I have new pennco gas boiler,series 15b. It reading the code sta 15. I don't understand the explanation for code 15 in book. I think it may have to do with thermostat?
15 - Wait for limit to close - Possible thermostat call for heat; limit switch is open.
The system is waiting for a safety switch to close. You (thermostat) may be calling for heat, but a limit (safety) switch is open; meaning something is becoming to hot during operation or a limit switch has failed and needs to be replaced.
Manuel resetting of the unit is accomplished by turning off/on the circuit breaker. Power cycling the unit, every so often, may provided you with some heat until the problem can be further diagnosed.
If after waiting a while for the unit to cool down and then you power cycle it, and it does not come back on, one of the limits has completely failed and must be replaced.
On a forced-air system, changing the filter may help you ride-out an intermittent failing high-temp limit switch. For either type of system, ensure that the exhaust flue is not blocked; check for leaves, birds nests, ect.
Penco 15b Series II PDF, page 27