Boiler pressure dropping is one of the most common faults with central heating boilers. Most (not all) modern boilers work with a sealed heating system.
What is the pressure
The heating system is filled with water from the cold mains with chemicals to reduce corrosion when it is installed.
The pressure on the pressure gauge is the water pressure in:
- the boiler
- the radiators
- the coil in a hot water tank (if you have one)
- the pipework leaking to the radiators, boiler and hot water tank.
That means leaks on any of those areas of your heating system can make the pressure drop.
What should the pressure be?
When cold the pressure should be at about 1 bar. It goes higher when the radiators are hot and should return to about 1 bar when it cools down.
What is the problem with pressure?
If the pressure regularly goes below 1 bar there may be a problem somewhere in the heating system. This needs to be fixed. If it goes below 0.5 bar some (but not all) boilers will display a fault that won’t clear until the pressure is topped up.
The most common causes of pressure dropping are:
- A leak from a radiator
- Bleeding air from radiators
- A leak in the boiler
- A fault with the expansion vessel
- Filling the system with too much pressure
- Mystery leaks that can’t be found
How often should the pressure drop on a boiler?
In a perfect world the pressure would never drop on a boiler. All joints and components should be 100% sealed and never fail.
However in the real world most central heating systems will have to have their pressure topped up.
This can range from never to once every few months.
If you are topping up the pressure every month you have a leak that needs to be at repaired.
What’s the problem with topping up the pressure?
Topping up the pressure does two things:
- Hides a leak or a potential fault.
- Creates corrosion and sludge in the heating system.
Any leaks and faults should be repaired as soon as possible.
Adding fresh water to the system causes the inside of radiators and boilers to slowly corrode. This can create faults, blockages and eventually rot components from the inside out.
A leak from a radiator
The water contained in the radiators contributes to the pressure displayed on the boiler. If there is even a tiny leak from a radiator the pressure will drop.
All leaks from radiators and pipework should be repaired as soon as possible to avoid a pressure drop.
Bleeding air from radiators
Bleeding a radiator is a normal part of central heating maintenance. If a radiator is cold at the top and warm at the bottom it has air in it. The radiator should be bled using a key.
However the air that comes out as you bleed the radiator contributes to the pressure reading on the boiler.
After bleeding the radiator the pressure should be topped back up to the correct level.
A leak in the boiler
Unless there is a puddle under your boiler its difficult to know if it is leaking.
Modern condensing boiler can leak through their heat exchangers down the condensate pipe. This means there can be quite serious leaks without showing any signs inside or outside the boiler.
When all other components (radiators, valves, pipework etc) have been ruled out think about getting the boiler checked.
A fault with the expansion vessel
As the central heating system is sealed and water expands when it heats up there needs to be somewhere for the expansion to go to.
The most typical method is an expansion vessel in the boiler itself. This has an air bubble contained in a rubber balloon which is compressed by the water as it expands.
Over time the air bubble leaks away which means there is no where for the water to expand to as it heats up.
This causes the pressure to rise too high which opens a safety valve causing the pressure to drop.
If the air bubble is gone then it either needs to be replaced by pumping it up. If the rubber balloon is burst the expansion vessel needs to be replaced.
A competent engineer is required to investigate and repair.
To confirm if the expansion vessel is at fault follow this procedure.
- Allow the central heating system to cool down.
- Set the pressure between 1 and 1.5 bar.
- Turn on the central heating and allow the radiators to get to their normal temperature.
- Watch the pressure gauge. The pressure will normally go up but to below 2.5 bar. If the pressure goes up beyond this to 3 bar or above then you have a fault with the expansion vessel.
Filling the system with too much pressure
Boilers are designed to work with pressure between 1 and 1.5 bar when they are cold. This allows plenty of space for the expansion of the water in the radiators as it heats up.
If the pressure is set too high when cold the pressure rises too high as the system heats up.
If the pressure goes too high a safety valve opens which will drop the pressure sometimes to 0 bar.
It is essential to set the cold pressure between 1 and 1.5 bar.
Mystery leaks that can’t be found?
If the boiler pressure is dropping, you have checked every visible part of the central heating system and there is no sign of a leak then it can only be in two places.
- Inside the boiler main heat exchanger.
- Hidden in an inaccessible area of your property where you can’t see it.
Leak inside the main heat exchanger.
If the leak is in the main heat exchanger it has to be replaced. They can’t be repaired and leak sealant very rarely works. Main heat exchangers are very expensive. Depending on age and condition of the boiler it can be more cost effective to replace the boiler.
Hidden or inaccessible leaks.
If the leak is in an inaccessible location you have two choices. Locate the leak in your property. It could be under floors in walls etc so finding it could be damaging and disruptive. The second option is to use central heating leak sealant.
Central heating leak sealer
Central heating leak sealer is designed to leak out of the system then harden up stopping the flow of water.
Central heating leak sealers only work on very slow leaks. If the pressure is dropping within minutes it won’t work. With a fast leak like that the water is spraying out of the leak too fast to allow the chemical to harden.
If the pressure is dropping over a few days to a week there is a good chance it will work.
Central heating leak sealant can be tricky to inject into the heatimg system. I would recommend getting a heating engineer to install it.
Read more about central heating leak sealant by following this link.
Use the filling loop to set the right pressure. Follow this link for advice on external filling loops.
If you have a Worcester boiler follow this link for advice filling your boiler up.
Summary
The pressure is dropping due to:
- A leak on a boiler component.
- A leak on heating pipework.
- A leak on a radiator.
- A faulty component on the boiler.
- A radiator has been bled without added water back into the system.
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