Troubleshooting hydronic systems can be challenging, as the pipes are hidden behind walls. Your imagination has to help you “see” the routing of the piping. In addition, a component in one part of the system could affect the operation in another. An example of this could be if the relief valve is leaking. A faulty relief valve may cause this, or it could be caused by a flooded expansion tank or a malfunctioning pressure-reducing valve. You would hate to replace the relief valve and see if you have the same issue.

To help diagnose system problems, a checklist like the one below could point you in the right direction. I hope this helps you on your next hydronic problem call.

The boiler is not firing

• Power — check the breaker, switch or door switch;
• Determine if there a call for heat;
• Verify boiler is not off on temperature;
• Look for closed manual gas valves;
• Flame failure: Only press the reset button once. If it trips again, diagnose the cause before restarting; 
• Determine if the gas pressure switches tripped; and
• Determine whether the limit control tripped.

Burner blower starts, but flame does not

• Check the burner air flow switch. Verify sensing tube is not plugged;
• Verify the flame detector is not dirty or defective;
• Verify the ignitor starts;
• The adjustable orifice on the gas valve could be closed;
• The gas train vent could be plugged;
• The pilot solenoid valve could be defective;
• The pilot flame signal may be weak. Check pilot gas pressure and flame signal;
• Excessive combustion air in boiler could blow the pilot away from the flame sensor; and/or
• Excessive draft could pull the flame away from the flame sensor.

Boiler pressure keeps rising

• The water feeder may be set too high or leaking through; and/or
• The compression tank could be flooded or undersized. 

The boiler makes a cracking noise when firing

• Scale could be forming on the water side of the boiler; and/or
• The system water content may be too low. If so, do not add water to the boiler until the boiler is shut off and cool and the problem diagnosed.

Relief valve leaks

• The boiler pressure could be too close to the relieving pressure;
• Verify the compression tank is not flooded or undersized;
• Verify the relief valve is not defective; and/or
• The water feeder could be set too high or is leaking through. 

Boiler going off on low water

• Check the low water cutoff;
• Verify the low water cutoff is operating properly;
• Check low system pressure; and
• Verify system pressure is high enough.

The boiler makes a moaning sound when firing

• Check flow and temperature rise through the boiler. The temperature rise should be 20° to 30° F, depending on the boiler.

Intermittent flame failures

• Check for loose wiring connections;
• Verify the electrical voltage is within the allowable range;
• Verify the boiler room temperature is below 140° F;
• Check the pilot solenoid valve;
• Determine whether the VFD is too close to the boiler electronics;
• Verify the draft-proving switch is working properly. Check for a blockage in draft-sensing tube; 
• Verify the gas pressure does not drop below minimum gas pressure setting when other gas-fired equipment operates;
• Check the flame safeguard operation;
• Check the flame signal on the pilot and when main flame starts. Verify the flame signal does not drop;
• Verify the boiler draft is not excessive;
• The flame sensor may be dirty and should be cleaned; and/or
• The flame detector could be defective.

Boiler losing water

• There could be a system water leak. Check hydronic coils in air handling units;
• Verify the boiler is not leaking internally;
• Verify there is a backflow preventer on the system makeup water pipe; and/or
• If the system pressure is too high, the water can leak from valve packing or cause the relief valve to weep. 

The flue gas temperature is higher than before

• Scale could be forming on the water side of the boiler; 
• If the fireside of the boiler is dirty, this could result in higher temperatures and fuel consumption;
• The burner may be over-firing in the boiler;
• The draft or flue gas velocity could be too high. This results in insufficient heat transferred to the boiler; and/or
• The water flow may be excessive and not capturing the heat from the boiler.

Burner short cycles

• The flame rod may need to be cleaned or replaced;
• Check the flame signal of flame detector;
• The burner air flow switch may be defective or the sensing tube is plugged;
• If the gas train vent is plugged, it could cause the valve or gas pressure regulator not to open;
• The differential on the temperature control could be too close. Widen the differential;
• The circulator or pump is not working;
• The temperature sensor for the reset control could be too close to the boiler outlet; and/or
• Boiler is piped backward.

Pipes are banging 

• Verify the system pressure is high enough on PTA gauge; and/or
• If a zone valve closes too quickly, it could create a banging sound.

The temperature rise through the boiler is too low

• The water velocity is too high and not able to transfer heat;
• The boiler draft may be too high;
• The heating surfaces inside the boiler could be scaled or dirty. Check the waterside and fireside; and/or
• The building is up to temperature.

The temperature rise through the boiler is too high

• Verify there is enough flow through boiler; and
• Verify the burner firing rate is correct.

Manual reset limit control trips

• The operating temperature control is set too close to the setting of the high limit control;
• Verify operating control shuts off at temperature setpoint;
• The limit control could be defective;
• Verify the isolation valve does not close too quickly after burner shuts off; and/or
• The manual reset control may be set too close to the operating control setting. It should be 20° to 30° F higher than the operating temperature control.

Entire building is cool, and the boiler is operating

• Check reset control for proper temperature;
• Verify thermostat operation and it does not have sun hitting it; and
• The outside air sensor could be in direct sunlight. Relocate it to another area.

One zone overheating

• Verify the flow is not going backward through the zone when off; and 
• Verify valves are not leaking through.

Some areas not heating

• Verify radiators not filled with air; and
• Verify system pressure is high enough to get to highest radiator.

Top floor cool, bottom floors overheating

• Verify the system pressure is high enough to reach top radiators; and
• Verify the flow is correct for all floors. The first floor flow may have to be lowered to force more above.

Discoloration of boiler jacket*

• Verify flue gases are not rolling out;
• An exhaust fan in the boiler room could pull the flue gases from the boiler and into the room;
• If the combustion air opening is blocked or restricted, it could cause the flue gases to roll out of the boiler and into the boiler room; and
• If the flue passages are plugged, the flue gases could spill from the boiler into the boiler room.

*Discoloration of jacket could signify dangerous back drafting of flue gases, and the boiler should not operate until checked.

Boiler sooting on fireside

• Verify the boiler is getting adequate combustion air;
• Verify there is not a negative condition in boiler room which could pull flue gases from boiler; and
• Operating a non-condensing boiler below the condensing temperature could cause sooting.

Boiler leaking

• Verify the water treatment system is working and is correct for boiler metallurgy;
• Verify the system is not leaking. This allows fresh, untreated water into the system; and
• Check for thermal shock occurs when colder water is introduced to warm water inside the boiler, damaging it.