What pipe diameter do I need for heat recovery ventilation
What pipe diameter do I need for heat recovery ventilation?
This question belongs to those that almost every builder or renovator asks when deciding on a mechanical ventilation system with heat recovery. The answer is not straightforward – it depends on the type of system, the building's layout, the required air flow rates, and whether it is a centralized or decentralized solution. In practice, I have seen cases where an overly small diameter was used, resulting in noise and insufficient performance, as well as the opposite extreme – oversized piping, unnecessarily high pressure losses, and unnecessarily high material costs. This article will guide you through the entire issue step by step, with specific numbers and examples from real projects.
Why is the pipe diameter so important?
The pipe in a heat recovery system is not just a "tube through which air flows". It is a key element of the entire air conditioning hydraulic circuit. An incorrectly chosen diameter has direct consequences on:
- Air flow speed – a too small diameter increases speed, leading to noise and increased pressure losses
- Pressure loss – every meter of pipe, every elbow, every branch creates resistance; small diameter = high resistance = weak flow or overloading the fan
- System noise – air flowing too fast generates noise in the pipes and at the outlets
- Energy efficiency – the fan must overcome the resistance of the network; oversized piping reduces the fan's energy consumption
- Air hygiene – slowly flowing air in oversized piping can condense moisture and create conditions for mold growth
Therefore, pipe sizing is not a matter of taste or chance – it is a calculation task that must be approached systematically. The following sections will show you how to do it.
Basic types of piping systems for heat recovery
Before we get into specific diameters, it is important to understand that there are essentially two different worlds of heat recovery ducting, and each works with different average dimensions.
Centralized systems – larger diameters, longer duct runs
A centralized heat recovery unit is located in one place (boiler room, technical room, attic) and distributes air to all rooms through a network of pipes. The main (collecting) pipe has a larger diameter, which gradually decreases towards individual outlets. Typical diameters for a single-family house:
- Main collecting pipe: 160 mm, 200 mm, 250 mm – depending on the total flow
- Branches to rooms: 100 mm, 125 mm, 150 mm
- Last sections to outlets: 75 mm, 90 mm, 100 mm
Decentralized systems – small diameters or no long duct runs
Decentralized units, such as Decentralized ventilation unit HRC E – electronic version 05 Master with a diameter of 160 mm, are placed directly in the wall of each ventilated room. The pipe here represents only a short core through the wall (usually 20–40 cm wall thickness) and the outside part is a simple outlet. Therefore, for decentralized systems, you do not need long duct runs – the pipe diameter is directly determined by the diameter of the unit itself (most commonly 160 mm).
How is the correct pipe diameter calculated?
Pipe sizing is based on two basic parameters: air volume flow (m³/h) and air flow speed (m/s). From these two values, we can calculate the required cross-section and from it the pipe diameter.
Step-by-step calculation procedure
The basic formula for calculating the diameter of a circular pipe:
d = √(4 × Q / (π × v)) × 1000
where: d = diameter (mm), Q = air flow (m³/s), v = air speed (m/s)
If we want to work with the more common unit m³/h, we convert: Q (m³/s) = Q (m³/h) / 3600
It is crucial to choose the correct design speed. For recovery systems in single-family homes, the following recommendations apply:
- Main collection pipe: 4–6 m/s (max. 6 m/s)
- Secondary branches: 3–4 m/s
- Last sections near outlets: 2–3 m/s
- For quiet operation requirements (bedrooms): max. 2 m/s
Practical example of calculation for a single-family home
Consider a single-family home with an area of 150 m² and the following rooms, where we need to design the air flow according to the standard (hygiene requires at least 0.5 times the room volume per hour, more for bathrooms and kitchens):
- Living room 30 m², height 2.7 m → volume 81 m³ → minimum flow 40 m³/h
- Bedroom 16 m² → volume 43 m³ → minimum flow 25 m³/h
- Children's room 12 m² → flow 20 m³/h
- Kitchen → flow 60 m³/h (extraction)
- Bathroom → flow 50 m³/h (extraction)
- WC → flow 25 m³/h (extraction)
Total flow: approx. 220 m³/h. For the main collection pipe at a speed of 5 m/s: d = √(4 × 0,0611 / (3,14159 × 5)) × 1000 = √(0,01553) × 1000 ≈ 125 mm → we choose the standard diameter 125 mm or 150 mm.
For the branch to the living room with a flow of 40 m³/h at a speed of 3 m/s: d = √(4 × 0,0111 / (3,14159 × 3)) × 1000 ≈ 69 mm → we choose 75 mm (the nearest standard size).
Standard pipe diameters for heat recovery – overview and use
Pipes are available on the market in the following standard diameters. Let's look at where each of them is typically used:
| Diameter | Typical use | Max. flow (4 m/s) | Note |
|---|---|---|---|
| Ø 63 mm | Last section to the outlet in apartments, seals | ~45 m³/h | For example tongue and groove circular seal pr. 63 mm |
| Ø 75 mm | Branches to individual rooms, apartments | ~64 m³/h | Common for smaller rooms, boosters for this diameter are available |
| Ø 90 mm | Branches to rooms, connectors of distribution cabinets | ~92 m³/h | Connector of modular distribution cabinet pr. 90 mm; also a booster for 75/90 mm |
| Ø 100 mm | Branches to larger rooms, kitchens | ~113 m³/h | Very common "medium" diameter |
| Ø 125 mm | Secondary collection pipes, kitchens in houses | ~177 m³/h | Good compromise for medium systems |
| Ø 150 mm | Main collection pipes of smaller houses | ~254 m³/h | For houses up to 150 m² |
| Ø 160 mm | Exhaust/inlet opening of the central unit, decentralized HRC | ~289 m³/h | External aesthetic grid for IVAR.HRC 160 mm |
| Ø 200 mm | Main collection pipes of larger houses | ~452 m³/h | For larger central units |
| Ø 250 mm | Main pipe of large objects | ~707 m³/h | Apartment buildings, administration |
Loop vs. Radial (Star) Distribution – Impact on Diameters
The way the pipes are arranged significantly affects what diameters you will need and where.
Radial (Star) Distribution
Each room has its own, separate pipe running directly from the distribution box (plenum box or distribution node). This system is the most hydraulically balanced, as each branch has the same characteristics. Diameters in a radial distribution are typically smaller and more uniform – 75 mm or 90 mm along the entire length from the distributor to the outlet. The distribution box then aggregates all these branches into one main pipe to the central unit.
Branch (Tree) Distribution
The main pipe runs from the unit and gradually branches off. In this arrangement, it is crucial to properly step down the diameters. With each branch, the flow decreases, and thus the pipe diameter must also be smaller. A branched distribution is cheaper in terms of material, but more difficult to hydraulically balance.
Special Cases – Flexible and Flat Ducts
Along with the classic round rigid or semi-flexible ducting, two alternatives are commonly used in heat recovery systems:
Flexible (Corrugated) Ducts
Used for the last sections before the outlet, where route adaptation is needed. They have higher pressure loss compared to rigid ducting (the corrugated surface creates more resistance), so it is reasonable to choose a larger diameter by one step or at least minimize their use to the last meter when dimensioning. Common diameters: 63, 75, 90, 100, 125 mm.
Flat (Oval) Ducts
They are advantageous in low construction space (e.g., in floor structures, behind drywall). They have the same flow area as round ducts, but a smaller installation height. Typical dimensions: 51×99 mm (equivalent Ø 75 mm), 55×110 mm (equivalent Ø 90 mm), 60×122 mm (equivalent Ø 100 mm). When dimensioning, use the hydraulic diameter: dh = 2 × a × b / (a + b), where a, b are the oval dimensions.
Influence of Duct Length and Shape Resistances on Diameter Selection
In practical calculations, it is not enough to simply choose a diameter based on flow and speed. You must also consider the total pressure loss of the entire loop, as the fan of the heat recovery unit has a limited static pressure (typically 100–300 Pa for domestic units).
The pressure loss of a straight duct is calculated as: ΔP = R × L, where R is the specific pressure loss (Pa/m) and L is the length (m).
The specific pressure loss depends on the diameter and speed. Approximate values for smooth ducts:
- Ø 75 mm, speed 3 m/s: R ≈ 1.8 Pa/m
- Ø 90 mm, speed 3 m/s: R ≈ 1.1 Pa/m
- Ø 100 mm, speed 3 m/s: R ≈ 0.8 Pa/m
- Ø 125 mm, speed 4 m/s: R ≈ 1.0 Pa/m
- Ø 150 mm, speed 4 m/s: R ≈ 0.7 Pa/m
To the pressure loss of the straight duct, you must add shape resistances. A 90° elbow on a duct of Ø 90 mm corresponds to an equivalent length of approximately 1.5–2.5 m of straight duct. A T-piece branch has an equivalent of 3–5 m. This means that if your route has 10 m of duct and 4 elbows, the actual equivalent length can be 20–25 m. For long routes, it is therefore advantageous to increase the diameter by one step.
For a better understanding, we recommend also reading the article How to Properly Design a Heat Recovery Ventilation System, where you will find a more detailed procedure for the overall dimensioning calculation including balancing the branches.
Duct Diameters in the IVAR PROFI-AIR CLASSIC System – A Practical Example
The IVAR PROFI-AIR CLASSIC system is a typical example of a well-thought-out modular distribution for centralized heat recovery ventilation in single-family homes. It works with two basic standard sizes: 90 mm for individual branches to the rooms and a larger diameter for the main collecting duct (usually 160 mm or more, depending on the total flow of the unit).
For example, the modular junction box for IVAR PROFI-AIR CLASSIC with a diameter of 90 mm is used to connect individual branches to the central duct, with each 90 mm branch being able to handle a flow of up to approximately 80–90 m³/h at an appropriate speed. This is sufficient for most rooms in a family house (bedrooms, children's rooms, living rooms).
If the flow in one of the branches is too low and there is a risk of air stagnation in the duct, the use of a booster for diameters 75 mm or 90 mm comes into consideration – this is a small inline fan that helps overcome the pressure loss of a long branch or compensates for hydraulic imbalance. In practice, this is an elegant solution for situations where one branch is 30–40 % longer than the others and would otherwise need to be oversized in a non-standard way.
Common dimensioning errors I see in practice
Over the years of contract work, the same mistakes keep repeating. Here is a summary of the most significant ones:
- Using the same diameter from the unit to the outlet – the installer takes Ø 90 mm and runs it from the unit through 15 m to the exhaust in the bathroom. Result: high pressure loss, suction noise at the outlet, insufficient flow.
- Too small diameter of the main collecting duct – if there are 6 branches of 90 m³/h in the distribution box, the total flow is 540 m³/h and a Ø 125 mm main section is not sufficient (only 177 m³/h at 4 m/s). A Ø 200 mm or larger is needed.
- Ignoring shape resistances – the project calculates only the duct length, forgetting about 8 elbows and 3 T-joints. The actual pressure loss is double, the fan is insufficient.
- Using flexible ducting on long sections – a corrugated hose for 5 m instead of a rigid pipe increases pressure loss by 50–80 %, and most people don't even test it.
- Forgetting thermal insulation in unheated spaces – ducts running in an unheated ceiling cavity must be insulated. Non-condensing air will cool below the dew point and moisture will condense in the duct – mold is guaranteed. The insulation thickness must be included in the construction solution (increasing the installation space).
- Undersizing the intake and exhaust opening through the façade – the external opening must have at least the same diameter as the connected duct, ideally larger. Grilles reduce the free cross-section, so it is good to choose, for example, a Ø 160 mm external grille even for a system working with a Ø 125 mm duct.
Practical scenarios from common projects
Scenario 1: New family house construction, 130 m², 5 rooms
A centralized heat recovery unit with a nominal flow rate of 200 m³/h is located in a technical room. A radial distribution with a central distribution box. Each branch (5 pcs) has a flow rate of 30–50 m³/h, branch length 4–10 m. Solution: branches Ø 90 mm, last meter flexible hose Ø 90 mm, from the unit to the distribution box Ø 150 mm. Total pressure loss of the most critical branch (10 m, 2 elbows): approximately 35–45 Pa, which is safely handled by any standard central unit.
Scenario 2: Apartment renovation, panel building, low structural height
Panel layout does not allow running round ducts in the ceiling cavity – the cavity height is only 8 cm. Solution: flat oval duct 51×99 mm (equivalent to Ø 75 mm) in the ceiling space, connected to round Ø 75 mm before the outlet and to round Ø 125 mm at the central unit. In total 3 branches, bathroom and WC served by special siphon outlets.
Scenario 3: Decentralized HRC units in a timber frame house
Timber frame house 120 m² with a wall thickness of 30 cm. 4 pairs of decentralized units (master + slave system). Each unit passes through a wall core Ø 160 mm. The outside is protected by an aesthetic grille. No long ducts, each room is ventilated separately. This configuration is the simplest in terms of duct dimensioning – the diameter of 160 mm is determined by the unit itself, the project only deals with the location of the cores and their coordination with the façade. More about this type of installation can be found in the article Installation of a decentralized HRC ventilation unit step by step.
Scenario 4: Large family house, 250 m², two floors
Here we encounter the need for vertical ducts and significantly longer routes. Central unit with a flow rate of 400 m³/h. Main pipe from the unit: Ø 200 mm (supply and return). On each floor, a distribution box connected with Ø 160 mm. From each distribution box, branches of Ø 90–100 mm lead to the rooms. Total pressure loss on the critical route (25 m, 4 elbows, T-piece): approx. 80–110 Pa – a unit with a minimum static pressure of 150 Pa is required.
Dimensioning of the external outlet – facade and roof
The external opening (supply of fresh air and exhaust of waste air) is the last in the hierarchy of dimensioning, but no less important. Grilles and covers on the external facade have free cross-sections of only 50–70 % of the total opening cross-section. Therefore, if you have a pipe of Ø 125 mm, the external opening should ideally have a diameter of 150–160 mm to avoid introducing additional pressure loss at the inlet/outlet.
Esthetically pleasing external grilles, such as external esthetic grille for IVAR.HRC 160 mm, also have the function of protection against birds, rain and insects, which is especially important for heat recovery – contamination of the intake opening has a direct impact on the quality of filtered air. Details on the choice of external outlet and its orientation on the facade vs. the roof can be found in the article Roof vs. facade heat recovery outlet – what to choose and when.
Condensate and pipe slope – a neglected factor
Condensation of moisture in the pipe is a real problem in heat recovery ventilation, especially in winter when the exhaust pipe runs through cold structures. The pipe must be dimensioned to ensure a minimum slope towards the condensate drain (minimum 2–3 % = 2–3 cm per meter). When the pipe is routed in the ceiling, this must be considered in the structural solution.
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