16 Dec Why Are My Radiators Cold? Simple Troubleshooting Guide
When a cold day hits London and your radiators stay stubbornly cool, it is more than just uncomfortable. It can be frustrating, particularly when energy bills are rising and you rely on your heating system to keep your home warm. The good news is that many common cold radiator problems have clear causes, and some have straightforward checks you can carry out before calling a professional.
At PEBS, we work on central heating systems, boilers, modern radiators, underfloor heating and full refurbishments and maintenance across London. We regularly see the same heating issues arise in both period properties and modern homes. This guide explains why your radiators might be cold, which problems you may be able to resolve yourself, and when it is time to involve a qualified heating engineer.
First check: is it the whole heating system or just one radiator?
The first step is to establish whether the issue affects the entire heating system or just one or two radiators. If all radiators are cold, or only slightly warm, the problem is more likely to relate to the boiler, system pressure, circulation or controls. If most radiators are heating normally but one remains cold, or upstairs radiators struggle to warm up, the cause is usually more local, such as trapped air, balancing issues or valve problems.
Understanding whether the issue is system-wide or limited to individual radiators helps you focus on the right checks and avoid unnecessary disruption.
Check boiler pressure and heating controls
If all radiators are cold, start with the boiler and heating controls. On a combi boiler or sealed central heating system, check the pressure gauge on the boiler. Most systems operate best at around 1 to 1.5 bar when cold. If the pressure is too low, the boiler may shut down or refuse to fire, leaving the radiators cold throughout the property.
Low pressure is commonly caused by bleeding radiators without topping up the system, minor leaks or previous maintenance work. Most boilers have a filling loop underneath, allowing you to safely top up the system with mains water. This should be done carefully, as over-pressurising the system can also cause faults. If pressure drops frequently or you are unsure how to top up safely, a Gas Safe registered engineer should investigate.
You should also confirm that the programmer or smart thermostat is calling for heat, that the room thermostat is set high enough, and that the boiler is actually firing. If you have hot water but no heating, this may indicate a diverter valve or control fault that requires professional repair.
One radiator cold: check the radiator valves
If most radiators are heating properly but one remains completely cold, the radiator valves are the next place to look. Each radiator has two valves: one is usually a thermostatic radiator valve (TRV) or manual valve, and the other is a lockshield valve.
Start by making sure the thermostatic or manual valve is fully open. TRVs can stick closed after long periods of inactivity, particularly after summer. Gently adjusting the valve head can sometimes free it, but if the internal pin remains stuck, the valve may need replacing.
The lockshield valve controls water flow through the radiator and is normally set during system balancing. If it has been accidentally closed, the radiator will stay cold. Carefully opening it a small amount can restore heat. If both valves are open and the radiator remains cold while others are hot, there may be a blockage or internal valve fault, and a heating engineer should take a closer look.
Cold at the top: trapped air and bleeding radiators
A radiator that is cold at the top but warm at the bottom usually contains trapped air. This air prevents hot water from circulating properly, leading to uneven heating and often gurgling noises.
Bleeding a radiator is a simple task in many homes. With the heating turned off and the radiator cool, use a radiator key to slowly open the bleed valve at the top of the radiator. Allow the trapped air to escape until water flows steadily, then close the valve. Always check the boiler pressure afterwards, as bleeding air can lower system pressure.
If air builds up frequently or returns soon after bleeding, this can indicate a deeper issue such as circulation problems or air ingress, which should be assessed professionally.
Cold at the bottom: sludge buildup inside the radiator
If a radiator is hot at the top but cold at the bottom, the most likely cause is sludge buildup. Over time, corrosion debris and magnetite settle inside radiators, particularly in older systems, restricting water flow and reducing heat output.
In these cases, bleeding will not solve the problem. Individual radiator flushing or a full system power flush is often required. Power flushing removes sludge and debris from the entire heating system, helping radiators heat evenly and improving overall efficiency. Left untreated, sludge can increase running costs and place additional strain on the boiler.
Some radiators hot, others lukewarm: system balancing
When radiators closest to the boiler heat up quickly while others remain lukewarm, the system may be unbalanced. Hot water naturally takes the easiest route through the pipework, leaving distant radiators under-supplied.
Balancing a heating system involves adjusting the lockshield valves so each radiator receives the correct flow of hot water. While it is possible for experienced homeowners to attempt balancing, it can be time-consuming and requires patience and careful temperature checks. Many people choose to have balancing carried out by a heating engineer as part of routine maintenance or after system upgrades.
Thermostatic radiator valves and room temperature control
Thermostatic radiator valves allow individual room temperature control, but they can also cause confusion if set incorrectly. A radiator will remain cold if the TRV is set very low or has failed in the closed position.
If adjusting the valve has no effect, the internal pin may be stuck. While gentle tapping can sometimes free it, leaking or seized valves should be replaced by a professional. Replacing a TRV involves draining part of the system and working with pressurised pipework, which should not be attempted without experience.
When the issue is not the radiator itself
Sometimes, cold radiators are a symptom of a wider system issue. A failing circulation pump may struggle to move water effectively, particularly to upper floors. Boiler faults, blocked filters or sensor issues can also limit heat distribution.
Regular boiler servicing helps identify these problems early. Servicing ensures correct pressure, clean components and safe operation, reducing the risk of breakdowns during colder months. Any work involving gas appliances or pipework must be carried out by a Gas Safe registered engineer.
Simple checks versus calling a professional
Many radiator issues have simple starting points. Checking boiler pressure, confirming controls are set correctly, ensuring valves are open and bleeding radiators where necessary are reasonable steps for most homeowners.
However, if radiators remain cold across the property, pressure drops repeatedly, cold spots return quickly, or you hear unusual noises from the boiler or pump, professional diagnosis is the safest option. Continuing to adjust valves or pressure without understanding the root cause can worsen the problem.
Final thoughts
Cold radiators are rarely random. Patterns such as cold tops, cold bottoms or uneven heating across floors usually point to specific underlying issues. Understanding those signs helps you decide whether a simple fix is enough or whether your heating system needs expert attention.
At PEBS, we specialise in diagnosing, upgrading and maintaining heating systems as part of wider refurbishment and improvement projects across London. If your radiators are not heating properly, your system feels unbalanced, or you are planning a renovation that includes new radiators or underfloor heating, our team can help you achieve reliable, efficient heating that works for your home.