Boiler pressure, trapped air and water flow can all affect your radiators. The pattern of a problem can often give you a useful clue about what is wrong.
What Is Boiler Pressure?
Many modern central-heating systems contain water in a sealed circuit.
The boiler heats this water. Then, a pump moves it through the pipes and radiators before it returns to the boiler.
The system needs suitable pressure to work properly.
You can normally see the pressure on:
- a gauge
- a digital display
- the boiler control screen
It is usually measured in bar.
This is the pressure inside the heating system. It is not the same as the water pressure at your taps.
What Should the Pressure Be?
There is no single correct pressure for every boiler.
Many sealed systems operate somewhere around 1 to 1.5 bar when cold. However, you should check the instructions for your own boiler.
The pressure normally rises when the heating gets hot.
This happens because water expands as it warms.
Then, as the system cools, the pressure should fall again.
So, some movement in the pressure reading is normal.
What If the Pressure Is Too Low?
If the pressure falls below the boiler’s operating range, the boiler may stop working.
For example, you might see:
- a low-pressure warning
- a fault code
- a boiler lockout
- heating that will not start
Low pressure does not automatically mean the boiler is faulty.
However, it is worth asking why the pressure has fallen.
Why Does Pressure Drop?
Possible reasons include:
- bleeding a radiator
- a leak
- work on the heating system
- a problem with the boiler or another component
For example, bleeding a radiator releases air and can reduce the system pressure.
However, pressure that keeps falling needs more attention.
Don’t simply keep adding water without finding out why.
Check for Leaks
If the pressure keeps falling, look for obvious signs of water around:
- radiator valves
- visible pipes and joints
- radiators
- the boiler area
Look for drips, damp patches, staining or corrosion.
However, not every leak is visible.
So, if the pressure repeatedly falls and you cannot find the cause, get the system checked.
How Do You Repressurise a Boiler?
Some heating systems allow you to restore the pressure yourself.
However, boilers use different filling systems.
For example, yours may have:
- a filling loop
- a filling key
- built-in valves
- another filling device
So, there is no safe universal set of instructions that applies to every boiler.
Use the instructions for your exact make and model.
Then, stop when the correct cold pressure for your boiler has been reached.
If you are unsure, get suitable help.
Don’t Keep Topping It Up
If you need to restore the pressure regularly, something may be wrong.
Adding more water restores the pressure. However, it does not repair the cause of the pressure loss.
So:
Pressure drops once after bleeding → may have a simple explanation
Pressure keeps dropping → investigate why
What If Pressure Is Too High?
Pressure also rises as the heating warms up.
A modest increase can be normal.
However, pressure that repeatedly becomes unusually high may point to a problem.
Don’t interfere with boiler safety devices or deliberately release water from the boiler to try to fix it.
Instead, check the boiler instructions and get professional advice if the problem continues.
What Can Radiators Tell You?
The pattern of heat across a radiator can give you useful clues.
A radiator does not need to feel exactly the same temperature everywhere.
However, clear cold areas can sometimes point towards a problem.
A useful starting point is:
Cold at the top → possibly trapped air
Cold at the bottom → possibly restricted circulation or deposits
One radiator completely cold → check its controls first
All radiators cold → check the wider heating system
These are clues rather than guaranteed diagnoses.
Radiator Cold at the Top
If the bottom of a radiator is warm but the top remains cold, air may be trapped inside.
That air can stop hot water filling the radiator properly.
The radiator may therefore need bleeding.
How to Bleed a Radiator
First, switch the heating off and allow the radiator to cool.
You will normally need:
- a radiator key or suitable tool
- a cloth
- a small container
Carefully open the bleed valve a little.
You may hear air escaping.
Once the air has gone and water starts to appear, close the valve.
Do not remove the valve completely.
Then, check your boiler pressure.
Bleeding can lower the system pressure, so you may need to restore it using the instructions for your boiler.
Do You Keep Bleeding It?
A radiator should not continually fill with air.
So, if you repeatedly need to bleed the same radiator, or several radiators, there may be another problem.
Rather than repeatedly treating the symptom, have the cause investigated.
Radiator Cold at the Bottom
If a radiator is hot at the top but has a large cold area at the bottom, trapped air is less likely to be the cause.
Sludge or other deposits can sometimes restrict the flow of water through a heating system.
Bleeding will not remove sludge.
So, if a radiator has persistent cold areas at the bottom, further investigation may be needed.
One Radiator Is Cold
If every other radiator works but one stays cold, start with the simple controls.
Check:
- the TRV
- whether the valves appear to be open
- whether the radiator contains trapped air
A stuck valve or poor circulation can also cause problems.
However, don’t dismantle valves or force components if you do not know what you are doing.
What Does a TRV Do?
A thermostatic radiator valve, or TRV, controls the flow of hot water through an individual radiator.
As the room reaches the selected temperature, the valve can reduce the flow.
So, a cooler radiator does not automatically mean something is wrong.
The room may simply be warm enough.
Learn more: Boiler and Heating Controls →
What Is a Lockshield Valve?
Many radiators have another valve at the opposite end from the TRV.
This is commonly called the lockshield valve.
It is not normally an everyday temperature control.
Instead, it helps control water flow through the radiator when the heating system is balanced.
So, don’t treat it like another TRV.
What Is Radiator Balancing?
Water does not always flow equally through every radiator.
Some may heat quickly while others take much longer.
Balancing adjusts the flow through individual radiators so the system distributes heat more effectively.
This is different from bleeding.
Bleeding → removes trapped air
Balancing → adjusts water flow
They solve different problems.
What If All Radiators Are Cold?
If every radiator is cold, the problem is less likely to be one radiator.
Check:
- Is the heating switched on?
- Is the timer calling for heat?
- Is the thermostat set correctly?
- Is the boiler running?
- Is there a fault code?
- Is the boiler pressure within its normal range?
If the boiler has stopped or locked out, follow the instructions for your particular boiler.
Next: Reset Your Boiler Safely →
Do Radiators Need to Be Very Hot?
No.
A radiator’s job is to provide enough heat for the room.
Water also cools as it travels through a radiator because heat is being released into the room.
So, the radiator does not need to feel equally hot everywhere.
Also, a heating system running at a lower flow temperature may have radiators that feel less hot while still keeping the rooms comfortable.
The important question is:
Does the room reach the temperature you need?
Keep Radiators Clear
Furniture, heavy curtains and poorly designed covers can restrict airflow around a radiator.
They can also affect a TRV if warm air becomes trapped around the valve.
So, where possible:
- keep airflow around radiators clear
- avoid covering TRVs
- don’t place large furniture directly in front
- don’t routinely cover radiators with wet clothes
This helps heat move into the room more freely.
When Should You Get Help?
Get suitable professional help if:
- pressure repeatedly falls
- pressure repeatedly rises too high
- you find a leak
- several radiators remain cold
- radiators repeatedly fill with air
- a radiator stays cold after basic checks
- the boiler repeatedly locks out
- you are unsure how to restore the pressure safely
For work on a gas boiler itself, use an appropriately qualified Gas Safe registered engineer.
Quick Problem Guide
Pressure low
Check the boiler instructions and look for obvious leaks.
Pressure keeps falling
The cause needs investigating.
Pressure rises excessively
Get the system checked.
Radiator cold at the top
Trapped air may be present.
Radiator cold at the bottom
Restricted circulation or deposits may be involved.
One radiator cold
Check its TRV and basic controls first.
Some radiators heat much faster than others
Balancing may be an issue.
All radiators cold
Check the boiler and heating controls.
Key Points
Boiler pressure normally changes somewhat as the heating warms and cools.
However, pressure that repeatedly disappears or becomes too high needs investigating.
A radiator cold at the top may contain trapped air.
A radiator cold at the bottom may have a circulation or deposit problem.
Also, remember:
Bleeding removes air.
Balancing adjusts water flow.
Finally, use the pressure range and filling instructions for your own boiler rather than assuming every boiler works in exactly the same way.
Next: Reset Your Boiler Safely →