Low Boiler Pressure? How to Repressurise Your Boiler

If you’re having problems with your boiler, like no heating or hot water, loss of water pressure might be to blame.

This article covers everything you need to know about low water pressure and how to fix it quickly.

Jump to a topic using the links below:

1. How to recognise low water pressure
2. What causes low pressure?
3. What is the correct pressure for my boiler?
4. Is low boiler pressure dangerous?
5. How to repressurise a boiler
6. How to prevent low water pressure

How to recognise low pressure

There are several signs that you may have low pressure in your boiler:

1. The pressure gauge on the front of the boiler is below the recommended level. Most boilers have a pressure gauge indicating the system’s current pressure. If the pressure on the gauge is below the recommended level, it may be an indication that you have low pressure in your boiler.
2. The boiler is not heating your home effectively. If the pressure in your boiler is low, the boiler may not be able to generate enough heat to warm your home.
3. The boiler is making unusual noises. Low pressure in a boiler can cause the boiler to make knocking or banging noises, which can be a sign of a problem.
4. The radiators are not getting hot. If the pressure in your boiler is low, the water may not be able to circulate through the radiators properly, which can cause the radiators to stay cold.

What causes low pressure in a boiler?

There are several possible causes for a drop in pressure in a boiler:

Leaks

A leak in the pipes or other parts of the heating system can cause a drop in pressure. This is because the leak allows water to escape from the system, reducing the overall water volume and lowering the pressure.

Air in the system

If air is trapped in the system, it can cause the water pressure to be too low. To fix this, you will need to bleed the air from the system. This typically involves opening a valve and allowing the air to escape until water flows from the valve.

Scale build-up: 

A build-up of scale (a hard, white deposit) on the inside of the boiler or the pipes can restrict water flow and cause low pressure. To fix this, you will need to descale the system. This can typically be done using a chemical descaler (power flush) or flushing the system with clean water.

Boiler Usage

If the boiler has been turned off for an extended period of time, the pressure may drop. This is because the water in the system will cool and contract, which can cause the pressure to fall.

Water usage

If the water in the system is being used for showers, washing dishes, or other purposes, the pressure may drop. This is because the water is being drawn from the system, which reduces the overall volume of water in the system and lowers the pressure.

Expansion vessel

If the expansion vessel on your boiler is not functioning properly, it may not be able to compensate for changes in the volume of water in the system. This can cause the pressure to drop.

If you are experiencing a drop in pressure in your boiler and are unsure of the cause, it is best to seek the assistance of a qualified heating engineer. They will be able to diagnose the problem and recommend the appropriate course of action.

What is the correct pressure for my boiler?

The correct pressure for your boiler will depend on the make and model of the boiler. Most boilers have a recommended pressure range printed on the pressure gauge or in the manufacturer’s instructions.

Typically, the recommended pressure range for a boiler is between 1 bar and 2 bar. Maintaining the correct pressure in your boiler is important to ensure that it is operating efficiently and safely. If the pressure is too low, the boiler may not work correctly and may not heat your home effectively. On the other hand, if the pressure is too high, the boiler may be at risk of overheating or causing leaks.

To check the pressure on your boiler, you can use the pressure gauge located on the front of the boiler. If the pressure exceeds the recommended range, you will need to repressurise the boiler by adding or bleeding water from the system as required. 

Is low boiler pressure dangerous?

Low pressure in a boiler can be a sign of a problem and, if left unchecked, could lead to long-term damage to your boiler. However, low boiler pressure is unlikely to cause you physical harm. This is because most modern central heating systems have safety features in place that will shut down the boiler in the event of a failure. 

However, if you suspect that your boiler pressure is low, it is vital to take action to resolve the issue before continuing to use it. If you aren’t confident about repressuring your boiler or are having trouble maintaining the correct pressure, it is best to seek the assistance of a qualified heating engineer. They will be able to diagnose the problem and recommend the appropriate course of action.

How to repressurise a boiler

Although it may sound technical, repressuring your boiler simply means allowing more water into the system. Most modern boilers can be topped up by using a filling loop.

To repressurise your boiler, you will need to follow these steps:

1. Locate the filling loop on your boiler. The filling loop is a pair of flexible hoses that are connected to the cold water inlet and the hot water outlet on your boiler.
2. Turn off the power to your boiler. This is usually done by turning off the boiler’s power switch or switching off the circuit breaker in the fuse box.
3. Locate the pressure gauge on your boiler. The pressure gauge is usually located on the front of the boiler and indicates the current pressure of the system.
4. If the pressure on the gauge is below the recommended level, you will need to add more water to the system to increase the pressure.
5. You will need to use the filling loop to add water to the system. First, open the valve on the cold water inlet side of the filling loop. Then, turn on the tap to allow water to flow into the system.
6. As the water flows into the system, you can use the pressure gauge to monitor the pressure. Once the pressure reaches the recommended level, close the valve on the cold water inlet side of the filling loop.
7. If the pressure on the gauge is above the recommended level, you will need to bleed some excess water out of the system. You will need to locate the bleed valve on one of your radiators to do this.
8. Turn the bleed valve clockwise to open it using a radiator key or a flat-head screwdriver. This will allow any excess water to escape from the system.
9. Once the excess water has been bled from the system, close the bleed valve by turning it counterclockwise.

When repressurising a boiler, it is essential to be careful, as the pipes may still contain hot water. If you need help with repressurising your boiler or are not comfortable doing it yourself, you should contact a professional.  

How to prevent low water pressure

Prevention is often much simpler than cure. The best way to prevent low water pressure in a boiler is to schedule an annual boiler service.

A boiler service will help to ensure your boiler runs efficiently and detect potential issues before they become a problem. As a result, you can gain peace of mind knowing your boiler is far less likely to breakdown when you need it.

About the author

PHS Trades Team are experts in Heating, Plumbing and Gas Services. We provide expert Boiler Repair and Boiler Servicing to homes and businesses across the Southeast.