JENERAL LEARNING / 03
See how your boiler works.
Choose a boiler and look inside. Follow the heat, see what the controls do, and learn why steam, hot-water heat and water at the tap need different paths.
Use the boiler sizing calculatorPlanning a replacement? Start with a heat-loss estimate, then confirm sizing at an in-home visit. Steam boilers need sizing from the radiators themselves.
Which boiler would you like to explore?
Educational illustrations only. Parts separate on screen to explain their roles. Real boilers involve fuel, electricity, hot surfaces and water or steam under pressure. Keep equipment closed and leave installation, adjustments and service to qualified professionals. Layouts vary; these models do not show a complete installation.
Choose a boiler to load its interactive model.
Your selected boiler will appear here.
Explore with scroll, hover, keyboard or the on-screen controls.How the four boiler types work
Read these explanations with or without the model. A boiler’s construction and the system it serves are separate questions: cast iron describes a material, steam describes the heat-carrying medium, and combi describes a second job supplying domestic hot water.
Steam boiler
This steam boiler holds water below a steam space. Heat from the burners makes steam, which travels through the supply piping to the radiators. As it gives up heat, the steam condenses back into liquid water and returns to the boiler. This example uses gravity return, not a hot-water circulator. It does not supply tap water.
- Jacket & insulation
- The jacket gives the boiler its familiar cabinet shape. Insulation reduces heat escaping through that enclosure. Neither is the vessel that holds boiler water.
- Cast-iron sections
- The sectional block transfers combustion heat to the boiler water. Unlike a hot-water boiler, this vessel has a water line with a steam space above it.
- Burners & gas control
- This atmospheric example draws combustion air from its surroundings. The gas valve controls fuel delivery to the burner assembly. Oil-fired and power-burner boilers have different arrangements.
- Steam out, water back
- The header receives steam from the boiler. Condensed water returns through the return piping. The wet return rises to a short Hartford-loop connection near the water line, joining the equalizer that runs down from the header. The sight glass shows the boiler water level.
- Flue collector & draft hood
- Combustion gases pass along the outside of the water-filled iron sections, collect above them and leave through the draft hood and vent. Exhaust gases do not belong in the steam piping.
- Pressure & water-level protection
- The pressure control limits burner operation as steam pressure rises. A low-water cutoff interrupts firing if the water level is too low. The gauge reports pressure, while the safety relief valve provides a separate path to release excessive pressure.
Follow the operating cycle
- The home calls for heat. A heat call asks the boiler to fire. Its operating and safety controls must permit burner operation.
- Burners heat the iron. The atmospheric gas burners heat the cast-iron sections. Combustion exhaust follows its own route to the vent.
- Water becomes steam. Water fills the lower passages. Steam collects above the water line and leaves through the riser and header.
- Air leaves; steam reaches the radiator. Main and radiator vents release air from this one-pipe example, then close as steam reaches them. Steam gives up heat in the radiator.
- Condensed water returns. The steam becomes liquid again. Condensate drains back through the return piping; this example uses gravity rather than a heating circulator.
- Pressure and water level are watched. The pressure control limits firing as pressure rises. A low-water cutoff stops firing if the sensed water level becomes too low. It has a different job from a water feeder.
Cast-iron hydronic boiler
This cast-iron hydronic boiler heats water without intentionally boiling it. A circulator moves that water through the heating system and back to the boiler. The same heating water repeats the loop; it is separate from the water at your taps. This example shows a conventional atmospheric gas design, not every cast-iron boiler.
- Jacket & insulation
- The painted jacket is the visible cabinet. Insulating panels sit around the hot internal assembly; they do a different job from the water-containing iron sections.
- Water-filled iron block
- The connected cast-iron sections provide water passages inside and combustion-gas passages outside. The water stays liquid in normal operation. This is a conventional noncondensing example.
- Burners & gas valve
- The burners in this atmospheric example use room air for combustion. The gas valve admits fuel only when the control system permits firing. This burner layout does not represent an oil boiler.
- Supply, return & circulator
- The supply carries heated water toward the heating zones. After releasing heat, the cooler water returns to the boiler. The circulator provides the pressure difference that moves water around this loop.
- Flue collector & draft hood
- The collector gathers combustion gases after they pass through the block. In this natural-draft example, a draft hood connects the boiler to its vent. Other boiler designs may use powered venting.
- Temperature, pressure & expansion
- The aquastat responds to boiler-water temperature. The combination gauge displays temperature and pressure. An expansion tank accommodates water expanding as it warms; the relief valve protects against excessive pressure.
Follow the operating cycle
- A heating zone calls. A thermostat requests heat. The boiler and zone controls coordinate the burner and circulation within their operating limits.
- The burner adds heat. Gas burns below the iron block in this atmospheric example. Exhaust leaves through the flue collector, draft hood and vent.
- The water stays liquid. Heat crosses the cast-iron sections into the heating water. The water passages are filled during normal operation, without a steam space.
- Supply out, cooler return back. The circulator moves heated water toward the emitters. Water releases heat in the rooms and returns cooler for reheating.
- Expansion needs room. As the heating water warms and expands, an expansion tank accepts the additional volume. Its gas cushion compresses on the other side of the diaphragm.
- The temperature control limits firing. The aquastat responds to boiler-water temperature. The pressure relief valve provides separate protection against excessive pressure.
High-efficiency boiler
This wall-hung condensing boiler heats a closed loop of water for the home. Its burner can vary its output. When operating conditions allow, the exchanger cools the exhaust enough for water vapor to condense, recovering more heat. Return-water temperature and the heating system affect that process. This heating-only illustration has no domestic hot-water exchanger or storage tank.
- Wall-hung cabinet
- The cabinet surrounds the boiler’s components. Space around real equipment is also needed for its approved connections and professional service; the small cabinet alone does not determine installation space.
- Condensing heat exchanger
- The exchanger separates the water from the combustion gases while transferring heat between them. It is designed to tolerate condensate formed on the exhaust side. Condensing performance depends on operating conditions.
- Blower, burner & gas valve
- The blower moves combustion air through this premix design. The gas valve and burner work with the controls to deliver heat at a firing rate suited to the demand and operating limits.
- Heating supply & return
- The circulator moves heating water through the boiler circuit. The supply and return connect that circuit to the system. Additional pumps, expansion control and other system components may be needed outside the illustrated cabinet.
- Air, exhaust & condensate
- This direct-vent example has a separate combustion-air inlet and exhaust outlet. Condensate collects on the exhaust side and leaves through a trapped drain. That liquid is distinct from the heating water.
- Controller, sensors & relief
- The controller uses sensor information and heat calls to manage firing. Temperature and other safety inputs can limit or stop operation. The pressure relief valve is a separate protective component for the water side.
Follow the operating cycle
- The controller receives a heat call. The control system considers heating demand and sensor information. An outdoor-reset control can adjust the requested water temperature as outdoor conditions change.
- The burner can vary its output. The blower, gas valve and burner work together to provide a controlled firing rate. Output changes within the appliance’s operating limits.
- The exchanger heats the water loop. Combustion gases transfer heat across the exchanger into the water. The heating-water passages remain separate from the combustion chamber.
- Cooler return water can help recover more heat. When conditions allow, water vapor in the exhaust condenses and releases additional heat. That condensate leaves through the trapped drain; it is not heating-loop water.
- The home receives the heat. The circulator moves water through the boiler circuit and the connected system carries heat to the rooms. Piping and other pumps depend on the installation.
- Sensors and limits remain active. The controller can reduce or stop firing in response to operating limits. A lower return-water temperature alone does not establish a particular efficiency or fuel saving.
Combi boiler
This combi uses a primary exchanger for combustion heat and a separate plate exchanger for domestic hot water. The heating water and drinking water do not mix. When a hot tap opens, the controls give domestic hot water priority and direct boiler water through the plate exchanger. Space heating resumes afterward. This example has no hot-water storage tank; available hot-water flow still depends on capacity and incoming water temperature.
- One cabinet, two jobs
- The wall-hung cabinet contains both heating and hot-water components. Combining those jobs does not combine their water supplies. The cover opens only in this educational view.
- Primary & plate exchangers
- The primary exchanger heats boiler water. For a hot-water call, that heated water passes along one side of the plate exchanger while domestic water passes along the other. Heat crosses the plates; the two water circuits remain separate.
- Controlled combustion
- The same combustion assembly serves space heating or domestic hot-water production. Its firing rate changes within the appliance’s limits; it is not a separate burner for each water circuit.
- Separate heating & tap-water paths
- The circulator moves boiler water. A diverter sends it toward space heating or the plate exchanger according to demand. Cold domestic water enters through a different connection and leaves toward the taps after gaining heat. This example gives domestic hot water priority.
- Combustion air, exhaust & drain
- The illustrated air inlet supplies combustion air separately from the exhaust outlet. The condensate trap serves the liquid formed on the combustion side. It is not a drain for either of the two water circuits.
- Demand sensing & protection
- The domestic flow sensor detects a hot-water draw. The controller coordinates that demand with the burner, circulator and diverter. Other sensors and limits protect operation, and the relief valve protects the boiler-water side against excessive pressure.
Follow the operating cycle
- One appliance receives two kinds of demand. A thermostat can request space heat. A domestic flow signal can request hot tap water. The controller decides which operating mode to serve.
- The primary exchanger heats boiler water. The burner transfers heat into the boiler-water circuit through the primary exchanger. Combustion gases do not enter either water circuit.
- Heating mode serves the rooms. The circulator and diverter route boiler water toward the heating supply and back through the return. The potable-water path remains separate.
- A hot tap creates a domestic-water call. The domestic flow sensor detects a draw. This illustrated combi gives hot-water production priority and can pause space heating during that call.
- The diverter changes the boiler-water route. For hot-water mode, the diverter sends heated boiler water through the domestic plate exchanger instead of the space-heating path.
- Heat crosses the plates; water stays separate. Cold domestic water gains heat across the plate exchanger and leaves toward the taps. It does not mix with heating water. Space heating can resume after the hot-water call ends.
The equipment around the boiler matters, too.
The appliance cutaways show selected parts. These system components may sit on the surrounding piping or elsewhere in the home. Some are optional or depend on the design; this list is not a complete installation specification.
- Automatic steam water feeder
- Where fitted, a compatible feeder adds water in response to its controls. It is separate from the low-water cutoff and is not a substitute for finding the cause of water loss.Steam
- Steam feed backflow protection
- Backflow protection helps prevent boiler water from entering the potable supply through a feed connection. The appropriate arrangement depends on the system and applicable requirements.Steam
- Main air vent
- A main vent lets air escape from the steam main so steam can reach the connected radiators. It closes when steam reaches it.Steam
- One-pipe radiator vent
- This vent releases air from the radiator as steam enters, then closes as it heats. A two-pipe system can use a different air-removal and trap arrangement.Steam
- One-pipe radiator & inlet valve
- Steam enters and condensed water leaves through the same radiator connection in this example. The radiator transfers heat to the room; the inlet valve and air vent have different roles.Steam
- Dry return, wet return & equalizer
- Returning condensate moves through piping above and below the boiler’s water line. The equalizer connects the header toward the boiler return and carries water separated in the header.Steam
- Hartford loop
- The wet return rises to a short connection near the water line before joining the equalizer. This protective arrangement has a different role from the low-water cutoff. The drawing gives no piping dimensions or leak-containment guarantee.Steam
- Pressure-control pigtail siphon
- Water retained in this loop protects the pressure-sensing control from direct contact with live steam. The loop and pressure control need assessment as part of the equipment’s service.Steam
- Skim connection & boiler drain
- These separate service connections provide access for work on the boiler water. Their locations and use depend on the boiler; this guide gives no skimming or draining procedure.Steam
- Heating expansion tank
- A diaphragm tank separates heating water from a sealed gas cushion. As warmer water expands into the tank, the cushion compresses; as the system cools, water can return to the loop.Cast iron · High efficiency · Combi
- Air separator & automatic vent
- The separator gathers small air bubbles from circulating water and the vent releases the collected air. Removing air and relieving excessive water pressure are separate jobs.Cast iron · High efficiency · Combi
- Pressure-reducing fill valve
- A fill regulator controls the pressure of water entering the heating system from its supply. It is different from a backflow preventer, and frequent water loss still needs investigation.Cast iron · High efficiency · Combi
- Hydronic feed backflow protection
- The feed connection needs suitable protection against heating water flowing toward the drinking-water supply. Water treatment, system conditions and local requirements affect the selected protection.Cast iron · High efficiency · Combi
- Water temperature & pressure indication
- A mechanical combination gauge or electronic sensors can report temperature and pressure. Those readings describe different conditions; the normal range depends on the equipment and system.Cast iron · High efficiency · Combi
- Isolation & purge valves
- These fittings let a professional isolate sections and remove air during filling or service. The shortened model piping does not show a complete arrangement for that work.Cast iron · High efficiency · Combi
- Zone valves or zone circulators
- A zone control directs heat to the area requesting it. Systems can use valves, separate circulators or other arrangements; one boiler can serve several heating zones.Cast iron · High efficiency · Combi
- Radiators, baseboard or radiant tubing
- The emitters transfer heat from circulating water to the rooms. Their output at the required water temperature is part of assessing which boiler and controls fit the home.Cast iron · High efficiency · Combi
- Optional indirect water heater
- A separate insulated tank can use boiler heat to warm stored domestic water through an exchanger. It is an optional system design, not a storage tank hidden inside the illustrated combi.Cast iron · High efficiency · Combi
- Gas supply shutoff & sediment trap
- Gas supply fittings sit outside the automatic gas valve’s role in firing the burner. Their arrangement belongs to the approved appliance installation, which is not fully shown here.Steam · Cast iron · High efficiency · Combi
- Condensate neutralizer, where needed
- Combustion condensate can be acidic. A suitable neutralizer raises its pH before disposal where the installation calls for treatment; it is separate from the condensate trap.High efficiency · Combi
- Condensate pump, if needed
- Where condensate cannot reach its approved drain by gravity, a suitable pump can move it. A pump is not required in every installation.High efficiency · Combi
- Outdoor reset sensor & control
- An outdoor sensor can help the control adjust its heating-water target with the weather. The control still has to meet the home’s heat demand and the appliance’s operating limits.High efficiency · Combi
- Hydraulic separation, when specified
- A suitable separator or closely spaced tees can reduce interaction between separately pumped boiler and system circuits. The selected boiler and system design determine whether and how separation is used.High efficiency · Combi
- Magnetic dirt separator
- A magnetic separator can capture circulating ferrous debris from the heating water. It serves a different purpose from potable-water treatment or combustion-condensate neutralization.High efficiency · Combi
Boiler guides and service stories
Compare boiler types, follow a heating fault, or read what a service visit found.
Questions about how boilers work
Is a steam boiler the same as a hot-water boiler?
Both add heat, but they move it through the home differently. A steam boiler makes steam that travels to radiators and returns as liquid condensate. A hydronic hot-water boiler circulates water through a heating loop without intentionally boiling it. The existing distribution system matters when considering a replacement.
Does cast iron mean the boiler makes steam?
No. Cast iron describes the heat exchanger material. Cast-iron boilers can serve steam or hot-water systems when designed for that use. This guide shows one example of each, with different controls and connections.
What makes the high-efficiency model different?
The illustrated condensing boiler can recover additional heat when water vapor in the combustion gases condenses. The return-water temperature and operating conditions affect how much condensing occurs. Its venting, condensate drainage and heating controls are part of the whole system; a nameplate rating alone cannot predict your bill.
Does a combi mix drinking water with radiator water?
In the illustrated combi, the heating water and domestic water stay in separate passages of a plate heat exchanger. Heat crosses the metal between them. The controls give domestic hot water priority during a draw; they can temporarily interrupt space heating. Designs and capacity limits vary by model.
Can I replace a steam boiler with a combi?
The illustrated combi serves a hot-water heating loop, not steam radiators. Moving from steam to hot-water heat is a system conversion, not simply swapping the boiler. An assessment must cover the distribution, heat emitters, heat loss, hot-water demand and installation conditions before recommending equipment.
Will this model tell me what is wrong with my boiler?
The model explains component roles. It cannot identify a failed part or establish whether your equipment is safe to run. Share the equipment model, symptoms and any complete fault message with a qualified professional. Leave panels, fuel controls and safety devices alone.
Bring the questions, leave the panels closed
Understand the system before choosing the work.
Tell Jeneral Plumbing what boiler you have, which rooms are affected and what has changed. We can assess the equipment and the heating system together. We serve homes within 20 miles of Westford, MA, in Middlesex & Worcester County and southern New Hampshire. Gas-fired installation and fuel-gas work are offered in Massachusetts only.
If you smell gas or a carbon monoxide alarm sounds, leave the building and call 911 from outside.