Boiler Losing Pressure London — Causes, Signs & Fixes

Leak in the heating system

Even a pinhole leak in a radiator, pipe joint or underfloor circuit loses enough water to drop pressure over days. Our engineers carry dye and acoustic sensors to find micro-leaks without opening walls.

Faulty pressure relief valve (PRV)

A weeping PRV discharges water continuously. The valve may need cleaning or replacing — a 30-minute job with the right parts on the van.

Failed expansion vessel

The vessel absorbs pressure changes as water heats. If its diaphragm fails, pressure spikes then drops. Visible as rapid cycling on the gauge. We re-pressurise or replace on the same visit.

Air in the system

Trapped air in radiators or pipework causes pressure fluctuation. Bleeding radiators and re-pressurising usually resolves this.

Filling loop left open or faulty

A filling loop that doesn't seal fully allows backflow and pressure loss. We check and replace if needed.

How often should I have to top up my boiler pressure?

Ideally never — a correctly sealed system with a healthy expansion vessel holds pressure indefinitely. If you top up more than once every few months, call an engineer.

Can I top up the pressure myself?

Yes — most boilers have a filling loop under the unit. Slowly open both valves until the gauge reaches 1.2 bar, then close. But this is a temporary fix. If pressure drops again within days, there's a fault that needs diagnosing.

Will a boiler run with low pressure?

Most modern boilers lock out below 0.5–0.8 bar and display a fault code. The boiler won't fire until pressure is restored to prevent pump and heat exchanger damage.

Is low boiler pressure dangerous?

Low pressure itself is not dangerous — it simply stops the boiler working. However, the underlying cause (a leak, faulty PRV) can cause water damage if left unaddressed.