water heated air curtain; lphw air curtain; hydronic air curtains; hot water heated air curtains; heated air curtains for doorsFlexiheat UK

Water Heated Air Curtain

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Water heated air curtain- designer, commercial and industrial range from Flexiheat UK


Water Heated Air Curtain – We do three ranges of water heated air curtain heaters or sometimes referred to as hot water over door heaters, a light commercial range, which is very aesthetically pleasing and tends to be used in shops, hair dressers, beauty salons or in hotel lobbies- this range is called the “Slim”, a commercial range – designated the “Airstream” and an industrial range- the “Elis G” range.

All of these water heated warm air curtains are suitable for taking and heating the air from the inside of the area via heated water from a wet heating system. They can be also be used without air heating in the summer months as an ambient air curtain. These commercial or industrial air curtains allow you to create barriers between two spaces, external and internal of the environment, which have different air temperatures.

These warm air curtains limit the access of cold air from the outside into the room, as well as the consequent temperature drops and they can result in an energy saving of 30%, when compared to premises that don’t have them installed. These also have a further advantage in the de-stratifying effect of the air that is moved from the top of the room and conveyed downwards, resulting in additional comfort for the users whilst reducing heating cost.

The main purpose of a water heated air curtain is to provide a heated curtain of air, which not only significantly increases the comfort of your building, but also helps reduce the energy loss that occurs when doors are opened and the heat escapes the room. To be fully effective as a thermal barrier they should cover the full width of the door, otherwise cold air may penetrate at the edges.

Over door heaters air curtains – This is a term that is sometimes used to qualify these units , in reality , they are two terms rolled into one – yes they are over door heaters , as they emit heat into the area , and yes whilst preforming this function they also emit or produce this warm air in ” curtain “form. This allows effective protection against heat, cold, dust and insects from the outside air from entering your building, whilst people or machinery are coming in or out of it .

A diagram showing the working principle of an air curtain by Flexiheat UK

Keep cold air out in the winter with low pressure hot water air barriers and keep cold air in from air conditioner system in the summer.

In the winter time , these hot water air curtains prevent the warm air from escaping from inside the building or room to the external environment, whilst in the summer and used in their ambient air mode ( fan only)  they prevent warm or hot air coming from the outside entering the building, allowing the room to maintain a comfortable temperature condition.

Hydronic Air Curtains

Hydronic air curtains – Hydronics is the technical term for a liquid heat transfer medium ,where the working fluid is typically water – thus the water gives off or transfers it heat to the air via some form of heat exchanger , and warms the air .In the UK this system is more commonly known as a ” Wet heating system” , which are very common especially in commercial and industrial heating systems, that are found in factories, schools, warehouses etc. Thus these water heated air curtains are a great solution for these types of warm air curtain applications.


LPHW Air Curtain

LPHW Air Curtain – This term “LPHW” is just an abbreviation for “ Low Pressure Hot Water” and traditionally in the UK this meant that the water from the boiler was distributed or flowed out at 82 °C , and was returned to the boiler at 71°C , giving up or losing 11°C of heat through it’s journey through the heating system. These temperatures have changed over the last 15 years or so, as condensing boiler technology has meant that we can now utilise the fuel use to power these low temperature boilers to the maximum efficiency, which reduces not only CO2, but also the heating system running costs. Nowadays flow and return temperatures of 70C/50C or 60C/40C are a lot more commonly used, and our hot water heated air curtains can operate and preform really well on these heating system temperatures.

We have a developed a new range especially for lower water temperatures that are often encountered with heat pump systems – these are detailed here – heat pump air curtains

Hot Water Heated Air Curtains

Be it a commercial over door heaters or over door heaters air curtains that you would like to qualify these units , our hot water heated air curtains will do both applications.

Commercial Air Curtain Heaters – Hot Water Fed

Firstly our aesthetically pleasing “Slim” range of low pressure hot water heated compact air curtain, which are the slimmest on the market, and come with a PIR integrated control system.

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The ” Slim ” range of warm water heated air curtains are aesthetically pleasing and provide an effective warm air curtain.


We start with our commercial air curtain heaters range of warm water heated air curtains -which are intended to be wall mounted or suspended from a ceiling, and available in different lengths being: 1 metre, 1.5 metre and 2 metre’s. They are most commonly installed at the entrances of supermarkets, theatres, meeting rooms, hotels, offices, storage areas etc. If the doorway or entrance is wider than the unit size available, then you can install two or more units in parallel, to cover the complete doorway or entrance. When used in a parallel configuration to encompass larger entrances, the units must have a 10-40 mm gap between the units.

Please also note that these water heated air curtains must have a 50 mm air gap between the top of the air curtain and the ceiling. All of this range of commercial air curtains are powered by single phase 240 volt electrical supply, and come as standard with a mounting panel and automatic door switch, to control the air curtain, so that when the door is opened , the unit comes on, and when the door is closed the unit switches off.

The maximum water temperature for this range is 110 C , and the maximum working pressure of the water is 16 Bar.

All of these over door heaters / air curtains come with a 1/2″ connection for the flow and return pipes, and have a 3 stage/speed fan.

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These hot water heated air curtains heat outputs will vary depending on your flow and return heating water temperatures and the air inlet temperature to the air curtain, we have full technical data charts that shows this data, like below shown for the various sized hot water heated air curtains – in the technical manual for this range of warm air curtains.

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Output Data for the ” Slim” range of LPHW Air Curtains at fan speed 3 – other fan speed data is also available


V – air flow

PT – heating power

Tp1 – air temperature at the inlet to the device

Tp2 – air temperature at the outlet from the device

Tw1 – temperature of the water on the flow (supply) of the exchanger

Tw2 – temperature of the water on the return from the exchanger

Qw – flow of the water in the exchanger

Δpw – pressure drop of the water in the exchanger

Slim Air Curtains range features and benefits brochure

Download the Flexiheat ” Slim “Light Commercial Hot Water Heated Air Curtain Brochure

Commercial buildings must now be designed to operate at a maximum flow temperature of 55°C because of changes to the Building Regulations see more here – Boiler flow temperatures Part L regulations.

We have a range of water fed lphw air curtains, which are designed for these lower heating temperatures and are fully effective with good leaving air temperatures. These are detailed here 

Airstream commercial hot water heated air curtains 

This range is suitable for low pressure hot water up to 95C and 16 Bar of pressure with a maximum installation height of 3.7 Metres. 

Airstream hot water heated air curtains from Flexiheat UK

Airstream low pressure hot water hydronic air curtains range for commercial premises with a maximum installation height of 3.7 Metres

The minimum distance between the unit and the ceiling should be 100 cm’s (4″) and these units are available in RAL 7016 Anthracite grey , or in RAL 9016 traffic white.

The heat output of these commercial air curtains is directly related to the flow and return temperatures of the hydronic heating system.

Internal picture of the water heater exchanger in our Airstream LPHW water heated air curtains - Flexiheat UK

Internal picture of the water fed heat exchanger for our airstream hot water heated air curtains

The 1 Metre unit has a heat output range from 4 to 17 kW , the 1.5 Metre unit has a heat output range of 10 to 32 kW and finally the 2 Metre has a heat output range of 17 to 47 kW with low pressure hot water up to 95C.

Airstream hot water commercial air curtains output data from Flexiheat UK

Airstream range product output and data


The airstream range is available with either “AC “fan motors that are 3 speed or with high efficiency  “EC” fans that can modulate from 0 to 100%.

Download Flexiheat brochure for Airstream air curtain range

Download the Airstream commercial air curtains technical manual Flexiheat UK

Commercial LPHW water heated warm air curtains – Accessories

The use of automatic controls dedicated to this range of commercial water heated air curtains from Flexiheat UK, gives great possibilities of adjusting the output of the warm air curtain in different applications and depending on your requirements, we can supply various components to fully automate the unit’s operation.

  1. A 3-stage speed regulator with thermostat – This Regulator is used to regulate our warm air curtains equipped with 3-stage fans. It has a 3-stage speed control and built-in thermostat that switches the air curtain off automatically when the pre-set temperature is reached. In addition, the unit controls the operation of the actuators installed on the control valve.
  2. The Programmable Control panel is used to regulate our lphw warm air curtains that are equipped with 3-stage fans. It controls them according to set program i.e. the required air temperature. There is also opportunity to connect the external air temperature sensor (in the set). The controller automatically controls the actuator of two-way valve. It has MODBUS communication too.
  3. The two-way valve with actuator is used to automatically regulate the flow of the heating medium i.e. the hot water .It is always installed on the return water connection from the heat exchanger of the warm air curtain.
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Hot water heated air curtains – Commercial – Accessories

Download the Flexiheat Technical Manual for Slim Water Heated Air Curtains


Hot water heated air curtains – LPHW – Industrial Range

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These industrial warm water heated air curtains are designed to maintain a constant temperature, in a given space, by preventing unwanted air exchange between the outside of a building or warehouse and the inside.

These hot water heated industrial air curtains generate an air screen either above the opening (be it a door or gate) , but can also be used on the side of the opening too, depending on the site requirements on which they are installed. In winter period, the heated air stream prevents the outdoor air from cooling down the room or area in which they are installed. The opposite applies during the summer period, when used in a fan only mode, the ambient air is blown to prevent the indoor air from heating up.

There are two models or modules of these industrial water heated air curtains, the first is a 1.5 Metre long unit and the other is a 2 Metre long unit. They are basically modules that can be coupled together to make up your desired width or height for your industrial or commercial overhead roller shutter type door. These Industrial warm water air curtains can be situated to the side or above industrial doorways and create a powerful heated air barrier across the open door way whilst providing total visibility and access. These air curtains in this configuration of “modules” tend to be easier to install, as they are lighter in weight and easier to manoeuvre into place than a dedicated single piece air curtain heater e.g. say a 4 meter long unit.”

“W ” donates – water heat exchanger – as standard the units have a 1 row heat exchanger or are also available with a 2 row exchanger ( designated the 2R ) which has larger heat outputs.

G-W-150G-W-150 2RG-W-200G-W-200 2R
Power supply [V/Hz]230/50230/50
Max. current consumption [A]2.84.2
Max. power consumption [kW]0.691
Airflow [m3/h]6,2005,7008,1007,600
Fan IP54
Acoustic pressure level [dB(A)]*6668
Sound power level [dB] **8183
Max. water temperature [o C]130130
Max. operating pressure [Bar]1616
Connection [”]¾¾
Weight of unit [kg]47.451.86266.4
Weight of unit filled with water [kg]49.756.464.371

* acoustic pressure level measured in the room with average sound absorption, capacity 500 m3, at a distance of 5 m from the unit.
**according to ISO 27327-2
*** according to ISO 27327-1

Please note that is a minimum distance of 400mm from the rear side of these industrial water heated warm air curtains required, whether they are installed in a horizontal or vertical orientation.

Download the Flexiheat Brochure for Water Fed Industrial Air Curtains

Below you can see an example installation picture of our industrial hot water fed air curtains

Industrial water heated air curtains uk


Accessories for Industrial Hot Water Fed Air Curtains –

A full range of accessories is available for these air curtains, they include –

  1. A door switch that increases the efficiency of the heating system by activating the industrial warm air curtain only when the door opens.
  2. The Control Box is the connection box that offers wiring points for all the essential controls for our hot water heated industrial air curtain range. The control box is essential when a highly efficient and energy saving heating system is required. They also offer additional current protection to the air curtains as well as offering the users the ability to install many more compatible controls such as; a door switch which increase the heaters efficiency by only operating the air curtain when the industrial door is open, and a speed controller to regulate the fan speed of these industrial air curtains. You can also install a thermostat into the control box, to regulate the temperature.
  3. Speed Controller – This piece of equipment offers users the opportunity to select 4 fan speeds that will control each control box, and thus the industrial water heated air curtain. This allows users the necessary function to control the temperature of these water heated air curtains to suit the environment.

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The Best Water Heated Air Curtains for Sale UK and Ireland

We think that we have the best range of Water Heated Air Curtains in the UK for either commercial and industrial applications. Should you require any further assistance with our Hot Water Air Curtains /Water Heated Air Curtain , please click here to contact us via email or call us direct on 01202 822221