A.O. Smith BFC-120, Installation, User And Service Manual

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5 Introduction
5.1 About the water heater
The SGS water heater is intended for heating water for sanitary purposes.
The SGS is a condensing gas–fired storage water heater with a fan in the air intake. The
flue gasses transfer their heat to the water through an efficient heat exchanger. The water
heater has a concentric venting connector and can function as an open or as a room-
sealed water heater.
The water heater is always supplied including a storage tank fitted with a heat exchanger
connected to a solar energy source.
5.2 The appliance's heating cycle
The appliance's heating cycle is activated when the measured water temperature (T
1
) falls
below the threshold value (T
set
). This threshold value depends on the currently selected
appliance operating mode. For example, if the appliance is in OFF mode (frost
protection), then this value is 5°C. If the appliance is in ON mode, this threshold value is
adjustable, for example to 65°C.
The heating cycle runs successively through the following states:
HEAT DEMAND;
PRE-PURGE;
PRESSURE SWITCH;
PRE-GLOW;
IGNITION;
RUNNING;
POST-PURGE.
5.3 Working principle
In this water heater, the cold water enters the bottom of the tank through the cold water
inlet (14).
Once the water heater and the storage reservoir are completely filled with water, they are
both constantly under mains water pressure. When hot water is drawn from the water
heater, it is immediately replenished with hot water from the storage tank, which in turn is
replenished with cold water. The water in the storage tank is heated by a heat exchanger
which is connected to the solar heating system.
The water heater is fitted with a modulating premix burner system with 1:1 gas–to–air
ratio regulation. The air required for combustion is delivered by the fan (18). The gas is
supplied via the gas control valve (16) and the venturi (30) on the intake side of the fan.
The 1:1 gas-to-air ratio always guarantees the most efficient gas/air mixture.
The heated tap water leaves the tank through the hot water outlet (2).
The special design of the heat exchanger (11) ensures that the flue gases are first led
downwards via the combustion chamber, then upwards via the heat exchanger, and
downwards again alongside the water in the tank. The flue gases gradually become cooler
in the process. Because the cooled flue gases flow alongside the cold water lower down in
the tank, they start to condense. This condensation causes latent heat energy to be
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0311753_SGS_28-120_II_UKUK_V2.0 , 2017-05-03 43