Boiler pressure dropping

The 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.

This means that when the heating system is installed it is filled with water from the cold mains with some chemicals to reduce corrosion.

When cold the pressure should stay at a pretty constant 1 to 1.5 bar. It goes higher if it is hot and should return to normal when it cools down.

If the pressure goes below 1 bar there may be a problem that 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

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.

A leaking radiator will drop the pressure on your boiler.
Leaking radiator

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 and should be bled using a key.

Radiator bleed keys

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.

Expansion vessel faults can cause the pressure to drop
Expansion vessel faults can cause the pressure to drop

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.

  1. Allow the central heating system to cool down.
  2. Set the pressure between 1 and 1.5 bar.
  3. Turn on the central heating and allow the radiators to get to their normal temperature.
  4. 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.

Can’t find the leak?

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.

  1. Inside the boiler main heat exchanger.
  2. Hidden in an inaccessible area of your property where you can’t see it.

If the leak is in the main heat exchanger it has to be replaced. As the main heat exchangers tend to be very expensive depending on age and condition it can be cheaper to replace the boiler.

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. This is designed to leak out of the system then harden up stopping the flow of water.

However they only work on very slow leaks. If the pressure is dropping within minutes it won’t work. If it dropping over a few days to a week there is a good chance it will work.

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.

If you have an Ideal boiler follow this link for advice on boiler pressure

Further reading

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