Unit Heaters,Hydronic Unit Heater,industrial unit heaters,hot water air heater,hot water fan heater,commercial unit heatersFlexiheat UK


Unit Heaters  – LTHW / MTHW or HTHW

Heat outputs from 6.5 kW to 344 kW

Unit Heaters, hydronic unit heater, industrial unit heaters, hot water air heater, hot water fan heater, commercial unit heaters

Unit Heaters -These Flexiheat water unit heaters are normally supplied for LTHW (Low Temperature Hot Water Heater, or Medium Temperature Hot Water –  MTHW – up to 150 C) and are a water-supplied forced air unit heaters for space heating, with high efficiency and fitted with a powerful axial fan.

These suspended warm air industrial unit heaters can discharge the air either horizontally (wall mounted) or vertically (ceiling mounted) for space heating. These hot water fan heaters have a powerful axial fan which means it is suitable for many commercial and industrial low temperature heating systems applications.

Flexiheat offers an extensive range of accessories allowing these warm water air heaters to be aligned to your site requirements. Our unit heater range offers cost effective and energy efficient water heating solutions suitable for heating warehouses, heating workshops, heating factories,exhibition halls,sports halls, changing rooms and many more applications.

We now supply two ranges of these Industrial Unit Heaters , as detailed below-

We also do an extensive range of steam unit heaters, which are detailed on it’s own web page –

Steam Unit Heaters, an extensive range with high heat outputs from 14.6KW to 459KW If you require more information we have a dedicated web page Steam unit heaters

or Electric unit heaters , which have their own dedicated webpage here – Electric unit heaters

Water Unit Heaters – Solutions

The heating hot water from your boiler flows through a radiator heating coil, there a powerful axial fan forces air through the radiator coil ( this is why these unit heaters are sometimes referred to as a forced air unit heater) ,the air passing over the heaters unit heat exchanger and absorbs the heat from the warm water, that is feeding the heater, culminating in a higher discharge air temperature. The cooler water leaves the heaters unit heat exchanger and is pumped back to the boiler to reheated, and the process starts again. Thus, often these water unit heaters are referred to as forced air unit heaters or fan coil heaters.

The integrated heat exchanger is made of copper/aluminium for hot water – low temperature water – as a heating medium. The units are splash water protected (protection class IP54).Operation with outside air, mixed air and recirculating air is also possible.

The multi-stage adjustable fans can be adapted to the requests on site. They assure draught-free, area-wide and comfortable distribution of heat and air. In order to calculate your requirements, our Flexiheat consultant team is at your disposal.

The two discharge options – downward (ceiling mounted) or horizontal – meet all likely space heating requirements. Downward discharge or ceiling mounted is particularly suited to complex areas where obstacles might impede airflows from a horizontal unit. However, the horizontal discharge units, with adjustable louvers, are ideal where directional heating (e.g., along production lines) is required. These unit heaters are of compact construction and can be mounted easily anywhere.

Our range of industrial unit heaters are designed for space heating in warehouses, distribution centres, showrooms, changing rooms, sports halls and car garages and are compatible with new and existing biomass heating systems, heat pumps heating systems or other traditional wet heating systems.

We do a few options – the first heater is encased in EPP. “EPP” is an abbreviation for Expanded Polypropylene, a material with some remarkable qualities, such as multiple impact resistance, an extraordinarily high strength to weight ratio, excellent water and chemical resistance, and, most importantly, EPP is 100 percent recyclable. These characteristics and advantages would be impossible to find in any other material.

This unit heater range is designated the “LEO” –

Leo hot water unit heaters EPP plastic cased for space heating Flexiheat UK

Leo unit heaters EPP Plastic cased from Flexiheat UK


The LEO series hot water unit heaters are available in eight different configurations with three different casing sizes, with a broad range of heating capacity from 0.7kW to 121 kW dependent on flow and return temperatures, and the entering air temperature. Sometimes these heaters are referred to as  LPHW unit heaters 

Normally used in a 100% recirculation warm air heating mode, in that the air in the area being heated is continually being drawn over the heater coil and expelled into the area. We also have the option to mount the air heaters with a mixer box, which enables fresh air to be added to the heater, resulting in a mixture of recirculated and fresh air being introduced into the space heating.

The Leo hot water unit heater range has by far the best controllability of all of our unit heater range, from basic controls all the way up to our T-Box controller, which will control up to 31 units, and is fully BMS compatible (used in larger HVAC systems)

Download the Flexiheat Brochure for Leo Unit Heaters

Download the manual for Leo unit heaters (includes NP and stainless steel casing option)

We also do a range of warm air unit heaters designated the “NP” range which are dedicated to operating with large difference between inlet and outlet heating temperatures i.e., A larger Delta T such as 60/30 or 70/40 etc. These units can be used to a maximum inlet water temperature of 70C so are ideal for commercial heat pump heating systems. These are detailed in the following dedicated brochure –

Download the Flexiheat Brochure for LEO warm air unit heaters for larger delta T – NP range

Benefits of a LPHW water unit heater in Industrial and Commercial Heating applications –

  • High reliability, as the only moving part is the powerful axial fan assembly and motor -and if the motor fails, it is a very simple task to replace.
  • Clean heat with no odours, and no by-products of combustion to flue away or a naked flame to worry about !
  • High Efficiency – aluminium fins , which ensure high heat outputs from this extensive range
  • Lightweight and compact details for simple and easy installation
  • Large range of heater sizes to match the heater to your space heating requirements. When selected properly, you don’t have to worry about under or over sizing the heaters
  • The fan speed can be adjusted on all models which allows you to have the perfect airflow with our powerful axial fan
  • Highly Durable. as all of our unit heater casings are treated for corrosion resistance and finished with a grey powered paint
  • Summer mode – The fans can run to the help circulate air in the summer period
  • Multiple unit heaters can be spaced strategically for zoning unique areas within the building, giving you maximum control and heating efficiency.
  • No oil or gas is required to be piped up to high level  as in the case of utilizing direct or indirect warm air heaters powered by those fuels

Unit Heater Technical Details

Firstly, we have the commercial unit heater range of these hot water fan heaters / hot water unit heaters. This range has an output range from 0.8 kW to 32.7 kW depending on the water flow and return temperature, and also the entering air temperature to these water to air space heaters.

They have a water heat exchanger, which is sometimes referred to as a heating coil which is manufactured from copper tube, with aluminium fins attached. The maximum working temperature for this range of commercial unit heaters is 120°C,with a maximum working pressure of 16 Bar, on a wet heating system.

The following output data table is based on various entering air temperatures to the heaters , and various flow and return water temperature to the warm air unit heaters

heating capacities for our industrial hot water unit heaters LEO S1 to Leo L1 data Flexiheat UK

Heating capacities for our industrial hot water unit heaters LEO S1 to Leo L1 at various flow and return temperatures and entering air temperatures


Then we move onto our industrial range of unit heaters. This range has an output from 4.9 kW to 121 kW, again depending on the water flow and return temperature, and also the entering air temperature to these water fed industrial air space heaters.

Again this data based on different entering air temperatures, and different flow and return temperatures feed the unit heaters, such as 90/70 degrees C, 70/50 C and 60/40 C etc, which is vital data should you wish to use these unit heaters on low temperature heating systems, that are common in heat pump installations. Get in touch with us if you require this additional information.



Industrial unit heaters hot water air heaters output data L2 to XL3 models from Flexiheat UK

Thermostat and controller options

We have various controller options for our Leo unit heater range, these being –

Controller options for our Leo unit heater range from Flexiheat UK

Controller options for our warm air unit heater LEO range


Firstly, we have our basic “TS” controller – Which is a room thermostat with 3-step manual fan speed regulation to control the heat outputs, with operation modes of heating or ventilation only.

The next stage up is our “HMI” programable controller – this controller has an inbuilt thermostat, 3-step automatic or manual fan speed regulation to control the heat outputs, weekly programmer, automatic antifreeze protection, operation modes of heating or ventilation only. This unit can be connected to a BMS (building management system) via MODBUS-RTU protocol.

Download the Flexiheat HMI Controller for unit heaters Manual

Our capable controller is the T-Box controller, this has all the capabilities of our HMI controller but with the addition of control of up to 31 units, local regulation of each unit operation (using optional temperature sensor), individual ModBus addresses for each unit as these BMS compatible unit heaters are fitted with a “DRV” drive module.

Download the T Box controller manual BMS compatible

Should you want to control the heaters in various heating zones – then you need to use the T-Box zone unit

Flexiheat Manual for T Box Zone Plus controller – BMS compatible

Air mixing chamber for the LEO hot water fed units

The KM mixing chamber is designed to work with LEO water unit heaters. It enables the delivery of fresh (from the outside) air into the building.

The mixing chamber is equipped with three air intakes: two for recirculation air and one for fresh air. It is possible to smoothly adjust the degree of damper opening from 0 to 100%. Thanks to this, it is possible to recover heat through recirculation.

Air mixing chamber attached to our LEO Water heater unit heaters

Air Mixing Chamber for our LEO water to air industrial unit heaters


When the mixer box is used in conjunction with a motorised damper, the air flow through these industrial unit heaters, can be set to either 100% recirculated air to 100% fresh air or anywhere in between these values.

The mixer box is usually connected to the external fresh air via a grill, which can be either wall mounted, which is the most common setup. Alternatively, you can also mount the grill in the ceiling or even below the heater. It is important to note , that when utilizing fresh air form outside the building, that these heaters either have a control system that has a frost protection system or that the heating system water is dosed with anti-freeze, to prevent the water freezing in the heaters coil or heat exchanger, when the units are not in use.

Our warm air unit heaters with mixing chamber are fitted with a coarse 80% class filter fitted to EN ISO 16890-1E, this is essential if the unit heaters are to be installed in a dusty environment, so that any build-up of dust on the heating coils is prevented, which would affect the correct operation and heat output of the units.

Download the KM Mixing Box for Leo water unit heaters brochure by Flexiheat UK

Optional accessories for our industrial unit heaters

Two options can be fitted to our “L” to “XL” unit heater range.

accessories for our warm air industrial unit heaters showing a confusor and a four way air outlet grille

Four way air outlet grille

The use of a four-way air outlet grille results in steady air distribution in four directions. These are highly recommended in low rooms where unit heaters are ceiling mounted.

Please note – The use of  a four-way outlet grille results in decrease of air flow and heating capacity by 10% in relation to nominal technical data of the LEO fan heaters.


Four way air distribution head option for our industrial unit heaters Flexiheat UK

Four way air distribution head option for our industrial unit heaters


Confusor air throw extender

The “confusor” is used to increase the air flow velocity from the unit heater to help push the warm air down to the ground level where it’s required. Therefore, they are highly recommended in high buildings where heaters are installed under the ceiling.

Again please note – Use of a confusor results in a decrease of air flow and heating capacity by 10% in relation to nominal technical data of LEO fan heaters.

Confuser to increase the air speed and throw for our warm air unit heaters

Stainless Steel casing option for our warm air unit heaters

The Leo warm air unit heaters are also available with stainless steel casings, which are highly resistant to corrosion, thus making these unit heaters appropriate for operation in high-humidity locations, such as food processing factories, gourmet buildings, greenhouses, and so on.

The casing and air distribution blades are manufactured from austenitic stainless steel, along with the mounting bracket.

The IP54 motor rating means these unit heaters will be protected against water sprays from all directions.

The outputs, etc., are still the same as the EPP-cased Leo unit heater range.

Stainless steel casing warm air unit heaters hot water fed from Flexiheat UK

Stainless steel casing option


Full detail on this optional stainless steel casing range of warm air unit heaters is in the following brochure

Download the Flexiheat Stainless Steel Leo warm air unit heaters brochure

We also supply a dedicated range, which have their own webpage of Unit heaters for Poultry and Pig farms or swimming pool heating applications

Heating and cooling unit heater option

We also do a hot water heater with cooling option, can only be installed on the wall – blowing air horizontal and can not be used in a vertical orientation i.e. ceiling mounted,as they come with a condensate drain pan. When the unit is cooling mode, the ambient humidity condenses on the coil. Thus, it is necessary to recover it by connecting a drain pan and hose.

Heating and cooling units, that are used in a HVAC system, for climate control by heating and air conditioning of commercial and industrial buildings. The space heaters side is powered by energy efficiency geothermal, ground source or air to water heat pumps, and the cooling aspect is provided by chillers or reversible heat pumps. This results in an accurate temperature controlled heating and air conditioning system with a seasonal energy efficiency ratio.

Heating and cooling units from Flexiheat UK

This unit / range has its own dedciated webapge here –

Heating and cooling unit heaters 

How do I calculate the heating load for my warehouse or commercial building?

ANSWERCall us as we are happy to run a heat loss calculation that will help you pick the perfect unit heater or heaters for your site conditions, as you do not want to under size, or over-size the heaters, PLEASE DO NOT FALL for unit heaters that are advertised as “will heat up to 1,200 square feet”. As heating a 1,200 square foot insulated warehouse in Kent, is an awful lot different to heating a 1,200 square foot un-insulated warehouse in Aberdeen. This one heater cannot “magically” service both 1,200 square foot spaces.

Our Leo unit heaters can operate with low temperature hot water as standard, but we can also supply units that use medium or high temperature hot water, please see the FHTERM series of industrial heaters below –


Heat outputs from 6.5 kW to 344 kW for high heat outputs

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This range of industrial unit heaters, which are designated the “FHTERM WB Range” are fitted with a bi-metal water heating coil / heat exchanger and can be fed with a water temperature of 150 Degrees C or lower and with the operational pressure of up to 16 Bar. This makes these industrial warm air unit heaters suitable for installation on heating systems that use MTHW – (Medium Temperature Hot Water) heating systems, which in the UK, is normally defined as a heating system with a maximum flow water temperature of 120 Degrees Centigrade, with high heat outputs.

They are also suitable for HTHW – (High Temperature Hot Water) heating systems, which again in the UK are normally defined as a heating system with flow water temperature of above 120 Degrees C.

The FHTERM- WB range are a water-supplied suspended warm air industrial unit heaters which can discharge air horizontally or vertically. These industrial unit heaters benefit from powerful axial fans which means they are suitable for many industrial and commercial warm air heating applications.

The heat exchanger or coil of these “FHTERM WB Range” industrial warm air unit heater range is made from steel pipes with an internal diameter of 12.4 mm and spirally rolled aluminium fins /ribbing with external diameter of 38 mm and with a rib spacing of 2.8 mm. These type of heat exchanger or coil is referred to as a bi-metal heating coil or heat exchanger. All of these commercial and industrial unit heaters are available with either a two row heating coil or a three row heating coil- the more heating coils , the larger the heat output available.

If you intend to use these unit heaters hanging from or attached to the ceiling, then they can be equipped with either with a slot diffuser, which distributes the warm air out of the heater via a four-sided air distribution head assembly, which we refer to as a slot diffuser or they can be fitted with a cone discharge nozzle, which extends the warm air throw of these forced air unit heaters.

Using the slot diffuser- When the units are used in a ceiling mounted or hanging unit heater configuration the models FHTERM – 0 and 1 units can be hung at the maximum height of 4 metres, and the FHTERM 2, 3 and 4 units can be hung at the maximum height of 6 metres

Air intake boxes can also be supplied on this range of industrial unit heaters. These are used to draw and / or mix the fresh and recirculated air in the building. In the basic version they consist of a fresh air damper and mixing chamber with recirculating air inlet grids, but you can also add extras such as a filter which is class “G3” or a recirculating air damper with actuators

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Industrial unit heaters – Hot Water – Fitted with air mixing chamber that allows you to bring in fresh air to the unit or mix the fresh air with recirculated air from the building

The first two units in the range, which are the model 0 & 1 are supplied only with single phase motors, which are 230 Volt / 50 Hz / 1 Phase

The model 2 unit heater can be supplied either with a three phase motor, which is 400 Volt / 50 Hz or with a single phase motor which is 230 Volt / 50Hz.

The models 3 and 4 industrial unit heaters are supplied only with the three phase motors which is 400 Volt / 50 Hz.

The three phase motors of models 2,3, and 4 can be connected in Star or Delta wiring configuration depending on your site requirements.

The model “0” comes with an IP40 rated fan motor as standard and an insulation class of “B”, whilst the rest of the industrial unit heaters come with an IP54 rating and an insulation class of “F”. At request we can supply these industrial unit heaters with the motors with an increased level of anti-explosive protection, for warm air industrial heating applications that require this level of protection.

These Industrial unit heaters cannot be used in areas or locations with relative humidity above 95% and air dust concentration over 3mg/m3. And the model “0” should not be used in high humidity locations at all.

Industrial unit heaters are invariably the most cost-efficient method of space heating a room or building, with warm air. They are a perfect heater for an industrial unit.

Model “0” Industrial Unit Heater Heat Output Data – 1 Phase Only

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Model “1” Industrial Unit Heater Heat Output Data – 1 Phase Only

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Model “2” Industrial Unit Heater Heat Output Data – 1 Phase or 3 Phase Available

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Model “3” Industrial Unit Heater Heat Output Data – 3 Phase Only

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Model “4” Industrial Unit Heater Heat Output Data – 3 Phase Only

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This unit heater range can also be supplied with stainless steel casing

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Industrial Hot Water Fed Unit Heaters available in Stainless Steel for corrosive atmospheres from Flexiheat UK


Download the Flexiheat “FHTERM” Unit Heater Brochure

Industrial Unit Heaters for Sale UK and Ireland 

So if you require a well made and high quality hydronic unit heater or water fed unit heater / forced air unit heater / hot water air heater our Industrial Unit Heaters are the perfect solution for your hot water warm air heating system and are particular suited to heating car garages and workshops , as well as any industrial unit or premises. They are often used in conjunction with biomass or air source heat pumps heating systems, as well as the traditional gas or oil fired heating systems. If you require any further assistance with our Industrial & Commercial Unit Heaters or any of our products, please click here to contact us via email or call us direct on 01202 822221