Boiler Losing Pressure? Causes, Quick Checks and When to Call a Derby Engineer
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Get a free quoteShort answer: A boiler losing pressure almost always means water is escaping from the sealed heating system. The most common causes are a leak in the pipework or radiators, a faulty pressure relief valve, or an expansion vessel that has lost its charge. Topping up the pressure is a safe short-term fix. If it keeps dropping, you need a Gas Safe engineer to find the cause.
A boiler losing pressure is one of the most common reasons Derby homeowners call us between October and February. The heating stops working, the boiler shows a fault code, and the needle on the pressure gauge has dropped into the red.
It is frustrating, but it is rarely dangerous, and the cause can usually be found quickly. This guide explains what normal boiler pressure looks like, why it drops, what you can safely check yourself and when it is time to book an engineer.
What should my boiler pressure be?
Most modern combi and system boilers run on a sealed system, which needs a certain amount of water pressure to work. You will find a pressure gauge on the front or underside of the boiler, either a dial or a digital display.
| Gauge reading | What it means | What to do |
|---|---|---|
| Below 1 bar (heating off) | Pressure is low | Top up using the filling loop, then watch it |
| 1 to 1.5 bar (heating off) | Normal for most boilers | Nothing |
| Up to around 2 bar (heating on) | Normal rise as water heats and expands | Nothing |
| Close to 3 bar | Pressure is too high | Call an engineer |
These are general guides. Always check your boiler manual, because some makes recommend slightly different figures. If you are not sure, a Derby engineer can confirm the right setting at your next service.
Why is my boiler losing pressure?
A sealed heating system should hold its pressure. If the gauge keeps falling, water is getting out somewhere, or a part that controls pressure is not doing its job. These are the causes we find most often in Derby homes.
1. A leak in the heating system
The most common cause is a small leak. It might be a weeping radiator valve, a joint under the floor or a pinhole in an old pipe. Small leaks often evaporate before you see a puddle, so look for staining, damp patches, rust marks or green deposits on copper pipes.
2. A faulty pressure relief valve
The pressure relief valve (PRV) is a safety part that lets water out if the pressure gets too high. If it is worn or has debris in it, it can let water out when it should not. Check the small copper pipe that runs from the boiler through an outside wall. If it is dripping, or the ground underneath is wet, the PRV may be the problem.
3. An expansion vessel that has lost its charge
The expansion vessel gives heated water room to expand. If it loses its air charge, the pressure shoots up when the heating comes on, the PRV releases water, and then the pressure falls when the system cools. A gauge that swings high when hot and low when cold is a classic sign.
4. You have recently bled the radiators
Bleeding radiators releases air, and some pressure goes with it. If the drop happened straight after bleeding, simply top up the pressure and it should hold.
5. A leak inside the boiler
Sometimes the leak is inside the boiler itself, from a seal, pump or heat exchanger. This must be checked by a Gas Safe registered engineer. Never take the casing off a gas boiler yourself.
Safe checks you can do at home
Before you book an engineer in Derby, these checks will often tell you what is going on. None of them involve opening the boiler.
- Read the gauge with the heating off and cold. This gives you a fair starting figure.
- Look for leaks. Check under radiators, around valves, under the boiler and anywhere pipes are visible. Look for damp patches on ceilings below pipe runs.
- Check the PRV pipe outside. Find the copper pipe coming out of the wall near the boiler. Is it dripping or wet at the end?
- Top up the pressure. Most combi boilers have a filling loop underneath. Follow the instructions in your boiler manual to bring the gauge back to around 1 to 1.5 bar, then close the valves fully.
- Write down the reading and watch it. Check again over the next day or two. How fast it drops tells an engineer a lot.
Safety note: Do not remove the boiler casing, adjust the expansion vessel or try to replace the PRV yourself. If you smell gas, turn off the supply, open the windows and call the National Gas Emergency Service on 0800 111 999. If water is leaking near electrics, turn off the power at the fuse box.
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View our price listHow fast is it dropping? What that tells you
The speed of the drop is one of the most useful clues. This is how our Derby engineers read it.
| Pattern | Likely cause | Next step |
|---|---|---|
| Dropped once, after bleeding radiators | Normal pressure loss from bleeding | Top up and monitor |
| Drops slowly over a few weeks | Small leak or weeping valve | Book a fault-finding visit |
| Drops within a day or two | Larger leak, PRV or expansion vessel | Book an engineer soon |
| Rises sharply when hot, falls when cold | Expansion vessel has lost its charge | Engineer to recharge or replace |
| Drops and the PRV pipe outside is dripping | PRV releasing water | Engineer to check PRV and expansion vessel |
Repair or replace? When pressure loss points to a bigger problem
Most pressure problems are a repair, not a reason for a new boiler. A leaking valve, a worn PRV or a flat expansion vessel can usually be fixed in a single visit.
It is a different picture if your boiler is older and the faults are stacking up. Our rule of thumb for Derby customers is simple. If a repair costs more than a third of the price of a new boiler, especially on a unit that is 10 to 15 years old or more, replacement usually makes more sense.
A modern condensing boiler is up to 40% more efficient than older models, and new boilers come with up to 12 years' manufacturer's warranty. Our guide to the signs you need a new boiler, not another repair covers how to weigh it up, and our boiler replacement cost guide explains what affects the price.
Not sure what your home needs?
Talk it through with a Gas Safe engineer. Call 01332 204533, Monday to Friday, 8am to 5pm, or send us a message.
Speak to our teamHow to stop your boiler losing pressure again
You cannot rule out every fault, but a few habits make pressure problems far less likely in a Derby home:
- Book an annual service. A yearly boiler service includes checks that catch worn parts and early leaks before they cause a breakdown.
- Check the gauge once a month. Especially in autumn and winter, when the heating works hardest.
- Deal with small leaks early. A weeping valve is a small job. Water damage under a floor is not.
- Keep the system clean. Sludge and corrosion inside the system can damage valves and seals over time. A power flush clears it out, and we include one with every new boiler installation.
When to call a boiler engineer in Derby
Call a Gas Safe registered engineer if:
- The pressure keeps dropping after you have topped it up.
- The gauge goes above 2.5 to 3 bar when the heating is on.
- The PRV pipe outside is dripping.
- You can see water leaking from the boiler itself.
- The boiler keeps locking out with a low-pressure fault code.
We have repaired and serviced boilers across Derby since 1997, and we work on gas, oil and LPG systems. We cover the city plus Amber Valley and Erewash, South Derbyshire, Uttoxeter and Ashbourne and Burton on Trent.
Boiler losing pressure: FAQs
Why does my boiler keep losing pressure?
Water is escaping from the sealed heating system. The most common causes are a leak in the pipework or radiators, a faulty pressure relief valve, or an expansion vessel that has lost its charge.
Is it dangerous if my boiler loses pressure?
Low pressure is not usually dangerous. Most boilers will simply lock out and stop heating to protect themselves. A leak should still be fixed quickly, because water can damage floors, ceilings and electrics.
What pressure should my boiler be at?
For most combi and system boilers, around 1 to 1.5 bar when the heating is off, rising to around 2 bar when it is running. Check your boiler manual for the exact figures.
Can I top up my boiler pressure myself?
Yes, on most combi boilers. Use the filling loop and follow the instructions in your boiler manual. If the pressure keeps dropping after you top it up, call an engineer to find the cause.
Why does my boiler pressure go up when the heating is on and drop when it is off?
A big swing between hot and cold usually points to the expansion vessel losing its charge. It needs a Gas Safe engineer to recharge or replace it.
Do you fix boilers losing pressure in Derby?
Yes. Etta Plumbing & Heating Services is a Gas Safe registered (166252) and OFTEC registered (C100278) heating engineer based in Derby, trading since 1997. We find and fix pressure faults on gas, oil and LPG boilers across Derby and Derbyshire.
Find the cause, not just the symptom
Topping up the pressure gets your heating back on, but it does not fix why the pressure dropped. If the gauge keeps falling, water is going somewhere, and finding it early saves bigger repairs later.
Do the simple checks, note how fast it drops and give a local Derby engineer a call if it will not hold. Most of the time it is a straightforward repair, and if it is not, we will give you honest advice on what makes sense.
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