There are a few reasons why your boiler pressure drops when not in use. First and foremost, low boiler pressure is not a good sign. When a gas boiler breaks down, it is typically because the pressure is too high or too low. Central heating systems are pressurized and often do not have vents because they are sealed. Without vents, there is no way to manage water contraction and expansion. This is why the system must be kept at a stable pressure. A boiler with stable pressure does a good job at moving water around your home.

The problem occurs when a boiler's pressure is too low. This can happen when the boiler is not in use for a period of time. You may notice that you no longer have hot water and your home's radiator does not warm up. This means your boiler has stopped working due to a drop in pressure. Let's take a look at the possible causes for low boiler pressure and what you can do to rectify them.

What Are the Reasons For Boiler Pressure Drops When Not In Use?

Bleeding Radiators

boiler pressure drops when not in use - bleeding radiators
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Bleeding radiators can cause boiler pressure drops when not in use from the loss of water. Bleeding your radiator removes water from the central heating system, thus lowering the boiler's pressure.

Bleeding your radiator is when you let out trapped air that is inside. The trapped air stops warm water from properly circulating. An indicator of trapped air is a radiator that is cold at the top and warm at the bottom. Trapped air can make your bills higher and significantly reduce the radiator's efficiency. It will take much longer to heat up your room. Bleeding your radiator is also good for solving clanking, gurgling and banging noises due to trapped air.

Water Leak

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When a water leak occurs, that means water is escaping your system and therefore lowering the boiler's pressure. Leaks can go undetected as they often are small. However, if the leak continues for long enough, you will see your boiler pressure drops when not in use. The best strategy is to check around your radiator and pipes for damp patches/water. You can even check your boiler for signs of a leak. However, never try to look inside your boiler. That needs to be done by a gas registered engineer.

How Do You Fix It?

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There are a few ways to fix low boiler pressure. You may be able to fix it on your own, however, you must check with the manual. Make sure you carefully read the manual before attempting anything.

In order to re-pressurize your system, the first step is to let more water in using a filling loop. Oftentimes a filling loop is built into your boiler or offered as an attachment. Each filling loop can be slightly different in design and process. Listed below is a general guide on how to re-pressurize your boiler. This method uses a filling loop.

  • Step 1: Completely turn off the boiler and let it cool.
  • Step 2: Locate the filling loop. If an attachment, attach it to the boiler.
  • Step 3: Ensure you have visibility of the pressure gauge while utilizing the filling loop.
  • Step 4: Open the valves for both sides. This will let water in.
  • Step 5: Wait until you see the pressure gauge reach 1.5 bar. Next, close both valves, one after the other.
  • Step 6: Turn your boiler back on. Press the reset button if needed.
  • Step 7: If your filling loop is an attachment, remember to remove it. Use caution.

What Are The Other Reasons My Boiler Pressure is Low?

In addition to the ones listed above, there can be other reasons why your boiler pressure drops when not in use. If any of these are the cause, there are solutions. Some require a professional while others can be done on your own.

Faulty Pressure Relief Valve

The majority of modern boilers operate at 1.0-1.5 bar. However, the pressure may climb up to 3.0 bar when there is an issue. This is most commonly seen with a boiler leak. The PRV (pressure relief valve) will detect this change and subsequently release it. In the event your PRV fails, the boiler will lose pressure.

Leaking Radiator Valves, Radiators & Towel Rails

When a combi boiler loses pressure, this is the most common cause. Even a minimal leak can cause boiler pressure drops when not in use. If you see water escaping when checking the radiators and valves, you have most likely located the problem. Even a small amount of water can make a big difference in pressure. Very loose connections should be carefully tightened up. This may solve the issue altogether. However, if it does not, replacing or repairing is needed.

An Auto Air Vent Leak

It is common for central heating systems to build up air pockets. This happens over time and when you refill them. The best choice is to bleed your radiator. However, the auto air vent is typically found inside the boiler casing (for newer combis). The auto air vent can help solve this problem which affects the boilers. If your auto air vent is damaged or faulty, pressure may be lost due to the leak.

If these steps do not fix the problem, it is best to contact a qualified heating and boiler installation professional. An engineer will know how to solve low boiler pressure and has the tools, training and expertise to repair it/replace it.

Is There a DIY Fix for Boiler Pressure?

While it is advised to contact a professional if the problem persists, there is a DIY method to fix boiler pressure drops when not in use (at least temporarily). Using a leak sealant such as the Fernox F4 can be a useful solution. The sealant will travel around the hot water system. This type of sealant will find its way into a small area that is leaking such as a small gap. The sealant will begin working in 1 to 24 hours. The goal of the sealant is to stop your boiler pressure from dropping.

If you are hungry to learn more about keeping your heating in tip-top shape, check out this article about central heating maintenance for spring! For more articles head over to my blog section!

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