Venturi And Gas Train Assembly - A.O. Smith 100 Series Service Handbook

VENTURI AND GAS TRAIN ASSEMBLY

The gas train includes a venturi connected to the inlet of the combustion blower. The outlet of the 24 VAC gas valve is also connected
to the venturi by a manifold gas line. There is a shutoff valve installed in the manifold gas line for start up and service procedures. The
venturi contains a convergent/divergent nozzle (cone shaped restrictor) that constricts the air passage to the blower. As air enters the
constriction point its velocity increases. A pressure drop occurs at this point and creates a negative (vacuum) pressure in the cavity
between the nozzle and the venturi housing. This negative pressure "pulls" gas from the outlet of the 24 VAC gas valve into the blower
where it is mixed with combustion air and then supplied to the burner, see Figure 6.
As the combustion blower speed is increased the velocity of air flowing through the venturi is also increased. This increases the
vacuum created by the venturi and more fuel gas is pulled from the 24 VAC gas valve and supplied to the burner. This increases the
firing rate (input Btu/hr) of the boiler/water heater. As the blower speed is decreased less fuel gas is supplied to the burner and the
firing rate is reduced.
BLOWER
DIFFERENTIAL MANIFOLD
PRESSURE TAP
14
MODULATING
GAS VALVE
FIGURE 6. VENTURI AND GAS TRAIN ASSEMBLY
Servicing should only be performed by a Qualified Service Agent
BURNER
FLAPPER
VENTURI
MANUAL GAS
SHUTOFF VALVE
GAS PRESSURE
SWITCH
FLEX GAS
MANIFOLDS