LH warm air unit heaterUnit Heaters LTHW/MTHW or STEAM –  The LH Series -Warm Air Unit Heaters

Heat outputs from 10kW to 118kW.

Unit Heaters -These Flexiheat supplied Kroll Unit Heaters are normally supplied for LTHW (Low Temperature Hot Water Heater, although we can supply them of MTHW or Steam if required)  and are a water-supplied suspended warm air unit heater which can discharge air horizontally or vertically. The units have a powerful axial fan which means it is suitable for many applications. Flexiheat offers an extensive range of accessories allowing these heaters to be aligned to your site requirements

Unit Heater Operating Principle

The heating hot water or steam from your boiler flows through a radiator heating coil, there an electric fan forces air through the radiator coil ,the air passing the coil absorbs the heat from the hot water or steam,culminating in a higher discharge air temperature. The cooler water  leaves the coil and is pumped back to the boiler to reheated , and the process starts again.

These unit heaters are of compact construction and can be mounted easily anywhere.

The LH range of unit heaters are designed for use in showrooms, changing rooms, sports halls and garages and compatible with new and existing biomass heating systems or other traditional wet heating systems

Benefits of hot water unit heaters and steam unit heaters in Industrial and Commercial Heating applications –

  • High reliability, as the only moving part is the 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 naked flame to worry about !
  • High Efficiency
  • Lightweight and compact for simple and easy installation
  • Large range of heater sizes to match the heater to your heating load required. 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
  • 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 withing the building, giving you maximum control and 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

The Kroll LH warm air unit heaters offers cost effective and energy efficient water heating solutions suitable for heating warehouses, heating workshops, heating factories and many more applications.

The two discharge options – downward or horizontal – meet all likely space configurations. Downward discharge is particularly suited to complex areas where obstacles might impede airflow’s from a horizontal unit. However the horizontal discharge units, with adjustable louvers, are ideal where directional heating (e.g along production lines) is required.

Unit heaters can operate with low temperature hot water as standard, but we can also supply units that use medium or high pressure hot water, or steam.

The integrated heat exchanger is made of copper/aluminium for hot 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 large range of different models with multi-stage fan permits an optimal solution to your requirements
  • On request, also special versions such as explosion-proof units or units for operation in chicken / pig farms /stables are available
  • Suitable for every room: they can easily be changed from wall-mounting to ceiling-mounting, which makes them extremely versatile
  • The low noise level ensures a silent operation and prevents noise pollution
  • In addition to the powder-coated versions LH (RAL 7035), all models are also available as versions LH-X with stainless steel casing
  • Ability to rotate the heaters through 180 ° – So the connections can be on the left or right, c/w air vents on both connecting nipples
  • With the right accessories, a comfortable room climate becomes the most natural thing

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

ANSWER: Call 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 oversize 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 can not “magically” service both 1,200 square foot spaces.


  • for mounting at the wall or the ceiling
  • operation with outside air, mixed air and recirculating air possible
  • for heating and ventilating
  • including air outlet louvres
  • heat exchanger made of copper/aluminium
  • fans with blade rotator


A full range of accessories are available.

Download the LH Heater Product Brochure

Download the LH Heater Technical Manual