The short answer
A boiler that fires but does not heat usually has a circulation problem — a failed circulator pump, a stuck zone valve, air in the system or low pressure — not a burner problem. Kettling (a rumbling, whistling kettle sound) means scale or sludge on the heat exchanger. Banging is water hammer or, on steam systems, wet steam and sagging pipes. Gurgling is air. Check the thermostat, the pressure gauge (12–15 psi cold) and one reset; beyond that, or with any gas smell or water on the floor, call.
Three things you can safely check
- Thermostat and power. Is the thermostat set to heat and calling (set it 5° above room temperature)? Is the boiler's service switch on and the breaker not tripped? On a multi-zone system, is only one zone cold? A dead thermostat battery has caused plenty of "no heat" calls.
- Pressure gauge. Most residential hot-water boilers should read 12–15 psi cold and climb to about 18–25 psi when hot. Under 10 psi the water may not reach upstairs radiators; at 30 psi the relief valve will discharge. If the pressure is low, the auto-fill valve or a leak is the question; if it is high, the expansion tank is usually waterlogged.
- One reset. If the boiler is locked out (a red light or error code), press reset once. If it lights and stays lit, note the code and mention it at the next service. If it locks out again within hours, stop — repeated resets on an ignition fault are how heat exchangers and flue collectors get damaged.
That is where homeowner checks end. Boilers involve gas, combustion and 180°F water; the rest is a licensed plumber's job.
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Boiler runs but the house is cold
The burner fires, the boiler gets hot, but the radiators or baseboards stay cool. That is a distribution problem, and the causes are ranked roughly by how often we find them:
- Circulator pump. The pump moves hot water out to the zones. When it fails, the boiler short-cycles on its high limit and the house never warms. You may hear it humming without the pipe near it getting hot.
- Zone valve or zone relay. One zone cold, others fine, almost always means that zone's valve is stuck or its relay has failed.
- Air in the system. Gurgling, cold spots at the top of radiators, upstairs cold while downstairs is warm. Bleeding radiators helps; recurring air means the air separator or expansion tank needs attention.
- Low pressure. Water cannot reach the top floor. See the pressure section below.
- Aquastat or limit control. The boiler never reaches temperature or shuts off too early. A controls repair.
- Frozen or blocked baseboard. In an addition or a room over a garage, a run on an exterior wall can freeze during a cold snap and block flow to the whole loop — see frozen pipes in a Massachusetts winter.
Boiler noises and what they mean
| Sound | Likely cause | Urgency |
|---|---|---|
| Kettling, rumbling, whistling like a kettle | Scale or sludge on the heat exchanger; poor circulation | Service soon — it damages the exchanger |
| Gurgling, bubbling | Air in the system, low pressure | Bleed radiators; service if it returns |
| Banging or clanking in pipes (hot water system) | Water hammer, pipes expanding against framing, loose hangers | Annoying; fix at next service |
| Loud bang at burner ignition | Delayed ignition — gas building before it lights | Call now; stop using the boiler |
| Humming or vibration | Circulator pump bearings, loose pump | Service soon |
| Hissing, steam or water at the relief valve | Over-pressure or a failed relief valve | Call now |
| Ticking or clicking as it cools | Normal expansion and contraction | None |
Pressure gauge too high or too low
Too low (under 10 psi): the automatic fill valve is stuck or shut, or the system is losing water through a leak — a radiator valve, a fitting in a wall, or the boiler itself. Adding water by hand fixes it for a day; finding the leak fixes it. Too high (25+ psi cold, relief valve dripping): the expansion tank has lost its air cushion, or the fill valve is letting water in continuously. The expansion tank is a $150–$350 replacement and one of the most common boiler repairs in older Lynnfield and North Shore homes. A relief valve that has discharged more than once should be replaced, not capped.
Steam boilers: banging pipes and wet steam
Pre-war homes in Boston, Lynn, Salem and Peabody still run steam. The classic steam complaints are banging pipes (water trapped in a sagging pipe being slammed by steam — a pitch or main-vent problem), spitting radiator vents (water level too high or a bad vent), and a sight glass that swings wildly (dirty water, skimming needed). Steam boilers need the water level checked weekly in heating season and a low-water cutoff blown down; letting one run dry is how sections crack. If your steam boiler is banging, do not add water while it is hot.
Call-now symptoms
- Gas smell anywhere near the boiler: leave the house, call 911 from outside, then call us.
- Water on the floor from the boiler body, not just the relief valve.
- A bang on ignition or visible soot around the burner or flue.
- Repeated lockouts after one reset.
- No heat below 40°F outside — pipes start freezing within hours in an unheated house.
Precision Plumbing Solutions repairs, services and replaces gas, oil and steam boilers across Lynnfield, the North Shore and Greater Boston, 24/7 in heating season — see boiler installation and repair. If the diagnosis lands on a cracked heat exchanger or a 20-year-old unit, boiler replacement cost in Massachusetts lays out the numbers and the Mass Save rebates that soften them.
Questions people ask
What pressure should my boiler be at?
A residential hot-water boiler typically sits at 12–15 psi cold and rises to 18–25 psi hot. Below 10 psi the system may not circulate to the top floor; at 30 psi the relief valve opens. Steam boilers run at 0.5–2 psi.
Can I reset my boiler myself?
Once. If the reset button restores heat and it stays on, fine. If it locks out again the same day, stop resetting — repeated lockouts point to an ignition, flame-sensing or venting fault that should be diagnosed.
Why does my boiler sound like a kettle?
Kettling is water boiling on scale or sludge inside the heat exchanger, or a circulation problem letting water sit too long on the burner. It gets worse, not better, and eventually cracks the exchanger. It is a service call.
How much does a boiler repair cost?
Common repairs (circulator pump, zone valve, igniter, aquastat, expansion tank, relief valve) run roughly $250–$900 in our area. A heat exchanger or sections failing is a replacement — see our boiler replacement cost guide.
