Gas-Fired Burner Turndown; Electronic Modulation; Available Turndown Control Options - LG GEN2 Series Installation And Operation Manual

Gas-Fired Burner Turndown

Furnaces are available in electronically modulated
configurations. Electronically modulated furnaces
have continuously variable firing rates. Turndown is
the capability of the furnace to operate at less than
full capacity, accomplished by reducing the amount of
gas flow when the unit is operating. The advantage of
turndown is that at partial-load conditions, the furnace
will not cycle as often.Turndown is expressed as a ratio
and is found by dividing the maximum BTU input by the
minimum BTU input.

Electronic Modulation

The most common electronic
modulation involves the use
of an electronically-controlled
gas valve which provides
a 4:1 turndown. The entire
furnace is sized appropriately
for the maximum heat output
ordered by the customer, but
it can operate as low as 25%
of its maximum capacity.
The electronically-controlled
gas valve will modulate the
combustion rate continuously,
dependent on the output from the furnace controller.
With this method, all the burners fire at the same time
but at a varying capacity. The Modulating Valve is
used in conjunction with a furnace controller and a
combination valve which provides an ON/OFF function.
The high turndown option uses the same functionality
as the 4:1 modulation described above in conjunction
with a split manifold to provide higher turndown
capabilities. When less heat is needed, a portion of
the manifold is turned off and the remaining portion
continues to modulate. This provides seamless
modulation at very high turndown ratios. High turndown
is classified as 8:1 turndown or greater per furnace.
Low Fire and High Fire
adjustments are made
on printed circuit board
behind removable cover
Combinations
Various combinations of the methods outlined may
be used to accomplish the turndown required by the
application.
Available turndown control options include:
4:1- uses modulating valve and furnace
Single Furnace controller
Unit
High turndown - uses 4:1 modulating valve
with split manifold and furnace controller
4:1 - uses two 4:1 modulating furnaces
controlled in parallel
Two Furnace
High turndown furnaces in a parallel
Unit
configuration - use two high turndown
furnaces, controlled in parallel
Electronic Modulation