P/N 42-9450 8/08 Copyright 2008 Hydrotherm
Page 2 KN INSTALLATION AND OPERATION INSTRUCTIONS
The Hydrotherm KN condensing cast iron boiler
Operation
Overview
e KN is a cast iron boiler designed and constructed to allow full condensing operation. e illustration
at le shows how the boiler is heated with a down- red premix gas burner mounted above the heat
exchanger. e KN blower pulls gas and air through a venturi assembly and pushes the mixture into
the burner. e power of the blower forces the ue products through the heat exchanger and out the
vent.
Boiler operation is controlled with the KN electronic control. e control modulates boiler ring rate
based on system demand, allowing input as low as 20% of maximum (5:1 turndown). Combined with
the indoor or outdoor reset option of the KN control, this allows maximum possible condensing mode
operation, with combustion e ciency up to 98%.
Cast iron heat exchanger
e KN cast iron heat exchanger uses Hydrotherm’s graphite port seals and ground joint t-up between
sections for maximum exibility and resistance to thermal expansion. e heat exchanger provides maxi-
mum heat transfer with its counter ow design and ow-controlling ba e assembly.
Premix burner
Premix means the air and gas are mixed before they enter the burner. e mixing of gas and air in the
blower and piping ahead of the burner causes a uniform gas/air mix in the burner, providing reliable
operation with low excess air. e result is exceptional combustion e ciency.
e burner consists of a high-temperature stainless mesh secured to a stainless steel distributor plate. e
design causes uniform ame across the entire surface of the burner and the ability to operate at very low
ring rates. e ame is ignited with direct-spark electrodes and an electronic ignition control. Flame is
proven using a ame rod ( ame recti cation sensing).
Because of its low excess air operation and controlled combustion, the burner also provides low NO
X
emissions and quiet operation.
Gas valve, blower and venturi
Most gas valves regulate at a positive pressure downstream of the valve (typically 3½ inches water
column). But the KN gas valve regulates to ZERO pressure downstream. is is called a “zero governor”
valve. e advantage is that gas will only ow if something lowers the pressure downstream of the valve
below zero. at is just what happens when the blower pulls air through the venturi. As the air speeds up
to pass through the narrow throat of the venturi, a vacuum forms in the throat. e more air that ows
through the throat, the greater the vacuum.
e gas valve raises the negative pressure in the venturi to zero by le ing gas ow. e lower the pressure
in the venturi throat, the more gas will that will ow. So gas ow automatically adapts to air ow, ensuring
consistent gas/air mix throughout the boiler’s modulating range. us, the KN HeatNet
TM
electronic
control regulates the blower speed to control boiler ring rate.
KN HeatNet
TM
control
e KNHeatNet
TM
electronic control consists of a microprocessor-based module and a keypad/display
unit for user interface. e control regulates boiler water temperature by adjusting boiler ring rate (by
varying the blower speed). e control considers how fast the temperature changes and how the
temperature varies over time to establish the best ring rate to meet demand. Options include an outdoor
sensor for outdoor reset operation and an indoor reset option. (Indoor reset monitors up to 8 zone
thermostats and determines a maximum ring rate needed to meet the demand. is keeps the boiler at
the highest possible e ciency.) In addition, the control can be set for multiple boiler operation, either
stand-alone or utilizing Hydrotherm’s on-board HeatNet
TM
technology. e control also automatically
cycles the factory-installed postpurge circulator to equalize residual heat in the boiler a er ring.
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