Boiler On But Radiators Cold? 5 Checks Before You Call
A boiler is a different animal from a furnace. It heats water and pushes it through pipes to radiators or baseboards, so "no heat" can be a boiler problem or a water-and-air problem in the pipes. These five checks sort out which.
Safety first: if you smell gas anywhere in the house, leave and call your gas company from outside. Don't touch a switch.
1. Thermostat first, always
Mode on HEAT, setpoint a few degrees above the room. Blank or dim screen? Fresh batteries. Boilers respond slowly — give it 15–20 minutes after a change before deciding nothing's happening.
2. Is the boiler actually firing?
Stand near it. A boiler that's working makes a soft whoosh or hum when it fires, and the pipes leaving it get warm within a few minutes. If the boiler is silent and cold, treat it like a furnace that won't turn on: check the light-switch-looking power switch on or near it, and the breaker.
If the boiler fires but the radiators stay cold, the problem is downstream — keep going.
3. The pressure gauge
Most boilers have a round gauge on the front. When the system is cold it should typically read somewhere around 12–15 psi; a bit higher when hot. If it's sitting at or near zero, the system has lost water and can't circulate heat — and many boilers won't fire at low pressure on purpose.
Adding water is a simple step on some boilers and not on others (there's often a small fill valve, but overfilling causes its own problems). If you're not sure how yours works, this is a quick technician call, and it's worth knowing why it lost pressure.
4. Air in the radiators
Hot water can't get past a pocket of air. If the top of a radiator is cold while the bottom is warm, it has air in it. Each radiator has a small bleed valve at the top corner; a radiator key or flat screwdriver opens it. Hold a cup under it, open it slightly until water (not air) comes out, then close it.
Baseboard systems can have bleeders at the ends or none at all — if you don't see an obvious valve, don't go looking inside the covers.
5. Zone valves and one cold zone
If some radiators are hot and a whole area of the house is cold, that area is probably its own zone with its own thermostat and a zone valve. Check that zone's thermostat first. A zone valve that's stuck closed is a common part failure; that's a technician fix.
When it's a pro
Boiler silent with power and gas confirmed, pressure at zero that keeps returning to zero after filling, a circulator pump that hums but doesn't move water, or a zone that stays cold after its thermostat checks out — call.
Before you do, run the free troubleshooter. It has a boiler path that captures what's hot, what's cold, and the gauge reading, and gives you a summary to read to the dispatcher.
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