For homeowners

Types of boiler: which one fits your home?

Choose Condensing hot water boiler (gas or oil) for replacing a traditional boiler in a hot water (hydronic) system while keeping its piping and radiators or baseboards, per the Building America Solution Center (BASC); Non-condensing hot water boiler (gas or oil) for a gas boiler swap where chimney (Category I) venting fits the house: DOE's boiler rule refers to "reduced costs associated with Category I venting in certain installations"; Steam boiler (gas or oil) for keeping the steam radiators and piping a house already has: HUD's Rehab Guide (1999) covers restoring an existing steam distribution system.

The right choice depends on whether the house is heated by hot water or by steam, whether your radiators or baseboards are sized for the low return water temperature a condensing boiler needs, and how a hot water boiler can be vented.

How we know: By InstaBids editorial team with AI research agents. · Sources read: 17 October 2026 · Updated 27 October 2026

AFUE (DOE Energy Saver Guide, 2022): "… annual fuel utilization efficiency, also known as AFUE, which is a measure of heating equipment efficiency."
Type	Best for	Main advantages	Main drawbacks
Condensing hot water boiler (gas or oil)	replacing a traditional boiler in a hot water (hydronic) system while keeping its piping and radiators or baseboards, per the Building America Solution Center (BASC)	BASC gives condensing boilers AFUEs of 90% to 96%; BASC gives Category IV (condensing) oil-fired boilers efficiencies of 90% to 93%; BASC: they can be vented through the wall or roof by a PVC pipe rather than a chimney	BASC: undersized radiators or baseboards return the water too hot, preventing condensing; BASC: the condensate water is highly acidic (pH of 2); BASC: some jurisdictions require the condensate to pass through a neutralizer before disposal
Non-condensing hot water boiler (gas or oil)	a gas boiler swap where chimney (Category I) venting fits the house: DOE's boiler rule refers to "reduced costs associated with Category I venting in certain installations"	no official source names one	BASC calls a gas-fired Category I (chimney-vented) boiler "not recommended": "If you choose a Category I or non-direct vented boiler (not recommended)"; BASC, on gas-fired Category I (chimney-vented) boilers only: "Backdrafting, when combustion gases spill down into the CAZ rather than going out the flue, can occur if the CAZ becomes depressurized" (the CAZ is the room where the boiler sits; BASC puts residential oil-fired boilers in Category III or IV, which vent through a sealed pipe)
Steam boiler (gas or oil)	keeping the steam radiators and piping a house already has: HUD's Rehab Guide (1999) covers restoring an existing steam distribution system	HUD Rehab Guide (1999) on restoring an existing steam or hot water distribution system (radiator valves and vents): "Low cost and least disruptive"	HUD Rehab Guide (1999): hot water systems are "inherently safer than steam systems"; a hot-water boiler can hardly be run "to dangerous pressures"; HUD Rehab Guide (1999), one-pipe steam systems: "If the single pipe is too small, steam will force condensate back up the pipe, causing noise"

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Sources for the facts on this page

Building America Solution Center (Pacific Northwest National Laboratory), Gas-Fired Boilers, national laboratory, read 17 October 2026.
Building America Solution Center (Pacific Northwest National Laboratory), Condensing Boilers, national laboratory, read 17 October 2026.
Building America Solution Center (Pacific Northwest National Laboratory), Oil-Fired Boilers, national laboratory, read 17 October 2026.
U.S. Department of Energy, Energy Saver Guide (2022), government, read 17 October 2026.
U.S. Department of Energy, Energy Conservation Standards for Residential Boilers, final rule (Federal Register, January 15, 2016), government, read 17 October 2026.
U.S. Department of Housing and Urban Development (HUD), The Rehab Guide, Volume 8: HVAC/Plumbing (August 1999; prepared for HUD by a building systems consulting firm), government, read 22 October 2026.