If your heating or hot water has suddenly stopped working, a drop in boiler pressure is a common cause, especially in modern sealed systems. Homeowners in Bearsden and nearby areas often face this in cold weather, and in many cases you can safely repressurise the boiler yourself if you follow the right steps.
Most sealed-system boilers are designed to run at around 1.0 to 1.5 bar when the system is cold. You can check this on the pressure gauge, usually found on the front of the boiler or just underneath it.
When the heating is on and the water is hot, it is normal for the pressure to rise slightly, often up to around 2.0 bar. It should not regularly sit near 3.0 bar, as that is where safety components like the pressure relief valve may activate.
Low pressure often shows up as one or more of these symptoms:
Pressure gauge sitting below 1.0 bar, sometimes near zero
Error codes on the boiler display relating to low pressure or “no flame”
Boiler frequently locking out or needing to be reset
Radiators not heating up, or hot water cutting out
If your gauge is low and the boiler is not working, repressurising via the filling loop is usually the next step, provided it is safe to do so.
Before you add water to the system, take a minute to check for any obvious problems. If you suspect anything more serious than a minor pressure drop, stop and contact an engineer.
Look around the boiler, pipework and radiators for visible leaks, dripping joints or staining on walls and ceilings. A constant drip suggests a system fault that needs professional attention, not just topping up.
Never attempt to repressurise if:
You can hear hissing or see water escaping from the boiler or pipework
The pressure rises very rapidly when you start filling
The gauge is already close to or above 2.0 bar when the system is cold
You smell gas or suspect any other safety issue
In any of these cases, turn the boiler off at the controls, isolate the gas at the main emergency control valve if you smell gas, and call a Gas Safe registered engineer.
The filling loop is the small arrangement of valves and a flexible or rigid pipe that connects your boiler to the mains cold water supply. Its job is to let you top up the system pressure when needed.
On many boilers, this is a silver braided flexible hose under the boiler, with one or two small black or blue levers. Others have an integrated filling key and valve built into the underside of the boiler casing.
If you are unsure where your filling loop is, check your boiler manual or look for labels such as “filling loop” or “system fill”. Do not start turning random valves, as you could isolate something important or over-pressurise the system.
The exact method depends slightly on the boiler brand, but the basic process is usually similar. Always refer to your manufacturer’s instructions if you have them.
Switch the boiler off at the control panel and, if it has been running, allow it to cool for at least 15 to 30 minutes. This gives you a more accurate cold pressure reading.
Check the gauge: if it is below 1.0 bar, topping up to around 1.2 to 1.5 bar is usually suitable for many homes, unless your manual says otherwise.
If you have a removable flexible filling loop, make sure it is securely connected at both ends and the fibre washers are in place to prevent leaks. Both valves at either end should initially be closed, usually with the levers at right angles to the pipe.
For an internal filling key style, insert the key and turn it to the “locked” or “fill” position as shown in the boiler instructions, but do not open the water valve fully yet.
Turn one valve a quarter turn so it is in line with the pipe, then gently open the second valve. You should hear water flowing into the system. Watch the pressure gauge constantly as you do this.
Allow the pressure to climb gradually. Do not walk away while the loop is open, as it is easy to overshoot the correct pressure.
Once the gauge reaches around 1.2 to 1.5 bar (cold), close both valves fully. Turn the levers back to their original closed position, at right angles to the pipe.
If the pressure is climbing quickly or shoots past 1.5 bar before you can react, close the valves immediately. Never deliberately take the pressure near 3.0 bar.
If your filling loop is removable, you can now disconnect it, catching any drips with a cloth. Store it safely nearby for next time. For integral loops, turn the key or control back to the normal position as per the instructions.
Turn the boiler back on, then reset it if required. After the system has run for a while, check the gauge again to make sure the pressure is stable.
Occasional topping up is not unusual, especially after bleeding radiators. However, if you find yourself repressurising every few days or weeks, there is likely an underlying fault.
Typical causes of ongoing pressure loss include:
Small leaks on radiator valves, pipework joints or towel rails, which can evaporate before you notice a wet patch. The boiler then slowly loses water over time.
A problem with the expansion vessel, the component that absorbs pressure changes as the water heats and cools. If its internal air charge fails, the system can over-pressurise when hot, push water out of the pressure relief valve, then sit low when cold.
Water discharging outside through the pressure relief valve pipe. This is usually a small copper pipe that terminates outside the property; staining or constant drips here suggest a fault that needs a Gas Safe engineer.
You do not need to take your boiler apart, but a few visual checks can be very helpful. Walk around your home and look closely at all accessible radiators and valves for green staining, rust marks or damp patches.
Check under the boiler and along any exposed pipe runs. If you see dried white deposits or rusty water marks, take a photo before it is cleaned off, as this can help your engineer locate the problem later.
Outside, find the copper pressure relief pipe and see if there are signs of recent discharge such as limescale streaks on the wall. Also check the condensate pipe outlet for any blockages or freezing issues in very cold weather.
Repressurising is only a safe DIY job when the system is otherwise healthy. You should stop and call in a professional if:
You have to top up the pressure more than a couple of times a year, or the pressure drops back down within days. This often points to a leak or expansion vessel fault.
The boiler keeps locking out or showing error codes even after the pressure is corrected. There may be sensor, pump or control issues that require proper testing.
Pressure shoots up towards 3.0 bar when the heating is on, or water is discharging from a pipe outside. The safety devices are doing their job, but the root cause must be fixed.
A Gas Safe engineer will typically check the expansion vessel, test the pressure relief valve, inspect for leaks, and verify that your boiler and controls are operating safely. They can also advise on whether repair or replacement is the most sensible option.
If you are uncomfortable repressurising your boiler, or the pressure keeps dropping after you top it up, it is time to get professional help. For more information on common faults, you may find our Common Boiler Issues guide useful.
When you are ready to get the problem sorted, you can book expert Boiler Repairs with our trusted local team. Robert Hill and Company (Electrical Engineers) Limited are available to diagnose and repair boiler pressure issues safely and efficiently.
To arrange a visit or discuss your boiler concerns, call Robert Hill and Company (Electrical Engineers) Limited on 01419562245 and speak to a friendly member of the team.
