Safety Of The Installation - A.O. Smith BFC-28 Installation, User And Service Manual

2
2.4.5
2.4.6
2.5 Safety of the
installation
2.5.1
2.5.2
2.5.3
16
Working principle of the water heater
Pressure switch
The pressure switch only guarantees the supply of air during pre-purging of the
water heater. If the pressure difference during pre-purging is sufficient, the
pressure switch closes. The table (3.4.2 "General and electrical specifications")
shows the trip point for each water heater.
Note
The trip point of the pressure switch is not adjustable.
Flame probe
To ensure that no gas can flow when there is no combustion, the water heater
is fitted with a flame probe (21). The controller uses this probe to detect the
presence of a flame, by means of ionisation detection. The controller closes the
gas control valve the instant it detects that there is a gas flow but no flame.
Excessive pressure in the tank can damage the enamelled layer (in the water
heater) or the tank itself. An inlet combination and pressure-reducing valve
prevents this. The inlet combination acts as a stop valve, non-return valve and
overflow valve. If the water mains pressure is too high (3.4.2 "General and
electrical specifications"), a pressure-reducing valve must be used. Both
components must be fitted in the cold water inlet (3.7 "Vented water
connections").
The use of an expansion vessel, expansion valve and/or pressure reducing
valve depends on the type of installation: unvented or vented.
Unvented installation
With an unvented installation, an expansion valve and expansion vessel prevent
the buildup of excessive pressure in the tank. This prevents damage being
caused to the enamel coating (in the water heater) or to the tank. A non-return
valve prevents excessive pressure build-up in the water supply system. This
valve also prevents water from flowing backwards from the tank into the cold
water supply system. The pressure-reducing valve protects the water heater
against excess mains water pressure (3.4.2 "General and electrical
specifications"). These components are installed in the cold water
inlet (3.7 "Vented water connections").
Vented installation
With a vented installation, excess pressure is taken up by the open cold water
head tank. The level of the cold water head tank determines the maximum
working pressure (3.4.2 "General and electrical specifications") in the tank. The
water heater must also be fitted with a vent pipe from the hot water pipe, which
opens into the cold water head tank.
Ideally, the vent pipe should discharge into a separate discharge channel/drain
or otherwise to the open cold water head tank. The water heater should also be
fitted with a stop valve (3.6.2 "Hot water side") on the hot water side.
T&P valve
A T&P valve is only mandatory in unvented installations. However, A.O.Smith
also recommends the use of a T&P valve in vented installations.
A T&P (Temperature and Pressure Relief) valve monitors the pressure in the
tank and the water temperature at the top of the tank. If the pressure in the tank
becomes too high (3.4.2 "General and electrical specifications"), the valve will
open. The hot water can now flow out of the tank. Because the water heater is
under water supply pressure, cold water will automatically flow into the tank.
The valve remains open until the unsafe situation has been rectified. The water
heater is fitted standard with a connection for a T&P valve (3.6.2 "Hot water
side").
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Instruction manual BFC