Few things are more frustrating than a boiler that keeps dropping pressure. You top it up, the heating comes back, and a week later the gauge has slipped towards zero again. In this guide you will learn what normal pressure looks like, the usual reasons a boiler loses it, how to top up safely and when to call an engineer. Our Gas Safe registered engineers fix pressure faults every week in homes across Surrey, Sussex and Kent, so this is familiar ground.
“Water cannot simply vanish from a sealed system. If you are topping up every few weeks, it is escaping somewhere, and finding that leak early protects your boiler and your ceilings.”
1. Understand boiler pressure — what the gauge is telling you
Most modern homes have a sealed, pressurised heating system. That covers nearly all combi and system boilers. The water circulating through the boiler, pipes and radiators is kept at a set pressure so it can move efficiently and so the boiler can run safely.
You will find a pressure gauge on the front of the boiler or on the pipework underneath. With the heating off and the system cold, most manufacturers recommend around 1–1.5 bar, though some say 1–2 bar. When the heating runs, the water expands and the reading rises, often to around 2 bar. Your boiler manual gives the exact figures for your model.
If pressure falls too low, the boiler locks out and shows a low-pressure fault. That is the boiler protecting itself, not a sign it has failed.
2. Look for leaks — the most common cause
Water cannot vanish from a sealed system. If the pressure keeps falling, water is escaping somewhere, and even a tiny leak will steadily drain it. Start with a careful look around the house.
- Radiator valves. Check for drips, damp patches or green and white crusty deposits around the valves and tails.
- Pipe joints and towel rails. Look at visible joints, especially in bathrooms and airing cupboards.
- Ceilings. Stains below upstairs radiators can point to a leak under the floorboards.
- Under the boiler. Look for drips on the pipework or a damp patch on the floor or cupboard base.
Leaks under floors or behind kitchen units are hard to spot. An engineer can pressure test the system to confirm a leak and narrow down where it is.
If you see water inside or coming from the boiler casing, do not open it. Turn the boiler off and call a Gas Safe registered engineer. If you ever smell gas, leave the property, avoid using switches or flames and call the National Gas Emergency number on 0800 111 999.
3. Check the pressure relief valve — and the pipe outside
Every sealed system has a pressure relief valve, or PRV. If pressure climbs too high, usually around 3 bar, the valve opens and lets water out through a small copper pipe that runs outside, often near the boiler flue. That is a safety feature doing its job.
The problem is that once a PRV has opened a few times, it can fail to reseal properly and let water through slowly. The tell-tale sign is a drip from that outside pipe, or a damp mark on the wall beneath it. Replacing a PRV is a job for an engineer.
A PRV that has lifted is often a symptom rather than the root cause. Something pushed the pressure high enough to open it in the first place, such as overfilling, a filling loop left slightly open or a flat expansion vessel. A good engineer will fix the valve and find out why it opened, so the problem does not simply return a few weeks later.
4. Suspect the expansion vessel — if pressure swings hot to cold
Inside most combi and system boilers is an expansion vessel. It is a small tank split in two by a rubber diaphragm, with air on one side, and it absorbs the extra volume of water as the system heats up.
Over time that air charge leaks away. When it does, pressure shoots up when the heating runs, the PRV opens to release the excess, and pressure drops right down when the system cools. If your gauge swings sharply between hot and cold, a flat expansion vessel is a likely cause. Recharging or replacing it is an engineer's job.
“Take a photo of the gauge when the heating is cold, then again after it has run for half an hour. If the needle climbs close to 3 bar when hot and falls well below 1 bar when cold, mention it when you call. It points us straight towards the expansion vessel.”
5. Rule out the simple causes — bleeding and recent work
Not every pressure drop means a fault. Two everyday situations explain plenty of calls.
- After bleeding radiators. When you let trapped air out, a little water usually escapes too, and the space the air took up is now gone. A small drop afterwards is completely normal. Baxi's guide on how to bleed a radiator walks you through it.
- After work on the system. If a radiator has been taken off for decorating or new pipework has gone in, expect some pressure loss. The system may need topping up and bleeding a couple of times as air works its way out.
In both cases, one top-up should settle things. If the pressure keeps falling after that, look for one of the causes above.
6. Top up the pressure safely — a step-by-step guide
Repressurising is a simple homeowner task that does not involve any gas parts. Check your manual first, as filling loops vary. Some are a braided hose with a valve at each end, others a built-in lever or key under the boiler. Turn the heating off and let the system cool before you start, because topping up while hot gives a false reading.
- Find the filling loop. It is usually beneath the boiler, linking the cold mains pipe to the heating pipework. If it is a removable hose, check both ends are secure.
- Open the first valve. Turn it so the handle lines up with the pipe.
- Slowly open the second valve. You will hear water flowing in. Watch the gauge closely.
- Stop at the right pressure. When the needle reaches around 1–1.5 bar, or the figure in your manual, close both valves fully.
- Double-check the valves are shut. A loop left slightly open can overpressurise the system and make the PRV discharge.
- Reset the boiler. If it locked out, press reset as described in the manual, then turn the heating back on.
- Remove the hose if required. Some manufacturers and water regulations expect a removable loop to be disconnected after use. Keep a towel handy for drips.
Overfilled it? If the gauge reads close to 3 bar when cold, Baxi explains how to bring high boiler pressure down by releasing a little water from a radiator. Pressure that climbs on its own with the heating off can point to a filling loop valve letting water by.
7. Know when to call an engineer — the clear signs
An occasional top-up after bleeding radiators is nothing to worry about. Call an engineer if:
- You need to top up more than once every few weeks.
- You can see water dripping from the outside PRV pipe.
- The pressure swings sharply between hot and cold.
- There is water inside or underneath the boiler.
- The boiler keeps locking out even with the pressure correct.
An engineer can pressure test the system, recharge or replace the expansion vessel, swap a failing PRV and trace hidden leaks. Our boiler repair engineers handle these faults daily, and you can check any engineer's credentials with the Gas Safe Register engineer search. National Gas also lists its official gas emergency contact details if you ever need them.
8. Prevent pressure problems — servicing and simple habits
Regular servicing is the best way to stop small faults growing. During an annual boiler service, an engineer checks the system pressure, the expansion vessel charge and the condition of the PRV. Our guide to what happens during a boiler service explains each step.
It also helps to glance at the gauge every week or two, especially at the start of the heating season, so you spot a slow decline early. Remember that every top-up adds fresh water, which dilutes the inhibitor protecting your system from corrosion. Our article on power flushing and chemical cleaning explains why that matters.
If you have an older system with frequent leaks, our certified heating engineers can advise whether repairs or an upgrade make more sense. We cover Crawley, Horsham, Reigate, East Grinstead and the towns in between.
Frequently asked questions
Is it dangerous if my boiler pressure is low?
Low pressure is not dangerous in itself. The boiler simply shuts down to protect itself until you top it up. The bigger concern is why the pressure is dropping, because a hidden leak can damage floors, ceilings and electrics, and repeated top-ups dilute the inhibitor that protects your system.
Can I use my boiler while the pressure is low?
If the reading is only slightly below the normal range, the boiler may keep working, but performance can suffer. If it falls too far, the boiler will lock out. Top it up to the level in your manual, usually around 1 to 1.5 bar when cold, as soon as you can.
How often should I need to top up my boiler?
On a healthy system, rarely. Once or twice a year after bleeding radiators is normal. If you find yourself topping up more than once every few weeks, water is escaping somewhere, through a leak, a passing pressure relief valve or a flat expansion vessel, and an engineer should find the cause.
Why does my boiler pressure drop overnight?
Pressure naturally falls a little as the system cools, but a big overnight drop often points to a flat expansion vessel or a relief valve that only lets water out when the system is hot. Note the hot and cold readings on the gauge and share them with your engineer to speed up diagnosis.
Written by the NW Heating Services team
NW Heating Services installs, services and repairs boilers, heat pumps and underfloor heating for homes and landlords across Surrey, Sussex and Kent, with more than ten years of local experience. Our guides are written from the jobs our engineers carry out every week. More about our team.