Boiler Components - ControlsBoiler Components - Controls - All hot water boilers must have an expansion tank. Water expands when it is heated. There must be a place for it to expand to so that the pressure in the boiler loop remains constant. Without a hot water boiler expansion tank, the pressures in the boiler loop system would vary wildly. Most likely, the pressure will exceed the maximum pressures designed for the boiler hot water loop system. This will result in setting off the pressure relief valve.

Boiler Components - Controls – Hot Water Boiler Expansion Tanks

There are two different types of expansion tanks that can be used on hot water boiler systems. The most modern is the bladder type expansion tank (see picture to the right). The other type of expansion tank is a basic cylinder. It looks like a barrel that hangs from the ceiling or is located in the attic above the boiler. These tanks need to be drained from time to time to make sure they are not full. If they fill up with water and have to be drained constantly there is a problem. A problem with your boiler that should be looked at by a boiler technician. The bladder type expansion tank does not need draining on a regular basis like the cylinder or barrel type tank.

Boiler Components - Controls - Boiler Aquastats

Boiler aquastats are like the typical HVAC thermostats. The exception is it controls the temperature of the water inside the boiler and is used mainly in residential applications. Again there are many different configurations for the way the boiler aquastat is hooked up. Keep it simple and stay with the basics. The aquastat has a sensing bulb that is inserted into a well in the side or front of the boiler. This bulb senses the temperature of the water inside the boiler. When the temperature drops below a certain set point (usually 140 degrees F.) it turns the burner on so that the water can be heated to the maximum temperature setting on the aquastat. The burners are then shut off until the temperature again drops below the set temperature.

These temperature settings are usually manual and are different from one type of boiler to the next. When you turn your thermostat up you are turning the loop circulator pump on (on hot water systems). This causes hot water to circulate through the pipes. The hot water leaves the boiler and circles the loop. By the time it gets back to the boiler it has lost some heat through the heat exchange process. The heat loss was in your radiators or baseboards. It also includes any un-insulated pipes under the house or in the loop (see the boiler piping insulation article). The cooler tepid return water is sensed by the bulb in the aquastat. The aquastat again turns the burner on to reheat the return water. That way it may make the trip through the loop again when the thermostat calls for heat.

Low Water Cut-off for Boilers

The purpose of low water cut-offs for boilers is to prevent a boiler from dry firing. Dry firing a boiler creates a serious safety hazard. Different code authorities require different regulation for low water cut-offs. It can be confusing for technicians to comply with boiler code when it comes to low water cut-offs. The International Mechanical Code (IMC) requires “all steam and hot water boilers shall be protected by a low water cut-off”. However, there are many circumstances where I have seen hot water boilers without low water cut-offs.

I have never seen a steam boiler without a low water cut-off and for good reason. If you have a loss of water in a steam boiler system where steam escapes from the system through steam traps and evaporation losses plus if the condensation return became plugged or failed (with condensation pump or gravity condensation return) and the make-up water failed, for whatever reason, the circumstances are prime for dry firing the steam boiler. A dry fired boiler is dangerous.


The American Society of Mechanical Engineers (ASME) seems to be the guiding light for many boiler installations. Whether it is for residential, commercial or industrial applications the ASME requires a low water cut-off for steam boilers of any size. And for any hot water boiler over 400,000 BTU input for residential hot water boiler installations. Anything else, for hot water boilers, must have a low water cut-off as stated by the ASME. This is required by most “the authority having jurisdiction or AHJ” otherwise known as the local code enforcement or municipal inspectors.

Newer condensing boilers have something altogether different for a low water cut-off. These types of boilers generally have a flow switch so when water is not flowing through the piping and into the boiler jacket where the water is heated the burners will not fire. This protects the boiler from dry firing and causing a safety hazard whether it is for residential, commercial or industrial.

Boiler Components - Controls – Automatic Water Feeders

Hot water boilers generally have a valve that remains open at all times to allow the system to have a ready source of makeup water. make-up water is needed in the event of a leak. However, steam boilers need a way to regulate the water level inside the boiler to maintain a level of water according to boiler design. Too much water and the mains become flooded and the boiler will not make steam with flooded mains. Generally, an automatic water feeder is used on the make-up water supply. This device has a float inside to regulate the level of water inside the boiler. Most steam boilers have a site glass located on the side of the steam boiler.

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Generally, these automatic water feeders are non-adjustable. They are installed where the makeup water is introduced to the steam boiler as designed by the steam boiler manufacturer. The automatic water feeder has a float inside that regulates the flow and the level of water inside the boiler. Problems occur when automatic water feeders becoming clogged. Plugged with debris, silt, and sand the automatic water feeder either needs a good flushing or replaced. Replacing it depends on how bad the automatic water feeder is plugged.

Hot Water Boiler Components - Controls | Combustion Control

Hot Water Boiler Components and Controls
Hot Water Boiler Components and Controls | Combustion Control - This photo shows the boiler cabinet open exposing controls and basic components of the boiler. The module on the left is the ignition control module which controls the sequence of operation for ignition and burner controls of the boiler. When the boiler is enabled to run, usually by a thermostat or controls, the ignition control module begins the process of firing the burners.

The process typically begins by having the ignition module check the safety circuit to ensure all safeties are normal and not in alarm. This includes energizing the inducer or forced draft fan motor and any limit switches and a flow switch to ensure the boiler has water flow.

Boiler Components - Controls | Induced or Forced Draft Systems

The inducer or forced draft fan motor causes some pressure switches to close proving that combustion air is available and that the combustion gases will be exhausted when the burners fire. After the safety circuit has been proven to be okay the ignition control opens the gas valve while simultaneously activating the spark ignition to ignite the gas now being supplied to the burner(s). Once the burners ignite the fire is proven, usually by a state of the art flame sensor, the ignition control module de-energizes the spark ignition.

As the water heats up it is sensed by an aquastat or temperature sensor. When the temperature of the water reaches the set point of main boiler controller the ignition control closes the gas valve and the main burners shut down. The inducer or forced draft fan motor continues to run for a period of time so that all the combustion gases in the system are purged up through the boiler flue or boiler stack.

Hot Water Boiler Components - Controls | Commercial Control Systems

In commercial systems, a DDC system can be integrated into the boiler. The DDC system simply monitors the boiler for boiler run status and temperatures of the water entering and leaving the boiler. If the boiler were to fail the DDC system would send an alarm to the main monitoring station where an operator could alert maintenance staff that there is a problem with the boiler or boilers.

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Boiler Components - Controls

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