How to size a cold-water booster set without oversizing it

Sizing a booster set is finding one point on the performance curve: the flow it must deliver, and the head it must deliver it at. That pair is the duty point. Fix it and the pump selection falls out of the curve. Miss it and no amount of specification saves the set.

The flow is the peak simultaneous demand, not the sum of every outlet. A building never draws all its fittings at once. The head is set by the index outlet, the tap or appliance highest and furthest from the set, because that is the outlet the set has to satisfy last. Get water to the index outlet at working pressure and every outlet below it is covered.

Then check where that point sits on the curve you are buying. Size the pump so the duty point lands near its best efficiency point (BEP). Left of BEP the set short-cycles and wastes energy. Far to the right of it the pump runs hot and cavitates. A duty point close to BEP is the clearest sign the sizing is right.

Sizing is a distinct job from selecting and specifying. You select the pump type against the medium and the duty, size the unit against the duty point, then specify the build. On a cold-water booster the type is settled before you start, so this guide is the middle step. The full sequence is in how to select a pump.

Four stages, in order. Each one feeds the next, so work them top to bottom rather than starting at the pump.

Oversizing is the default mistake on booster sets, and it arrives by margin stacking rather than by carelessness. A booster set is specified through a chain, and every link in it adds a little. The customer states a flow rate. The consultant adds margin, reasonably, for a future duty nobody can yet describe. The contractor adds margin. The subcontractor doing the installation adds a bit more flow, or a bit more pressure, because the last one was tight. Nobody in that chain is being careless. The set that comes out of the end of it is far too big.

Building services is where this bites hardest, because a booster set passes through every link of that chain. A process pump bought direct by the engineer who will run it skips most of them. A packaged set on a new-build block goes through all of them, and each allowance is applied to a figure that already had one in it.

Then the economics. Power rises with both flow and pressure, so the allowances multiply in the motor rather than simply adding. Put an illustrative figure on it and the shape is familiar: a duty that needed 3 kW arrives on site with an 11 kW motor on it. That is uneconomical on the day it is commissioned and it stays uneconomical for the life of the installation, because nobody replaces a working booster set to save running cost.

Stripping every allowance out is not the answer either. Hold one, once, at a level someone can name, and put the duty point on the drawing, so the next link in the chain adds its margin to a stated figure rather than to a guess.

An oversized booster set costs more to run, and the hydraulic damage is the bigger problem. A centrifugal pump run off the end of its performance curve needs back pressure it is not getting, so it tries to draw more than the supply can deliver. Size for 10 m³/h when the break tank or the incoming main will only give you 5 m³/h and you are pumping air.

That is cavitation. Vapour forms at the impeller eye where the pressure drops below the vapour pressure of the water, then collapses as the pressure recovers across the impeller. It pits the impeller, drops the head the set can make, and takes the mechanical seal with it. It also puts vibration and noise into the pipework.

Noise carries more weight here than on an industrial duty, because the set is not standing in a works yard. A booster set sits inside an occupied building, usually in a plant room with a flat or a ward on the other side of the wall. A cavitating set is audible, and an audible set becomes a complaint, then a call-out, then an acoustic enclosure bolted on to fix a decision that was made at sizing. The oversized set is noisier in normal running too, because it spends its life throttled and cycling.

Compare the available NPSH (NPSHa) at the pump inlet against the pump's required NPSH (NPSHr) at the duty point, and keep a margin between them. When NPSHa falls below NPSHr, the pump cavitates. That is the whole check.

In practice it almost never goes wrong in the arithmetic. It goes wrong in the suction conditions: a suction tank that is too small, or a suction head that is too small. A flooded suction from a break tank sitting above the set hands you margin before you start. A long, undersized suction run, or a tank whose working level sits at or below the pump centreline, spends that margin before the pump has done any work. Check NPSHa at the worst case, which is peak flow with the tank at its lowest working level, not at the level it happens to sit at on the day of the survey.

Decide how the set is fed before you finalise the pump, because the answer changes the sizing. Boosting straight from the main is limited by what that main can actually give, and it is notifiable: under regulation 5 of the Water Supply (Water Fittings) Regulations 1999, a pump or booster drawing more than 12 l/min (0.2 l/s) from a supply pipe, directly or indirectly, has to be notified to the water undertaker, and the work cannot start without consent. A break tank removes that ceiling, protects the main from backflow and buffers demand, at the cost of plant space, a second set of level controls, and a stored volume somebody has to turn over.

Most of what goes wrong on a booster set is a system fault presenting as a pump fault. The set gets specified as though the incoming main, the break tank and the riser were somebody else's problem, and often they genuinely were: installed by others, or inherited with the building. It is the second most common mistake we see, after oversizing.

Dry running is the plainest example. Somebody shuts an isolating valve and forgets it is shut, then the set runs. On a centrifugal the usual result is a failed mechanical seal, because a mechanical seal needs liquid to lubricate it. Take the liquid away and the seal goes. There is no design fault anywhere in that sequence, and a conventional mechanical seal will not survive it. Low-level and low-pressure cut-outs exist to stop it, and they only do their job if they are commissioned and left enabled.

So size the set against the system it will sit in, not against the pump on its own. Send us the peak simultaneous flow, the pressure needed at the index outlet, the number of storeys, and how the set is fed: straight from the main, or from a break tank, and what the working level and static head at that tank actually are. We will size the set to the duty and specify the build to match, and we will tell you if the duty you have been handed does not add up. See the packaged booster sets we supply, and how a booster set holds pressure for the mechanism behind the numbers.