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Working principle of the appliance
16 Instruction manual BFC
2
gis
2.4.6 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 electronic controller uses this probe to
detect the presence of a flame, by means of ionisation detection. The electronic
controller closes the gas control the instant it detects that there is a gas flow but
no flame.
2.5 Safety of the instal-
lation
In addition to the appliance's standard built-in safety monitoring, the appliance
must also be protected by an expansion vessel, expansion valve, pressure
reducing valve, non-return valve and a T&P valve.
The use of an expansion vessel, expansion valve and/or pressure reducing
valve depends on the type of installation: unvented or vented.
2.5.1 Unvented installation
With an unvented installation, an expansion valve valve and expansion vessel
prevent the buildup of excessive pressure in the tank. This prevents damage
being caused to the enamelled coating (in the appliance) or to the tank. A non-
return valve prevents excessive pressure buildup 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 installation
against an excessively high water supply pressure (> 8 bar). These components
are fitted to the cold water pipe (3.7 "Water connections, Vented").
2.5.2 Vented installation
With a vented installation, excess pressure is taken up by the open cold water
head tank. The height of the head tank determines the working pressure in the
water heater, which may not exceed 8 bar. The installation must also be fitted
with a vent pipe from the hot water pipe, that opens into the cold water tank.
Ideally, the vent pipe should discharge into a separate tundish/drain or
otherwise to the open cold water head tank. The water heater should also be
fitted on the hot water side with a stop valve (3.6.2 "Hot water side").
2.5.3 T&P valve
A T&P valve is only mandatory in an unvented installation. 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 excessive (> 10 bar) or the water temperature is too high (> 97°C), the
valve will open. The hot water can now flow out of the tank. Because the
appliance 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 appliance is fitted standard with a connecting point for a T&P
valve (3.6.2 "Hot water side").