Normal hot water heating boiler operation

All you need to know about maintaining and operating hot water boilers

January 2020 — Hot water boilers are similar to steam boilers in many ways. The major difference is that hot water boilers are always filled with water. Unless the system is drained or work is performed on some part of the system, a hot water system should not require makeup water. The system should not be drained during the off season, since this would introduce oxygen into the system and accelerate corrosion. Thus the first step in starting up a hot water boiler is to flush and fill the boiler with feedwater.

Filling hot water heating systems

Before starting a hot water heating boiler, make sure that the boiler and heating system are completely filled with water.

To fill a hot water heating system, use the following procedure:

  1. Review the manufacturer’s recommendations.
  2. Make sure that the air vents on all radiators and/or convectors are closed.
  3. Open all valves in the circulating lines. This includes the valves on the supply and return lines at the boiler, the suction and the discharge valve on the circulating pump, and the shut-off valves in the various branch lines.
  4. Open the vent valve on top of the boiler, open the makeup water valve, and fill the boiler.
  5. When the boiler is filled, close the vent valve. Begin on the lowest floor of the building and open the vent on each radiator or convector, one at a time, until the water flows out. Close the vent.
  6. Repeat this throughout the building until all radiators and convectors are full.
  7. If the system is equipped with an open expansion tank, continue filling it until the tank is about one-third full, and then close the makeup water valve. If the system is properly filled, the altitude gauge will show the minimum system pressure. If the operating pressure drops below the minimum system pressure at any time during operation, there is a loss of water in the system.
  8. If the system is equipped with a closed expansion tank, make sure that the valve in the connecting line between the tank and system is open. When the system is completely filled, the water should just be showing in the lower part of the gauge glass on the tank.
  9. On closed systems with an automatic fill valve, leave the valve open after filling. The minimum pressure in the heating system (shown on the altitude gauge) will be maintained in accordance with the pressure setting of the automatic fill valve.

Filling the hot water boiler only

On some boilers, the heating system may remain full of water, even though the boiler was drained for service during the off season. If the boiler was isolated from the heating system by shut-off valves on the supply and return, the boiler should be refilled before it is reconnected to the rest of the system. This process will remove as much air as possible.

To fill only the boiler, use the following procedure:

  1. Review the manufacturer’s recommendations.
  2. Open the vent on the boiler, and then open the makeup water supply. When water comes out of the vent, close the vent valve.
  3. Open the shut-off valves in the supply and return lines to the boiler, making sure that all the shut-off valves in all zones and branches of the system are open.
  4. One by one, open each vent valve in the system until water comes out, then close the valve. To ensure that as much air as possible is bled from the system, start at the lowest level and work up.
  5. Check the water level in the expansion tank.
  6. When the system is full and the air has been bled from each of the vents, close the makeup valve, unless the system has an automatic makeup valve. If there is an automatic makeup valve, the valve can remain open.

Pre-Startup checks

Before the boiler is restarted after a long shutdown, several items and procedures must be checked. Start by reviewing the logbook for any previous problems or areas of concern. Before proceeding further, make sure that any noted maintenance returned the equipment to a satisfactory operating condition. If everything is in order, use the following procedure to continue the pre-startup checks:

  1. Review the manufacturer’s recommendations for boiler startup.
  2. Inspect the boiler to make sure that the blow-off valve is closed and handhole covers and/or washout plugs are in place and tightened.
  3. Fill the boiler (as instructed previously).
  4. Check the altitude gauge and expansion tank to make sure the system is properly filled.
  5. Make sure the manual shut-off valves in the fuel lines to the main burner and pilot are closed.
  6. Make sure a fuel supply is available to the burner fuel shut-off valves.
  7. Check to make sure that the fresh air supply to the boiler room is unobstructed.
  8. Set the boiler control switch to the “off” position, and then switch on the power supply to the boiler and circulating pump.
  9. Check the temperature settings of the operating and high-limit control.
  10. Check the manual reset (if provided) on the low-water fuel cut-off and high limit controls.

Starting up a hot water boiler

After the system has been completely filled with water and the pre-startup checks have been made, the boiler should be ready to start.

Use the following procedure to start the boiler:

  1. Review the manufacturer’s recommendations.
  2. Open the manual fuel shut-off valve to the pilot light, then light the pilot on gas-fired boilers with a standing pilot.
  3. Open the shut-off valve in the fuel line to the burner.
  4. Turn the boiler control switch to the “on” position. The boiler should start up.
  5. Slowly raise the water temperature and monitor it closely throughout the startup.
  6. Check the boiler frequently for leaks and make sure that the burner is operating properly.
  7. Do not leave the boiler until the operating control reaches the cut-off temperature. Make sure the control shuts off the burner.
  8. When the burner shuts off, check the water pressure on the altitude gauge and water level in the expansion tank.
  9. Check the safety relief valve for evidence of leaking. Perform a check on the test lever by releasing a small amount of water.
  10. Enter the following information into the logbook:
  • Date and time of startup
  • Irregularities observed and corrective action taken
  • Time when controls shut off burner at established temperature
  • Tests performed
  • Operator’s signature

What to do in case of abnormal conditions

If any abnormal conditions occur during the initial startup or temperature build-up, close the manual fuel shut-off valve immediately and switch off the power supply to the boiler. Do not attempt to restart the boiler until the problem has been identified and corrected.

Bringing an additional boiler online

When placing a boiler on the line with other boilers already in service, start the boiler using the above procedures. However, keep the supply shut-off valve and the return shut-off valve closed. Bring the boiler to the same temperature as the operating boiler(s) and partially open the supply valve. If there is no unusual disturbance (such as noise or vibration), continue opening the valve. Open the valve in the return line.

This article is adapted from BOMI International’s Boilers, Heating Systems, and Applied Mathematics course, part of the SMA and SMT designation programs. More information regarding this course or BOMI International’s new High-Performance Sustainable Buildings credential (BOMI-HP™) is available by calling 1-800-235-2664. Visit BOMI International’s website,