Pressure Keeps Dropping on Combi Boiler: Troubleshooting Guide

Read Time: 11 minutes

The short answer: a combi boiler that keeps losing pressure is either losing water somewhere in the sealed heating circuit, or it is pushing water out through the pressure relief valve because the expansion vessel is no longer cushioning the heat. The pattern of the drop tells you which one you are dealing with, and the table below will point you in the right direction in under a minute.

Combi boiler pressure dropping happens on every popular make, including Ideal Logic, Baxi, Vaillant and Worcester Bosch. If you have been topping the boiler up every few days, treat that as a warning sign and not a routine chore.

Based in London and not sure which problem you have? Our Gas Safe engineers for boiler repair in London can trace the fault and explain it before any work starts.

What is the normal pressure for a combi boiler?

With the heating off and the system cold, the gauge should sit between 1 and 1.5 bar, usually in the green zone. When the heating runs, the water expands and the needle will creep up to roughly 2 to 2.5 bar, which is normal. If it climbs towards 3 bar, the relief valve is designed to open and discharge water outside.

Below 1 bar the boiler is running short of water. Many models will lock out somewhere between 0.5 and 0.8 bar to protect the heat exchanger and pump. Your manual shows the exact figures for your model.

Which pattern matches your boiler?

What you notice	Most likely cause	Next step
Slow drop over days or weeks, no puddles anywhere	Small leak at a radiator valve, pipe joint or under a floor	Do the checks below, then call an engineer if nothing shows
Needle rushes up to 3 bar when the heating runs, then reads low by morning	Expansion vessel has lost its air charge or failed, and the relief valve is venting	Gas Safe engineer
Pressure falls overnight or while the heating is off	A leak that carries on when the system is cold, a weeping relief valve, or an internal boiler leak	Check the outside discharge pipe, then call an engineer
Dropped straight after you bled the radiators	Normal, you let water and air out	Repressurise once and watch it
Gauge reads zero but the boiler still heats	Faulty gauge or pressure sensor	Engineer to confirm

Why does a combi boiler lose pressure? Seven causes

1. A small leak in pipework or radiators

The sealed circuit holds only a modest amount of water, so a leak too slow to leave a puddle can still empty half a bar in a fortnight. Hot pipes and radiators dry the drips before you see them, so look for staining, rust, crusty white or brown deposits and damp patches on ceilings below bathrooms and kitchens. Radiator valve spindles, tails and old compression joints are the usual suspects. Corrosion and sludge make older systems more prone to pinhole leaks.

2. A relief valve that is weeping

The pressure relief valve protects the boiler by discharging water through a copper pipe that exits the wall, normally close to the boiler. If grit or limescale stops it seating properly, it drips slowly and the system drains without any sign inside the house. A damp wall below that outside pipe, or a steady drip, points straight at it.

3. A failed or flat expansion vessel

The expansion vessel is a small tank with a rubber diaphragm and an air cushion. It absorbs the extra volume as water heats. When the diaphragm splits or the air charge leaks away, pressure spikes, the relief valve opens, and the gauge reads low again once the system cools. Frequent top-ups and a needle that hits 3 bar are the classic signs.

4. Recently bled radiators

Bleeding releases air and a little water, so pressure drops by design. Bleed first, then repressurise to 1 to 1.5 bar. If you bleed the same radiator again and again, air is getting in from somewhere, and that is worth investigating.

5. A filling loop that is not fully closed

The filling loop is the braided hose with two small valves under the boiler. If a valve passes slightly, pressure can creep up or wander unpredictably. Check both are properly shut, and if you do not need the hose, disconnect it once you finish topping up.

6. A leak inside the boiler

Several internal parts can fail and let water escape: the automatic air vent, washers and ring seals, the diverter valve and, in the worst case, a cracked heat exchanger. Internal leaks can show up as pressure loss with no wet floor, and some only leak while the boiler is firing. These repairs need a Gas Safe engineer.

7. A faulty pressure gauge or sensor

Sometimes the water is fine and the reading is wrong. If the pressure reads low but radiators are hot and nothing drips anywhere, the gauge or sensor may be at fault.

Combi boiler losing pressure but no leaks: what is going on?

When you cannot find any wet patch, the cause is usually one of three things: the expansion vessel venting excess pressure through the relief valve, a very slow leak that evaporates, or a leak hidden under flooring or inside the boiler casing. Since the water rarely appears where you expect it, the quickest clue is the gauge itself. Write down the reading cold in the morning, hot after the heating has run, and cold again the next morning. A big jump when hot means the vessel. A steady fall with no jump means a leak.

Combi boiler losing pressure overnight or when the heating is off

A combi boiler that loses pressure overnight usually has something that leaks or vents whether or not the heating is running: a weeping relief valve, a slow pipework leak, or an internal fault such as a tired automatic air vent or diverter valve seal. Cold water also takes up slightly less space, so a flat expansion vessel can make the morning reading look worse than it really is.

Note the gauge at bedtime and first thing in the morning for a few nights. If the heating stays off all day and the needle still falls, rule out the filling loop and inspect the outside discharge pipe before you book an engineer.

Checks you can safely do yourself

1. Look at the discharge pipe outside. Find the copper pipe leaving the wall near the boiler. Water dripping from it, or a dark damp stain below it, means the relief valve is opening.
2. Check the filling loop. Both valves should be fully closed, with the hose connected at both ends or removed altogether.
3. Do the tissue test. Wipe dry the radiator valves, pipe joints and the area under the boiler, then press a dry tissue to each. Any dampness shows up on the paper even when you cannot see water.
4. Scan the house. Look under sinks, along skirting boards and on ceilings below radiators for stains or soft plaster.
5. Track the gauge. Note cold, hot and next-morning readings for two or three days.

Do not remove the boiler casing, and do not attempt to recharge or replace an expansion vessel yourself. That involves isolating and draining the boiler and working beside gas components.

How to repressurise a combi boiler

Always check your manual first, as layouts differ slightly between models.

1. Switch the heating off and let the boiler cool down fully.
2. Make sure the filling loop is connected at both ends.
3. Open both valves a little and listen for water flowing in. Keep your eyes on the gauge.
4. Stop at 1.2 to 1.5 bar by closing both valves tightly.
5. Restart the boiler. If the reading falls again within a few days, do not keep topping up, because something is wrong.

Repeated top-ups feed fresh oxygen into the system, which speeds up corrosion and sludge and can hide a leak that is quietly damaging a ceiling or floor. One or two top-ups a year is normal. Weekly top-ups are not.

When should you call a Gas Safe engineer?

Book a visit if any of these apply:

• You need to top up more than once every few months
• The gauge goes above 2.5 bar when cold or reaches 3 bar when hot
• Water drips from the outside discharge pipe
• You can see water inside or underneath the boiler
• The boiler locks out or shows a low pressure fault code

Codes differ between models, so check your manual. Common examples include F1 on Ideal Logic, E118 on many Baxi models and F22 on Vaillant boilers. Worcester Bosch models use model specific codes or a lockout light.

Low pressure on Ideal Logic, Baxi, Vaillant and Worcester Bosch combi boilers

The causes above apply to all four makes, but the recommended pressure and the lockout point differ by model, so always read the figures in your own manual. If you own a Vaillant, our guide to common Vaillant boiler problems and simple solutions covers other faults that can show up alongside pressure loss. If your pressure problems have turned into a replacement decision, our Worcester Greenstar 1000 review and price guide shows what to expect from a new combi.

If you ever smell gas, turn off the supply at the meter, open windows and call the National Gas Emergency Service on 0800 111 999 before anything else.

A proper diagnosis normally includes isolating the boiler from the heating circuit to see whether it holds pressure on its own, testing the expansion vessel charge, checking the relief valve and tracing the pipework. That is how an engineer separates a boiler fault from a leak in the system, which saves you from replacing a boiler that was never the problem.

Before anyone works on your boiler, ask to see their ID card and look them up on the Gas Safe Register.

What does it cost to fix?

Prices depend on access, the make of boiler and how long the leak takes to find. As a rough UK guide, recharging an expansion vessel often sits around £30 to £120, and replacing one is commonly £130 to £420. Replacing a relief valve is usually a smaller job than a vessel, while leak tracing is charged by time on site. Ask for a written quote that explains the fault before any work starts.

Need a hand finding the cause?

If your combi boiler pressure keeps dropping and the checks above have not found the problem, our Gas Safe registered engineers can trace the fault and give you a clear quote before any work begins. Call us on 07877 767776 for free advice, or book a visit through the website.

Looking for boiler repair in London? Visit Boiler Services London to see what we cover, or call 07877 767776 to speak to an engineer.

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