Troubleshooting a Gas-Fired Hot Water Boiler

  • 01 of 08

    Introduction

    Technician servicing boiler
    SolStock / Getty Images

    Modern boiler systems for home heating are called hydronic systems and include a boiler that heats water for circulation throughout the house. Pipes carrying the hot water radiate heat through either steel radiators or baseboard convectors (sometimes called "fin-tubes"). Once the radiator or baseboard convector is heated with the hot water, the water is returned back to the boiler to be reheated, and the water circulation loop continues. Understanding the components of a hot water boiler...MORE helps you with basic troubleshooting steps. 

    Continue to 2 of 8 below.
  • 02 of 08

    Components of a Hot Water Boiler

    Gas Fired Boiler
    The main components of a gas-fired hot water boiler. © Home-Cost.com 2007

    A boiler is a bit more complex than a forced air furnace in that it has more parts, valves, and controls. However, gas-fired boilers are fairly reliable and, when problems do occur, they are usually related to the expansion tank or water circulation pump(s).

    Here are the major components of the boiler for a hot water (hydronic) system:

    Hot Water Supply Side

    • Aquastat: Thermostat that regulates the boiler's temperature.
    • Gas valve and burners: Combustion assembly that heats the water chamber.
    • Combina...MOREtion pressure/ temperature gauge (Tridicator): Monitors water temperature and pressure.
    • Water feed valve: Controls water flow to the boiler.
    • Pressure-reducing valve: Automatically maintains correct water level and pressure at about 12 to 15 psi (pounds per square inch).
    • Air vent: Automatically purges unwanted air from the hydronic system.
    • Pressure-relief valve: Safety valve that automatically opens if pressure gets too high inside boiler.
    • Expansion tank: Allows expansion of water as it heats; there are two types of expansion tank: horizontal-style (older, larger) and diaphragm-style (newer, smaller).
    • Flow control valve: Regulates flow of hot water to the system.

    Hot Water Return Side

    • Circulation pump: Electric pump that circulates water through the system.
    • Drain valve: Valve that opens to allow draining the boiler.
    Continue to 3 of 8 below.
  • 03 of 08

    PROBLEM: Boiler Produces No Heat

    Possible Causes and Fixes

    • Boiler has no power. Circuit breaker or fuse controlling the furnace is tripped or blown. Reset tripped circuit breaker or replace blown fuse.
    • Water level low. Maintain water level in boiler at half-full. The boiler's automatic filling system, controlled by the pressure-reducing valve, should maintain proper water level at 12 to 15 psi of pressure. If there is no pressure-reducing valve, you can manually feed the boiler by opening up the water feed valve until the...MORE boiler pressure reaches 12 psi.
    • Natural gas or propane control valve to boiler burner closed. Make sure the valve is open. 
    • Pilot light out or malfunctioning. Relight standing pilot.
    • Electronic burner ignition malfunctioning. On boilers with no standing pilot; troubleshoot electronic ignition system. 
    • Thermostat malfunctioning. Check that thermostat is in heat mode and has an appropriate temperature setting. Try moving the thermostat setting for the temperature up or down a few degrees. If this does not work, troubleshoot thermostat.
    Continue to 4 of 8 below.
  • 04 of 08

    PROBLEM: Boiler Heats Poorly

    Pressure Gauge and Thermometer
    pixonaut / Getty Images

    Possible Causes and Fixes​

    • Improper water level. More likely if the change has been sudden. Check reading of the Tridicator (combination pressure/ temperature gauge). If water pressure is low (below 12 psi), the system needs to have water added. The boiler's automatic filling system controlled by the pressure-reducing valve should maintain proper water level at 12 to 15 psi. If there is no pressure-reducing valve, you can manually feed the boiler by opening up the water feed valve until...MORE boiler pressure reaches 12 psi.
    • Mineral deposits accumulating in boiler and heat exchanger. More likely if the change has been gradual. Flush the boiler or call a boiler pro. 
    • Expansion tank has too much or too little water. Tank must be properly charged with air to prevent boiling (see next slide).
    Continue to 5 of 8 below.
  • 05 of 08

    PROBLEM: Expansion Tank Has Excess Water and Inadequate Air

    Oil fired central heating boiler.
    Stephen Rfferty / Getty Images

    In older boiler systems, the steel expansion tank may be found in the attic or suspended between joists in the basement. In newer systems, the diaphragm expansion tank may be attached to the boiler piping near the boiler. The expansion tank must be properly charged with air to prevent water in the system from boiling and exceeding the desired 12 psi of pressure.

    Continue to 6 of 8 below.
  • 06 of 08

    PROBLEM: Water Leaking Around the Boiler

    Possible Causes and Fixes​

    • Faulty circulator (pump). Most repairs to the circulator require a service technician.
    • Leaking circulator. May be possible to replace the pump seal. 
    • Leaking pressure-relief valve. Can be caused by expansion tank being filled with water. Otherwise, the valve may have sediment preventing it from closing. To check this, turn the boiler off and let it cool. Lift the manual pressure relief lever and discharge some water for 3 seconds, letting the lever snap back into the...MORE closed position. The water should discharge strongly and be relatively clean. If the valve leaks slightly afterward, this could be due to sediment trapped in the seat. Open the valve again and discharge a second time. 
    • Faulty pressure-relief valve. If no water at all discharges from the valve, or If the valve will not close at all, shut off the boiler water feed valve, and replace the pressure-relief valve. 
    • Leaking water pipe connection. If water is leaking or dripping from a pipe, follow the leak back to its source and repair the connection where the leak is originating. This requires turning off the water supply to the house and draining the boiler system.
    Continue to 7 of 8 below.
  • 07 of 08

    PROBLEM: Some Radiators Do Not Heat

    Radiator for hot water heating system
    Steven Beijer / EyeEm / Getty Images

    Possible Causes and Fixes​

    • Air trapped in lines or in radiator. Bleed air from a cool radiator by opening the radiator bleed valve at top of radiator. When water squirts from the radiator, close the valve.
    • Faulty zone valve. Check zone valve for proper operation. Water pipe should be hot up to and beyond the zone valve. If the valve is faulty or stuck, lines will be hot up to the valve but will cool off slightly beyond the valve. Have a faulty valve replaced by a pro.  
    • Faulty circulator. Check...MORE circulator for proper operation (motor runs). There may be dedicated circulators for different heating zones in the home. Have the circulator replaced if it is faulty. 
    Continue to 8 of 8 below.
  • 08 of 08

    PROBLEM: Noisy Pipes

    Possible Causes and Fixes

    • Faulty circulator. Check the circulator: There is a spring-loaded coupling that connects the pump to the motor; when it breaks after the pump jams, the coupling will make a loud noise as the motor runs.
    • Water trapped in return lines. Check to make sure the pitch of the return lines slopes back toward the boiler. Adjust pitch with new pipe hangers, if necessary. You may also need to adjust the pitch of the radiator with a shim so that it slopes back toward the return pipe.