Pentair MiniMax Plus Service Manual page 32

Troubleshooting
PRESSURE SWITCH
If heater fires when you jumper the pressure switch,
the switch is suspect. Before replacing the switch,
make sure wire terminals are clean and tight, the
pressure switch tube is clear and the filter is clean. If
all the tests and checks prove positive, replace pressure
switch.
THERMOSTAT
If you attach a jumper wire to the thermostat terminals
or wires and the heater fires, the thermostat is suspect.
Before replacing, check wire terminals, make sure they
are tight and clean, and make sure the thermostat
sensing bulb is inserted into the inlet/outlet header.
HI-LIMITS - OFF/ON SWITCH -
THERMAL CUTOFF
Use the same jumper wire procedure on the High-
Limits, Off/On Switch and Thermal Cut/Off.
SLOW CYCLE
LOW GAS PRESSURE
If the gas pressure is too low, the pilot will light,
because the pilot doesn't require much gas. When the
main burners fire, they require a lot of gas, so they
steal gas from the pilot. The pilot flame diminishes
and the pilot generator loses power or the millivolts
decrease and when the millivolts reach gas valve drop
out, the valve shuts off.
P/N 38033
MILLIVOLT
When main burners shut down, the gas pressure
increases in the pilot tube, until the pilot flame heats
the generator enough to cause the gas valve to open
and the main burners refire.
This sequence may take as long as 5 - 10 minutes.
You need a millivolt meter to diagnose this problem.
The meter will read 250 MV when heater fires, but as
the gas is robbed from the pilot, the millivolts will
drop lower and lower until the gas valve shuts down
and then the millivolts will increase until the generator
has enough power to open the valve again.
IF FIRE BOX IS DAMAGED
When the fire box is damaged enough to allow the
heat to leak out onto the thermostat wire, the
thermostat sensing bulb will tell the heater to shut off
prematurely.
You can spot this problem by watching the millivolt
meter. When the heater fires the reading should be
close to 250 MV. As the thermostat wire heats up and
travels to the sensing bulb. The millivolts increase
slowly until they reach 500-600 MV or open circuit
and the heater shuts off. Also if you turn the thermostat
up one or two degrees, the heater will refire and then
shut down. This sequence will repeat until you cannot
turn the thermostat up any further.
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Rev. C 12-1-00