A.O. Smith Dre 120 - Boil Dry Protection Mechanism

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1.2.1 Boil Dry Protection
Fig. 2 - oat switch
The boil dry protection mechanism
consists of a oat switch which monitors
the water level approximately halfway
up the tank above the elements. If there
is insufcient water present in the tank,
then the oat (1) hangs down. If the tank
is lled with water then the oat is moved
upwards by the water. In the electrical
diagram the oat switch is reproduced
diagrammatically. The actual process
is as follows: the oat arm moves a
magnet with respect to the reed-contact
(2) which is located in the black plastic
tube at the end of the oat switch. The
movement of the magnet changes the
magnetic eld and the reed-contact is
closed.
The exposure height (3) of the reed
contact is very important.
This is 14 mm. This setting may not
be changed!
Explanation:
With an exposure height of 14 mm the
switch point is approximately halfway
along the stroke of the oat.
If the exposure height is set lower, then
the switch point shifts upwards. If the
exposure height is too small the contact
will no longer connect even if the oat
is completely above it. In this case the
device will not come into operation.
If the exposure height is set lower than
14 mm , then the switch point shifts
downwards. This means that the contact
will already connect upon a small
upwards movement of the oat. If the
exposure height is set too high then the
contact is permanently closed and the
safety mechanism does not work. This
means that the elements can come into
operation without there being sufcient
water present in the device. This will lead
to the elements burning out in a very
short time and short circuit and leakage.
So never change the exposure height.
The reed contact and the oat must
always be vertical. The arrow on the
clamping plate on the front must point
upwards. The oat switch is secured in
the tank with nut (5). By loosening nut
(4) the oat switch can be set vertically.
Nut (4)
is sealed with an o-ring with respect to
nut (5). The oat switch must be taken
out of the device as a complete unit
for checking or replacement. (The oat
cannot be passed through the opening in
nut (5).) For this purpose the device thus
has to be drained.