Water boilers

Water boilers

KNS boilers at GRASO are constantly developed in order to obtain the cleanest and the most environmentally-friendly combustion of straw. The units manufactured are a new generation of boilers that meets these requirements. Low temperature water boilers are designed for the preparation of hot water for heating purposes, namely heating of accommodation, office space, workshop, plant and production facilities, farms, dryers, plastic tunnels, greenhouses, etc.

The boilers can be used anywhere where it is possible to use renewable and waste fuels, often inutilely lost. Water boilers can only be used in open system installations protected according to PN-91/B-024413 “Heating and heat engineering. Protection of open system water heating installations. Requirements” not exceeding a pressure of 0.15 MPa, i.e. 1.5 bar (15 m of water column).


DetailsUnitKNS -200KNS -250KNS -300KNS -400KNS -500KNS -750KNS -1000
Power rating *kW2002503004005007501000
Surface of heated room **m21200140018002200280042005600
( poorly insulated 60)
(well insulated - 40W )m21800210025003400420063008400
h- 3m
Fuel consumption ***kg / h~ 60~ 80~ 90~ 120~ 165~ 225~ 350
( Average)
Boiler heating surfacem2273540536887120
Water capacitym3308.mar03.kwi608.lip05.sie05.wrz
Combustion time ***h333305.lut05.lut3
Fuel loadpc1 bale fi 120x1201 block 200x120 x80fi 1bale 150x1202 bales 120x120 fi2 bales fi 150x1202 bales fi 150x1203 bales fi 150x120
Boiler weight without waterkg340045005500700090001000012000
Operating pressureMPa0.150.20
Required exhaust draftPa50405055556575
Chimney heightm12141415161718
Chimney sectionfi~ 350~ 400~ 400~ 450~ 500About 600~ 700
Thermal efficiency%75do80
Minimum return temperatureoC50
Maximum flow temperatureoC90
Accumulation tank capacity ( 60l/1kW )l~ 12000~ 15000~ 18000~ 24000~ 30000~ 45000~ 60000
Boiler lengthmmA2100280026003800380038004900
( without fan )
Boiler widthmmB1600170020001800210021002100
Boiler heightmmH2200220025502300260028002900


Research on the construction of boilers has led to the development of boilers with a so-called counter-current combustion system, where thermal efficiency reaches 75-85%. Such boilers have two combustion chambers. Straw is loaded into the first chamber where it undergoes gasification (primary combustion). The second chamber is the proper combustion chamber where the combustion of gases takes place. The fan forces air into the boiler. The air is divided into two streams: primary and secondary. The primary stream is directed to the chamber with the burning straw. There the process of gasification of the straw takes place with the insufficient amount of oxygen, and the resulting gas containing mainly CO and other combustible organic components is directed in counter-current to the space where it meets the secondary air stream providing the sufficient amount of oxygen for combustion. This is the place where CO and CO2 are mixed together and organic compounds are burnt. Exhaust is directed to the heat exchanger and then to the chimney. To ensure a good straw combustion process, excess air of 25-50% should be provided. The optimal and completely clean combustion of straw is obtained only when all the gases are burnt which takes place at a temperature of about 850°C. The boilers are equipped with a control system that measures the temperature of straw combustion according to which it controls the flow of fresh air for combustion.

Through the use of a so-called accumulation tank we provide the continuous supply of Thermal Energy. Boilers manufactured by our Company require minimum operation and maintenance. KNS boilers are not subject to Technical Inspection and registration at the Inspectorate of Technical Inspection.


The basic fuel for firing KNS water boilers is straw with a moisture content of up to 15% in the form of bales with dimensions of fi150x120 cm. Alternatively, bales, for example, with dimensions of fi120x120 cm or blocks with dimensions of 40x50x80 cm can be used.

The combustion of straw with higher moisture will reduce the thermal efficiency of the boiler and substantially accelerate the wear of the boiler and the chimney. The combustion of straw with a moisture content of above 30% is energetically unacceptable and economically unjustified. In the case of the use of alternative fuels, such as chips, pieces of wood, the boiler efficiency may decrease.


  • efficiency of about 80-85%,
  • cheap fuel (straw, hay, etc.) 4 kg straw= 1 litre of heating oil
  • easy operation
  • compact design
  • environmentally-friendly combustion of straw