16 Model VSU Make-Up Air ®
IMPORTANT
Proper air velocity over the burner is critical on direct-
fired gas units. If the air velocity is not within the unit
specifications, the unit will not operate efficiently,
may have sporadic shutdowns, and may produce
excessive carbon monoxide (CO) or other gases.
The proper static pressure should be between 0.625
and 0.675 in. wg (155.68 and 168.64 Pa). If needed,
evenly adjust the baffles on both sides of the burner,
keeping the burner centered in the opening until the
required pressure is obtained.
Set the Burner Air Pressure Differential
With the fan running and discharging 70ºF (21ºC) air,
connect a U-Tube manometer to the outer sensing
probes (see below) and measure the static pressure
across the burner.
Adjustable
Baffle
Airflow
Burner
Adjustable
Baffle
Airflow
Burner and Baffles
9
8
7
6
0.625 - 0.675 in. wc
Outer Sensing Probes
Measuring the Pressure Drop
U-Tube
Manometer
NOTE
The pressure drop was set at the factory and may not
need adjustment.
NOTE
When required pressure is obtained, be sure to
reconnect the outer sensing probes.
IMPORTANT
This process may need to be repeated until the proper
pressure is achieved. This adjustment will change the
air quantity delivered by the unit and therefore the air
quantity delivered should be rechecked. Refer to the
Blower Start-Up section.
NOTE
To increase the static pressure decrease the opening.
To decrease the static pressure increase the opening.
Set the Maximum Firing Rate
Monitor the unit’s actual temperature rise by placing
a thermocouple in the unit’s inlet and a second in the
discharge, three duct diameters downstream of the burner.
Send the unit to maximum fire by disconnecting and
isolating the wire connected to Terminal 3 on the Maxitrol
14 or the Maxitrol 44. See images on page 18.
While monitoring the units temperature rise, set the
maximum firing rate by adjusting the regulator until the
designed temperature rise is achieved. After setting the
maximum firing rate, reconnect the wire to the amplifier.
NOTE
Do not set the burner maximum firing rate based on gas
pressure. It should be set based on the unit’s designed
temperature rise shown on the direct gas label.
Direct Gas Nameplate
“ WC
“ WC
F
PSI
“ WC
MIN GAS
PRESSURE
PRESSION DE GAZ
MIN BURNER
PRESSURE DROP
PERTE MIN DE PRESSION
DANS LE BRULEUR
TYPE OF GAS
NATURE DU GAZ
MAX BURNER
PRESSURE DROP
PERTE MAX DE PRESSION
DANS LE BRULEUR
MAX GAS
PRESSURE
PRESSION DE GAZ
MAX
DESIGN ∆T
∆T NORMALE
EQUIPPED FOR
CONCU POUR
SCFM “ WC
EXTERNAL STATIC PRESSURE
PRESSION STATIQUE EXTERIEURE
AGAINST
CONTE
Maximum firing
rate adjustment
Combined Regulator Valve