Steam and Hot-Water Space Heating Systems

Boilers supply hot water or steam to heat buildings. The boilers that we see in residences are low-pressure steam and hot-water (hydronic) space heating boilers. These boilers are fired up with fossil fuels. They have an insulated steel jacket on the outside, a lower chamber where combustion takes place, and an upper chamber where the heat exchanger is located. That is where the water is heated or turned into steam. The steam or hot water is then circulated throughout the building through distribution boiler pipes.

Steam boilers are not completely full of water. They are only about 75% full. A hot-water boiler is completely filled with water.

Steam boilers run at about 2 psi. Hot-water boilers operate at about 12 to 15 psi.

All steam boilers are required to have a low-water cutoff device. Hot-water boilers are usually not required to have this.

Steam boilers require regular fresh water to be added to the system because steam boilers lose water due to evaporation and steam. Hot-water boilers typically do not require the regular addition of fresh water to the system.

The construction and design of steam and hot-water boilers are similar. The combustion chamber will differ for each type of boiler with the different types of fuel that the boiler is designed to burn. Oil boilers have oil burners that are mounted on the outside of the combustion chamber. Gas boilers have gas burner assemblies that are located inside the combustion chamber.

The upper chamber of a boiler contains cast-iron sections or steel tubes. Water is inside the cast-iron section or steel tubes. This water gets heated by the combustion taking place below in the lower chamber (the combustion chamber). The heat from the combustion taking place in the lower chamber is transferred to the cast-iron sections or steel tubes that contain water and to the water inside. In a hot-water boiler, the water completely fills the cast-iron sections or steel tubes. In a steam boiler, only the lower two-thirds are filled with water.

In a steam boiler, water is heated rapidly to the point when steam is created. That hot steam rises up through the distribution pipes and supplies heat to the building.

Boiler Efficiency

Boiler efficiency is based on several factors, including the type of fuel used, the method of firing, and the control settings. The efficiency rating of a boiler is measured by its annual fuel utilization efficiency (AFUE). The minimum established by the U.S. government for boilers is 78%. Mid-efficiency boilers have a range of 78 to 82%. High-efficiency (condensing) boilers range from 88 to 97%. Conventional (non-condensing) steam and hot-water space heating boilers have AFUE ranges of around 60 to 65%.

Describing Boilers

Low-pressure boilers and hot-water space heaters can be identified and described in variety of ways. Such boilers can be categorized by:

cast iron or steel;
type of exchanger;
type of fuel used; and
one or two pipes.

Most boilers are made of cast iron or steel. A few are aluminum. Cast-iron boilers typically last longer because cast iron resists corrosion better than steel. The heat exchanger of many boilers is made up of sections of cast-iron pieces that are joined together either in layers horizontally (“sandwiched” or “pancaked” together), or vertically joined together, like a tight stack of dominos. The water moves from section to section in a zigzag pattern and gets heated.

For boilers with a tubular heat exchanger, the water either circulates within the tubes and gets heated by the combustion gases, or the water circulates around the tubes that contain the hot combustion gases.

A hot-water copper fin tube operates slightly differently than cast-iron and steel boilers. Water flows across the fins and gets heated very quickly.

Boilers can be fired by various fuels, including electricity. The most common boilers tend to be gas-fired or oil-fired.

Steam Boiler Valves and Controls

Boilers used for steam heat have a variety of valves and controls. They can be divided in two broad categories:

safety and measurement; and
controlling.

Steam boilers might have installed on them the following components:

a water level gauge, which measures the water level in the boiler;
a low-water cutoff, which is a device that turns the boiler off if the water level gets too low;
a pressure gauge, which measures the operating pressure inside the boiler;
a pressure-relief valve, which discharges excess steam when the pressure in the boiler exceeds a pre-set limit; and
a high-pressure limit switch, which shuts off the burner when the boiler reaches a high-pressure limit.

Pressure Safety Relief Valves

On low-pressure boilers that you might inspect in a residential home, the pressure-relief valve is set to open at a pressure of 15 psi in the boiler. The pressure-relief valve should not be rated higher than 15 psi. The pressure valve should not have a rating higher than the maximum working pressure of the boiler. You don’t want the boiler to rupture under high pressure; you want the valve to activate before that happens.

Pressure-relief valves that are installed on hot-water heating boilers will open under two conditions:

water thermal conditions; and
steam-pressure conditions.

These relief valves are used to protect against damage to property and injury to people. These relief valves will start to open up at a set pressure and will open fully at a maximum pressure.

If temperature limits are reached in the boiler, the valve will start to open. If pressure in the boiler reaches a limit, the valve opens as a water-relief valve discharges a small amount of water that has expanded inside the boiler.

If both water and steam are present in a hot-water boiler, there is a problem with the boiler. The boiler is reaching steam-forming temperatures and steam pressure is being created. The relief valve will open under this steam-pressure condition. It acts as a steam-pressure relief valve under this condition in the boiler.

These safety-relief valves open under excessive water pressure conditions and under excessive steam pressure conditions.

Pressure-Reducing Valve

Pressure-reducing valves are sometimes referred to as boiler feed valves. Pressure-reducing valves are designed to fill a hot-water boiler automatically if the pressure in the system drops below the setting of the valve. Its primary function is to keep the system automatically filled with water at the desired operating pressure. Older systems have valves that are operated manually. The valve is usually installed on the cold-water supply line.

Combination Valves

Combination valves are used in hot-water boiler systems. They combine a pressure-reducing valve and a relief valve in one device. They provide pressure regulation and automatic filling of water under certain conditions.

Blowdown Valve

Sediments will settle at the bottom of the boiler. Sediments can be removed by using a blowdown valve. The valve is installed at the bottom of the boiler. The blowdown valve should be operated periodically to drain the sediments from the boiler.

Water Gauges

A water gauge is used to visually check the level of the water in the boiler. The water level should be at a level somewhere between 60 to 75% full. A safe operating level of water varies slightly between different manufacturers of boilers. If the water level appears to be low or empty, then do not operate the boiler. Turn it off, and recommend that it be serviced, including re-filling the water, using caution. Adding water to a hot boiler could cause damage.

Backflow Preventer

The unwanted mixing of the water supply to the boiler and the domestic water supply to the house is prevented with a backflow preventer device.

Circulating Pumps

Circulating pumps are used to circulate hot water in a hydronic forced hot-water heating system. They push the water through the piping system. The pump should be placed in the correct position on the heating system in order to be effective. Manufacturers will describe the proper location of the pump in the installation manual.

Hartford Return

A steam heating system should have a Hartford return connection, often referred to as a Hartford loop. The Hartford return is installed on the condensate return line. It prevents total loss of water from the boiler if there is a water leak in the return line.

An equalizer connects the lower outlet to the steam outlet. The Hartford return is connected to the equalizer about 2 inches below the normal water level of the steam boiler.

Air Separator

An air separator will trap and remove air bubbles from the water in the boiler system. When a hot-water heating system is filled with cold water, the water contains some air. Air bubbles are created when the water heats up and moves rapidly through the pipes and radiators. The bubbles make noise when they pass through these components. Sometimes a radiator will not heat up because there is a lot of air trapped inside it. An air separator is designed to remove those air bubbles from the system.

Some boilers have air separators designed into them. The air is trapped, separated, and diverted to an automatic air vent at the top of the boiler’s separator. Sometimes the air is diverted into the expansion tank above the boiler.

Expansion Tanks

A hot-water heating system will have an expansion tank installed on it. There are two types of tanks:

steel tanks; and
diaphragm tanks.

The primary function of the expansion tank in a hot-water heating system is to provide a space for the expanding water to move into. When the water in the system is heated, its volume might increase by as much as 5%. Tanks limit increases in pressure to the allowable operating/working pressure of the system. They also maintain minimum operating pressures.

The maximum pressure in the boiler system is maintained by the pressure-relief valve. Minimum pressure is maintained by a water-fill valve.

In residential installations, closed steel tanks and diaphragm tanks are used to control the expansion of heated water inside a hot-water heating system.

Closed Steel Tanks

The closed steel tank has no diaphragm and no moving parts. It is simply filled with water (about two-thirds) and air (about one-third). When the boiler heats up the water, the water expands and enters the tank. As the water enters the tank, the air inside the tank gets compressed. The compression of the air inside the tank results in an increase of system pressure. That pressure can be measured at the pressure gauge of the boiler.

When the water cools down, the water in the system contracts, and the water in the tank releases back into the system. The rise and fall of pressure in the system is related to the expansion and contraction of the air in the expansion tank.

Diaphragm Expansion Tanks

Inside a diaphragm expansion tank, there is a flexible rubber membrane. The function of this membrane is to prevent air from becoming absorbed into the water, a process that could cause the expansion tank to lose its ability to act as a sort of shock absorber. If, over time, the bladder begins to leak some air, a Schrader valve, identical to the fill valve found on bicycle and car tires, can be used to add more air.

A diaphragm tank is much smaller than a closed steel tank. Most come from the manufacturer pre-charged at 12 psi.

A diaphragm tank that is waterlogged will cause the boiler pressure to reach excessive levels. Under this condition, the relief valve will likely begin to drip water. A general rule of thumb for the size of an expansion tank is that there should be one gallon for every 3,500 BTUs of radiation at heat-emitting units.

Water-Tempering Valves

Water-tempering valves are used at hot-water heating systems that heat domestic hot water at the boiler. The water-tempering valve mixes hot and cold water to the desired, safe temperature, thus preventing scalding at the fixtures. Without a water-tempering valve, the scalding hot water that is produced at the boiler would be supplied directly from the boiler to the fixtures. These valves are installed at tankless heaters, boiler coils, water heaters, and space heating systems.

Balancing Valve

A balancing valve is used to balance a heating system. You can also use the valve to determine how much water is flowing through the valve.