It can be frustrating: your radiators seem to warm up when the heating is off, but when the heating finally switches on, they cool down instead. Many homeowners around Farnham, Bordon, Whitehill and the wider GU35, GU34 and GU9 areas report this confusing behaviour, especially during seasonal temperature changes. Fortunately, the causes are usually simple to diagnose, particularly if you understand how thermostatic radiator valves (TRVs) and circulation work.
Understanding why radiators behave this way
Most cases come down to two core issues. First, a stuck TRV may let hot water pass through even when the heating is off. Second, poor circulation can cause radiators to cool when the system starts, particularly if the boiler prioritises hot water or if the pump setup encourages reverse flow.
Before assuming the worst, it’s worth carrying out a few simple checks. Many homeowners in Farnham and nearby Liphook and Alton notice improvements just by checking valve positions or gently freeing up a stiff TRV pin.
Quick checks you can do now
- Check all radiator valves are fully open, especially the lockshield.
- Confirm your hot water is not set to constant.
- Feel the pipework: is heat creeping into the radiator even when the heating is off?
- Gently tap or loosen a stuck TRV head to release the pin.
- Check your boiler display for demand indicators.
Common causes: radiators warm when off
When you notice radiators warming up unexpectedly, the issue often lies with one of the following.
1. A stuck TRV allowing unwanted flow
TRVs control the flow of hot water into a radiator. Inside each TRV is a small spring-loaded pin. When the room reaches the set temperature, the pin pushes down to restrict flow. When the TRV gets stuck in the open position, hot water can circulate even if the heating is technically off.
This is especially common after summer, when pins can seize due to inactivity. Homeowners in Bordon and Whitehill often report this happening during the first cool week of autumn.
How to check a stuck TRV
- Unscrew the TRV head carefully.
- Look for the small pin sticking out of the base.
- Press it gently: it should spring up and down smoothly.
- If it is stuck, tap lightly around the valve body and try again.
Do not force the pin. If it refuses to move, a Gas Safe engineer can free it safely or advise replacement.
2. Reverse circulation issues
This happens when hot water returns from the cylinder or bypass circuit and drifts into the radiators. It can cause warmth even when the heating mode is off. When the heating then activates, pump direction or balancing issues can make those same radiators cool down instead of heating up.
Reverse circulation is more common in older systems around Farnham and Alton with gravity-fed hot water circuits or partial upgrades.
3. Hot water priority drawing heat away
On many traditional systems, when the hot water cylinder calls for heat, it takes priority over the radiators. If your radiators were warm earlier due to residual flow or a bypass circuit, they can cool down the moment the cylinder demands heat.
You may also hear the boiler firing even though the heating is off, which often confuses homeowners. This is normal when hot water is calling for heat—but if radiators get warm at the same time, something may be misconfigured.
4. Lockshield imbalance
The lockshield valve controls how much water flows through a radiator. If one radiator is getting more flow than intended, it may heat up when others don’t, particularly when the pump is only circulating small volumes of water. When the heating system starts properly, flow is redirected and that radiator cools.
Balanced lockshields ensure that every radiator gets the right share of heat. Poor balancing is a common cause of radiators behaving oddly in properties across GU35 and GU34.
Checklist: signs your system needs attention
- Radiators warm up during hot water cycles.
- TRV heads feel loose, stiff or unresponsive.
- Some radiators heat but others stay cold.
- Radiators cool down when the heating finally comes on.
- Pipes vibrate or feel unusually hot when they shouldn’t.
5. A bypass valve that’s set too high
Modern systems include an automatic bypass valve (ABV). If the ABV is set too high, it can allow hot water to circulate unnecessarily through certain radiators during low-demand periods. When the heating turns on and demand increases, the ABV closes slightly, causing those radiators to cool instead of heat.
In many homes around Farnham and Liphook, we see ABVs that have never been adjusted since installation, even though the system has changed over time.
6. Cylinder coil heating the system unintentionally
If the coil inside your hot water cylinder is poorly insulated or the valves around it are not closing correctly, the stored hot water can cause passive heat transfer into nearby pipework and radiators. This can make them feel warm even when the heating is switched off.
This issue tends to show up after hot water has been running for long periods, such as baths or long showers. When the heating starts later, circulation patterns shift and the radiators that were warm go cool again.
When it’s time to call an engineer
If you have checked your TRVs, confirmed valve settings and considered balancing or bypass issues but the problem continues, it’s wise to have the system assessed professionally. Persistent reverse circulation or TRV faults can lead to uneven heating, wasted energy and strain on the boiler.
Homes across Farnham, Bordon and GU9 often benefit from a system balance, TRV replacement or bypass valve adjustment—quick jobs that restore proper heating performance.
Next steps
If your radiators warm when the heating is off then cool when it comes on, we can diagnose the issue quickly and safely—get in touch at (01420) 558993.