Many homeowners find that their heating works perfectly until someone runs a hot tap or the shower, at which point the radiators cool down or stop heating entirely. If you live in Liphook or nearby areas such as Bordon, Whitehill or Alton, this guide will help you understand what’s happening and what you can safely check before calling for support.
Understanding Why Heating and Hot Water Compete
Most UK homes with system or regular boilers rely on diverter valves, pumps and timers to decide whether heat goes to the radiators or the hot water cylinder. When something within this sequence isn’t working correctly, the radiators may cool down while the boiler prioritises hot water without switching back properly.
Quick checks you can do now
Before assuming there is a major fault, a few simple checks can help you narrow down the issue.
- Check the thermostat is set high enough for heating to run.
- Ensure the programmer or controller is calling for heating as well as hot water.
- Confirm the boiler pressure (if you have a sealed system) is within the green zone.
- Listen for the pump – does it run when heating is selected?
- Check for any error lights on the boiler or control panel.
Common causes of cold radiators after hot water demand
The phrase cold radiators is one homeowners often use when diagnosing problems on older systems. Here are the most common technical reasons behind it.
Faulty or sticking diverter valve
In many homes, especially those around Liphook, Farnham and GU35 areas with older plumbing layouts, the diverter valve is a frequent culprit. It directs hot water either to the cylinder or to the heating circuit. If it sticks in the hot water position, the radiators won’t receive heat even after the taps stop running.
- Radiators stay cold but hot water works normally.
- Heating may start only after the boiler cools down.
- You might hear clicking or humming from the valve body.
Heating pump not circulating properly
A worn or seized pump may allow enough flow for hot water but not enough for the radiators. This is common in older properties in GU34 and GU9 where original pumps are still installed.
Typical signs include:
- Pump feels excessively hot or unusually quiet.
- Radiators heat only at the top or very slowly.
- Boiler overheats and shuts down intermittently.
Airlocks in the heating system
Air trapped in the system can prevent circulation and cause radiators to cool quickly when the boiler switches modes. Bleeding radiators may help, but persistent air returning could mean a system leak or failing vent.
Cylinder priority settings causing delays
Some systems are configured to heat the hot water cylinder first. While normal, the switchover should be smooth. If the cylinder stat or motorised valve is faulty, the radiators may not resume heating until the cylinder is satisfied, which can take far longer than it should.
Checklist: When to call an engineer
- You smell gas or suspect a leak.
- The boiler displays fault codes repeatedly.
- The pump or diverter valve is making loud mechanical noises.
- Pressure drops daily or won’t stay in the green zone.
- Hot water temperature fluctuates alongside radiator issues.
How professionals diagnose heating and hot water interaction faults
A Gas Safe engineer will test the boiler’s response to both heating and hot water demand, check pump performance, inspect valve travel, measure flow temperatures and confirm correct wiring at the controls. In towns like Liphook and Bordon where many homes have legacy systems, small upgrades or a single replacement part often restore stable operation.
Preventing future problems
Regular servicing helps keep pumps lubricated, valves moving freely and control components calibrated. Powerflushing older systems in Whitehill, Alton and Farnham can greatly improve circulation and reduce the likelihood of airlocks or pump strain.
Next steps: If the issue persists after basic checks, book a professional inspection via https://www.embassygas.com/book