It is a frustrating scenario to wake up on a cold morning, walk past rooms with functional radiators, and realize some are cold despite the boiler working and the hot water flowing perfectly through your taps. This specific issue, where radiators not working but hot water ok, points to a system imbalance rather than a complete breakdown of your heating infrastructure. Understanding the physics of your central heating loop is the first step in diagnosing why your rooms are failing to warm up.
Identifying the Core Problem: System Imbalance
The most common reason for radiators not working but hot water ok lies in the hydraulic balance of your system. Your boiler heats the water, and a pump circulates it. The radiators are designed to dissipate this heat, while the hot water cylinder stores heat for your taps. If the pressure, flow rates, or air content within the system are not optimized, the path of least resistance often favors the hot water cylinder, starving the radiators of the energy they need to function.
Checking the Simple Culprits
Before diving into complex system bleeding, you should verify basic operational settings. Ensure the thermostat is functioning correctly and is calling for heat. Check that the TRVs (Thermostatic Radiator Valves) on the affected radiators are turned up and not accidentally set to "Off." Additionally, if your system has lockshield valves, ensure they are not fully closed, as these manual valves regulate the flow of water through specific radiators.
The Role of Air and Blockages
Air trapped in the system is a prime suspect when radiators are not working but hot water ok. Air collects at the highest points, typically in upstairs radiators, creating a pocket that prevents water from filling the unit completely. This results in a cold top or middle section. Conversely, sludge or magnetite buildup—a mixture of rust and debris—can block the narrow channels within the radiator or the valves, effectively closing off the flow of water to that specific unit.
How to Bleed a Radiator
Bleeding is the process of releasing trapped air to restore proper water levels. You should only bleed the system when the boiler is off and the radiers are cold to prevent burns and pressure spikes. Place a cloth beneath the valve to catch drips, insert a radiator key into the bleed valve, and turn it anti-clockwise slightly. You will hear a hissing sound; once water drips out steadily, tighten the valve immediately. This often resolves isolated cold spots.
| Symptom | Likely Cause | Initial Action |
|---|---|---|
| Cold top of radiator | Air trapped | Bleed the radiator |
| Completely cold radiator | Closed valve or blockage | Check TRV and lockshield valves; bleed if pressure drops |
| Cold bottom of radiator | Sludge buildup | Power flush or manual cleaning |
Addressing System Pressure
Your heating system relies on pressure to function. If the pressure is too low, usually found below 1 bar on the pressure gauge, the pump struggles to push water through the radiators, creating a scenario where radiators not working but hot water ok persists because the cylinder is on a different, gravity-fed loop. Conversely, over-pressurization can cause the system to release water through the safety valve (PRV), which also disrupts the balance. Regularly checking and maintaining the pressure within the manufacturer’s recommended range is vital for consistent performance.
Advanced Diagnostics: Motorized Zones and Pumps
If you have a zoned heating system with motorized valves controlling the flow to different areas or rooms, a failure in the actuator or wiring can isolate specific radiators. These valves might click loudly but fail to open, stopping the water flow entirely to that section. Similarly, a failing circulator pump losing its prime or suffering from internal wear might generate enough noise but lack the pressure to distribute hot water effectively to the extremities of your heating circuit.
While identifying the cause of radiators not working but hot water ok can often be managed with bleeding and basic checks, some issues require specialized tools. If the problem persists after addressing air and simple settings, it is time to call in a heating engineer for a power flush or to inspect the pump and motorized valves.