Proportional polyphosphates dosing unit: how does it work?

What are the advantages over a polyphosphates dosing pump and magnetic scale inhibitor?

Why install a polyphosphates dosing unit?

Calcium and magnesium are naturally contained in the water of domestic water systems. During the heating of the water, a chemical encrustation process occurs: the calcium bicarbonate precipitates and settles on the heat exchange surfaces of the hot water production system.

To prevent this problem and on the recommendation of various national standards and by boiler manufacturers, it is necessary to install a proportional dosing unit. This will prevent the build-up of scale but also corrosion by dosing a food grade chemical solution which will not alter the potability of the water according to the Commission Directive 98/83/EC and its latest amendments 2015/1787/EC.

How does a proportional polyphosphates dosing unit work?

A proportional polyphosphates dosing unit uses the Venturi effect to work. It reduces the fluid pressure that results when a fluid flows through a constricted section of the pipe guaranteeing a direct proportional quantity of the chemical solution allowing it protect the system from lime-scale deposits.


A constant dosage allows the polyphosphates concentration to remain within the limits permitted by the regulations and at the same time to keep its anti-scale and anti-corrosive properties active.


The dosing unit is ideal for use with gas / electric boilers, and water heaters.

Advantages of DOSPRO dosing unit with DOSTABS universal refill tablets

DOSPRO dosing unit is equipped with a floating red sphere that – once it reaches the bottom of the bowl – will indicate the solution has depleted and must be refilled with two new tabs. DOSPRO is the only dosing unit with a proportional double control dosing system, Vortex and Venturi.

Advantages of DOSPRO dosing unit

  1. The smallest on the market
  2. Simplified replacement
  3. Proportional dosing with minimum tap opening
  4. Incorporated by-pass
  5. Horizontal, vertical and 45° assembly
  6. Bi-directional assembly with water entry from right or left

The DOSTABS tablets consumption is proportional to the water usage. The refill life of 80 m³ is influenced by the temperature, frequency of use and by the sampling frequency. DOSTABS blister contains 6 tabs, suitable for a one-year protection of an average domestic system.

Advantages of DOSTABS universal refill tablets

  1. The smallest on the market
  2. Easy to use
  3. High concentration
  4. Long-lasting
  5. Form a protective film against corrosion and the build-up of calcium carbonate deposits and also corrosion

Differences between proportional dosing units and proportional dosing pumps

Dosing units


No moving parts and less prone to breakage


Constant and continuous dosage


DOSTABS are supplied already compressed, not diluted with water. The powder dissolves and becomes a liquid solution before being injected, ensuring a longer life.


Dosage starts as soon as hot sanitary water is required from any tap of the system

Dosing pumps


Moving piston with mechanical intermittent operation which is prone to crystallization formation that interrupts the protection of the system and the boiler

Intermittent dosage


Dosage uses a pre-mixed liquid solution that has shorter life



Dosage starts from a few litres of hot sanitary water, at least 3L/min


Differences between proportional dosing units and magnetic scale inhibitors

Magnetic scale inhibitors work by preventing and/or reducing scale formation by applying static magnetic fields to water. These devices are tested by law to ensure their compatibility with biological parameters and therefore that they are not harmful to humans.

Health compatibility does not imply an effectiveness of these devices, whose anti-scaling and descaling efficiency has not yet been widely proved outside science labs. The application of this scale-inhibiting effect to specific systems is in fact very difficult because:

  1. the fluid flow should be orthogonal to the magnetic field;
  2. the exposure to the field should be prolonged or the solution should be recirculated.

Neither of these two requirements takes place inside a domestic hot water production system.