the more air travels around the exchanger through the cracks, and the less of the unit you actually get.
Five myths worth killing
Almost every argument about heat recovery rests on five claims. Each has a real kernel, it is simply assembled wrongly.
| Myth | Where it comes from | How it actually is |
|---|---|---|
| It replaces the heating | From early passive houses, where heating through the air was discussed | Air can carry only a small amount of power, because the airflow is set by hygiene, not by heat loss. Heat recovery reduces the ventilation loss, but it replaces neither the heat source nor the emitters |
| It dries the air out | From genuine winter experience, houses do feel dry in a frost | What is dry is cold outdoor air once it is warmed. The unit removes no moisture, it merely exchanges the air faster than necessary. The fix is a lower winter airflow, and if needed an enthalpy exchanger that returns part of the moisture |
| You must never open a window | From sales arguments | Open the window whenever you like. The system is bypassed for a while, nothing breaks. The only difference is that you do not have to |
| It is noisy | From installations with no attenuators and undersized ducts | Noise follows air velocity and missing attenuators, not the principle. With correct duct diameters and silencers on both supply and extract, the unit is inaudible in a bedroom on the night setting |
| It only makes sense in a passive house | From tying heat recovery to certification | What decides is airtightness, not the label. In an ordinary airtight new build it makes the same sense; in a leaky old house the payback is questionable |
| Set it once and forget it | From the fact that the unit runs even when set up badly | Filters, airflow checks and the summer mode are operation, not failure. A neglected unit quietly ventilates less and consumes more |
Commissioning the airflows: the single most common cause of dissatisfaction
When a client tells me the ventilation does not work, in the overwhelming majority of cases nothing is broken. The unit runs, nobody ever measured how much air goes where. Commissioning is not plugging the thing in. It is measuring the flow at every valve and grille, adjusting the restrictors until the measured values match the design, and only then issuing a report.
Without that you get the classic picture of an unhappy house. The upstairs bedroom is stale in the morning, because half the intended air arrives there. The bathroom mirror stays fogged longer than it should. One valve blows too hard and that room hisses. The sum of supply does not equal the sum of extract, so the house sits slightly under pressure and pulls air in through cracks, the letterbox and the drains. The owner reasonably concludes the technology is bad and turns it down to the lowest setting. That completes the problem: the house now officially has ventilation that does not ventilate.
So the contract with the installer needs a clause about a handover report with measured airflows room by room, compared against the design and signed by whoever measured them. Handover should also include setting the summer and winter stages, verifying that the summer bypass works, setting the filter reminder, and fifteen minutes of explaining the controls to the owner. Commissioning is the cheapest line in the whole system and the one that decides whether the client will be satisfied.
The real maintenance calendar and what it costs
Maintaining heat recovery is simple, but it has to happen. This is not a service contract worth thousands of euros, it is a handful of tasks a year, most of which the owner can do alone.
| Task | How often | Who | What happens if it is skipped |
|---|---|---|---|
| Changing the filters in the unit | Usually twice a year, more often in pollen season and on a dusty site. Looking at the filter decides, not the date | Owner | Airflow falls, fan consumption rises, the air starts to smell |
| Washing the valves and grilles | Once a year, more often on the kitchen extract | Owner | Grease deposits change the flow and undo the commissioning |
| Checking the condensate drain and trap | Before the heating season | Owner or service | Water inside the unit, drain smell through a dried-out trap |
| Removing and rinsing the heat exchanger core | Every one to two years as the manual says | Owner or service | Lower efficiency and higher resistance |
| Cleaning the intake and exhaust grilles on the facade | Twice a year, certainly after autumn | Owner | Leaves and insects choke the intake, the unit gets louder |
| Testing the summer bypass | In spring, before the first warm nights | Owner | The exchanger keeps adding heat in summer and the house overheats |
| Re-measuring the airflows | After a change of layout or occupancy, otherwise roughly every five years | Service | The system quietly drifts and ventilates where nobody lives |
| Camera inspection of the ducts, cleaning if needed | Roughly every ten years, sooner only on suspicion | Service | In a well designed and filtered system almost nothing settles in the ducts |
On costs my position is this: ask the supplier for the price of a filter set before you sign, together with whether it is a standard size or a cassette locked to one manufacturer. A set tends to be in the tens of euros, though it varies with the unit and the filter class, and twice that is your annual material cost. The second item is electricity for fans that run continuously. That is why it is worth asking about the specific fan power at the design airflow, and why oversizing the system generously is a bad idea.
Ducts and plant space belong in the drawings before the structure
The expensive mistakes in heat recovery are not made when choosing the unit. They are made when the ductwork is drawn into a house that is already built. At that point the route is found wherever it fits, not where it should go. The result is long bends, needless transitions and reductions, all of which raise the pressure loss. The fan compensates with speed, speed makes noise and consumption, and the owner learns that heat recovery is a noisy thing.
One point about flexible ducting deserves to be said out loud. Corrugated aluminium hose is the cheapest and the worst way to move air. Its inner surface multiplies the resistance compared with smooth duct of the same diameter, sagging runs above a ceiling collect condensate, and they cannot be cleaned. As a short flexible connection at the unit it has a place. As the distribution through a house it does not.
| Duct run | Pressure loss and noise | When it is acceptable |
|---|---|---|
| Corrugated aluminium flexible hose | Highest resistance, hard to clean, risk of sagging | Only as a short flexible connection to the unit |
| Semi-rigid round duct on a manifold system | Low resistance, each room gets its own branch with no reductions | The standard answer in a new build, easy to commission |
| Rigid galvanised duct with fittings | Lowest resistance at higher flows, demands accurate drawing | Main runs, larger houses, the plant room |
| Flat ducts in the floor or ceiling void | Acceptable if the cross section and the number of bends are respected | Renovations and shallow construction depths |
So the drawings need the plant space as a room with dimensions, not a hatched rectangle. The unit needs room to pull out filters and the core, a condensate drain to the sewer, a power supply and distance from a bedroom wall. The routes need ceiling void depth and penetrations through the structure that the engineer has to see before the slab is poured. And every penetration of the airtight layer needs a proper collar, otherwise the leaks show up in the blower door test and worsen the n50 value, which nobody then repairs without demolition.
Retrofitting into a house that never planned for it
It can be done, the balance of costs simply changes. In a new build the unit is the expensive part and the ducts are labour in an open structure. In a retrofit it is the other way round: the cost is dropped ceilings, chases, work in the floor, redecoration and cleaning, while the unit itself is the smaller part of the bill. The gap against installing during the shell stage tends to be a multiple rather than a percentage, which is why retrofitting is worth pairing with another large job, typically new floors, rebuilt ceilings or a roof renewal.
In an old house the realistic routes are three: a dropped ceiling in the corridor as the spine, the roof space for the upper floor, and a service shaft next to the bathroom. If none of that works, decentralised through-wall units come into play, working room by room, reversing their direction and needing no ducts. They reach neither the efficiency nor the balance of a central system and they are audible in a bedroom, but in a flat or a single extension they are a sensible compromise. The deciding question is always the same one: does this house have an envelope that will keep the air where I am sending it?
Summary
Heat recovery is neither a miracle nor a waste. It is a piece of equipment whose outcome rests on three things: routes drawn before the slab is poured, commissioning with measured airflows at handover, and filters changed twice a year. People who have all three never talk about their ventilation, because they never notice it. People who have none of them write online that it does not work. If you are weighing up the whole concept of the house rather than the ventilation alone, carry on with Passive House from A to Z.