Boiler Pressure Dropping? Causes, Safe Checks and When to Call

The short answer

Falling boiler pressure usually means water is going somewhere it should not. Check the gauge when the system is cold, and read your boiler manual. If the pressure keeps dropping, a Gas Safe registered engineer needs to find the cause, because gas parts sit inside.

What the gauge tells you

Most sealed boilers have a dial or a screen that shows pressure. Look at it when the heating has been off for a while. Many dials mark a safe zone, while the manual gives the right range for your model.

We do not quote a single figure here, because boilers differ. Always follow the maker’s manual. If the needle sits below the marked zone, the system may be low on water.

Why pressure drops

A sealed heating system is a closed loop. It should not lose water, so a steady fall is a warning sign. Since the causes differ, these are the usual ones.

• A small leak. Look at radiator valves, pipe joints and the area below the boiler.
• A weak expansion vessel. This part helps the system cope as water heats and cools.
• A leaking safety valve. It can release water through a discharge pipe outside.
• Recent radiator bleeding. Bleeding lets out a little water, so pressure dips afterwards.

If the discharge pipe ends at an outside gully, check that the gully is clear. Our guide to why drains block explains the signs, because a drip there is easy to miss.

Safe checks you can make

Start with the simple things, since none of these involve opening the boiler.

1. Look for drips, damp patches and stains near pipes and radiators.
2. Check the pressure gauge when the system is cold.
3. Read the manual for the right range and for how to top up.
4. Note how fast the pressure falls, and write it down.

Many sealed systems have a filling loop for topping up. If yours does, follow the manual step by step. Never open the boiler casing, because gas parts sit behind it.

When to stop and call

Do not keep topping up, because water added to a leaking system only moves the problem. Call an engineer in any of these cases.

• The pressure falls again within days.
• You see water leaking from the boiler or pipes.
• The boiler shows an error code. Our guide to boiler fault codes explains what to do.
• You smell gas or suspect carbon monoxide. Read carbon monoxide at home at once.

The Health and Safety Executive says gas appliances need a Gas Safe registered engineer. Its domestic gas FAQ also advises an annual service. Our engineers are Gas Safe registered, and you can ask to see the ID card.

Pressure, leaks and emergencies

A leak in the heating circuit can look like a burst pipe. Our guide to the signs of a burst pipe lists the clues. If water is pouring out, follow the first 10 minutes of a plumbing emergency.

Radiators that stay cold can point to the same fault. Our guide to radiator cold spots explains each symptom. An old boiler that keeps losing pressure may need replacing, so read what a new boiler costs first.

Landlords and buyers

Landlords must keep gas appliances safe in rented homes. Our notes on commercial plumbing cover planned maintenance for several properties. If you are buying a home, ask about the boiler as well as the drains. A pre-purchase CCTV survey covers the drains, while the boiler needs its own check.

A drain survey also helps if a leak runs into the ground, and our guide to CCTV drain surveys explains how it works.

Frequently asked questions

Can I top up my boiler pressure myself?

On many sealed systems you can, using the filling loop. Follow the manual. However, call an engineer if the pressure drops again.

Is low pressure dangerous?

Low pressure usually stops the boiler from firing, so you lose heat rather than safety. Yet a leak still needs fixing.

How often should a boiler be serviced?

The HSE advises an annual service by a Gas Safe registered engineer.

Get boiler help

Our boiler repair service and radiator and central heating service cover these faults. Use the request form and describe what the gauge shows. Mark it urgent if the boiler is leaking. You can also see all our services and the areas we cover. Add your town and a short description, because that helps an engineer get back to you.

Near Brixworth, see plumbers in Brixworth. In Long Buckby, see plumbers in Long Buckby. Around Earls Barton, see plumbers in Earls Barton. In Northampton, see boiler help in Northampton. These pages list local details, since coverage varies from town to town.