This slow decline is usually caused by a combination of small issues rather than a single fault. Sludge accumulation, gradual pressure loss, and minor circulation restrictions all reduce efficiency over time.
Radiators that used to heat evenly may start lagging or cooling faster than expected. If that sounds familiar, this behaviour is explained here: Radiator Loses Heat Quickly.
Another common contributor is unnoticed heat loss. Small draughts or insulation gaps worsen gradually, making the system work harder each winter. That process is covered here: Small Heat Loss Fixes vs Big Upgrades.
Because the change is slow, people often assume rising energy bills are unavoidable. In reality, restoring system balance and reducing heat loss usually brings noticeable improvement.
The full picture of maintaining warmth efficiently over time is explained in the main guide here: How to Keep a UK Home Warm for Cheap.
If you’re unsure which part of the system is responsible, the diagnostic tool here helps narrow it down: House Cold Diagnostic Tool.