Gas Fired Burner Turndown - LG AR-DR12-05A Installation And Operation Manual

INDIRECT GAS FIRED FURNACES

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 conjunc-
tion 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 mani-
fold 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.
Combinations
Various combinations of the methods outlined may be used to accomplish the turndown required by the application.
Available turndown control options include:
Due to our policy of continuous product innovation, some specifications may change without notification.
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