Installing a Heat Recovery Ventilator

The void closes on Friday

Ventilation ductwork is the largest, least compressible service in a ceiling, and it arrives on site with the weakest claim on the space. Structure was there first. Drainage has gravity on its side and cannot be talked out of a fall. Cable trays can dogleg around anything. By the time the ventilation contractor unrolls a drawing, the easy runs are gone and the remaining route is the one nobody else wanted.

Losing that argument does not produce a redesign. It produces a squeeze: a flexible connector where a rigid sweep was drawn, a branch flattened to clear a beam, a manifold hung where nobody will ever reach it again, two extra bends to dodge a pipe that turned up late. Each of those is a permanent resistance, paid for by two fans running continuously for the life of the building, and every one of them disappears from view when the plasterboard goes on.

So the ventilation route is booked at first fix or it is not booked at all. Mark the unit position on the joists with its service envelope drawn around it. Set the two external sleeves through the wall with their falls cut. Drop the branch stubs into every room that will get a terminal, capped and labelled. Get the manifold locations agreed with whoever owns the void, in writing, while there is still a void to agree about. Photograph the whole run before the ceiling closes, branch by branch, with a tape in shot for scale. Six months on, when one bedroom will not hit its design flow, those photographs are the difference between a diagnosis and a hole cut in a finished ceiling.

Which side of the core each room sits on

Every room in the dwelling sits on exactly one side of the heat exchanger, and the choice is made by what the room produces. Rooms that make moisture, grease or odour are on the extract side. Rooms where people sleep and sit are on the supply side. Circulation space is on neither: it is the transfer path that lets air move from one column to the other.

Those two columns have to sum to the same total, and they have to sum to the dwelling rate. That constraint is what turns the room split into arithmetic instead of preference. Add a second bathroom to the extract column and something on the supply side grows to match, or the unit runs unbalanced and the building starts moving air through its own fabric.

The transfer path is the part that gets built last and designed never. Air arriving in a bedroom has to leave it, which means a door undercut with a measured free area or a transfer grille through the wall, sized and not guessed. Carpet fitted after commissioning routinely halves an undercut, and a door that seals against a threshold strip converts a balanced system into a pressurised room with a whistle.

Boost is a separate conversation from the continuous rate. It exists for events — a shower, a cooking session, a laundry load — and it should be triggered by something the occupant cannot casually defeat: humidity, occupancy, an overrun timer wired to the light. A boost switch that only works when somebody remembers it is a boost rate that does not exist.

Where the box hangs, and what has to reach it

Put the unit inside the heated envelope. A recovery unit in a cold loft spends winter warming its own casing, condenses on surfaces the manufacturer never intended to be cold, and freezes its drain on the night it is needed most.

Where a loft position is genuinely unavoidable, the unit and every metre of duct around it get wrapped as though they were outside, because thermally they are.

The service envelope is bigger than the box. Filters come out annually at least, the core comes out for washing, and both come out forwards. A unit hung tight against a rafter with a water cylinder in front of it will be serviced once, by you, and never again. Leave the manufacturer's stated clearance on the access face and leave a way to reach it that does not involve a stepladder balanced on a joist.

Weight and vibration go together. Hang from structure, not from ceiling battens, on anti-vibration mounts if the manufacturer supplies them, and keep the casing off the plasterboard so it does not use the ceiling as a soundboard. A unit fixed to the party side of a bedroom wall will be audible in the bedroom no matter what the sound data says.

Height is a drainage decision as much as an access one. The condensate spigot needs fall from where it leaves the casing to wherever the water is going, and that fall has to exist over the whole route. Hanging the unit as high as the void allows costs nothing at the time and buys the drain the gradient it will need.

Duct type decided before duct route

Three families of duct show up on this work and they are not interchangeable. Rigid galvanised spiral is the lowest resistance per metre, holds its shape, cleans out and takes a proper sealed joint. Semi-rigid radial ducting runs continuously from a manifold to a terminal with no joints in the middle, which removes most of the leakage risk and most of the crawling. Flexible ducting is the cheapest and the worst, and its place is the final short connection where the manufacturer allows one.

Radial and branched are different systems, not different layouts. A radial system runs one unbroken duct per terminal from a supply and an extract manifold, which makes commissioning tractable because a valve setting affects that room and nothing else. A branched trunk-and-spur system uses less duct and gives you cross-talk between rooms sharing a spur, plus a balancing exercise where every adjustment moves every other reading.

Bend radius is where the design either survives contact with the void or does not. A tight bend in semi-rigid duct is a permanent restriction and there is no way to see it afterwards; the manufacturer publishes a minimum radius and it is not advice.

Two long sweeps will beat four short elbows over the same displacement every time, and a route that adds a metre of straight duct to delete a pair of bends is the shorter route in the only sense the fan measures.

Keep runs to a similar length within a manifold where the layout allows. Wildly unequal branches can still be balanced with valves, but you balance them by throttling the short ones down to match the long one, which means the fan works at the pressure the longest branch demands while most of the system throws the difference away.

Terminals, throw, and the bed underneath

A supply terminal is a directional device. Air leaves an adjustable cone valve or a linear diffuser as a jet, attaches itself to the ceiling if it is close enough to it, and travels across the room losing velocity as it entrains the air around it. Get that throw right and the room mixes with no draught anywhere. Get it wrong and a cold jet drops out of the ceiling directly onto whoever is sitting under it.

Position accordingly. Keep supply valves away from the head of a bed and away from a desk chair, and let the throw run the length of the room instead of across its width. Extract valves go high in wet rooms where the moisture is, and away from the door, so the room's air crosses the space on the way out instead of short-circuiting from the undercut straight up the duct.

Valves have to stay adjustable to be commissioned. A cone valve painted into a ceiling is a fixed orifice at whatever setting the decorator left it; a valve fitted with its locknut already seized is the same. Fit them after painting or fit them with a protective cap, and check every one turns before the commissioning day starts.

Free area matters as much on the terminal as it does on the external grille. A decorative plate chosen by the customer can have a fraction of the free area of the valve it replaced, and the reading at that room drops without anything else in the system changing.

The two holes in the outside wall

The external terminals decide the quality of the air the system distributes, and they get sited late, by whoever is holding the core drill. Intake and exhaust have to be far enough apart that the unit does not breathe its own exhaust; the manufacturer publishes a separation and the prevailing wind decides whether that separation is generous or optimistic. Put the intake on the side the wind arrives from where you can.

Keep the intake away from everything that produces what you do not want indoors: flue terminals, plumbing vents, dryer ducts, bin stores, parking spaces, the ground itself. Height above finished ground level matters for splash, for snow and for road dust, and height above a flat roof matters for the same reasons plus standing water.

Grilles are chosen by free area, not by outside dimension. A weather louvre with an insect mesh can have well under half the free area of the duct behind it, and the mesh is the part that blocks. Choose a grille whose free area suits the design flow, then make the mesh accessible so it can be cleaned, because it will need it.

Sleeve both penetrations with a fall to the outside, seal the sleeve to the air barrier and the weather barrier on their own terms, and carry the duct insulation right through to the grille. A sleeve cut level is a rain path; a sleeve sealed only on the inside face is a wall cavity that fills up quietly.

What the second winter shows

Recovery ventilation degrades quietly and on a predictable schedule. Filters load and airflow falls, which the occupant experiences as a house that feels stuffier without anything having broken. External meshes clog with pollen and dust. Kitchen extract valves accumulate a film that changes their free area. None of this triggers an alarm, and by the second winter a system commissioned perfectly can be delivering a fraction of its design flow.

That is why the access decisions made at first fix matter more than any other. If filters can be changed from a landing in two minutes with no tools, they will be. If the change involves a loft hatch, a plank and a torch, the system will run on the original set until something else forces the issue.

Hand over more than a remote control. The occupant needs to know what the boost does, why the unit runs continuously, what a filter change looks like, and — most importantly — why switching it off in winter to save money produces condensation on the windows within a fortnight. A one-page sheet at the unit outlasts every verbal explanation. Leave the commissioning record where the next person will find it: valve settings, measured flows, fan speeds, filter sizes, and the date. A future imbalance is trivially diagnosed against a record of what the system did on the day it worked, and nearly impossible to diagnose without one.