The Short Version

A boiler heats water (or turns it into steam) using a gas burner, then circulates that heated water or steam through pipes to radiators, baseboard heaters, or radiant floor tubing throughout your home. The heat from the water or steam transfers into the room, and the cooled water returns to the boiler to be reheated. It is a closed loop.

Boilers are common in Chicago's older housing stock — bungalows, two-flats, greystones, and vintage apartment buildings — because they were the standard heating technology when most of these buildings were constructed.

The Two Types: Steam and Hot Water

There are two fundamentally different types of residential boilers, and they work differently:

• Steam boilers heat water until it boils and turns to steam. The steam rises through pipes to radiators, where it condenses back to water and releases heat. The condensate drains back to the boiler by gravity. Steam systems operate at low pressure (typically 0.5–2 PSI) and require no pump.
• Hot water boilers heat water to 140–180°F but do not boil it. A circulator pump moves the hot water through pipes to radiators or baseboard heaters. Hot water systems are more efficient and easier to zone than steam systems.

Most boilers installed in Chicago before 1950 are steam systems. Most boilers installed after 1960 are hot water systems. If you have old-style cast iron radiators with a single pipe, you almost certainly have a steam system. If you have baseboard heaters or two-pipe radiators, you have a hot water system.

Key Components of a Boiler System

• Burner: Burns natural gas (or oil) to produce heat. Modern boilers use electronic ignition; older boilers use a standing pilot light.
• Heat exchanger: Transfers heat from the combustion gases to the water without mixing them. The heat exchanger is the most critical component — a cracked heat exchanger is a safety hazard and typically means replacement.
• Aquastat: The boiler's primary temperature control. It monitors water temperature and turns the burner on and off to maintain the set temperature. On a steam boiler, a pressurestat monitors steam pressure instead.
• Circulator pump (hot water only): Moves hot water through the distribution pipes. Steam systems do not need a pump — steam rises on its own.
• Expansion tank: Absorbs the pressure increase as water heats and expands. Without it, pressure relief valves would open constantly.
• Pressure relief valve: A safety device that opens if pressure exceeds a safe limit. It should never be dripping — if it is, the system has a pressure problem.
• Low-water cutoff: Shuts down the burner if the water level drops too low. Critical on steam boilers, where water level is constantly changing.

How a Hot Water Boiler Heating Cycle Works

Here is the sequence for a typical hot water boiler heating cycle:

1. The thermostat calls for heat and sends a signal to the boiler control.
2. The circulator pump starts, moving water through the distribution pipes.
3. The burner ignites and heats the water in the heat exchanger.
4. The aquastat monitors water temperature. When it reaches the set point (typically 160–180°F), the burner shuts off.
5. Hot water circulates to the radiators or baseboards, where it releases heat into the room.
6. The cooled water returns to the boiler through the return pipe.
7. The cycle repeats until the thermostat is satisfied.

How a Steam Boiler Heating Cycle Works

1. The thermostat calls for heat.
2. The burner ignites and heats the water until it boils.
3. Steam rises through the supply pipes to the radiators.
4. In the radiators, steam condenses back to water and releases heat into the room.
5. The condensate drains back to the boiler through the return pipes (or a condensate pump if the return is below the boiler).
6. The pressurestat monitors steam pressure. When it reaches the set point (typically 0.5–1.5 PSI), the burner shuts off.
7. The cycle repeats until the thermostat is satisfied.

Condensing vs. Non-Condensing Boilers

Modern high-efficiency boilers are condensing boilers. They extract so much heat from the combustion gases that the gases cool below the dew point and condense into water vapor. This condensate (slightly acidic water) drains away through a neutralizer. Condensing boilers achieve 90–98% AFUE efficiency.

Older cast iron boilers are non-condensing. They vent hot exhaust gases up a chimney and achieve 80–85% AFUE at best. Most boilers in Chicago's older housing stock are non-condensing cast iron units.

Why Boilers Are Common in Chicago

Chicago's older housing stock — built primarily between 1900 and 1960 — was designed around steam and hot water heat. Boilers were the dominant residential heating technology of that era. Many of these systems are still in service today, some 50–80 years old. Cast iron boilers are extremely durable; it is not unusual to find a 40-year-old cast iron boiler still operating reliably.

D's Air & Heating services, repairs, and replaces boilers throughout Chicago and the surrounding suburbs. We work on steam and hot water systems, cast iron and condensing boilers, and all major brands.