Serving Aberdeen and surrounding areas
You check the pressure gauge on your boiler and the needle is sitting below 1 bar. Again. You topped it up last week, maybe the week before that. Now you’re wondering whether something is actually wrong or if this is just what boilers do.
It’s one of the most common calls we get at Kelgas, especially from homeowners across Aberdeen and Aberdeenshire. The good news: most causes of dropping boiler pressure are straightforward to diagnose and fix. The not-so-good news: if you’re topping up every few days, something in your heating system needs attention.
Here’s what causes boiler pressure to keep dropping, how to repressurise your boiler safely, and when it’s time to call a Gas Safe registered engineer.
Your boiler operates within a sealed heating system. Water circulates through the boiler, into your radiators, and back again. For this cycle to work properly, the water needs to be at the right pressure.
Most combi boilers are designed to run between 1.0 and 1.5 bar. You can check this on the pressure gauge, usually located on the front panel of your boiler. Some newer models display pressure digitally.
Below 0.5 bar: Your boiler will likely lock out and stop working altogether. Most modern boilers have a low-pressure safety cut-off to prevent damage.
Between 0.5 and 1.0 bar: Your heating may still work, but it won’t be efficient. Radiators might not heat evenly, and your boiler is working harder than it needs to.
Between 1.0 and 1.5 bar (when cold): This is the sweet spot. Your system is running as designed.
Above 2.5 bar: Too high. The pressure relief valve will release water to protect the system. If you see water dripping from the overflow pipe outside, this is likely why.
A one-off pressure drop isn’t unusual. But if your boiler pressure keeps dropping after you top it up, your system is losing water somewhere. The question is where.
This is the most common cause by far. The leak might be tiny, just a weeping joint under a floorboard or a pinhole in an old radiator. You might not see any visible water, especially if the leak is slow enough to evaporate or drip into a space you can’t easily check.
What to look for: Damp patches near radiators, staining on ceilings below upstairs radiators, wet spots around pipe joints, or a slightly damp area under your boiler. Check radiator valves too. A small drip from a valve can lose enough water over days to drop your pressure noticeably.
The pressure relief valve (PRV) is a safety device. When pressure gets too high, the PRV opens and releases water through the overflow pipe (the copper pipe that exits through an outside wall, usually near the boiler).
A faulty PRV can release water even when the pressure is normal. You’ll notice water dripping from the overflow pipe outside your property. The valve might need replacing, or there could be debris stopping it from sealing properly.
Inside your boiler, there’s an expansion vessel. It’s a small metal tank divided into two halves by a rubber diaphragm. One side holds water, the other holds air (or nitrogen). As the water in your heating system heats up and expands, the air in the expansion vessel compresses to absorb the extra volume.
Over time, the air charge in the vessel can decrease. When that happens, there’s nowhere for the expanding water to go, so the pressure rises when the heating is on and the PRV releases water. The result? Pressure drops once the system cools.
Recharging the expansion vessel is a routine job during a boiler service. Our engineers at Kelgas check the vessel charge as part of every service we carry out.
Recently bled your radiators? A small pressure drop afterwards is completely normal. Bleeding releases trapped air, which reduces the total volume in the system. Just repressurise the boiler using the filling loop (instructions below) and you should be fine.
If the pressure stays stable after you top it up, this was the cause. No need to call anyone.
Some boilers have automatic air vents that release small amounts of air from the system over time. This is normal, but if the vent is leaking water as well as air, it can cause gradual pressure loss. Similarly, if your system has a filling valve that isn’t sealing properly, water can slowly escape.
These are internal boiler components that a Gas Safe registered engineer needs to inspect. They’re not something you should try to fix yourself.
Boiler pressure dropped and you need to top it up? You can usually do this yourself using the filling loop. It takes about two minutes.
Before you start: Check your boiler’s manual for the exact method. The steps below are general guidance for most combi boilers, but the filling loop location and operation can vary by manufacturer.
First, switch your boiler off and let the system cool down. Repressurising a hot system gives an inaccurate reading because hot water has higher pressure.
Find the filling loop. On most combi boilers, this is a braided metal hose with a valve at each end, located underneath the boiler. Some modern boilers have a built-in filling key instead.
Open the valve(s) slowly. You’ll hear water flowing into the system. Watch the pressure gauge as it rises.
Close the valve(s) when the gauge reaches 1.0 to 1.5 bar. Don’t go above 1.5 bar on a cold system.
Turn your boiler back on and check the pressure again once the system has heated up. It will rise slightly, which is normal.
Important: If you need to repressurise your boiler more than once every few months, something is wrong. A healthy, sealed heating system should hold its pressure with only occasional top-ups. Topping up every week, or even every few weeks, means water is leaving the system and you need an engineer to find out where.
Heating systems across Aberdeen and Aberdeenshire face conditions that make pressure issues more likely than in many other parts of the UK.
Older pipework in granite properties. Many homes in Aberdeen, particularly the granite tenements and older builds in areas like Rosemount, Torry, and the city centre, have heating systems that have been retrofitted into buildings originally designed without central heating. Pipework runs through thick stone walls and under floors, making leaks difficult to spot. A small leak behind a granite wall can go undetected for months, slowly draining your system pressure.
Harsh winter temperatures. Aberdeen regularly sees sub-zero temperatures from November through March. When temperatures drop sharply, heating systems cycle harder and more frequently. This puts additional stress on joints, valves, and the expansion vessel. Freezing conditions can also affect condensate pipes (the plastic waste pipe that runs from your boiler to an outside drain). If the condensate pipe freezes, the boiler locks out entirely, which is a separate problem but one we deal with regularly during Aberdeen winters.
System age. Homes in Westhill, Cove Bay, Banchory, and Stonehaven that were built in the 1980s and 1990s may still have original heating pipework. After 25 to 30 years, corrosion and sludge build-up inside the system can cause pinhole leaks in radiators and pipes. If your system is that old, it’s worth considering whether a new boiler installation would be more cost-effective than repeated repairs. A power flush can clear out the sludge, but if the pipework itself is corroded, sections may need replacing.
Hard water deposits. While Aberdeen’s water supply is relatively soft compared to the south of England, mineral deposits can still build up in heating systems over many years. This affects valves and seals, making them more prone to small leaks.
Some boiler pressure problems are a quick fix at home. Others need a qualified professional. Here’s when to pick up the phone.
Call an engineer if:
At Kelgas, our engineers are Gas Safe registered (registration number 547324) and trained to diagnose pressure faults across all major boiler manufacturers, including Ideal, Worcester Bosch, ATAG, and Baxi. A diagnostic visit starts with a full visual inspection of the boiler and system, pressure testing, and a check of the expansion vessel, PRV, and all accessible pipework and connections.
If you’re due for an annual service, a pressure investigation can often be carried out at the same time. Our standard boiler service is £69.30 +VAT, with a loyalty rate of £60 +VAT for returning customers. No call-out fees. No per-hour charges.
In a healthy sealed system, you shouldn’t need to repressurise more than once or twice a year. If you’re topping up monthly or more frequently, water is escaping from somewhere in the system and it needs investigating.
Low pressure alone isn’t dangerous. Your boiler has a safety cut-off that stops it firing if the pressure drops too low. However, the underlying cause of the pressure drop, such as a leak, can cause water damage to your property if left unchecked. And a boiler that won’t fire means no heating or hot water, which is a real problem during an Aberdeen winter.
Yes. During a standard service, our engineers check the expansion vessel charge, inspect the pressure relief valve, test system pressure, and look for visible leaks. Many pressure problems are caught and fixed during routine servicing before they become a bigger issue. Read our full guide to what a boiler service includes for the complete breakdown.
If the pressure drops only when the heating is off overnight, the expansion vessel is the most likely cause. As the water cools and contracts, a weak expansion vessel can’t maintain the correct pressure balance. Your engineer can recharge or replace the vessel. If the pressure drops regardless of whether the heating is on or off, a leak is more likely.
It depends on the cause. Recharging an expansion vessel or replacing a pressure relief valve is a relatively quick, affordable repair. Tracking down a hidden leak in pipework under floors or behind walls takes longer and may involve some investigation. At Kelgas, we’ll always give you a clear explanation of the problem and the cost before we carry out any work. No surprises.
A one-off pressure drop is usually nothing to worry about. Repressurise the boiler, keep an eye on the gauge, and move on. But if you’re reaching for that filling loop every week, your system is telling you something.
The five most common causes are leaks, a faulty pressure relief valve, a flat expansion vessel, recently bled radiators, or a failing internal component. Some are a quick fix at home. Others need a qualified engineer to diagnose and repair properly.
If your boiler pressure keeps dropping and you’re in Aberdeen or Aberdeenshire, give us a call on 01224 961610 or text 07897 012244. We’ll find the cause, explain your options, and give you a fixed price before any work starts.
Book a boiler service today and get your system checked before winter.
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