If your heating isn't working properly and you've noticed the pressure gauge on your boiler showing a low reading, don't panic. Low boiler pressure is one of the most common issues we see, and it's often something you can fix yourself.
What is Boiler Pressure?
Your boiler system is a sealed circuit filled with water. The pressure reading shows how much force is in this system. Most boilers work best at a pressure between 1.0 and 1.5 bar.
- Below 1 bar = Too low, may cause problems
- 1.0 to 1.5 bar = Normal operating range
- Above 2.5 bar = Too high
Symptoms of Low Pressure
You might notice:
- Heating not working or radiators staying cold
- Hot water not reaching desired temperature
- Pressure gauge showing below 1 bar
- Some boilers display a low pressure error code
Common Causes
1. Natural Pressure Loss
It's normal for pressure to drop slightly over time. If it's gradual (over weeks or months), it's usually nothing to worry about.
2. Bleeding Radiators
Every time you bleed a radiator, you release some water from the system, reducing pressure.
3. Leaks
A leak anywhere in the system will cause pressure to drop. Check:
- Under radiators
- Around pipe joints
- At the boiler itself
- Around the pressure relief valve
4. Faulty Expansion Vessel
The expansion vessel absorbs pressure changes when water heats up. If it fails, pressure can fluctuate or drop.
How to Repressurise Your Boiler
Most combi boilers have a filling loop that lets you add water to increase pressure. Here's how:
Step 1: Locate the Filling Loop
Look under your boiler for a flexible hose with valves at each end. It connects two pipes together.
Step 2: Turn Off the Boiler
Switch off your boiler and let it cool for safety.
Step 3: Open the Valves
Slowly open one valve, then the other. You'll hear water flowing into the system.
Step 4: Watch the Gauge
Keep watching the pressure gauge. Stop when it reaches 1.0 to 1.5 bar.
Step 5: Close the Valves
Close both valves tightly. Check for any drips.
Step 6: Restart the Boiler
Turn your boiler back on. The pressure might rise slightly when heating is on - this is normal.
If Pressure Keeps Dropping
If you're having to repressurise frequently (more than twice a year), there's likely an underlying problem:
- Check for visible leaks around radiators, pipes, and the boiler
- Look for wet patches on ceilings or walls
- Listen for dripping sounds when the system is quiet
A persistent leak needs professional attention. Our Gas Safe registered engineers can:
- Locate hidden leaks
- Repair or replace faulty valves
- Check and recharge the expansion vessel
- Service your boiler to prevent future issues
Safety Warning
While topping up pressure is safe for homeowners, never:
- Open the boiler casing
- Attempt to repair internal components
- Ignore persistent pressure problems
- Adjust the pressure relief valve
If in doubt, call a professional.
When to Call an Engineer
Contact us if:
- Pressure drops again within days of topping up
- You can't locate the filling loop
- There's a visible leak
- The boiler displays error codes
- You're uncomfortable doing it yourself
Book a Boiler Repair
Our Gas Safe registered engineers can diagnose and fix boiler pressure problems quickly.
Call 0330 822 7695 for a quote, or contact us to arrange a visit.