LG GEN2 Series Installation And Operation Manual page 79

High Turndown Electronic Modulation
IMPORTANT
*High turndown furnaces use a split burner manifold
with a shutoff and a modulating valve on each
manifold section. High and low fire manifold pressure
must be checked and properly set on each manifold
section
*High turndown furnace patent pending.
Furnaces with electronically modulated high turndown
control use two combination valves for primary gas
control – one valve controls each manifold section.
They control the high fire gas supply and act as on/off
switching devices. The combination valves are normally
closed and require 24 VAC to hold them open.
A modulating valve is located just after each
combination valve. The modulating valve is the device
that modulates or changes the gas volume that is
being supplied to the furnace manifold. Its function is
described in the 4:1 Turndown Electronic Modulation
section.
When a manifold section is initiated, the furnace
controller will first provide a 10 VDC signal to the
modulating valve so that it will always start in a high fire
condition. After ignition, the controller will change its
output signal, causing the volume of gas to be reduced
to as little as 25% of full flow (4:1 turndown) based on
demand.
If less heat is needed, a manifold section is turned
off and the remaining manifold section is allowed to
modulate based on demand.
Adjustment of the high turndown modulation system
follows the same steps as the 4:1 modulation system.
The same settings must be performed on each manifold
section. Refer to the 4:1 modulation section for proper
adjustment of each section of the high turndown
modulating furnace.