How a boiler heats a house
A boiler heats water (or turns it into steam) and moves that heat through pipes to radiators, baseboard convectors or tubing under the floor. Nothing blows air. That is why boiler heat feels even and quiet, and also why a boiler problem shows up as cold rooms, odd noises in the pipes or a pressure gauge that has wandered out of its normal band.
Most homes have one of two kinds. A hot water (hydronic) system keeps the loop full of water at roughly 12 to 15 psi when cold, uses a circulator pump to push it around, and relies on an expansion tank to absorb the swelling as it warms. A steam system boils water at very low pressure, lets the steam rise to the radiators on its own, and returns the condensate by gravity. Steam systems have a sight glass, a pressuretrol and a low-water cutoff instead of a circulator and an expansion tank.
Boilers also split by efficiency. Older cast-iron units with an atmospheric burner and a chimney flue typically rate somewhere in the 80 to 85 percent AFUE band and can last decades. Newer high-efficiency condensing boilers, often modulating, wring extra heat from the exhaust, vent through plastic pipe out a side wall, and drain acidic condensate. A combi boiler is a condensing boiler that also makes domestic hot water on demand, so when it fails you can lose both heat and hot taps at once. Plenty of houses with a heating-only boiler also run an indirect tank off it, which works the same way: no boiler, no hot showers.
What is usually wrong
Match the symptom to the likely cause. These are starting points for the conversation with a technician, not a substitute for a diagnosis.
| What you notice | Likely cause | Usually |
|---|---|---|
| Boiler fires but radiators stay cold | Failed circulator pump, stuck zone valve or air in the loop | Repair |
| Pressure gauge reads near zero | Leak somewhere in the loop or a failed fill valve | Repair |
| Pressure climbs past 25 psi and the relief valve drips | Waterlogged expansion tank or a fill valve passing water | Repair |
| One zone cold, others fine | Zone valve motor or end switch, or that zone's thermostat | Repair |
| Gurgling or trickling sounds upstairs | Air trapped at high points in the piping | Bleed radiators |
| Steam boiler shuts off with water in the glass dirty or low | Low-water cutoff tripping or clogged probe | Repair or skim |
| Banging pipes on a steam system | Water trapped in sagging pipe or a radiator pitched the wrong way | Adjust piping |
| Condensing boiler shows a lockout code | Flame sensing, ignition, blocked condensate drain or venting fault | Repair |
| Water pooling under the boiler | Leaking relief valve, pump flange or fitting; sometimes the heat exchanger | Repair or replace |
Air is the most common nuisance in a hot water system. It sneaks in when someone drains a zone, when the fill valve tops up the loop, or through a tiny leak. A single radiator that is cold at the top and warm at the bottom is the classic sign. Bleeding it with a radiator key at the small valve near the top, with a rag ready, often fixes it in a minute. If the same radiator needs bleeding every week, something is pulling air in, and that deserves a service visit.
What a boiler technician checks
- The gauge and the glass. On a hot water boiler, cold pressure and hot pressure. On steam, the waterline in the sight glass and whether it bounces while firing.
- Burner and ignition. Whether the pilot or hot surface igniter lights, whether the flame sensor proves flame, and what the control board has logged.
- Venting and combustion. Draft at the flue on a chimney-vented boiler, or the intake and exhaust pipes on a sealed condensing unit. Many techs run a combustion analyzer and check carbon monoxide in the flue gas.
- Circulation. Whether the circulator is running, warm and quiet, and whether each zone valve opens when its thermostat calls.
- Safety controls. The pressure relief valve, the high limit, the low-water cutoff and, on steam, the pressuretrol setting, which should usually be low.
- The condensate path. On condensing boilers, the trap and drain line, which can clog and shut the boiler down.
A tech who works on boilers regularly will ask whether you have steam or hot water before they arrive. Not every heating company services both, and steam in particular rewards someone who has spent years on old one-pipe and two-pipe systems.
Typical boiler repair prices
| Repair | Typical range | What moves the price |
|---|---|---|
| Diagnostic visit and minor adjustment | $100–$250 | After-hours rates and travel distance |
| Circulator pump replacement | $300–$800 | Standard vs variable-speed ECM pump |
| Zone valve or zone valve head | $200–$500 | Head only vs full valve body with draining |
| Expansion tank replacement | $250–$600 | Tank size and whether the loop must be drained |
| Pressure relief valve | $150–$350 | Access and condition of the old fittings |
| Low-water cutoff (steam) | $250–$650 | Probe vs float type and boiler tapping |
| Igniter, flame sensor or thermocouple | $150–$450 | Brand and whether the part is in stock |
| Control board or gas valve | $400–$1,200 | Condensing boiler boards cost more |
Most repair calls land between $150 and $700. Parts for big-name condensing boilers from Navien, Viessmann, Lochinvar or Buderus can run higher than parts for a basic cast-iron Weil-McLain or Burnham, and a few must be ordered. Ask whether the tech carries common parts for your brand before booking.
Repair or replace
Cast-iron boilers commonly run 20 to 30 years, and some well-kept ones go longer. Condensing boilers tend to land closer to 15 to 20 years, depending on water quality, venting and how often they were serviced. Age alone is not a reason to replace a boiler that still runs clean and dry.
The case for replacing gets strong when the heat exchanger leaks, when a cast-iron section has cracked, when the same unit needs a major part for the second winter in a row, or when parts for an old model are no longer made. It also gets strong when the boiler is badly oversized for the house, which is common after insulation and window upgrades. Our page on boiler replacement covers sizing, venting and costs.
A useful test: if a single repair costs more than about a third of a new installed boiler, and the current one is in the last quarter of its expected life, get a replacement quote alongside the repair quote before deciding.
Safety first: carbon monoxide and gas smells
If you smell gas, leave the house now. Do not flip switches, start a car in an attached garage or use a phone inside. Once outside, call your gas utility's emergency line or 911, then call for repair.
A boiler that is burning badly or venting poorly can push carbon monoxide into the house. Headaches or nausea that ease when you go outside, soot around the burner or flue, a yellow lazy flame on an older boiler, or a CO alarm going off are all reasons to shut the boiler down at its service switch and get out. Every home with fuel-burning equipment should have working CO alarms near sleeping areas and on each level.
Do not cap, plug or tighten a dripping pressure relief valve. It drips for a reason, most often excess pressure caused by a waterlogged expansion tank or a fill valve that will not shut, and it is the part that stops the boiler from building dangerous pressure.
Seasonal maintenance that prevents most calls
- Annual service before the cold arrives. Cleaning, a combustion check, a look at the relief valve and the venting. Condensing boilers need their heat exchanger and condensate trap cleaned on the manufacturer's schedule.
- Bleed radiators in a hot water system at the start of the season, top floor last, then check that cold pressure is back where it should be.
- Steam owners: check the sight glass weekly in winter, and flush the low-water cutoff as the manual describes if yours is a float type.
- Keep the area clear. No boxes against the boiler, nothing stored near the burner, and clear snow away from side-wall vents on condensing units.
- Test CO alarms when you change the clocks.
What to ask the heating contractor
- Are you licensed for HVAC or gas work where I live, and are you insured?
- Do you service steam, hot water or both, and how often do you work on my brand?
- Do you carry common parts on the truck, or will this need a second trip?
- What does the diagnostic fee cover, and is it applied to the repair?
- What warranty do you give on the part and on your labor?
If the problem is a furnace or a heat pump rather than a boiler, see furnace repair or the broader heating repair page.
Questions people ask
How much does boiler repair usually cost?
Most repairs run $150 to $700 including the visit. Pumps, zone valves and expansion tanks sit in the middle; control boards and gas valves on condensing boilers can push past $1,000.
What pressure should my boiler gauge show?
Many residential hot water systems sit around 12 to 15 psi cold and rise somewhat when hot. The right number depends on how tall the house is, so check the manual or ask the tech who services it. Steam boilers run at a fraction of a psi and use a separate gauge.
Why do I keep having to bleed the same radiator?
Repeated air means air is getting in, often through a small leak, a failing automatic air vent or a fill valve topping up a leaking loop. Bleeding treats the symptom; finding the leak fixes it.
Is a dripping relief valve dangerous?
The drip is a warning that pressure is too high or the valve is worn. Never plug it. A failed expansion tank is a frequent cause and is usually a modest repair.
My combi boiler gives hot water but no heat. What is wrong?
Combi boilers prioritize hot water, so a fault in the heating side, such as a diverter valve, the circulator or a thermostat, can leave the taps hot and the radiators cold. A technician can usually trace it in one visit.
Can someone fix a boiler tonight?
Phones are staffed 24/7, and during heating season a lot of boiler contractors dispatch after hours. After-hours rates cost more, so if the house is safe and not at risk of freezing, a morning visit may save money.