A radiator not heating up in an otherwise warm house is annoying, but it's usually a small problem with a clear cause. The trick is narrowing it down before you start turning wrenches: one cold radiator points one way, every radiator cold points another, and a radiator that's cold only at the top points a third way.
| What you see | Most likely cause | Where to start |
|---|---|---|
| Cold at the top, warm at the bottom | Trapped air | Bleed the radiator |
| One radiator fully cold, others fine | Closed or stuck valve | Check both valves |
| One radiator lukewarm, others hot | Balancing | Adjust the lockshield valve |
| Every radiator cold | Boiler or system-wide issue | Check boiler pressure and settings, then call a pro |
| Radiator cold after system work | Air introduced during the work | Bleed, then check pressure |
Feel the radiator before you touch anything: top, bottom, and the pipes on both ends. That thirty-second check tells you which section below applies, and it saves bleeding a radiator whose real problem is a valve.
Air rises, so it collects at the top of a radiator and blocks hot water from getting there. Cold top, warm bottom is the signature. The fix is bleeding: open the bleed valve until water runs in a steady stream, then close it. The radiator bleeding guide covers the steps, including the pressure check afterward — bleeding drops system pressure a little, and a system that runs too low won't heat well either.
Bleed one radiator at a time, starting with the cold one. If you have to bleed the same radiator again and again, that's a symptom, not a routine — air keeps getting in somewhere, and a pro should find out where.
Every radiator has two valves: the control valve (often a thermostatic TRV with numbers on it) and the lockshield valve on the other end, usually under a cap. Both need to be open for water to flow.
Turn the TRV to its highest setting and check the lockshield cap is not screwed fully shut. TRVs stick after a summer off — the little pin under the TRV head can seize in the closed position. Pop the head off, and if the pin doesn't spring back when you press it, work it gently with a cloth until it moves. Don't force it with pliers: a snapped pin turns a five-minute job into a valve replacement.
If a valve itself leaks or won't move at all, that's pro territory. Draining a system to swap a valve is not a job to improvise.
Find the pressure gauge on the boiler. Most residential systems want about 12–15 psi when cold. Below that, water may not reach the far radiators, and the top floor suffers first. If pressure reads low, top it up through the filling loop following your boiler's manual, then watch the gauge over the next few days.
Pressure that keeps dropping after top-ups points to a leak or an expansion issue somewhere in the system. That's not a homeowner fix — call a heating pro, and mention the pressure trend when you call, since it narrows their search.
If one radiator runs lukewarm while the rest run hot, the system may be out of balance: the near radiators hog the hot water and the far ones starve. The fix is the lockshield valve — the one under the cap. Open it a quarter turn on the cold radiator and give the system an hour to settle before judging. Small moves, long waits: balancing is tuning, not a race.
Uneven heat across rooms has other causes too, and why one room runs hotter than others covers the wider picture, from sun exposure to duct and airflow issues in forced-air homes.
If every radiator is cold, the problem is usually upstream: the boiler isn't running, isn't reaching temperature, or has locked out on an error. Check the obvious — thermostat setting, boiler power, and any error code on the display — and write the code down before you reset anything. Then call a pro.
Don't open the boiler case or touch anything inside it. Boiler internals, gas connections, and combustion parts are pro-only work, and guessing there can make a no-heat problem into a safety problem. The boiler vs furnace comparison explains how boiler systems differ from forced-air ones, which helps when the tech talks through options.
Most cold radiators come back with air bled, a valve opened, or pressure topped up — in that order. Work the table from the top, one check at a time, and give each fix time to show. If every radiator is cold, or the same radiator keeps failing, the problem is bigger than a homeowner check, and a heating pro is the next step.
Trapped air. Air rises to the top of the radiator and blocks hot water from getting there. Bleed the radiator until water runs in a steady stream, then check boiler pressure.
Usually a closed or stuck valve, trapped air, or a balancing issue. Check both valves are open, bleed the radiator, and if it runs lukewarm rather than cold, try the lockshield adjustment.
Most residential systems want about 12–15 psi when cold. Below that, far radiators may not heat. Top up through the filling loop per your boiler's manual, and call a pro if pressure keeps dropping.
No — bleed the cold ones. Bleeding radiators that already heat fine just drops system pressure for no reason. After bleeding, always check the boiler gauge.
When every radiator is cold, when pressure keeps dropping, when a valve leaks or won't move, or when the boiler shows an error code. Anything inside the boiler case is pro-only work.