Practical sizing guide

Gather the inputs that determine the right system

Sizing begins with the water you have, the water you need and the way it will be used. This guide helps prepare a technical enquiry for a new installation, an equipment replacement or a change in site demand.

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The data that determine the size of a system

A water treatment system is sized on four inputs: the quality of the incoming water, the quality required at the outlet, the flow it must deliver at peak times and the volume it treats each day. When one of them is missing, the system usually ends up undersized, or oversized as a precaution.

Input	How to obtain it	What it determines
Water analysis	Report from the water supplier or a laboratory: hardness, iron, manganese, conductivity, chlorine	Choice of technology and pretreatment
Required outlet quality	Water specification of the equipment manufacturer, or process requirement	Treatment sequence and residual hardness
Peak flow	Highest simultaneous demand, in m³/h or L/min	Hydraulic size of the valve and vessels, pressure drop
Daily consumption	Water meter readings, or an estimate from occupancy and equipment	Exchange capacity, regeneration frequency and salt consumption
Operating conditions	Operating hours, continuity required, pressure range, water temperature	Simplex, duplex or multiplex configuration
Installation	Available space, pipe diameter, drain and electrical supply	Physical format and connection of the equipment

Peak flow versus daily consumption

The two figures answer different questions. Peak flow is the highest demand at any moment, for example when kitchens, warewashing and hot water run at the same time; it sets the hydraulic size of the equipment. Daily consumption, combined with the hardness to be treated, sets the exchange capacity the softener needs between regenerations.

Worked example

A restaurant uses 4 m³ of water per day. The incoming hardness is 30 °f (16.8 °dH) and the equipment requires softened water at 0 °f. The daily hardness load is 30 × 4 = 120 °f·m³. With a typical exchange capacity of about 5 °f·m³ per litre of resin, this corresponds to roughly 24 litres of resin per day of autonomy. The model is then checked against the peak flow, for example 3 m³/h, or 50 L/min.

Hardness unit conversion

Unit	°f	°dH	mg/L CaCO3	mmol/L
1 °f (French degree)	1	0.56	10	0.1
1 °dH (German degree)	1.78	1	17.8	0.178
1 mmol/L	10	5.6	100	1

Softener sizing calculator

Enter your water data to estimate the daily hardness load and the indicative resin volume. The result prepares your enquiry; the final model is confirmed after the technical study.

Incoming water hardness

Unit

Target residual hardness (same unit)

Daily consumption of water to be softened (m³/day)

Peak flow (m³/h)

Hardness to be treated: 0 °f
Daily hardness load: 0 °f·m³/day
Equivalent calcium carbonate: 0 kg/day
Indicative resin volume per day of autonomy: 0 L
Indicative salt consumption: 0 kg/day
Peak flow to compare with manufacturer data: 0 L/min

Assumptions: 1 °dH = 1.78 °f, 10 mg/L CaCO3 = 1 °f, an exchange capacity of about 5 °f·m³ per litre of resin and about 150 g of salt per litre of resin per regeneration. Actual values depend on the resin, the salt dose and the equipment; Kinetico twin-tank systems regenerate according to the volume actually used.

Frequently asked questions about sizing

Which residual hardness should I aim for?

It depends on the use. Boilers, steam ovens and some process equipment require water close to 0 °f, while drinking water circuits often keep some residual hardness with a mixing valve. Equipment specifications and local regulations take precedence.

How is a reverse osmosis system sized?

On the permeate volume needed per hour and per day, the feed-water conductivity and the water temperature. Rated production is given at a reference temperature, often 15 °C, and decreases in colder water, so the reference conditions must be checked.

How is a UV system sized?

On the peak flow and the UV transmittance of the water. A lower transmittance reduces the dose delivered, so a larger reactor may be needed for the same flow. Pre-filtration and the water quality limits given by the manufacturer must also be respected.

Is a twin-tank softener sized differently?

The hardness load is calculated in the same way, but a twin-tank system always has one column in service, so it supplies softened water continuously and regenerates according to the volume actually used. Sizing focuses on the capacity per column and the peak flow.