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How to Size Ventilation Ducting and Balance MVHR Systems
Correct duct sizing and careful system balancing are what separate a quiet, efficient MVHR or MEV system from one that is noisy, underperforming and difficult to commission. However, many designs in Irish homes still treat ductwork and ventilation duct systems as an afterthought.
This guide explains, in practical terms, how to size ventilation ducting and how to balance residential MVHR/MEV systems so they achieve the design flow rates and comply with Irish ventilation standards, including TGD Part F, TGD Part L and S.R. 54. Importantly, I.S. EN 14134:2019 sets out how airflows must be measured and calculated during commissioning, and NSAI ventilation validators use this standard when checking test results and certificates.
Why duct sizing and balancing matter
When ventilation duct dimensions are too small or badly routed, you will quickly see several problems:
- higher resistance and fan power
- higher noise levels at valves and in ducts
- lower actual flow rates than the design requires
- difficulty achieving balanced airflow and proving compliance
Conversely, well-sized and well-balanced ducting ventilation delivers clear benefits:
- design flow rates at sensible fan speeds
- quieter bedrooms and living spaces
- better heat-recovery efficiency in MVHR systems
- easier commissioning and clearer certificates for building control
In short, getting MVHR ductwork right from the start is one of the most effective ways to avoid callbacks and noise complaints in Irish homes.
Key concepts – air flow rates, velocity and pressure loss
Before deciding how to size ventilation ducting, it helps to keep three basic concepts in mind:
- Airflow (Q) – the volume of air moved, typically in litres per second (l/s) or cubic metres per hour (m³/h).
- Velocity (v) – the speed of air in the duct, in metres per second (m/s).
- Duct area (A) – the internal cross-sectional area of the ventilation duct, in m².
They are linked by a simple relationship:
Q = v × A
For residential ventilation duct systems, you typically aim for:
- main ducts: roughly 2–3 m/s
- branches to rooms: roughly 1–2 m/s
Higher velocities increase pressure loss and noise. Lower velocities, on the other hand, require larger standard ventilation duct sizes, which may not fit in ceilings, joists or service zones. Therefore, duct sizing is always a balance between acoustic comfort, available space and fan performance.
Step 1 – Start with the room-by-room air flow rates schedule
Duct sizing must always start with a ventilation schedule, not with a favourite duct diameter. Accordingly, the first step is to define the target flow rates.
- Use the design documentation or TGD Part F examples to determine extract and supply rates for each room.
- Sum flows along each path to identify:
- total supply flow
- total extract flow
- maximum flow in each section of ductwork
These numbers define how much air each piece of ventilation duct must handle. Only after you have this airflow schedule does it make sense to select standard ventilation duct sizes.
Step 2 – Choose a duct layout strategy
Before doing detailed calculations, decide on the overall duct layout. In Irish homes, two main approaches are common.
Radial / manifold systems
In radial ventilation duct systems:
- A central manifold distributes to multiple small semi-rigid ducts (for example, 75 mm or 90 mm).
- Each room may have one or more branches, depending on required flow rates.
- Pressure losses are relatively predictable and system balancing is straightforward.
This layout is often used for new builds where semi-rigid ducting ventilation is preferred for speed and coordination.
Branch / tree (rigid) systems
In a branch or “tree” system:
- Larger main ducts (typically 150–200 mm) run through the house.
- Branches (generally 100–150 mm) serve individual rooms.
- The system uses tees, reducers and bends, giving a more traditional ductwork layout.
Both options can work well in Irish dwellings. However, each has different implications for how to size ventilation ducting and how easy system balancing will be on site.
Step 3 – Sizing ducts in radial (semi-rigid) systems
For radial systems, manufacturers usually provide tables that link:
- duct diameter (for example, 75 mm)
- maximum recommended airflow per branch
- maximum recommended branch length
A typical approach is as follows:
- Decide a target airflow per branch (for example, 15–20 l/s per 75 mm line, depending on manufacturer guidance).
- For each room, calculate the number of branches required.
- Bedroom supply 15 l/s → 1 × 75 mm branch
- Living room supply 30 l/s → 2 × 75 mm branches
- Keep individual branch lengths within manufacturer limits.
- Group branches logically on the manifold (for example, upstairs bedrooms on one manifold, downstairs living on another).
Balancing is then achieved by adjusting the valves and, if necessary, the manifold regulators. Because each room has its own branch or pair of branches, interaction between rooms is limited and MVHR flow rates are relatively easy to fine-tune.
Step 4 – Sizing ducts in branch (rigid) systems
With traditional rigid MVHR ductwork, you have more freedom but also more responsibility. Consequently, a systematic approach is essential.
4.1 Size the main ducts
First, identify the maximum total airflow that the main supply duct must carry (for example, 120 m³/h). If required, convert to l/s:
Q (l/s) ≈ Q (m³/h) ÷ 3.6
Next, choose a diameter from standard ventilation duct sizes that keeps velocities within the target range.
Example:
- Main supply airflow: 120 m³/h ≈ 33 l/s
- 150 mm round duct: area ≈ 0.0177 m²
- Velocity v ≈ Q / A = 0.033 / 0.0177 ≈ 1.9 m/s → good
If you used 125 mm for the same flow rate, velocity would increase significantly, and so would noise and pressure losses. Therefore, main duct sizing is a key control on both acoustics and fan duty.
4.2 Size branch ducts to rooms
For each branch:
- Use the room design flow rate from the schedule.
- Select a duct diameter that keeps velocity around 1–2 m/s.
Typical guideline ranges (always confirm with manufacturer data):
- 100 mm branch: comfortable up to approximately 25–30 m³/h for bedrooms.
- 125 mm branch: suitable for higher flows to main living areas.
Avoid running many small branches off an already heavily loaded main without increasing the main size. Otherwise, pressure losses increase and system balancing becomes much harder.
For rectangular ducts, which are often used in ceiling voids or bulkheads, you should also consider the hydraulic diameter or equivalent area so that performance matches the intended round size. As a broad rule of thumb, a 204 × 60 mm flat duct is roughly comparable to a 125 mm round duct at typical domestic flow rates, although this “equivalence” varies with airflow and pressure loss. Therefore, it is always wise to check manufacturer data or use an air duct sizing calculator when substituting rectangular for round duct sizes.
4.3 Consider fittings and equivalent length
Bends, tees and reducers all add pressure loss. Many manufacturers provide “equivalent length” data where, for example, a 90° bend might equal 1–3 m of straight duct.
Accordingly, you should:
- Add the equivalent length of fittings to your straight runs.
- Identify the “index run” – the path with the highest total resistance from the unit to the valve.
- Check that this index run remains within the fan’s pressure capability at design flow rates.
If it does not, you may need to adjust the layout, increase duct sizes or review the chosen unit. In the Irish context, it is also good practice – and is generally expected under NSAI-aligned validation – to select an MVHR or MEV unit with at least +25% reserve capacity above the calculated general ventilation rate. This additional 25% headroom means the fans are not running at 100% output just to meet normal demand, which is crucial for noise control, energy efficiency and long-term reliability.
Step 5 – Plan for silencers and noise control
Noise issues in Irish MVHR systems are very often linked to:
- undersized ducts
- excessive air velocities
- lack of internal or branch silencers
When deciding ventilation duct dimensions, you should plan noise control at the same time:
- Reserve space for attenuators near the MVHR unit and, where required, near sensitive bedrooms.
- Keep velocities lower in bedroom branches and at bedroom valves.
- Avoid placing valves directly over beds wherever possible.
If you integrate acoustic thinking into how to size ventilation ducting, you reduce both commissioning challenges and post-occupancy complaints.
System balancing – getting from “roughly right” to “actually right”
Once the MVHR ductwork is sized and installed, the system needs proper system balancing. This is the fine-tuning process that ensures real flow rates match the design schedule.
There are two main levels:
- Unit-level balance – total supply equals total extract.
- Room-level balance – each valve delivers its design flow rate within a reasonable tolerance.
Tools you will need
To balance ventilation systems correctly, you will typically require:
- a calibrated flow hood or vane anemometer to measure supply and extract at valves
- access to MVHR or MEV fan settings (percentage, duty or speed)
- commissioning sheets to record readings and final settings
Typical duct sizing and balancing mistakes (and how to avoid them)
Even experienced teams encounter recurring issues. The most common mistakes include:
- Starting with duct sizes instead of airflow.
Always start from the ventilation schedule and flow rates; duct sizes come second. - Using one duct size everywhere.
This may be convenient, but mains, branches and terminals usually need different ventilation duct dimensions. - Long, narrow ducts serving high flows.
These create excessive resistance and fan noise. Increase diameter or shorten the route. - Too many rooms on one branch.
In rigid systems, a long branch with multiple tees can be almost impossible to balance. Split it into more sensible sub-branches. - Balancing only at the unit.
Turning fans up and down cannot correct local imbalances. You must adjust valves and branch regulators. - No allowance for future filter loading.
If you commission the system on the limit of the fan curve, performance will drop as filters load. Always leave a margin and, in Irish practice, aim for at least +25% reserve capacity above the general ventilation rate.
Avoiding these errors is often the difference between a system that quietly meets its design flow rates and one that struggles from the first day.
Practical Rules for Irish residential MVHR/MEV ductwork
These simple rules are not a substitute for full design, but they are useful during early sketches and coordination:
- Keep velocities in main ducts around 2–3 m/s, and in bedroom branches closer to 1–2 m/s.
- Use manufacturer guidance for semi-rigid radial systems and respect maximum branch lengths and standard ventilation duct sizes.
- In rigid systems, avoid long runs of 100 mm duct serving multiple high-flow rooms.
- Treat bedrooms as noise-sensitive zones: use smoother bends, larger ducts and silencers where necessary.
- Make sure every valve is safely accessible for measurement, balancing and future cleaning.
When you apply these rules consistently, ducting ventilation becomes much easier to integrate with structure and finishes in Irish homes.
Air Duct Sizing Calculator
You do not have to rely on manual calculations alone. For early-stage sketches and quick sense-checks, you can use an online air duct sizing calculator to estimate suitable diameters and velocities before you produce detailed drawings. Tools such as the Lindab Vent Tools duct sizing app and the H2X Engineering duct calculators are particularly useful for comparing different ventilation duct dimensions, visualising pressure losses and checking whether proposed duct runs are realistic. These calculators should not replace a full design in accordance with Irish regulations, but they can significantly speed up option studies and help you select sensible starting sizes for MVHR/MEV ductwork.
When to call in a specialist
You should consider bringing in a dedicated ventilation designer or commissioning specialist when:
- the dwelling is complex or has multiple wings and split levels
- there are strict acoustic or architectural constraints on ceiling height and duct routes
- you are unsure whether the chosen MVHR unit can overcome total system resistance at design flow rates
- you need an independent MVHR commissioning certificate and validation documentation for building control
Eco Vent can assist with duct design checks, MVHR ductwork reviews, system balancing, and full commissioning reports for dwellings across Ireland. This support helps ensure that your ventilation duct systems are not only compliant on paper but also quiet, efficient and robust in day-to-day use.