What is a boiler?
Updated August 2026
A boiler is a heater that warms water and pumps it through pipes to radiators, baseboard convectors, or tubing buried in the floor, instead of blowing hot air through ducts. Most residential units never boil anything: a hydronic system circulates water at 140–180F, and only a steam boiler, common in pre-war buildings, truly makes steam. Fuel can be natural gas, oil, propane, or electricity. A conventional boiler runs 80–86% AFUE through a metal flue, while a condensing boiler reaches 90–95% by harvesting heat from its own exhaust, which is why it vents in PVC and needs a drain for acidic condensate.
Where it is
A boiler sits where the pipes converge, nearly always a basement or utility room. Newer condensing models hang on the wall at about the size of a carry-on suitcase; older ones are floor-standing cast-iron blocks weighing several hundred pounds. Around it you will find a pressure gauge, an expansion tank, one or more circulator pumps, zone valves or a manifold of loops, and on steam systems a low-water cutoff with a sight glass. Learn the pressure gauge first. A cold hydronic system should read 12–15 psi, and a needle above 25–30 psi or sitting near zero both point at something worth chasing.
What it does
The burner heats water in the vessel, a circulator pushes it out through the supply loop, each radiator gives its heat to the room, and cooler water returns to be reheated. Failures center on water and air more than fire. A seized circulator leaves one zone cold and costs $300–600 to replace. Air trapped at a high point makes a radiator hot at the bottom and cold at the top. A waterlogged expansion tank drives pressure up until the relief valve dumps water on the floor, a $200–400 fix. On condensing units a plugged condensate trap shuts the burner down on a fault code, and a fouled flame sensor does the same. The one that ends a boiler is a cracked cast-iron section.
Maintenance
Book a professional service every fall at $150–300, covering a combustion test, burner cleaning, expansion tank charge, relief valve test, and on steam systems the low-water cutoff. Your own share is small but it matters: read the pressure gauge monthly through heating season, and bleed the radiators once at the start of it, top floor first. Watch for rust staining beneath the unit, the early tell of a leak. Boilers are the long-lived member of the HVAC family at 15–30 years, so replacement planning is usually about efficiency rather than failure.
What it costs to replace
A replacement boiler runs $4,000–9,000 installed for a standard gas boiler, and $7,000–12,000 for a high-efficiency condensing unit with new venting.
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Frequently asked questions
What is the difference between a boiler and a furnace?
A furnace heats air and delivers it through ducts and registers. A boiler heats water and delivers it through pipes to radiators or floor loops. The practical differences show up daily: hydronic heat feels steadier and quieter, moves no dust around the house, and keeps radiating after the burner stops. It also cannot share ductwork with central air, so a boiler home needs a separate cooling plan, usually ductless mini-splits or a high-velocity duct system.
What pressure should a home boiler run at?
Cold, a typical two-story system reads 12–15 psi, and it rises to roughly 18–22 psi once hot as the water expands. Below 12 psi cold, upper-floor radiators will not fill and the boiler may lock out. Above 30 psi the relief valve opens and spills. Both directions have plain causes: low pressure usually means a leak or a closed feed valve, and high pressure usually means a waterlogged expansion tank or a feed valve stuck open.
How do I bleed a radiator?
Run the heat until the system reaches temperature, then shut it off and let everything settle for 20 minutes. Start with the radiator highest and farthest from the boiler. Hold a cup under the small bleed valve at the top, open it a quarter turn with a radiator key or flat screwdriver, wait until the hiss becomes steady water, and close it. Work downward floor by floor, then recheck the boiler gauge and top the pressure back to 12–15 psi.
Can a boiler heat the shower water too?
Yes, in two ways. A combi boiler heats domestic water on demand through a second internal exchanger, which saves the space and standby loss of a tank. An indirect-fired water heater is a separate insulated tank the boiler heats through its own zone, holding 40–80 gallons for households that draw a lot at once. Both arrangements are common in boiler homes, and both let you retire a standalone water heater when it finally gives out.