How the two differ in kind
Air takes the path of least resistance, so a duct system delivers too much to the branches nearest the fan and too little to the ones furthest away, unless something equalises them. There are two ways to equalise, and they are not equivalent.
BALANCING WITH DAMPERS equalises by adding resistance where there was too little. The commissioning engineer measures the flow at each terminal and closes dampers on the generous branches until every branch delivers its design flow. It works, it is necessary on every real system, and it is the only way to deal with the differences that remain after the design has done what it can.
The property that matters is what a damper cannot do: it never REDUCES the resistance of the worst branch. So after balancing, the fan still has to overcome the index run — the highest-resistance path — and every other branch is being throttled up to match it. The system's total resistance is set by its worst path plus whatever the dampers are adding everywhere else.
That resistance is paid for every hour the system runs. A fan working against artificial resistance consumes energy producing nothing, and on a variable-speed system it cannot turn down as far as a well-proportioned system would allow. It is also noisy: a heavily throttled damper is a significant noise source, generating turbulence right at the point where the air is about to enter a room.
SIZING THE DUCTS so the branches start out comparable reduces how much has to be thrown away. Shorter runs to distant terminals, larger branches where the run is long, a layout that does not hang a short branch off the plenum next to a long one, and on larger systems static regain sizing, which deliberately equalises the static pressure at every take-off. The dampers are then trimming small differences rather than compensating for large ones.
Both are needed. The question is how much work the dampers are being asked to do.
The factors that actually differ
Show
| Balancing with dampers | Sizing the ducts to balance | |
|---|---|---|
| How it equalises | Adds resistance to the easy paths until they match the hard one. | Makes the paths comparable in the first place, so less has to be added. |
| Effect on the index run | None. The fan still has to overcome the worst path. | Can reduce it directly, by enlarging the branch or shortening the run. |
| Fan energy | Paid continuously, for the life of the system — a heavily throttled system wastes fan power every hour. | Lower, permanently. |
| Variable-speed operation | Limits how far the fan can turn down, since the artificial resistance remains at part load. | Allows a lower minimum speed and greater turndown savings. |
| Noise | A heavily throttled damper generates turbulence close to the terminal, which is where it is heard. | Quieter, since the air is not being forced through a restriction near the room. |
| When it can be done | At commissioning, and adjustable afterwards — which is its real advantage. | At design. Once the ducts are in, resizing is a rebuild. |
| Adaptability | High. A layout change, a new terminal or a changed load is re-balanced rather than re-ducted. | Low. The system is proportioned for the design it was built for. |
| Cost | Commissioning time, plus the energy penalty forever. | More sheet metal and more design effort, once. |
| Is it optional | No — every system needs balancing, and one that was never balanced is delivering the design flow nowhere in particular. | No — a system that relies entirely on dampers to correct a badly proportioned layout will be noisy and wasteful however well it is balanced. |
| The honest framing | The trim. | The proportioning. Do this well, and the trim is small. |
Which one, and when
Choose balancing with dampers when…
- Commissioning any system — this is not optional and no design removes the need for it.
- Correcting for the differences that remain after a good design, which is what dampers are for.
- An existing system that is out of balance, where re-ducting is not available.
- A system whose use changes, where re-balancing is far cheaper than re-ducting.
Choose sizing the ducts to balance when…
- At design stage, where proportioning the branches costs sheet metal rather than energy.
- Where the layout is badly unbalanced by geometry — a very short branch beside a very long one.
- Where fan energy matters, which on a system running continuously it does.
- Where noise at the terminals is a criterion, since throttled dampers are heard in the room.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does throttling cost energy permanently?
- Because the resistance a damper adds does not go away, and the fan overcomes it every hour the system runs. A partly closed damper is a deliberate restriction, converting fan energy into turbulence and heat rather than into delivered air — so the fan is doing work that produces nothing useful. On a constant-volume system that penalty is fixed. On a variable-speed system it is worse than it looks: fan power falls steeply with speed, so the savings from turning down are large, and a system carrying a lot of artificial resistance cannot turn down as far before the worst branch is starved. That means the throttling costs not only its own share of the energy but part of the turndown saving the variable-speed drive was installed to capture.
- Can't the design just make all the branches equal?
- Not exactly, and not usually anywhere near. A real layout has terminals at different distances with different numbers of fittings, serving rooms with different loads — so the resistances are genuinely different and no sizing makes them identical. What design can do is reduce the SPREAD: avoid hanging a very short branch off the same plenum as a very long one, size distant branches generously so their extra length costs less pressure, use a layout that reaches the far rooms by a more direct route, and on larger systems use static regain sizing, which deliberately proportions each section so the static pressure at every take-off is comparable. That turns a large imbalance into a small one, which the dampers can trim without heavy throttling.
- What is the index run and why does it matter here?
- The path from the fan to a terminal that suffers the greatest total pressure loss, and it sets the fan's duty because every other branch is throttled up to match it. That is the key to why dampers cannot help the fan: closing a damper on a generous branch raises that branch's resistance toward the index run's, but nothing about the index run itself changes. So the fan must always deliver at least the index run's pressure, and the only way to reduce the system's demand is to reduce that path — by shortening it, enlarging it, improving its fittings, or moving the terminal it serves. Identifying it is therefore the first step in any attempt to make a system cheaper to run, and it is not always the longest run: fittings and a large static rise can make a shorter path worse.
- How is a system actually balanced?
- By measuring, adjusting and re-measuring, working from the worst branch outward — not by setting dampers by eye. The method is to measure the flow at every terminal, identify the branch delivering the lowest proportion of its design flow, open it fully, and then throttle the others in proportion until all reach their targets, accepting that each adjustment changes the others and repeating until it converges. That iterative property is why balancing takes time and why it is a specialist activity with instruments rather than a walk round with a screwdriver. The output is a record: the measured flow at each terminal against its design flow, the damper positions, and the fan's operating point — which is the document that proves the system delivers what it was designed to.
- Are there dampers that balance themselves?
- There are, and they solve a specific problem rather than the general one. Constant-flow regulators — mechanical devices that maintain a set flow across a range of upstream pressures — hold a branch's flow steady as conditions change, which is useful where pressure varies, such as on a variable-volume system or where terminals open and close. Pressure-independent terminal units do the same at the terminal. What they do not do is reduce the system's resistance: they achieve their constant flow by throttling, exactly as a manual damper does, so the energy argument is unchanged. Their value is in maintaining the balance over time and under changing conditions, not in avoiding the penalty of a poorly proportioned system.
- How does this compare with the duct sizing methods?
- It is the consequence of them. Equal-friction sizing holds pressure loss per unit length constant, which on a symmetrical layout produces branches of comparable resistance and a system close to balanced before any damper is touched; on an asymmetrical one it does not, and the dampers make up the difference. Velocity sizing controls noise in specific sections and says nothing about balance. Static regain sizing is the method that targets balance directly, proportioning sections so the static pressure at each take-off is similar — which is why it is used on larger and longer systems where the alternative is heavy throttling of the near branches. The choice of sizing method is therefore also a choice about how much balancing work the dampers will have to do.
- What if an existing system is badly out of balance?
- Balance it first and measure what that costs, because the measurement identifies whether re-ducting is worth considering. A proper balance with a record of the damper positions and the fan's operating point tells you how much resistance is being added and where — and a system with several dampers nearly closed is telling you that its proportioning is poor. At that point the targeted interventions are worth pricing: enlarging the index run's branch, improving a particularly bad fitting, or re-routing one long branch, each of which reduces the system's resistance directly. Replacing all the ductwork is rarely justified; fixing the worst path usually is, because it is the path the fan's duty is set by and therefore the one whose improvement benefits every hour of operation.
- Does any of this apply to a domestic system?
- Yes, in a simpler form, and it is where the problem is most often left unaddressed entirely. A domestic ducted system typically has no commissioning stage at all, so the near rooms are over-served and the far ones under-served for the life of the house — which is the familiar complaint of one bedroom that is never warm enough. The interventions are the same in kind: dampers at the branch take-offs, adjusted with a measurement rather than by feel, and where the layout allows it, a larger branch or a shorter route to the distant rooms. The design-side lesson is also the same: a layout that runs one short branch off the plenum and one long one to the far end of the house will be unbalanced whatever is done afterwards.