How to Size a Variable Frequency Booster Pump: Flow, Head and Duty Point

Step 1 — Required flow: count what runs at the same time

Flow is the volume the site must deliver at the same moment, not the total of everything connected. A house with four taps, two showers and a washing machine rarely runs them all at once, so size for the realistic peak: the outlets that are actually open together at the busiest time of day, for example two showers plus a kitchen tap.

Write the peak down in litres per minute. If the site already has a meter or a bucket test, measure the flow at the busiest outlet group rather than guessing. An inflated flow figure is the most common sizing mistake: it pushes the selection to a larger pump that then runs throttled, noisy and far from its best efficiency point.

Step 2 — Required head: lift, friction and outlet pressure

Head is the height the water must be lifted, plus the friction the pipes add, plus the pressure the highest outlet needs. Measure the vertical lift from the water surface to the highest outlet, then allow for pipe friction — long runs, small bores and many bends all add head — and for the outlet pressure itself. Head is normally written in metres, and 1 bar is about 10.2 m of water column.

For a VFD booster the outlet pressure is the constant-pressure set point, so the required head is the set point plus lift plus friction. If the pump is fed from a pressurised mains line rather than a tank, the inlet pressure counts toward the total, and the pump only has to add the difference.

Step 3 — Plot the duty point on the Q-H curve

Every model page carries a Q-H curve: head on the vertical axis, flow on the horizontal axis. The curve falls as flow rises, so the two headline figures on a model table are the ends of the curve, not a combination — maximum flow belongs to the lowest head, and maximum head belongs to the smallest flow. A model fits when the curve passes above the duty point with a small margin, ideally near the middle of the curve rather than at either end.

The worked example below uses a two-storey house whose peak demand is 40 L/min at 28 m of head. That is the duty point: 40 L/min, 28 m.

• SPS-90, Stage-5 (0.38 kW): the published curve runs from 32 m at zero flow to 0 m at 60 L/min, passing 25.9 m at 30 L/min and 15.9 m at 45 L/min. At 40 L/min it delivers about 19 m — below the 28 m the house needs. The SPS-90 does not cover this duty point.
• ICP100F (0.75 kW): the published curve runs from 38 m at zero flow to 10 m at 87 L/min, passing 32 m at 34 L/min and 29 m at 47 L/min. At 40 L/min it delivers about 30.6 m — above the 28 m required, with roughly 2.6 m of margin. The ICP100F covers the duty point.
• SCS100P-3, Power 1 (0.55 kW): the lowest of its five power stages already reaches 115 L/min at up to 46 m of head, and its Power 5 curve passes about 63 m at 40 L/min. It covers the duty point with far more head than the house needs, so it is oversized for this duty — the extra head would be throttled away.
• Verdict: the ICP100F is the fit. It covers the duty point near the middle of its curve, while the SPS-90 falls short and the SCS100P-3 is more pump than the house can use.

Step 4 — Check voltage and suction before ordering

A pump that covers the duty point still fails if the site cannot power it or feed it. Check the nameplate voltage and frequency against the supply: the ICP100F runs on single-phase 170–250 V at 50/60 Hz, so it suits mains supplies that swing, but it is not a 110 V model — sites on 110 V / 60 Hz use the PYS-E and PYPZ-E families instead.

Then check the water source. The ICP100F is self-priming with 8 m of suction lift, so it can draw from a cistern or shallow well below the pump, provided the suction pipe is short, fitted with a strainer and a non-return valve, and free of high points that trap air. If the source is a rooftop tank above the pump, the inlet is flooded and suction lift does not apply. Dry-run protection matters on any suction-lift installation, so the pump stops when the source runs empty.

What oversizing costs

Choosing the next model up feels safe, but an oversized pump reaches the pressure set point quickly and then runs at low speed or cycles, turning the extra head into noise and wear rather than useful flow. It also costs more to buy and to run. The SCS100P-3 in the example above is the right pump for a villa that genuinely needs 46 m or more of head — not for a house that needs 28 m.

The same logic applies across the range: the SPS-90 suits apartments and small homes up to 32 m of head, the ICP100F suits tank and well supply up to 38 m, and the SCS100P-3 suits multi-storey and plant-room duty up to 75 m. Match the duty point first, then let the model follow.

Questions buyers ask

Can I use the maximum flow and the maximum head together? No. They are opposite ends of the same curve: maximum flow is delivered at the lowest head, and maximum head at the smallest flow. Any duty you need sits somewhere between the two, so always read the curve, not the two headline numbers.

What if my duty point falls between two models? Take the smaller model that still covers the point with a margin of a few metres. If the point sits exactly on a curve, check the next power stage of the same model first — the SPS-90, SCS100P-3 and ICP100A families all offer several stages in one body — before jumping to a larger pump.

Should I size for future outlets? Size for the outlets that run at the same time, not for every outlet in the building. If a genuine extension is planned, send the future duty point with the inquiry and we will confirm whether the same model still covers it.

Next steps

Work through the selection guide for a shortlist, then open a model page for the full specification and the Q-H curve. Send the four numbers — flow, head, voltage and suction — with the inquiry, and our team confirms the model and replies with a quotation.