Heat-recovery ventilation in an old apartment: unit location and ducts

What does heat recovery give an old flat and is it worth it?

A heat-recovery ventilator brings in fresh air and extracts stale air, passing heat between the two flows at 75 to 90 percent efficiency. Windows can stay shut: CO2 does not build up in rooms, glass does not mist and the street outside stays quiet. In an old building on a busy Barcelona street that is a clear plus.

It is worth it when you are doing a full renovation and the ceilings are being opened anyway. Fitting it into a finished flat means losing finishes. In the new scheme ventilation does not compete with air conditioning but complements it: the ventilator deals with air and the climate system deals with temperature. More on indoor air is in our article on ventilation and air quality.

Where does the air intake go if the facade cannot be touched?

The most convenient places are an inner courtyard (patio de luces), the roof or an existing ventilation shaft of the building. Intake and exhaust are separated by at least 1.5 to 2 m so stale air does not return, and the intake is kept away from the kitchen flue, the garage and courtyard bins.

If the building is protected, no facade grille is planned at all: it needs a permit and spoils the view, as an outdoor air-conditioning unit does. Exit through a shared shaft is only possible with the comunidad's agreement. We covered the facade question in our article on protected buildings.

When the flat is on lower floors, the run to the roof is long, and a duct of that length eats noticeable ceiling height and fan power. Then the courtyard or a short wall exit into an enclosed space is considered.

How much space does the unit need and where does it go?

A horizontal unit for a 100 to 150 m2 flat is about 70 by 60 by 30 cm and moves 200 to 350 m3/h. It goes in the ceiling void of a corridor, bathroom or utility room, like ducted air conditioning, with an access hatch of at least 60 by 60 cm for filters and servicing.

The spot is chosen for size and also for noise: it is not hung over a bedroom. The hatch walls and the hanger itself get vibration isolation, and silencers go beside the unit on supply and extract. Condensate drains to the sewer with a fall, because it always appears in cold weather.

In the 250 m2 children's psychology centre in Eixample we did climate control with heat recovery, and the places for units, hatches and routes were settled at the same time as the hidden doors. In a flat the approach is the same: scheme first, finishes after.

How are the ducts routed in the ceiling void next to the air conditioning?

Supply and extract ducts run parallel to the air-conditioning routes but separate: one network cannot replace the other, because they have different flows and filters. Main ducts are round, 125 to 160 mm, branches 75 to 90 mm, and together they take 20 to 30 cm of ceiling height.

In an old building timber or steel beams get in the way. They cannot be cut: the structural designer decides, and ducts normally run along the beams, with crossings made only by calculation. So the route is drawn not on paper but on site, with the floors opened.

Supply goes to bedrooms and living room, extract to bathrooms, kitchen and utility room. The cooker hood stays separate with an outside outlet: grease clogs a heat-recovery unit in one season.

How often are filters changed and how is the unit maintained?

Supply and extract filters are usually changed every 6 to 12 months; in a city with polluted air and pollen, every six is better. A clogged filter raises noise and electricity use, and the air in rooms stagnates.

The heat exchanger is cleaned every year or two as the manufacturer states, and in summer the bypass operates, diverting air round the exchanger so the flat is not warmed. An alert signals filter changes by timer or by pressure.

The ceiling hatch must be convenient: if the unit can only be reached on a stepladder after removing light fittings, servicing is postponed and the system performs worse. It is wise to buy filters and spares in advance for several years while the model is still on sale.

Can heat recovery be combined with air conditioning and cooling?

Yes: they are two different systems in the same ceiling volume. The ventilator supplies fresh air and the air conditioner cools. There are schemes where a ducted unit mixes in fresh air from the ventilator, but they are designed separately so flow is not lost.

In hot weather recovery alone barely cools, but it reduces the load from open windows and lets you air the flat at night with cool air through the bypass. Together with dehumidification it also helps a cooling floor.

All routes are planned at the same time as electrics, plumbing and lighting: in the ceiling they share one volume and each system has its priority. Details on hidden services are in our article on installations before the walls close.

How can BCN YARD help with ventilation in an old building?

We visit the flat, look at floors, shafts and courtyard and propose a scheme that can be agreed with the comunidad. Routes and hatches are planned alongside the other services. We work with our own crew and trusted subcontractors, one person is responsible for the site, and we speak Russian and Spanish. The estimate is prepared after the visit and measurements.