This situation usually shows up gradually. The boiler runs for longer periods, energy use increases, and rooms may feel slightly warm near radiators but never properly comfortable. Because the system is still operating, it is easy to assume the issue lies with energy prices alone rather than how heat is being used.
A common reason is that the boiler is compensating for heat that is escaping faster than it can be retained. When heat loss is high, the boiler keeps firing to maintain temperature, but the warmth never settles. This creates longer run times and higher bills without improving comfort.
Circulation problems can also drive costs up quietly. If hot water is not moving efficiently through the system, the boiler has to work harder to achieve the same result. This is especially noticeable in colder weather, when demand increases and any inefficiency becomes more expensive.
Pressure and cycling behaviour matter here too. A boiler that fires repeatedly or runs continuously without delivering strong heat output will burn more fuel than necessary. Even without obvious faults, this pattern can significantly increase running costs over a heating season.
If it is unclear whether the problem is heat loss, distribution, or boiler behaviour, the quickest way to narrow it down is to step through the house-wide diagnostic tool. It helps identify where energy is being lost and which fixes are most likely to reduce running costs. You can start here: house cold diagnostic.
When higher bills are linked to heat escaping rather than boiler failure, this broader guide explains how different issues combine to drive costs up: How to Keep a UK Home Warm for Cheap.
If the boiler is running for long periods without delivering enough warmth, this behaviour is explored in more detail here: Boiler Runs Constantly but the House Never Warms.
Where pressure changes are part of the pattern, this guide explains how they affect efficiency and fuel use: Boiler Pressure Drops Overnight.