How to properly design a ventilation ductwork with heat recovery

Heat recovery ventilation is now standard in low-energy and passive houses, but it only works properly if it is backed by a well-thought-out ventilation system. The unit itself – centralized or decentralized ventilation unit HRC E – is only the heart of the system. Without a properly designed ductwork, correctly dimensioned components, and well-thought-out outlets, even the most expensive heat recovery unit will remain just an expensive fan with poor performance, high noise levels, and frustrated residents.

In this article, we will walk through the entire process of designing a ventilation ductwork step by step – from assessing the building, through calculating airflows, dimensioning the ducts, selecting the route, placing the outlets, to typical mistakes that appear in practice again and again. If you are planning a renovation or new construction, this article will save you from several expensive lessons from practice.

Why the ductwork design is as important as the unit selection

Over the years of practice, we have seen dozens of installations where the customer invested in a premium heat recovery unit, but saved on the ductwork – using unsuitable dimensions, ignoring minimum bend radii, placing the outlets wherever it was "convenient", and the result was that the system never reached the designed performance. The resulting airflows were 30–40 % lower than calculated, in some rooms the air did not circulate at all, and the noise level was unacceptable.

The ventilation ductwork must be viewed as a whole – every bend, every branch, every leak, every incorrectly placed outlet affects the overall hydraulic resistance of the network and thus the actual airflow through each branch. A heat recovery unit operates with relatively low pressure difference (usually 100–250 Pa static pressure), so every unsuitable fitting or unnecessary bend will be much more noticeable than in industrial ventilation with larger fans.

Step 1 – Assessing the building and determining hygienic airflows

The first step is a thorough analysis of the building. It is not enough to just count the rooms – you need to know how the rooms are arranged, where the load-bearing structures are (through which it is difficult to run the ducts), where the technical room or a suitable place for the unit is, and what the layout of the stairs, corridors, and entrance hall is, through which air naturally flows.

Hygienic airflows are determined according to the current standard STN EN 15251 and related regulations. Basic rules for residential buildings:

• Bedroom (1 person): minimum 15 m³/h, recommended 25–30 m³/h
• Children's room: 15–25 m³/h according to the number of people
• Living room: 25–50 m³/h according to the size and occupancy
• Kitchen (without extractor): 50–90 m³/h
• Bathroom: 25–50 m³/h
• Toilet (separate): 15–25 m³/h
• Laundry room / technical room: 25–50 m³/h

These values are minimums for standard occupancy. For passive houses or houses with low infiltration of the building envelope, we recommend considering values at the upper limit, because heat recovery is the only source of fresh air in these cases.

The total airflow of the system is obtained as the sum of all outlets on the supply side (fresh air into the living rooms) or on the exhaust side (exhaust air from wet and polluted areas) – both sides must be balanced, with a tolerance of ±10 % in favor of exhaust (mild negative pressure prevents condensation in the building envelope).

Step 2 – Choosing the ductwork topology: star vs. classic tree

There are two basic approaches to routing the ductwork, and each has different space, regulation, and hydraulic balance requirements.

Star system (radial distribution)

Each outlet is connected by a separate branch directly from a distribution box located in the center of the layout. This system is preferred in modern residential buildings today, because:

• Each branch has almost the same hydraulic resistance (it is enough to balance by length and diameter)
• Balance is simpler and more stable
• Small diameter ducts (63, 75, 90 mm) are easily routed in the structure
• Modular distribution boxes allow for expansion – for example, through a modular distribution box connector for 90 mm the system can be expanded without reconstructing the box

Branch system (tree)

A single main duct of larger diameter branches out gradually. The advantage is shorter duct length and less material, the disadvantage is much more complex hydraulic regulation – branches closer to the unit have significantly lower resistance than distant ones, and without thorough balancing using regulating dampers, the nearby outlets will have double the airflow compared to the designed and the distant ones will have almost nothing.

In practice, a radial star system with a central distribution box is almost always recommended for single-family homes. The branch system is justified in apartment buildings or larger buildings, where the number of individual branches would be disproportionately high.

Step 3 – Pipe sizing: diameter, speed, resistance

This is the most commonly underestimated part of the design. Many installers use one diameter "for everything" – usually 90 mm – without considering the actual flow in a given branch. The result is either too high speeds (noise, high resistance) or too low speeds (air does not settle, and the system operates at low efficiency).

Basic rule: air speed in the duct should not exceed 3.5–4.0 m/s in the main branches and 2.0–2.5 m/s in the end branches at outlets in residential buildings. Exceeding these values causes aerodynamic noise that spreads through the entire duct and cannot be easily eliminated without changing the dimensions.

Approximate table of the relationship between diameter, flow, and speed:

Diameter [mm]	Max. flow [m³/h] at 3.5 m/s	Typical use	Specific resistance [Pa/m] at max. flow
63 mm	39 m³/h	End branch – bedroom, WC	approx. 1.5–2.5 Pa/m
75 mm	56 m³/h	Bathroom, kitchen, living room	approx. 1.0–2.0 Pa/m
90 mm	80 m³/h	Collection branch, bathroom+WC together	approx. 0.7–1.5 Pa/m
125 mm	155 m³/h	Main duct after the unit	approx. 0.5–1.0 Pa/m
160 mm	255 m³/h	External connection, large buildings	approx. 0.3–0.8 Pa/m

In the design, always calculate with the equivalent length of fittings. Each 90° bend in a 90 mm diameter corresponds to approx. 3–5 m of straight pipe, a T-junction is even more. Therefore, the rule is: minimize the number of bends, use large-radius bends (min. 1.5× diameter), and maintain sealing at every joint – leaks in the negative pressure section of the suction duct cause uncontrolled air to be drawn from the structure, which degrades the energy balance of the entire system.

Step 4 – Duct routing: where to run and where not

The duct routing must meet several conditions simultaneously: be as short as possible, be hidden in the structure or in the ceiling, allow access to inspection points, and not interfere with the structure or the thermal insulation envelope of the building.

Horizontal routing

The most common solution in new buildings is routing in a drywall ceiling. The minimum height of the ceiling for a diameter of 90–125 mm is approx. 180–200 mm (including hangers and installation space). For a diameter of 160 mm, calculate with at least 230 mm. Be careful when crossing with electrical wiring and water pipes – ventilation has priority in routing, as it cannot be bent as easily as cables.

Pipes in unheated areas (basement, technical rooms) must be thermally insulated – otherwise condensation of moisture occurs on the cold wall of the pipe, which causes biological growth inside the duct system. Minimum insulation thickness: 20 mm for diameters up to 90 mm, 30 mm for larger diameters.

Vertical routing

Vertical routing through floors (e.g., from a ground-floor technical room to a floor above) requires coordination with other trades – usually installation shafts or hidden spaces behind built-in furniture are used. Minimum opening for a diameter of 90 mm in a concrete slab: 120 mm (including insulation and installation clearance).

Pass-throughs through the building envelope

Each pass-through through the exterior wall or roof must be sealed with a vapor barrier and thermally insulated to prevent thermal bridging. The external termination must be protected against rain, insects and birds – for this purpose, external grilles are used, such as external aesthetic grille for IVAR.HRC pr. 160 mm, which also aesthetically hides the pass-through and ensures protection against weather conditions.

Step 5 – Placement of supply and exhaust outlets

The position of the outlets directly determines the quality of ventilation in the room. Poorly placed outlets cause so-called short circuit – air goes directly from the supply to the exhaust without ventilating the living zone.

Supply outlets (fresh air)

They are placed in living rooms – bedrooms, children's rooms, living room, or office. The optimal position is in the ceiling or in the upper part of the wall, ideally opposite the window or door, so that fresh air passes through the entire room before reaching the exhaust. The minimum distance between supply and exhaust outlets in the same room should be at least 2.5 m – otherwise there is direct mixing without ventilating the zone.

Ceiling outlets are acoustically advantageous – air flows downward diffusely and no draft is created. Wall outlets are suitable where there is no ceiling, but attention must be paid to the direction of the outlet – it should not point directly at a seating area or bed.

Exhaust outlets (exhaust air)

They are placed in the bathroom, on the WC, in the kitchen (not instead of a range hood, but as additional ventilation). In the bathroom, the ideal position is in the ceiling above the bath or shower area – where the highest humidity occurs. In the kitchen, the exhaust outlet must be as far away from the cooking surface as possible, because fat and grease from cooking will clog the filter and duct much faster.

Important: exhaust outlets must be equipped with a check valve to prevent air flow from flowing back between rooms (e.g. odors from the WC to the kitchen via a common duct).

Step 6 – Hydraulic balancing of the system

Even with the best design, the final ductwork will have branches of different lengths and resistances. Without balancing, the flows in the individual branches will differ from the design values, sometimes dramatically. There are two methods of balancing:

• Passive balancing: Correctly selected pipe diameters and branch lengths so that each loop has a comparable hydraulic resistance. Requires accurate hydraulic simulation (e.g. in the program TRACE, LINDAB Ductsize or a simpler tabular calculation).
• Active balancing: A regulating damper is inserted into each branch, which after installation adjusts the individual flows by measurement (e.g. with a digital anemometer or flow collector). This procedure is more reliable and recommended whenever the branches are longer than 10 m or the number of branches is more than 5.

Measuring flows after installation is not a luxury – it is a requirement. The standard tolerance is ±15 % from the designed flow. If after balancing the flow is insufficient in any branch even with the damper fully open, the error is in the sizing or in something that increases the local resistance (dirt in the duct, broken bend, hidden leak).

With long branches (over 15 m with a diameter of 63–75 mm), it may be necessary to use flow booster for diameter 75 or 90 mm, which compensates for the pressure loss in the end branch without the need to overdimension the entire system.

Step 7 – Exhaust to the exterior: location, direction and protection

The recuperation unit requires two penetrations through the building envelope: fresh air intake from the exterior and exhaust of used air out. The location of these two openings has a decisive influence on the efficiency and hygiene of the entire system.

Basic rules for the placement of external outlets:

• Minimum distance between intake and exhaust: at least 1.5 m, ideally 2.0–3.0 m. If they are too close, the extracted moist air is sucked back into the intake opening – in winter this causes frost in the penetration and degrades the thermal performance of the recuperator.
• Fresh air intake should be located on the north or northeast side of the house, in the shade, away from the path and the neighbor's chimney. Minimum height above ground: 0.5 m (protection against dirty snow and leaves).
• Exhaust of used air can be on any side, but ideally below the intake or at least not above it (warm moist exhaust does not rise into the intake opening).
• Wall outlets must have a slope of at least 5° downward to prevent rainwater from entering. Roof outlets require a certified roof penetration with fastening into the roofing material.

More about choosing the location of the outlet can be found in the topic Roof vs. wall outlets for recuperation – what to choose and when in this Knowledge Center.

Decentralized systems – what changes in the duct design

With decentralized units, such as the decentralized ventilation unit HRC E – electronic version Master, the design philosophy is different. Each unit has its own opening through the wall, its own fan and its own recuperation insert. This eliminates the need for an extensive internal duct system.

Despite this, there are still several design rules for decentralized systems:

• Units operate in pairs or groups with a phase cycle (one blows in, the other out, then they reverse) – they need to be connected to a synchronized network, otherwise they interfere with each other's thermal performance.
• Wall openings must be arranged so that each room receives sufficient airflow without the need for air to flow through the door from an adjacent room.
• Mounting the unit in the wall must respect the insulation layer – the unit must not create a thermal bridge through the insulation.
• In larger houses, decentralized units can be supplemented with a simple interconnected ventilation network (so-called hybrid system), which better coordinates air flow between rooms.

More about choosing between centralized and decentralized systems can be found in the article Centralized vs. decentralized recuperation units – what is better in this Knowledge Center.

Typical errors in the design and implementation of the duct system

Over the years of practice, we have encountered several errors that repeat themselves again and again – sometimes even among more experienced installers who do not work with recuperation for a long time. Here is a list of the most common ones:

• Overdimensioned diameter on short branches: A branch with a flow of 25 m³/h in a 90 mm diameter has a speed of only 1.1 m/s – the air cannot spread sufficiently at the outlet and drops down directly as a cold stream. The correct diameter for this flow is 63 mm.
• Elbows with a small radius: Industrial air duct elbows with a resistance factor ζ = 1.5 are a disaster in residential ventilation. Use only shaped ducts from system manufacturers with a long elbow radius.
• Missing seals: Every unconnected and unsealed joint is a potential leak. In the supply section, it is an unnecessary pressure loss, in the exhaust section it is a hygiene problem (moist air from the bathroom penetrates into the structure). Use lipped circular seals for each diameter consistently at every joint.
• Too long supply branches: A branch longer than 20 m with a 75 mm diameter has a specific resistance of about 2 Pa/m, which gives 40 Pa just on the piping. With shaped ducts, you get to 60–80 Pa – that is a third of the total available pressure of a standard unit. Shorten the branch, increase the diameter or add a booster.
• Forgotten inspection openings: A duct network hidden in a false ceiling must have access for cleaning at least every 10 m and at every turn over 45°. Without them, the system cannot be hygienically maintained after 5–7 years.
• Unbalanced after installation: The system is physically complete, but no one measures or balances it. The customer thinks everything works – in reality, one room gets 60 m³/h and another 10 m³/h.
• Intake and exhaust too close to each other (exterior): A classic mistake with wall outlets – both openings are next to each other on the same wall. In winter, the extracted air freezes on the intake grille and blocks the suction.

Practical example: duct design for a family house 4+1, 180 m²

Consider a two-story family house with a ground floor (living room, kitchen, study, bathroom, WC) and an upper floor (3 bedrooms, children's bathroom, WC). The technical room is on the ground floor near the entrance.

Flow calculation (rounded):

Room	Type of outlet	Flow [m³/h]	Designed diameter
Living room	Supply	50	90 mm
Study	Supply	25	63 mm
Bathroom (ground floor)	Exhaust	50	90 mm
WC (ground floor)	Exhaust	20	63 mm
Kitchen	Exhaust	30	75 mm
Bedroom (2×)	Supply	2 × 30	75 mm
Children's room	Supply	25	63 mm
Children's bathroom	Exhaust	40	75 mm
Total supply		160 m³/h
Total exhaust		170 m³/h

The main duct behind the unit has a diameter of 160 mm (both directions – supply and exhaust). On the floor, a distribution box (supply and exhaust) with outlet necks of 63 and 75 mm is installed. Branches are routed in the suspended ceiling of the corridor on the floor. On the ground floor, the distribution box is suspended in the technical room. The total length of the pipes (both directions) is approximately 85 m, of which 40 m is on the supply side and 45 m on the exhaust side.

For this building, the required static pressure of the unit is approximately 180–220 Pa – this is a typical value for standard centralized units with a capacity of 150–250 m³/h.

Documentation, handover and regular maintenance

After the installation and hydraulic balancing are completed, it is essential to hand over complete documentation to the customer: an actual layout drawing with dimensions and the location of all components, measured flow values at each outlet, setting of control dampers, and an operating and maintenance manual.

The heat recovery system requires regular maintenance: filters on the outdoor air inlet should be cleaned or replaced every 3–6 months (depending on the dustiness of the environment), heat recovery

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