Ideal Boiler Pressure: What It Is, Why It Drops, and When to Worry
You’ve just glanced at your boiler’s pressure gauge, and now you’re worried. Is 2.5 bars normal? Should it be higher? Lower? Do you need to call someone?
We get these questions a lot at KD Jones Heating Engineers, especially when customers are facing a chilly home or an error message on their display.
In this blog, we’ll explain the ideal boiler pressure, what causes it to rise or fall, and how to respond safely if things don’t look quite right. If you’re searching for answers to questions like "why does my boiler keep losing pressure" or "how to reduce the pressure on a boiler", you’re in the right place.
What is the ideal boiler pressure?
For most combi boilers and modern systems, the ideal boiler pressure when the system is cold is between 1 and 1.5 bars. Once your heating is on and the system warms up, the pressure may rise slightly, typically no higher than 2 bars.
If your gauge reads below 1, your boiler may not fire up. If it’s above 2.5 bars, you could be heading into the “boiler pressure too high” danger zone.
Always check your boiler manual for model-specific guidance, but if you're unsure, a good (if obvious) rule of thumb is:
- Green = good
- Red = needs attention
Why does boiler pressure go up?
Pressure increases naturally as water heats and expands, which is normal. But if it rises too much, it could mean:
- Too much water was added when repressurising
- A fault with the expansion vessel
- Blocked pipes or radiators
- A stuck pressure relief valve
Not sure what to do? We’re always on hand to help, and if your system needs upgrading, 0% boiler finance is available to make the cost more manageable.
Why does my boiler keep losing pressure?
The most common reasons your boiler loses pressure are:
- A leak in the system, often small and hidden under floors or behind radiators
- A faulty pressure relief valve
- Air trapped in the system
- Bleeding a radiator without topping up the system afterwards
If your boiler keeps losing pressure regularly, it’s time to get it checked. Constantly refilling the system can cause long-term damage.
How to increase boiler pressure
If your boiler pressure is low, here’s what to do:
- Turn off the boiler and let it cool.
- Locate the fill loop (usually a silver braided hose beneath the boiler).
- Open the valves to slowly let cold water into the system.
- Watch the pressure gauge until it reaches 1.5 bars.
- Close the valves and turn the boiler back on.
Be careful not to overfill it. If you’re unsure, call a Gas Safe registered engineer like us, and we’ll talk you through it or come out to help.
How to lower boiler pressure
If the pressure is too high, you can usually:
- Bleed your radiators slightly to release excess water
- Use the drain valve to remove a small amount of water
- Check the fill loop is fully closed (many people forget this)
Not sure how to lower boiler pressure without bleeding radiators? You can release water from a drain-off valve near the boiler or lowest radiator, but only if you’re confident in doing so. If in doubt, don’t touch it; that’s what we’re here for.
Is it dangerous?
A boiler with slightly high or low pressure isn’t usually dangerous, but it will stop working safely and efficiently. Prolonged pressure issues can damage internal components, shorten your boiler’s lifespan, and increase running costs.
That’s why regular servicing matters. If your system is showing its age, a full upgrade could be the smartest move. And thanks to our 0% boiler finance offer, you can spread the cost of a brand-new, energy-efficient boiler without paying any interest.
Book your boiler check today
Whether your boiler pressure is too high, you’re constantly losing pressure, or you just want peace of mind before winter, KD Jones Heating Engineers is here to help.
📞 Call us on 01737 211376
🌐 Visit our contact page to request a quote online
📍 Based in Redhill, we cover homes across Surrey and the surrounding areas
And remember — if your boiler’s on its last legs, ask us about our 0% finance deals for new installations.
FAQs
1. What should be the pressure in a boiler?
Ideally between 1 and 1.5 bars when cold, and under 2 bars when running.
2. How do I increase boiler pressure?
Use the fill loop to add cold water slowly until you reach the right level.
3. How do I reduce the pressure on a boiler?
Bleed your radiators or drain a small amount of water. Make sure the fill loop is closed.
4. Why does my boiler keep losing pressure?
Likely causes include a leak in the system, trapped air, or a faulty valve.
5. Can I fix pressure issues myself?
Some you can, but if you’re unsure, it’s best to call a Gas Safe registered engineer.
6. Is boiler replacement covered by finance?
Yes! We offer 0% boiler finance to make your upgrade more affordable.