A.O. Smith TC-099 User Manual

AOS WPC - Tech Training

54 of 72

Ashland City, TN © 2007

Servicing should only be performed by a Qualified Service Agent

The state of the contacts for the High BPS can be 
viewed in real time using the System Status Menu. 
An asterisk will appear next to the “BlwHi Prv” menu 
item when the switch contacts are closed. The 
asterisk is not present when the switch contacts are 
open.

Normally Open Low Blower Prover or High Blower 
Prover pressure switch contacts are not closing 
during the pre or post purge operating states - or 
within 5 minutes of call for heat being activated.

Normally Open High Blower Prover pressure switch 
contacts did not re-open after pre purge operating 
state was completed. The High Blower Prover switch 
is used to prove adequate purging of the combustion 
chamber during pre and post purge cycles only. The 
blower runs at approximately 5000 rpm during pre 
and post purge operating states and at lower speeds 
during all other operating states. High blower prover 
switch contacts must close during the pre and post 
purge states ONLY and open during all other 
operating states.

Normally Open Low Blower Prover pressure switch 
contacts have opened during any operating state 
when the blower should be running. IE: This can be 
caused due to blower failure during the heating cycle 
or if the pressure sensed by the switch rises above 
the switches activation pressure for any reason. The 
normally open contacts on both Blower Prover 
switches close on a fall in pressure - the pressure 
sensed must be in a vacuum.

See blower prover switch information on page 24 
and the Sequence of Operation on page 15.

Determine if High BPS is causing the lock out. 

•

The normally open High BPS (BPS = Blower 
Prover Switch) switch must close its contacts 
during the Pre and Post purge operating states 
only. This is when the blower is running at 
approximately 5000 rpm. When blower speed is 
reduced for ignition and throughout the heating 
mode the contacts of the High BPS must re-
open and remain open. The contacts of the High 
BPS switch must close and open as described 
here during normal operation to avoid the 
“Blower Prov Stg1” lock out. See the Sequence 
of Operation on page 15.

Turn power off; disconnect the two wires to the 
High BPS. See panel layout on page 24 to 
determine which is the High BPS.

Prepare a jumper wire to jumper between the 
two wires just disconnected. Turn the boiler 
back on, ensure a call for heat is present. 
AFTER the blower motor starts jumper the two 
(24 VAC) wires together carefully. Do not let the 
wire ends touch any grounded surface.

When the blower reduces speed at the begin-
ning of the ignition period disconnect the jumper 
wire and leave the two wires from the High BPS 
apart. Do not let the wire ends touch any 
grounded surface.

Notice whether or not the boiler will continue 
operation to the igniter warm up period when 
the igniter is energized or locks out again on 
“Blower Prov Stg1.” Clip an amp meter around 
one of the igniter wires to see when the igniter 
warm up period begins.

If the boiler will not continue on to the igniter 
warm up period and locks out again on “Blower 
Prov Stg1” before the igniter is energized; the 
High BPS is not causing the lock out. Go to: 
“Determine if Low BPS is causing the lock out” 
in the table on page 53.

If the boiler operation continues on to the igniter 
warm up period without lock out, the High BPS 
is causing the lock out condition. Continue on to 
the: “Pressure Reading Test” on page 55.