Heating water

Proheat II

The water for heated pools requires an external power supply in order to maintain a pleasant temperature as the natural tendency of water is to reach the same as that of its surroundings. If the outdoor temperature is lower, the temperature of the water will gradually drop depending on external conditions: temperature of the air, pool walls, floor, etc.
The natural tendency of water to reach that of its surroundings results in heat loss, which depending on the circumstances can take place through radiation, conduction or convection.

Types of heat loss in pools


This is the energy that is lost because the water in the pool shell goes from a liquid state to a gaseous state. The amount of water evaporated is determined by the water’s temperature, the ambient air’s thermo-hygrometric conditions and the water’s partial saturation pressure in the case of an indoor pool. For an outdoor pool, the wind speed would have to be taken into consideration.
Evaporation is the phenomenon through which most heat is lost.


Convection contributes to heat loss due to the movement of air over the surface of the water. As it is in constant movement, currents are created that make heat transfer increase as movement builds up. This is further added to by the temperature difference between the surface of the water and the air, and wind speed (in outdoor pools).


This energy loss takes place between the water in the pool shell and the solids that surround it, that is, the walls and floor. It increases when there is a high temperature difference between the water and these solids.


This is due to the energy lost through radiation waves that all bodies emit due to their temperature. During the daytime, the solar radiation on water will make the pool water gain energy, whilst at night time the pool will radiate energy and, therefore, lose it.

Technologies for heating pool water


A heat exchanger on a boiler will supply the right amount of heat to raise and maintain a pool’s temperature. There are several types of boiler depending on the fuel that runs them.

Gas boilers: fed by the mains gas supply or tanks.

Oil-fired boilers: fed by a tank.

Biomass boiler: manually loaded from a hopper or silo.

Heat exchangers

At AstralPool we have a full range of heat exchangers for pools, of which the following can be highlighted:

Plate heat exchangers (ETNA). They are made up of a number of plates through which two fluids flow without mixing but rather by the exchange of heat from the highest to the lowest temperature. They can either be made of AISI-316 stainless steel or titanium, the latter of which is more resistant to corrosion.

Shell and tube heat exchangers (Waterheat). They are made up of tubes inside a shell whose axes are parallel to each other. One of the fluids flows through the tubes and the other through the shell, and they exchange heat without mixing. These exchangers are made of titanium.

Because of the way they are built, shell and tube heat exchangers can hold a greater amount of fluid than plate heat exchangers with the same load loss. By contrast, plate heat exchangers perform better in terms of the heat produced.

Joule effect

This system only uses electricity as a source of power. The Joule effect is a physical phenomenon whereby a conductor increases the temperature when electricity flows through it.

In this range of heaters, AstralPool’s Compact ElectricHeat model stands out.

Solar energy

Solar irradiance is the source of energy that solar panels use to absorb the heat energy that warms up the water in a pool. There are three types of collectors.

Polypropylene collectors: low performance when air temperatures are low.

Glass collectors: higher performance thanks to the greenhouse effect that occurs between the glass and the collector.

Vacuum tube collectors: the highest performing ones due to the vacuum between the glass tube and the collector that makes heat loss extremely low.

A heat exchanger must be fitted in glass and vacuum tube collectors that divide the circuit in two in order to prevent corrosion in the collectors, and anti-freeze must be added to the primary circuit.

Heat pumps

Heat pumps use a cooling circuit to transform the energy in the air or water so that the pool water can be heated. The properties of a cooling circuit mean that the performance of these pumps is in the order of five times above the electric power they consume. This factor makes this technology one of the most efficient around and the one that results in the greatest energy savings whilst maintaining a pleasant temperature.

Among heat pumps for pools, the most widely accepted are air source heat pumps. They work by extracting ambient air and can turn up to 80% of their energy into heat, so this energy can be classed as free. The energy that a pump needs to run its compressor is the only energy that users will have to pay for (around 20% of the total thermal energy required to heat pool water).

The European Union considers that such installations run on renewable energy, provided they comply with the requirements set out in directive 2009/28/EC.

Proheat II

The Proheat II heat pump enables the bathing season of medium-sized and large pools to be lengthened. It has been specially designed to work outdoors at external air temperatures as low as 0 ºC to heat pool water to a pleasant temperature.

Every component was carefully selected in the design of this unit so that it would adapt perfectly to user requirements. Furthermore, energy efficiency was made one of the focal points of the design so that power consumption would be as low as possible. Thus, the performance of this heat pump has earned it a B rating on the energy labels used by leading manufacturers in the pool sector (compliant with EU standard 14511-4 on FPP classification).

The Proheat II heat pump has been designed so that the built-in cooling circuit has totally optimised components that result in the compressor, evaporator, condenser, expansion valve and the piping being just the right size to deliver maximum power. This means that the refrigerant gas (R-410A, which does not damage the ozone layer) load is very low in comparison with other models available in the market..

Its main features are:

  • Robust, lightweight housing made of a sunlight-resistant aluminium-magnesium alloy.
  • High-performance evaporator coil made of copper piping and fins with a special aluminium coating for corrosive and seaside environments.
  • Axial fan/s.
  • Scroll compressor.
  • Water condensers in G2 titanium. Guaranteed against corrosion.
  • R-410-A refrigerant gas.
  • High- and low-pressure switches (HP/LP).
  • Thermostatic expansion valve with external regulator.
  • Reverse cycle defrosting with a four-way valve.
  • Two-way dehydrating filter.
  • Hydraulic circuit with water inlet flow switch.
  • Control of purifying system.
  • Circuit breakers for the control and drive systems.
  • Easy-to-use control panel with a display of the actual and default temperature settings, and alarm messages.