Cold radiators are one of the most common calls the network takes. In most cases the fix is straightforward. The trick is working out which of the usual suspects is actually causing it.
Before you do anything else, pay attention to where the radiator is cold. This one detail tells you most of what you need to know.
Cold at the top, warm at the bottom
This is almost always air in the system. When a radiator fills with air, it rises to the top and pushes the hot water out. The top half sits stone-cold while the bottom stays hot.
The fix: bleed the radiator. You'll need a radiator key (a couple of quid from any hardware shop). Turn the heating off, let the system cool for ten minutes, hold a cloth under the bleed valve at the top corner, and turn it slowly anti-clockwise until water starts to come out. Close it back up. Job done.
Cold at the bottom, warm at the top
The opposite problem, and unfortunately a more serious one. Sludge and corrosion debris have settled in the bottom of the radiator, blocking hot water circulation. The water heats the top via radiation but can't get underneath the blockage.
The fix: flushing. A single radiator can sometimes be flushed on its own. If several radiators have the same symptom, the whole system needs a proper power flush.
One radiator cold, others fine
Usually a balancing problem or a stuck TRV. Open the valve at the bottom of the cold radiator fully. If it warms up, the system needs balancing so flow distributes evenly across all radiators.
If opening the valve does nothing, the TRV pin may be stuck closed. Lift the TRV head off and tap the pin gently with a spanner. If it springs back up, the TRV is working. If it stays stuck, it needs replacing, a 30-minute job for an engineer.
All radiators lukewarm
When the whole system is sluggish, the cause is usually the pump, a system-wide sludge problem, or the boiler itself not firing to full output. Check the boiler pressure first, it should sit between 1.0 and 1.5 bar when cold. If it's too low, top it up via the filling loop.
If pressure is fine but the system is still cold, you're into engineer territory.
Six common causes, ranked
- 01 Air trapped in the radiator, the most common cause, and the easiest to fix.
- 02 Sludge and corrosion debris blocking circulation at the bottom of the radiator.
- 03 System needs balancing, one radiator starves the others of flow.
- 04 A faulty thermostatic radiator valve (TRV) stuck closed.
- 05 A pump that's weak, airlocked or failing entirely.
- 06 A whole-system sludge build-up that can only be cleared by power flushing.
When to call someone
Bleeding a radiator and balancing the system are comfortable DIY jobs. Anything that involves draining down the system, replacing a TRV or dealing with sludge is usually faster and cleaner as an engineer visit. It's rarely worth a day of your own time to save ninety quid.
If you're in Belfast, Lisburn, Bangor or twenty miles around, request a quote or send an emergency request and an installer on the network will confirm whether it's worth a call-out.
If it's a full-system job
A proper power flush is usually the right fix.
When the problem is sludge across multiple radiators, nothing short of a power flush clears it. Installers flush from £450 including inhibitor and warranty certificate.
Read about power flushing