Cold at the top, warm at the bottom: air
Air rises and collects at the top of the radiator, so hot water cannot fill it. You may also hear gurgling. Bleeding releases the air: with the heating off and the radiator cool, put a cloth under the bleed valve at the top corner, open it a quarter turn with a radiator key until air hisses out and water appears, then close it.
Afterwards, check the boiler pressure gauge. Bleeding lets a little water out, and if the pressure has fallen below the level marked in your boiler’s manual, it needs topping up. If radiators need bleeding again and again, air is getting in somewhere, and that needs a technician.
Cold at the bottom, warm at the top: sludge
Over the years, rust and debris from inside the system settle as dark sludge at the bottom of radiators, blocking the flow. Bleeding will not help. The system needs cleaning: a flush, sometimes with a cleaning chemical, then a filter and an inhibitor so it does not come back.
Sludge is a sign the water inside the system has been neglected. Left alone, it reaches the pump and the boiler’s heat exchanger, so it is worth dealing with early.
Cold all over
First the simple things: is the heating actually on and calling for heat? Are both valves on the radiator open? A thermostatic radiator head that has sat at one setting all summer can stick; turning it fully up and down a few times often frees it.
If other radiators are hot and this one stays cold, the system may be out of balance: the radiators nearest the boiler take most of the hot water. Balancing means adjusting the valves so each radiator gets its share, and it is best done by a technician with the system running.
Several radiators cold at once
When a whole floor or most of the house stays cold, the problem is usually at the heart of the system: low pressure, a pump that is not running, a zone valve that is stuck, or a boiler fault. Check the boiler’s display and pressure, and if nothing obvious shows, call a technician rather than opening the boiler.
Quick diagnosis
| Likely cause | What to do | |
|---|---|---|
| Cold at the top | Trapped air | Bleed it, then check pressure |
| Cold at the bottom | Sludge | System clean, filter, inhibitor |
| Cold all over | Valve, stuck head, balance | Check valves; then balancing |
| Many cold | Pressure, pump, boiler | Check display; call a technician |
When to call us
Bleed a radiator yourself if you are comfortable doing it. Call a technician for sludge, repeated air, balancing, low pressure that keeps coming back, or anything inside the boiler.
Conversion EG has designed, installed and maintained heating in Egypt since 2010, with one team from design and supply through installation and maintenance. We also service systems installed by others.
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