Noticed your boiler pressure dropping overnight? While it might seem like a minor issue, persistent pressure loss can signal underlying faults that reduce efficiency, comfort, and safety. Here's a practical guide to help you understand the causes, how to identify the source, and what steps to take next.
Why Does Boiler Pressure Drop Overnight? Causes, Fixes & Prevention
Common Causes of Overnight Boiler Pressure Drops
Leaks in the Heating System
One of the most frequent causes of overnight pressure drops is a leak. As your heating system cools at night, metal components contract, making existing leaks more noticeable.
Signs to look out for:
- Dripping sounds at night
- Damp patches around pipes, joints, or radiators
- Water stains on floors or ceilings near pipework
Expansion Vessel Problems
The expansion vessel manages water expansion when your system heats up. A faulty one can’t balance pressure effectively, leading to fluctuations or sudden drops overnight.
Warning signs:
- A bouncing pressure gauge
- Pressure rises quickly when the boiler fires up, then falls again
- Repeated pressure loss after repressurising
Faulty Pressure Relief Valve
This safety component is designed to release excess pressure. If it’s faulty, it might leak even when pressure is within the safe range.
Check for:
- Drips from the pressure relief pipe
- Rust or corrosion around the valve housing
Air Locks
Trapped air in your system can disrupt water flow and reduce pressure. You may hear gurgling or notice radiators staying cold at the top.
Quick fix: Bleed your radiators, starting from the lowest level in the house.
Incorrect Filling Loop Use
Leaving the filling loop open after topping up your system can cause a gradual loss of pressure. Conversely, overfilling can trip the safety valve.
Tip: Always close and, if possible, remove the filling loop after use.
How to Identify the Source of the Problem
Step-by-step diagnosis:
- Turn off specific parts of the heating system to isolate the fault (e.g. cylinder or heating zones).
- Attach a plastic bag to the pressure relief pipe to check for leaks overnight.
- Visually inspect accessible joints, pipes, and radiators for moisture or damage.
When to Call a Heating Engineer
Some issues, such as internal leaks or faulty components, need professional tools and experience. A qualified engineer can:
- Use thermal imaging to detect hidden leaks
- Perform pressure testing
- Safely inspect and replace boiler components
If your boiler continues losing pressure despite basic checks, it’s time for a professional diagnosis.
Solutions to Fix Pressure Drops
Repairing Leaks
Minor leaks in pipe joints or radiator valves can often be fixed with repair tape or joint tightening. For radiator issues, draining and resealing may be needed. Boiler leaks should only be handled by Gas Safe engineers.
Replacing Faulty Components
If your expansion vessel or pressure relief valve is to blame, professional replacement is essential. These parts regulate safety and system performance, and incorrect installation can lead to further damage or void warranties.
Bleeding Radiators and Dealing with Air Locks
Turn off your system and allow it to cool. Use a radiator key to bleed each radiator starting from the lowest point, releasing trapped air until water flows steadily.
For systems prone to air locks, consider installing automatic air vents to reduce the need for manual bleeding.
Preventing Future Pressure Drops
Regular Maintenance
Schedule annual servicing with a Gas Safe registered engineer to catch small issues before they escalate. Routine checks extend the life of your system and keep it running efficiently.
Monthly visual inspections can help spot early signs of:
- Corrosion
- Moisture around joints
- Unusual noises
Monitor Your Boiler’s Pressure
Know your boiler’s ideal pressure range (usually 1 to 1.5 bar when cold) and check it weekly. A sudden drop is a signal that something isn’t right, address it before it leads to more serious issues.
Experiencing boiler pressure problems in Blackheath, Charlton, or Greenwich?
McGrath Heating Services provides expert diagnostics, repairs, and system servicing to keep your boiler running smoothly. Call us on 07812 739696 or fill out our contact form to book a visit today.
