A.O. Smith BFM 50 ユーザーズマニュアル

6

The water heaters are equipped with a gas control

valve which regulates the flow of gas to the burner. The
combination gas control is equipped with a safety valve,
gas control valve, pilot safety valve and burner control
(on a standard natural gas setting)

For safety purposes, water heaters are always fitted

with three thermostats: a control thermostat, a high-limit
thermostat and an overheat thermostat.

The desired water temperature is set with the control

thermostat. To protect against freezing, the control
thermostat is also fitted with frost protection which
ignites the burner when the water temperature drops to
20

o

C, irrespective of the thermostat’s temperature

setting. The overheat thermostat prevents the water
reaching too high a temperature.

Should both control and high-limit thermostats

malfunction, the overheat thermostat will automatically
shut down the water heater.

The control thermostat can be set at settings 1-4

(approx. 40°C to 73°C). The frost protection ignites the
burner if the water temperature drops below 20°C. The
switch temperature of the high-limit thermostat is 84°C;
the overheat thermostat cuts in at 93°C. The sensor for
the control/frost thermostat is located at the level of the
control switch; the sensors for the high-limit and
overheat thermostats are located near the top of the
water heater.

The control box is fitted with an ON/OFF switch (I/0).

At setting I, the gas control operates on the basis of heat
demand from the control thermostat. At setting 0 the
water heater is switched off. The burner will only ignite
on the basis of heat demand from the frost protection.

When the control thermostat calls for heat, the water

heater checks all controls are in the off position. Power
is then switched to the ventilation fan. Immediately the
air pressure switch opens. The purge cycle starts. This
runs for between 30 and 60 seconds after which the
safety valve opens and the spark generator is operated.

Gas flows to the pilot burner where it is ignited by the

spark generator. The ionization probe now detects the
flame, switches off power to the spark generator and
opens the main gas valve.A softlite staged ignition
control prevents explosive ignition by allowing a small
percentage of total gas rate into the combustion
chamber initially followed by the full rate when
combustion is established. When the water temperature
reaches the set point the water heater is turned off.

If the flame extinguishes whilst in operation both the

safety (pilot) valve and the gas valve will close
immediately. If the control thermostat is still calling for
heat the burner automation will repeat the ignition
procedure.

-

If no flame is detected within the safety time of
50 seconds, the burner automation will interlock and
the reset knob will light up.

-

As long as the air proving switch does not operate
(remains closed), the burner automation will remain
in the waiting position, while the ventilation fan

continues to work.

-

An interlocking of the burner automation can be
cancelled out by pressing the reset knob. If the
procedure does not succeed immediately, wait at
least 15 seconds before pressing the interlocking
knob again.

If the flame detection falls out during normal

operation, the burner automation will automatically
repeat its ignition procedure. As long as a flame is
detected within the safety period of 50 seconds, the
automation will continue to repeat the ignition proce-
dure.

As soon as the water temperature has reached the

set value, the burner automation will close the gas
valves.

The control column has a potential-free contact for

external error message.

The ventilation fan is installed in the top-box. When

heat is required the ventilation fan is switched on by the
burner automation. The fan switches off when the main
burner has extinguished. The fan can be removed from
the top box via the access panel.

The air proving switch ensures that there is sufficient

air flow for the safe operation of the water heater. If the
airflow falls below the preset value this will be detected
by the air proving switch and the gas supply to both the
ignition and main burners will be closed. The air proving
switch is preset and cannot be adjusted.

Air proving switches BFM-units

BFM 30

250

220

BFM 50

250

220

BFM 80

230

205

The air proving switch must always be fitted with the

connection points downwards!
The air pressure differential switch must always be fitted
with the connection points at the bottom, while the pipe
with the letter H must be connected to the point P1. Pipe
L goes to connection P2.

The water heaters are provided with a gas valve that

regulates the gas supply to the burners.

The gas control has a safety valve and a gas valve

and hence has a B/D-configuration. The position of both
valves can be regulated by the burner automation. To
ensure improved ignition the gas control valve opening
mechanism is fitted with a delay (softlite).