Boilers Explained (2026): Gas, Oil, Combi and Hot Water Boiler Systems

A boiler heats your home by heating water, not air. Hot water (or steam) travels through pipes to radiators, baseboard, or tubing in the floor, and the rooms warm up without a blower or ductwork. Boilers are the standard heating system across much of the Northeast and Upper Midwest, in older homes everywhere, and in many new homes built with radiant floors.

This guide walks through gas, oil, combi, and steam boilers, the parts that make a system work, and what tends to wear out. HVAC MORE does not sell complete boilers, but we stock the controls and replacement parts that keep them running.

The quick answer: A boiler system heats water in a gas or oil-fired heat exchanger and uses a circulator pump to move it through radiators, baseboard, or radiant floor loops; a steam boiler skips the pump and lets steam rise on its own. Gas boilers come in non-condensing models (roughly 82-85% AFUE) and condensing models (90-95%+ AFUE). Oil-fired boilers are common where natural gas is not available, especially in the Northeast. A combi boiler is a single wall-hung unit that provides both space heating and on-demand domestic hot water with no storage tank, which saves space but limits how much hot water you can draw at once.

How a hydronic boiler system works

"Hydronic" just means heating with water. A residential hot water boiler keeps a sealed loop of water at roughly 12-15 psi and heats it to a setpoint, typically 140-180°F for baseboard and radiators, or 90-120°F for radiant floors (usually through a mixing valve). When a thermostat calls for heat, the burner fires, the zone opens, and a circulator pump moves the water. The main components:

• Boiler - the heat exchanger (cast iron, steel, copper, stainless, or aluminum) plus burner and controls, usually shipped as one packaged, jacketed unit. These factory-cased units are what people mean by "boxed" or packaged boilers, as opposed to older field-assembled sectional boilers.
• Circulator pump - a small wet-rotor pump (Taco 007 and Grundfos UPS15-58 are the classic examples). Some systems use one circulator per zone; others use a single pump with zone valves.
• Zone valves - 24V motorized valves that open one heating zone at a time so different areas can be controlled separately.
• Expansion tank - absorbs the expansion of heated water so system pressure stays steady instead of lifting the relief valve.
• Pressure relief valve - set at 30 psi on residential hot water boilers. If it drips, the expansion tank is usually the first suspect.
• Heat emitters - cast iron radiators, fin-tube baseboard, panel radiators, or PEX radiant floor loops.

Hot water boilers vs steam boilers

Most boilers installed today are hot water boilers, but steam boilers are still common in pre-war homes and apartment buildings. A steam boiler is only partly full of water; it boils that water, the steam rises through the piping at very low pressure (usually under 2 psi), condenses in the radiators, and drains back by gravity. There is no circulator pump.

Steam systems have their own controls: a sight glass for water level, a low-water cutoff that shuts the burner off if the level drops, a pressuretrol that limits pressure, and radiator air vents or steam traps. A steam boiler is normally replaced with another steam boiler; converting to hot water means new piping and radiators.

Gas boilers: non-condensing vs condensing

Non-condensing (atmospheric or power-vented) gas boilers use a cast iron or steel heat exchanger, send hot flue gases out a chimney or metal vent, and land in the low-to-mid 80s for AFUE. They are simple, durable, and happy with the high return-water temperatures of old cast iron radiators.

Condensing boilers use a stainless steel or aluminum heat exchanger that pulls extra heat out of the flue gas until the water vapor in it condenses, reaching 90-95%+ AFUE. They vent through PVC or polypropylene pipe, need a condensate drain (often with a neutralizer), and most modulate the burner to match the load instead of cycling on and off. The catch: a condensing boiler only condenses when return water is cool, roughly below 130°F. Radiant floors and oversized radiators get the full benefit; baseboard run at 180°F all winter will not see the rated efficiency.

Gas boilers light with a standing pilot (older units, with a thermocouple), an intermittent spark pilot, or a hot surface igniter, all working with a 24V gas valve. If a gas boiler will not fire, the igniter, flame sensor, and gas valve are the usual suspects. If your home has ductwork and you are weighing forced air instead, see our gas furnaces.

Oil-fired boilers

Oil boilers burn No. 2 heating oil from a tank in the basement or yard and dominate in rural New England, upstate New York, Pennsylvania, and other areas without gas mains. Modern oil boilers typically rate 84-87% AFUE and are known for long service lives.

The heart of an oil boiler is the pressure-atomizing gun burner (Beckett, Riello, and Carlin are the common brands). A fuel pump pressurizes oil to around 100-150 psi, a nozzle sprays it in a fine cone, and an ignition transformer or solid-state igniter lights it with a spark across two electrodes. A cad cell flame sensor proves the flame to the primary control, which locks out the burner if ignition fails. Each nozzle is stamped with a flow rate in gallons per hour and a spray angle that must match the boiler's firing rate.

Oil equipment needs an annual tune-up more than any other heating system: replace the nozzle, oil filter, and pump strainer; clean the heat exchanger and flue passages; check the electrodes and cad cell; and run a combustion test.

Combi boilers and indirect water heaters

A combination (combi) boiler does two jobs in one wall-hung cabinet: it heats the home through the hydronic loop and heats domestic hot water on demand through a built-in secondary heat exchanger. Open a hot tap and a flow sensor switches the boiler to hot water priority; close it and the boiler goes back to heating. There is no storage tank, so you save floor space and standby losses.

The trade-off is flow rate. A combi can only heat about 2-5 gallons per minute depending on its BTU rating and how cold the incoming water is: one shower plus a sink, not two showers and a dishwasher at once. Combis suit apartments, townhouses, and small to mid-size homes, and nearly all are condensing, modulating gas units. In hard water areas the domestic heat exchanger scales and needs periodic descaling or a softener.

If you want the boiler to make your hot water with more capacity, the usual answer is an indirect water heater: an insulated tank (typically 30-80 gallons) with an internal coil heated by boiler water, piped as its own zone. Recovery is fast because the boiler's full output is available, and well-built indirect tanks often outlast conventional water heaters.

Controls: aquastats, outdoor reset, and thermostats

The aquastat is the boiler's own thermostat. It senses boiler water temperature and controls the burner: a high limit shuts the burner off at the setpoint (say 180°F), and a differential lets it drop a set amount before refiring. Triple aquastats (common on boilers with a tankless coil) add a low limit that keeps the boiler warm even when no zone is calling. A failed aquastat can leave a boiler that never fires, or one that overheats and trips the relief valve.

Outdoor reset lowers the target water temperature as the weather outside gets milder. On a 45°F day the house does not need 180°F water, so the boiler runs cooler, cycles less, and, on a condensing boiler, condenses more.

Room thermostats on a boiler system are ordinary 24V stats, one per zone, wired to a zone valve or a zone control relay panel. Nearly any thermostat in our thermostats collection that supports a conventional heat call will work; smart thermostats need a common (C) wire or the maker's adapter.

Lifespan, maintenance, and the parts that fail

Cast iron gas and oil boilers routinely last 25-30 years or more with annual service. Condensing boilers, with thinner heat exchangers and more electronics, are generally planned around 15-20 years. Annual service should check pressure and the expansion tank, test the relief valve, inspect venting, clean the burner and heat exchanger, and bleed air from radiators. The parts that most often need replacing:

• Igniters and flame sensors - hot surface igniters crack, spark electrodes erode, flame rods and cad cells get dirty. Browse igniters and flame sensors.
• Gas valves - 24V combination valves that stop opening or fail a leak check. See gas valves.
• Aquastats and limit controls - drifting or dead temperature controls.
• Circulator pumps - seized or noisy pumps and failed cartridges.
• Pressure relief valves and expansion tanks - a waterlogged expansion tank is the most common reason a relief valve drips.
• Transformers and relays - the 24V control transformer that powers the thermostat circuit and zone valves, plus switching relays. See transformers.

Efficiency figures are typical ranges for new equipment; older boilers still in service may be lower.

FAQ

What is a combi boiler?

A combi boiler is a single gas-fired unit that heats your home through radiators or radiant floors and also heats domestic hot water instantly as it flows through, with no storage tank. It saves space but can only supply a limited flow of hot water at one time.

How long does a boiler last?

Cast iron gas and oil boilers often run 25-30 years or more with annual service. Condensing and combi boilers are typically planned around 15-20 years because their heat exchangers are thinner and they carry more electronics.

Why is my boiler's pressure relief valve dripping?

Usually because the expansion tank has lost its air charge or failed, so pressure spikes when the water heats. A stuck fill valve or a leaking tankless coil can also push pressure past the 30 psi relief setting. Replace the expansion tank first; if the relief valve has opened repeatedly, replace it too.

Does HVAC MORE sell boilers?

No. We do not sell complete boilers, but we stock boiler controls and replacement parts, including aquastats, igniters, flame sensors, gas valves, transformers, relays, and circulator components, plus thermostats for zoned hydronic systems.

Shop at HVAC MORE

When a boiler quits in January it is almost always a part, not the whole unit. Our boiler parts collection covers aquastats, igniters, gas valves, transformers, relays, and circulator components for common Honeywell, Taco, Robertshaw, and OEM applications. If you are not sure which part matches your boiler, send us the model number through our contact page and we will help you track it down.