Kroll LH Air Unit Heaters,heater unit,commercial unit heaters,hot water air heater,hot water fan heaters,industrial unit heaters

LTHW Air Unit Heaters

LTHW air unit heaters

Many industrial buildings prefer to run their heating from a piped hot water circuit. Where heat is available in this form then the use of LTHW air unit heaters fed with hot water is invariably the most cost-effective method of space heating a room or building. As with electric fan heaters, there are a variety of types and options available and we aim to make choosing the right model as easy as possible. The following guidance is written around our LTHW unit heater extensive range that we call the LH series. They are also referred to as lphw unit heaters – (low pressure hot water).

They are great heating installation solution for large areas such as warehouses, exhibition halls, factories etc.



LTHW air unit heaters

Applications for Water LTHW air unit heaters / lphw unit heaters

There is no doubt that if you have a good source of heated water on a piped circuit then these unit heaters are a very cost-effective way to discharge the heat in to the areas in which it is needed. Each model has a large range of heat output depending on the flow temperature, return temperature, ambient air temperature, and water flow rate. This means that with the addition of a recirculation air / fresh air mixer cabinet and suitable controls, these ranges are an ideal compliment to heat sources that are bit less controllable than straight oil gas or electricity fired systems. For biomass boilers, solar fed thermal stores and wind turbine heated thermal stores it is useful to have not only a quick and effective means of dumping excess heat, but also a system that can deliver heat from quite low temperatures and be easy to integrate with a secondary heating system when water temperature is too low.


If you are using a water fed LTHW air unit heater to space heat an area, the implication is that you are using available surplus heat and this will be the principle heating for the area.  As with any heating requirement the initial stage is to identify the size of the heat output requirement. Please contact us at Flexiheat UK for advice


It is seldom the case that a required heat output for a system of water fed unit heaters can be met neatly with a single heater. In most cases there are a number of configurations using a number of models from a range to achieve the overall space heating requirement for an application. Sometimes there is a choice between a number of unit heaters of a large size or a greater number of unit heaters at a smaller size.

Generally the smaller number of larger unit heaters will be more cost-effective in terms of unit heater cost, control costs and installation cost. But the choice of unit heaters should also take into consideration the shape of the room and air throw of heaters, perhaps using larger unit heaters for larger uniform areas, and using smaller unit heater for smaller dead areas of the main area.

Uses of Fresh Air in Space heating

LTHW air unit heaters

Because a water fed unit heater usually uses a circuit of hot water that is often ‘surplus’ it is quite common that these unit heaters are not merely mounted on wall brackets, in 100% re-circulation mode, but mounted on mixer boxes, which themselves are mounted over a vent in the outside wall, enabling heated air to be a mixture of re-circulated and fresh air.

With the addition of options available such as either manual or motorized dampers , the mixer boxes allows the flow to through the unit heater to be set at any level from 100% fresh to 100% re-circulated and anywhere in between.

Mixer boxes are useful because in addition to being able to take air straight from the room through a grill, the grill can be removed leaving an upstand (spigot) for ducting connection, so that the mixing box can be part of a more complex ducted ventilation system.

The key element to connection to a mixing box where outside air is brought in, is that the heater control system incorporates frost protection functionality, to prevent any chance of water freezing in the coils when the heater coils are not being used for general heating.

As well as mixer box options, there are also filter box options allowing a G3 efficiency filter to be fitted. This fitting is essential if the unit heater are to be used in dusty environments in order that build up of dust on the coils is prevented.


Whilst many will consider conventional boilers to be the heat source of choice for a hot water circuit, they are not necessarily the most flexible source of heat. Water fed unit heaters are not fussy about what heats the water up and will operate perfectly well from a wide range of water feed temperatures. This means that as well as being ideally suited to conventional large boilers, the units can also use relatively low-grade heat produced by chiller/heat pump/ boiler combinations, where the chiller or heat pump preheats water to say 40-50°C  before the boiler adds top temperature to get it to normal heating circuit flow of say 80°C. The benefit of this type of arrangement is that typically the winter heating costs are lower and that in summer the chiller can be used to produce cool water for the heating circuit. Even if you have a heat pump or chiller producing 60°C this is hot enough to be used to feed LTHW air unit heaters, the only difference being that you will get fewer kilowatts out of it than if the flow temperature was higher. The beauty of LTHW air unit heaters is that even for lower temperature flows, they are still a very cost-effective way of discharging heat.

Our LH range of unit heaters from Kroll(A German manufacturer ) offer a wide range sizes and are noted for larger sizes with nominal heat outputs up to 118kw. They are competitively priced and at the bigger end of the range they offer size and output which is beyond the range of most of the other manufactures.

LTHW air unit heaters

The larger Kroll lthw air unit heaters in this range have two fans and at the output at 90/70°C and an ambient air temperature of -15°C the largest LH range heater produces a whopping 203kw. Other useful features in our range are that there are extensive range of options for filter boxes, louvers, and side outlets. There are also options the heaters being supplied with stainless steel casing as an alternative to the standard powder coating. These air unit heaters can also be supplied with explosion proof fan motors, where required.

For more information please visit our  LTHW Unit Heater page , or contact us on 01202 822221  or  visit our website