One safety rule before anything else: if you ever smell gas, leave the house and call your gas utility from outside — don't troubleshoot anything. And never open the boiler's pressure-relief valve to "test" it; they often fail to reseat and then drip forever. With that said, here's the symptom-by-symptom rundown.
No heat at all
Likely causes: the boring stuff first — a thermostat that isn't calling (dead batteries, wrong mode, schedule mishap), a tripped breaker or a service switch someone bumped, or a gas supply issue. Beyond that: a boiler locked out on a fault (low pressure, ignition failure, safety trip), a failed circulator, or a frozen condensate line on a condensing boiler in cold weather.
What you can safely check: thermostat batteries and settings, the breaker panel, the emergency switch (often a light-switch-style toggle near the boiler or at the top of basement stairs), the pressure gauge (about 12–15 psi cold is normal), and the boiler's display for an error code — write the code down before resetting. One reset is fine; repeated resets to limp along are not, because the boiler is locking out for a reason.
Concern level: call soon — same day in freezing weather, since a heatless house risks frozen pipes. What a pro will do: read the fault history, test ignition, safeties, sensors, and the circulator, and fix the actual cause rather than clearing the code.
One room or zone is cold
Likely causes: if the rest of the house is fine, the problem is local — a thermostat not calling for that zone, a stuck or failed zone valve or zone circulator, air trapped in that loop, or a radiator valve someone closed and forgot.
What you can safely check: the zone's thermostat (batteries, setting, mode), whether radiator valves are open, and — with the zone calling for heat — whether the pipes serving it warm up. A radiator that's warm at the bottom but cold at the top just has air in it; bleeding it with a radiator key is a normal homeowner task. Check the system pressure afterward and top up if it dropped.
Concern level: relax — this is an inconvenience, not a hazard. If bleeding and thermostat checks don't fix it, a pro will test the zone valve or circulator and purge the loop properly.
Boiler losing pressure
Likely causes: a small leak somewhere, a failed expansion tank, a weeping relief valve, or normal pressure loss after bleeding radiators. Topping up occasionally is normal; topping up weekly means water is escaping and fresh fill water is quietly corroding the system from the inside.
Concern level: keep an eye on it if it's occasional; call soon if it's frequent. This one has enough nuance that we gave it its own page — see Why is my boiler losing pressure? for what's normal, how to top up safely, and when repeated refilling means a real problem.
Banging or gurgling pipes
Likely causes: gurgling or trickling sounds are almost always air in the system. Sharp banging when zones open or close is often water hammer or pipes expanding against framing. A deep rumbling or "kettle about to boil" sound from the boiler itself is kettling — mineral scale or sludge on the heat exchanger causing water to flash-boil in spots.
What you can safely check: for gurgling, bleed the radiators and verify system pressure. Note when the banging happens (startup, shutdown, a specific zone) — that detail helps a technician enormously.
Concern level: gurgling — relax, bleed and move on. Occasional expansion ticking — relax. Kettling — call soon: it stresses the heat exchanger and shortens boiler life, and the fix (a chemical cleaning and addressing water quality) is far cheaper than a new boiler.
Boiler short-cycling
Likely causes: the boiler fires, runs a few minutes, shuts off, and repeats. Usually this means the boiler is oversized for the current heat demand, flow through it is restricted (clogged strainer, failing circulator, too-small zone calling alone), or controls are set with too narrow a temperature differential.
What you can safely check: honestly, not much beyond noticing it and confirming there's no error code. This is a diagnosis-and-adjustment job.
Concern level: keep an eye on it. Short-cycling isn't dangerous, but it wastes fuel and wears out ignition components and controls early. A pro will check flow rates, adjust control differentials or firing rate, and sometimes add buffering or re-zone the system so the boiler can run in longer, efficient cycles.
Radiant floor heats unevenly
Likely causes: warm here, cold there usually means an unbalanced manifold (some loops getting more flow than others), air trapped in the cold loops, a partly closed loop valve, or — remember before assuming a problem — the normal slow response of a high-mass slab, which can take hours to catch up after a setback. Rugs and furniture also insulate the floor above them.
What you can safely check: find the manifold and see whether all loop valves are open and, if it has flow indicators, whether every loop shows flow. Give the slab time — judge evenness after the system has run steadily for a day, not an hour after a thermostat change.
Concern level: relax, then call soon if it persists — chronic cold loops usually need a proper purge and manifold balancing, which is routine work for a hydronic tech but fiddly to do well without the right pump and gauges.
Snowmelt system not melting
Likely causes: a sensor or control issue (the slab or moisture sensor not triggering), the system not given enough lead time — snowmelt slabs take hours to come up to temperature — an undersized or overwhelmed heat source during a heavy storm, low glycol level or pressure in the snowmelt loop, or a failed circulator or heat exchanger between the boiler and the glycol loop.
What you can safely check: whether the control is actually calling (many have an indicator or a manual-run mode), whether the boiler fires when it does, and whether the snowmelt loop's pressure gauge shows normal pressure. During an extreme storm, understand that many systems are designed to keep up with typical snowfall rates, not record ones.
Concern level: keep an eye on it if it's slow in one storm; call soon if it's not running at all, since a dead snowmelt system in winter is exactly when you need it. A pro will test sensors and controls, verify glycol concentration and pressure, and check the dedicated circulator and heat exchanger.
When to shut it off and call now
Most hydronic problems are comfort problems, not emergencies. But turn the boiler off at its switch and call immediately if you see steady water discharging from the relief valve pipe, water pooling around the boiler, scorch marks or a burning smell at the boiler, or — for any gas smell — skip the switch, leave the house, and call the gas utility first. None of those are wait-and-see situations.
If you're in Utah and one of these symptoms sounds like your house, see our guide to finding a Utah hydronic installer, where we recommend Phillips Hydronics for readers along the Wasatch Front and beyond.