With the heating season in full force, it is a good time to review best practices in heating troubleshooting. The following article covers boilers. The following information is a summarized section from HVAC Troubleshooting, a guide book published by BSRIA, a test, instruments, research, and consultancy organization headquartered in the United Kingdom.
The full guide covers the most common building heating, cooling, and controls systems and assumes a sound knowledge of the investigation/check procedures to be carried out. As well as the summary below, the full boiler troubleshooting section of the guide can be downloaded from www.bsria.co.uk/goto/boiler-trouble.
Boiler Fails to Meet System Load
One possible cause is that the boiler thermostat is set incorrectly. Corrective action is to check the thermostat setting. Another possible cause is poor combustion. Recommended action is to check flue, flame, and assess combustion quality.
Boiler and System Overheat
Possible cause is that system controls are faulty. Corrective action is to check voltage, temperature settings, and safety switches.
This problem may also be caused because the control thermostat failed and high-limit thermostat activated, or both controls may have failed. Action to be taken is to check temperatures — if they are higher than the high-limit setpoint, replace the high-limit thermostat.
Fumes in Boiler Room
Check for incorrect combustion. Perform a combustion analysis and correct.
Another possible cause is inadequate airflow for combustion. In this case, inspect the boiler, chimney, and boiler room fresh air intake for the source of the blockage.
One more possibility is test holes in the flue that are unsealed or flue leakage. To correct, cover the holes and check for leakage.
Finally, the boiler room fans may have stopped. In this instance, check the boiler room fans.
Boiler Does Not Ignite
There are six possible causes to look for here.
Check for a boiler internal controls failure. Investigate this possibility and reset or replace.
The boiler may have previously overheated and the overheat cut-out has not reset. Determine the cause of the overheat condition. Once it has been corrected, reset.
The supply fuse may have blown or the supply isolated. Establish the cause of the blown fuse and replace.
There may no pilot flame. Possibilities for this condition are: (1) a dirty or incorrectly installed pilot nozzle — check the size of the pilot nozzle and check for blockage; (2) insufficient gas supply to pilot — check the gas supply to gas safety valve; (3) pilot flame ignites but becomes faulty immediately — check the position of the flame detector probe and check the voltage of the flame detector; and (4) thermocouple voltage is too low — replace the thermocouple.
If the main burner does not light, investigate the loss of fuel to the burner, repair, and light.
There may be no ignition spark. Possibilities in this case are: (1) high tension leads not connected at the ignition transformer or electrodes — check connections; (2) short circuit or ground fault on high tension leads — check leads; (3) carbon deposit on the electrode insulation or between the electrodes — check and clean the electrodes; (4) cracked electrode insulators — check insulators; (5) excessive electrode gap — measure electrode gap; (6) no supply to the ignition transformer — check ignition relay operation: (7) secondary winding of transformer open circuited or short circuited — test and inspect transformer, fix any defects found or replace; (8) primary winding of transformer grounded, open circuited, or short circuited — check continuity and insulation resistance of primary winding.
Boiler Will Not Purge or Ignite
This problem has 12 possible causes.
First, there could be a control problem. Check the building management system control signal.
The circuit breaker could be tripped or there could be a blown fuse. Look for a possible ground fault and reset the breaker or replace the fuse.
There may no pilot flame. Possibilities for this condition: (1) a dirty or incorrectly installed pilot nozzle — check the size of the pilot nozzle and check for blockage; (2) insufficient gas supply to pilot — check the gas supply to gas safety valve; (3) pilot flame ignites but becomes faulty immediately — check the position of the flame detector probe and check the voltage of the flame detector; and (4) thermocouple voltage is too low — replace the thermocouple.
The gas and/or electricity supplies may be off. Check the voltage and gas supply.
The thermostats could be set incorrectly. Check the thermostats and reset, repair, or replace as necessary.
The flue gas fan may not be functioning. Check the electrical power to the fan as well as the control signal.
There may be no ignition spark. Check the ignition coil, the electrical supply to the ignition coil, and the electrode gap.
There could be no main flame. Check the voltage to the safety valves, and check the ionization current, the setting of the ionization probe, and the wiring of the ionization probe.
The automatic control box may have gone to lockout because of a fault. Press the button on the control box. If normal start-up does not occur, check whether the boiler’s manual gas valves are fully open, and whether the boiler control thermostat is set at the correct value.
The flue air pressure switch may not be switching to the safe position. Replace the switch.
The boiler room emergency stop could be activated. Check the cause of this condition and reset.
Finally, the thermal links may have melted or broken. Check the cause and replace the thermal links.
Boiler Fires Continuously
A potential cause is a thermostat fault. Investigate and either repair or replace the thermostat.
There could be a wiring fault. Check the wiring and make the required repair.
One more possibility is a control valve that is faulty or sticking. Replace the valve.
Boiler Overheats and Is Noisy
There could be poor circulation in the primary circuit. Vent the system.
Another possibility is low shunt-pump discharge pressure. There could be an obstruction in the piping; open a piping joint and check for foreign materials. There may be an air pocket in the line; purge the system.
Boiler Starts but Flame Goes Out Soon After
A possible cause is flue blockage. Check the flue for blockage. Check dilution fan operation.
Another potential cause is primary low-pressure hot water (LPHW) circuit failure. Check operation of the high-temperature cut-out.
Fault in Flue Gas System
In this case, a cause may be that the flue gas fan does not run during the start sequence. Check the control circuit.
It is also possible that the fan has stopped during operation. Check the fan, make any necessary fixes, and restart.
High Water Temperature
A possible cause is that the system control thermostat is set too high. Check the calibration and reset.
Another likely cause is that the circulation system pump is not working. There could be an obstruction in the piping; open a piping joint and check for foreign materials. There may be an air pocket in the line; purge the system.
Low Water Temperature
In this instance, one possible cause is the system control thermostat is set too low. Recommended action is to check the calibration and reset.
Another cause is air in the system. Corrective action is to vent the system.
Editor’s Note: The complete HVAC Troubleshootingguide is available for purchase from BSRIA’s Bookshop at https://www.bsria.co.uk/information-membership/bookshop/publication/hvac-troubleshooting-bg252014/ in electronic or hard-copy format. It is also available as part of BSRIA Bookshop’s set of five guides that enable you to buy a suite of facility maintenance guide books at a discounted price. Go to https://www.bsria.co.uk/information-membership/bookshop/publication/maintaining-facilities-guide-set/.
Readers of the ACHR News can purchase the HVAC guide or the set for a 10 percent discount. Enter BOOKACHR upon checkout.
Publication date: 3/2/2015