Boiler pressure is a bit like Goldilocks — it needs to be just right. Not too high, not too low. So, when your pressure gauge is playing yoyo — shooting up when the heating’s on, then crashing down to zero when it’s off — it’s your boiler’s way of saying: “Something’s not right here, mate.”
Let’s break down what’s going on, what might be causing it, and whether you need a boiler service or a repair visit.
What’s Normal Boiler Pressure?
Most boilers like to sit between 1.0 and 1.5 bar when cold, and around 2.0 to 2.5 bar when hot. If it’s going above 3 baror dropping to zero, that’s a red flag and needs attention.
What Causes the Pressure to Go Up and Down?
Here are the most common reasons this happens.
1. Expansion Vessel Issue
The most likely culprit is a faulty expansion vessel. This part of your boiler absorbs the pressure as water heats and expands. If it’s lost its air charge or the internal diaphragm has failed, it can’t regulate the pressure properly.
What happens:
- Pressure rises too high when the heating’s on
- Then drops right back to zero when the system cools
- You might also get water discharging from the overflow pipe outside
This is one of the most common faults we get called out to. It’s fixable, but needs a professional to test and recharge or replace the expansion vessel.In short, it’s smart, solid, and built to last.
2. Faulty Pressure Relief Valve (PRV)
When the pressure gets too high, the PRV is meant to release water to keep things safe. But if it’s:
- Let off too many times,
- Not reseated correctly, or
- Just worn out
…it might continue to leak even when the pressure is normal. This causes pressure to drop, especially once the heating is off.
You can often spot this one by checking the pipe that sticks out the wall behind your boiler — if it’s dripping or wet, the PRV might be leaking.
3. Filling Loop Left Open
If your filling loop (usually a silver braided hose under the boiler) has been left slightly open after topping up the pressure, water will continue to enter the system.
This raises the pressure too high, the boiler dumps the excess water through the PRV, and you end up right back where you started — pressure at zero.
Always make sure both ends of the filling loop are fully closed after topping up.
4. Leak or Blockage in the System
In rarer cases, a hidden leak somewhere in the pipework or a blockage in the system can cause pressure fluctuations. If your pressure is constantly dropping and you’re having to top it up every few days, this might be the case
Should You Book a Service or a Repair?
That depends on what exactly is going wrong. Here’s a quick guide:
| Symptom | Best Option |
|---|---|
| Pressure rises above 3 bar when heating is on | Repair visit (likely expansion vessel issue) |
| Pressure drops to 0 when boiler is off | Repair visit (check PRV and expansion vessel) |
| Constant topping up required | Repair and possible system inspection |
| No heating or hot water | Emergency repair visit |
| Boiler still running fine, but pressure acting odd | Book a service and mention the issue – we’ll check it thoroughly |
Why You Shouldn’t Ignore It
Topping up the pressure every few days might seem like an easy fix, but it’s only masking a bigger issue. Left untreated, it can lead to internal leaks, water damage, or even boiler failure.
Pressure that goes too high puts strain on valves and joints, and pressure that drops too low can shut the boiler off completely — often when you need it most.
Book a Repair or Service with DTS Plumbing & Heating
We handle these kinds of pressure problems all the time — it’s usually a straightforward fix when caught early. If you’re based in South Yorkshire or nearby, our engineers can come out, diagnose the problem properly, and get your boiler back on track.
You can book online here:
www.dtsplumbing.co.uk/book-an-appointment
Or call the office on:
0114 248 3011
Cover Future Problems with Our Cosy Care Plan
If you don’t fancy dealing with pressure problems, leaks, or breakdowns in the middle of winter, our Cosy Care Plans offer full boiler cover with annual servicing, priority callouts, and parts and labour included (depending on the plan you choose).
Even better — if you take out a Cosy Care Plan before your visit, you’ll get 10% off this repair or labour covered plan dependant .
Take a look at the plans here:
www.dtsplumbing.co.uk/boiler-cover-plans
Final Thought
A pressure gauge that won’t sit still is your boiler’s way of saying it needs a bit of TLC. Whether it’s a tired expansion vessel or a leaky valve, our team can fix it before it turns into something more serious.
Just pop the kettle on and we’ll handle the rest.