If your boiler keeps losing pressure or cuts out when the gauge drops below 1 bar, the system is telling you something is wrong. Our Gas Safe registered engineers across Cardiff diagnose the underlying cause of pressure loss rather than simply topping the system up so it drops again a week later. Boiler pressure naturally needs to sit between roughly 1 and 1.5 bar when cold. Persistent loss usually means water is escaping somewhere — through a small leak on a radiator valve, a pipe joint, or the boiler itself — or it is being expelled through the pressure relief valve because of a faulty PRV or a failed expansion vessel. Bleeding radiators incorrectly, or repeated bleeding, can also lower pressure. We carry out a systematic check of the whole system, inspecting radiators, visible pipework, the filling loop, the expansion vessel, and the PRV to find exactly where pressure is being lost. Once the cause is found, we repair it and re-pressurise the system correctly. Every visit begins with a fixed-price diagnostic from £85 with no call-out fee, across all common boiler brands in Cardiff homes.
We read the current pressure, inspect the gauge behaviour, and assess whether pressure is being lost gradually or expelled suddenly through the relief valve.
We check radiator valves, visible pipework, the filling loop, and boiler connections for the small leaks that most commonly cause gradual pressure loss.
We test the expansion vessel and pressure relief valve, the two parts most likely to cause repeated pressure loss when they fail, and confirm which is at fault.
We repair the cause, re-pressurise the system to the correct level, and confirm the gauge holds steady before completing the job.
Boiler pressure-loss diagnostics start from £85 fixed price with no call-out fee. Minor fixes — tightening a fitting, replacing a radiator valve, or re-pressurising after a small repair — are typically completed within the diagnostic visit. Replacing a faulty expansion vessel or pressure relief valve is a larger job that we quote at a fixed price before proceeding.
Most domestic boilers should sit between 1 and 1.5 bar when the system is cold, rising slightly when the heating is running. If yours repeatedly falls below 1 bar, there is usually a leak or a faulty component that needs investigating rather than just topping up.
The most common causes are a small leak somewhere in the system, a failed expansion vessel, or a faulty pressure relief valve expelling water. Repeated radiator bleeding can also reduce pressure. We trace the exact cause so the problem is fixed permanently.
Yes — most boilers have a filling loop you can use to top up pressure, following the manufacturer's instructions. However, if you are topping up frequently, that is a sign of an underlying fault. Repeatedly re-pressurising masks the real problem, so it is worth having it diagnosed.
Low pressure itself is rarely dangerous, but it often stops the boiler from working, leaving you without heating or hot water. If that happens in cold weather we treat it as a priority and aim to attend the same day across Cardiff.