What Cabinet Size Based on Number of Circuits

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Why the number of circuits is the starting point for choosing a cabinet

When choosing a manifold cabinet, most people make the same mistake - they first pick a cabinet based on where it "somehow fits" into the wall or the available space, and only afterwards check whether the manifold with all its circuits will fit inside it. The correct approach is exactly the opposite. First you need to know how many heating or water circuits will be connected to the manifold, use that to work out the manifold's actual width with its fittings, and only then choose a cabinet size with an adequate margin. If the order is reversed, it very often happens that the manifold literally doesn't fit into the chosen cabinet, or it does fit but with no room for servicing - bleeding, replacing a head, working the valves.

In this article we'll show how the number of circuits translates into the manifold's width, what margins to allow for, and how to choose a specific cabinet size accordingly. We'll base this on the actually available surface-mounted cabinets in three common widths - 385 mm, 485 mm and 615 mm - which cover most flat and family-house installations.

How the number of circuits relates to the manifold's width

A manifold (a hydraulic heating manifold, often also called a collector) is built from modular segments - each circuit represents one supply/return pair connected to the shared manifold body. For common manifolds used in both underfloor and radiator heating, a spacing of roughly 50 mm per circuit is assumed between the individual circuits. To that you need to add the end elements - the inlet and outlet valve, ball valves, and possibly an air-vent and drain valve at the ends - which together take up a further roughly 100 to 150 mm of extra width.

A simplified formula that has proven useful in practice for a quick estimate looks like this:

manifold width (mm) ≈ 100 to 150 mm (end fittings) + (number of circuits × 50 mm)

This is an approximate calculation - specific manifolds from different manufacturers can differ in detail (ball-valve width, the presence of flow meters on individual circuits, thermostatic heads instead of simple shut-offs), so you always need to allow for the margin discussed below. For orientation, though, let's show how this width grows with the number of circuits, using the specific examples we most commonly see in practice.

As you can see, going from 4 to 12 circuits roughly doubles the manifold's own width. That's exactly why cabinets are made in several width steps too - a single universal size would mean a needlessly big and expensive cabinet for small installations, and conversely, for large ones, a manifold that simply wouldn't fit at all.

Why it isn't enough to calculate "to the millimetre"

A common mistake when ordering a cabinet is to calculate the manifold's width completely tight and choose a cabinet whose interior is only a few millimetres bigger. In practice this causes several real problems we regularly see during installations:

Thermostatic heads and actuators - if actuators for a room thermostat or thermostatic heads on individual circuits are fitted to the manifold later, these elements stick out from the manifold and need free space that at first glance "isn't needed".
Lever ball valves - the levers of shut-off valves have to be able to turn a full 90°, which in a tightly fitted cabinet often hits the frame or door.
Flow meters - if the manifold has flow meters fitted on the supply (common in multi-zone underfloor heating), they increase not just the manifold's width but also its depth.
Future expansion - in both flats and houses, the layout changes over time, a bathroom towel radiator gets added, a heating circuit for an extension, or the underfloor heating zoning changes. A cabinet with a margin for at least 2 extra circuits saves a complete replacement a few years later.
Servicing access - bleeding a circuit, replacing a valve gland, or cleaning a filter needs room for tools around the manifold, not just "just enough for the door to close".

For this reason, we recommend always allowing a margin of at least 100 to 150 mm on top of the calculated manifold width when choosing a cabinet, ideally more if it's already clear the number of circuits will increase in the future.

Step-by-step cabinet selection process

The whole selection process has four steps in practice, following on from each other. We recommend proceeding in exactly this order - from the number of circuits toward the cabinet size, not the other way round.

Step 1 - count the actual number of circuits

The circuit count includes absolutely everything that will be connected to the manifold - not just underfloor heating zones, but also radiator branches if they run through the same manifold, and any circuit for a bathroom towel radiator or an additional fan coil. We recommend making a simple list of rooms and assigning each one the number of separate circuits, including a margin for rooms that might be finished later (for example an unheated attic or garage).

Step 2 - estimate the manifold's width

Using the formula above (100 to 150 mm plus 50 mm per circuit), you can calculate an approximate width. If you've already chosen a specific manufacturer's manifold, the most accurate approach is to check its actual datasheet - manufacturers commonly state the total body width for a given number of circuits directly in mm.

Step 3 - add a margin for fittings and servicing

Add the margin discussed above to the calculated manifold width. For small manifolds (up to 6 circuits), 100 mm is usually enough; for larger ones with thermostatic heads or actuators, it's safer to allow at least 150 mm.

Step 4 - choose the corresponding cabinet size

Only at this step does choosing the specific cabinet come into play. Let's look at how this choice works out for the actually available surface-mounted cabinets.

Comparison of cabinet sizes - approximate recommendation

The table below sums up which width suits which approximate range of circuit numbers. This is a general recommendation based on common manifolds on the market - for a specific product, we always recommend also checking against the datasheet of the manifold you plan to use.

These ranges already include the margin described above - so they're not just "mathematically tight" boundary values, but sensible recommendations that leave room for minor future adjustments too.

385 mm cabinet - smaller installations

A cabinet with a 385 mm interior width is the smallest of the commonly used surface-mounted sizes. It suits situations where the manifold is genuinely small - typically bathroom heating with one or two circuits, a smaller flat where underfloor heating only covers the living room and kitchen, or simpler systems without many separate zones.

Manifold cabinet 385 mm - surface-mounted - the most compact of the available sizes, ideal for manifolds up to roughly 6 circuits including a fittings margin. Suitable for smaller flats, bathrooms, or anywhere cabinet space is limited and the installation doesn't have many separate zones. Price from €63.10.

With this size we recommend being especially careful if even one extra circuit is planned for the future - the 385 mm cabinet has the smallest margin, so in practice it's worth considering the medium size instead if there's even a small chance of the system being expanded.

485 mm cabinet - the most versatile choice

A cabinet with a 485 mm interior width is, in practice, the best-selling size, for a simple reason - it covers the most common range of family houses and larger flats, that is, manifolds with 6 to 10 circuits. This is a typical situation where a house has underfloor heating split by room (living room, kitchen, bedroom, children's room, bathroom, hallway) plus possibly one or two circuits for radiators or a bathroom towel radiator.

Manifold cabinet 485 mm - surface-mounted - a medium size suitable for an ordinary family house or larger flat with a manifold on 6 to 10 circuits. It offers enough margin for actuators and for adding an extra circuit later. Price from €67.53.

This size offers the best balance of price versus margin - in practice we recommend it even in cases where the calculation comes out just under the threshold for the smaller cabinet (for example exactly 6 circuits), since the price difference is minimal but the margin is significantly larger.

615 mm cabinet - larger and more complex systems

The largest of the commonly available surface-mounted cabinets, 615 mm wide, is designed for manifolds with a higher number of circuits - typically 10 to 14. This applies where a house has more heating zones, perhaps combining underfloor heating across several floors through one central manifold, or where flow meters and thermostatic heads are fitted on every circuit, requiring more space.

Manifold cabinet 615 mm - surface-mounted - the largest of the available sizes, suitable for manifolds with 10 to 14 circuits, multi-zone systems, or manifolds with flow meters and actuators on every circuit. Price from €75.15.

For larger manifolds it's also worth thinking through the cabinet's depth and height, not just its width - with 10 or more circuits, additional components such as a mixing group or a circulation pump are usually added too, and these need their own space beyond the manifold itself. If these components aren't meant to fit inside the cabinet at all (they're often placed alongside it, not inside the cabinet), you only need to plan for the manifold itself, but it's good to confirm this in advance with a designer or installation company.

A practical example from everyday installation

To see how the whole procedure plays out in a real situation, let's use a simple example of a family house with underfloor heating. The house has five rooms with their own underfloor zone (living room, kitchen, two bedrooms, bathroom), plus one circuit for a radiator in the entrance hall and one for a bathroom towel radiator - 7 circuits in total.

Using the formula: 100 to 150 mm plus 7 × 50 mm = 450 to 500 mm. Add a margin of 100 mm for actuators planned to be added later for room thermostats, and the resulting recommended cabinet width comes out at 550 to 600 mm. In this case, the 615 mm cabinet is the safer choice, even though the strictly narrowest calculation (450 mm) could theoretically be covered by a 485 mm cabinet with no margin. This gap between the "calculated minimum" and the "actually needed space with actuators" is exactly why, in practice, we always recommend rounding up, not down.

Adding together the base, the width for 7 circuits, and the margin for actuators gives an approximate manifold width of roughly 575 mm - in practice rounded up to a 615 mm cabinet.

A second example - a smaller 2-room flat with underfloor heating only in the living room and kitchen, that is, 2 circuits. The calculation: 100 to 150 mm plus 2 × 50 mm = 200 to 250 mm. Even with a margin of 100 mm, that comes to 300 to 350 mm, which the smallest 385 mm cabinet comfortably covers with a still-reasonable margin to spare.

The link between the number of circuits and other cabinet dimensions

Besides width, it's worth keeping in mind that as the number of circuits grows, the required cabinet depth often grows too, especially if flow meters are fitted on the manifold, or a mixing group is planned right inside the cabinet. The cabinet's height, on the other hand, usually doesn't change with the number of circuits - it changes mainly based on whether it's a surface-mounted or recessed version, and the height of the manifold body itself (single-row or multi-row manifold, though this is less common in flat installations).

For the surface-mounted cabinets discussed in this article, it's the width that changes the most significantly, and it's therefore the main selection criterion based on the number of circuits. If you're more concerned with whether to choose a surface-mounted or recessed version, we cover that topic in detail in a separate article (linked below).

What happens if the cabinet is too small

Practical installation experience shows several typical consequences when a cabinet is chosen too tight:

The cabinet door can't close fully, because valve levers or actuators stick out.
Bleeding or servicing requires removing the cabinet frame first to comfortably reach the manifold - which extends even routine annual maintenance.
Adding even a single extra circuit later (for example finishing an attic or extension) requires replacing the whole cabinet, not just adding to the manifold.
Condensation and air circulation in an overly tight cabinet is worse, which increases the risk of condensation forming on some materials.

Conversely, a cabinet that's too large isn't a technical problem - it just takes up unnecessary wall space and costs a bit more. So when deciding between two neighbouring sizes (for example 485 mm vs. 615 mm), we recommend going for the larger one if in doubt.

Summary of recommendations

When choosing a manifold cabinet size based on the number of circuits, remember these main points:

First count the actual number of circuits, including a margin for future expansion.
Estimate the manifold's width using the formula of 100-150 mm plus 50 mm per circuit, or from the datasheet of the specific manifold.
Add a further margin of 100 to 150 mm to the calculated width for fittings, actuators and servicing space.
The 385 mm cabinet suits roughly up to 6 circuits, the 485 mm up to 10 circuits, the 615 mm up to 14 circuits - always with a margin.
If in doubt between two sizes, choose the larger one - the price difference is negligible compared to the cost of replacing the cabinet later.

Frequently Asked Questions

How do I find out exactly how many circuits my manifold will have?

The number of circuits is determined by the heating design or the installation company based on the number of heated zones - usually one room equals one circuit for underfloor heating, and larger rooms are sometimes split into two circuits for more even heat distribution. If you don't have a design available, consult an installation company before ordering the cabinet.

Can I find the manifold's width directly from the manufacturer?

Yes, and it's more accurate than the approximate formula given in this article. Most manifold manufacturers state the body's total width for a given number of circuits, in millimetres, in the datasheet - we recommend always using this figure if available, and using the approximate formula only when a specific manifold hasn't been chosen yet.

What if I'm exactly on the boundary, for example 6 circuits - 385 mm or 485 mm?

At a boundary value, we recommend choosing the larger cabinet, especially if there's even a small chance of future expansion, or if adding actuators to individual circuits is planned. The price difference between neighbouring sizes is usually only a few euros, while replacing the whole cabinet later means breaking out and reworking the wall.

Can a manifold with more circuits be squeezed into a smaller cabinet with "a bit of a tight fit"?

Technically it sometimes works, but we don't recommend it - it restricts access to the fittings, makes servicing harder, and in many cases the cabinet door can't close fully. The short-term saving on purchase often turns into complications at every yearly maintenance visit.

Does the number of circuits also affect the cabinet's depth, not just its width?

For common manifolds without flow meters, depth changes only slightly. However, if flow meters are fitted on every circuit, or a mixing group or circulation pump is placed inside the cabinet too, the depth can be considerably larger regardless of the number of circuits - in that case we recommend consulting an installation company before choosing a specific cabinet model.

Is there a difference in size selection between a surface-mounted and a recessed cabinet?

The principle of calculating based on the number of circuits is the same for both types - the difference is only that a recessed cabinet is set into the wall, with only the frame around the door visible from outside, while a surface-mounted cabinet sits entirely on the wall surface. You'll find a detailed comparison of both types in a separate article, linked below.

Have a question about this topic?

Not sure what to choose, or dealing with a specific situation in your home? Write to us - we're happy to help.