Homeowners Professionals

How to repressurise a combi boiler

If you’ve noticed that you don’t have any hot water or your radiators aren’t getting as hot as they should, the problem could be your boiler pressure. If the pressure of your boiler has dropped, you may need to re-pressurise it to ensure your boiler continues to heat your home and provide hot water effectively.

You can check the pressure of your boiler and re-pressurise it yourself without having to call an engineer out. This post explains everything you need to know when it comes to re-pressurising a boiler and how to do it.

How do I check my boiler pressure?

The good news is, checking boiler pressure is a simple task and does not require the help of a specialist heating engineer. You can check the pressure by simply looking at the gauge which is usually located on the front of your boiler. These typically hydraulic pressure gauges indicate the standard operating pressure with a green zone and the maximum safe operating pressures with a red zone. Of course, some modern boilers have digital gauges rather than traditional hydraulic ones. A digital gauge will show if the pressure is too high or too low by flashing the reading on the guage’s screen.

To ensure that you are confident locating your boiler’s pressure gauge and taking a reading safely, below we run through the four simple steps you will need to take when checking boiler pressure.  

  1. Check your boiler’s optimum pressure range - before doing anything else, it can be a good idea to check what your boiler’s pressure gauge should read. As we discuss below, the ideal pressure for most boilers in the UK is typically between 1 and 1.5 bars (when not in use). However, some boilers can differ. For this reason, it is important to consult your boiler’s manual or owner’s handbook to ensure you are clear on the optimum pressure range for your make and model.
  2. Find the gauge - next, it’s time to locate the pressure gauge. These small measuring devices are typically found on the front of your boiler unit. Standard hydraulic gauges usually look like a small circular analogue dial, not dissimilar to a clock or speedometer. Digital gauges, on the other hand, will typically take the form of small electronic displays.
  3. Take the current reading - to take a reading, simply look at the gauge and note what it says. If the needle/digital reading falls between the high and low pressure range - as specified in your boiler’s manual - nothing needs to be adjusted. If the reading is outside the optimum range, you will need to re-pressurise.

What pressure should combi boilers run at?

As a rule, the pressure of your combi boiler should be between 1 and 1.5 bar.

If your pressure reads at less than 1 bar, it’s possible that you have lost water from the system via a leak. If the pressure reads at 2.75 bar or above, you may need to bleed a radiator to release the pressure and bring it back down to a suitable level. If you attempt to bleed a radiator, you should make sure that you have a way of catching the water to prevent the walls or floor from being ruined. Take caution when undertaking this job as the water can be very hot. If you’re unsure about anything, it’s a good idea to call an engineer who can help you.

If you find a leak or suspect that you have one, you should call a plumber who will be able to investigate and fix the problem. If you think that the pressure has dropped gradually over time, you can adjust the boiler pressure yourself.

How do I re-pressurise my boiler?

Before getting stuck in, it’s a good idea to check your boiler manual for instructions on how to re-pressurise your combi boiler and whether you can do this yourself. Every type and model of boiler is slightly different so double check you know what to do.

Once you’re ready, you can use the following steps to complete the process:

  1. Switch off your boiler and allow it to cool completely
  2. Find the filling loop and check that both ends are securely attached. A leak in a valve could be a reason for losing pressure so check for water around the valve
  3. Open both valves to allow cold water to enter the system, keeping an eye on the pressure gauge
  4. Close both valves when the pressure gauge reaches 1.5 bar
  5. Remove the filling loop if it isn’t a built-in one
  6. Turn the boiler back on and recheck the pressure
  7. There are two types of filling loop: external and built-in. An external one will need to be attached by you, so you should attach it during Step 2 and then remove it before turning the boiler back on in Step 6.

By re-pressurising your boiler, you’re allowing more water to enter the system from the mains supply. If you accidentally over-pressurise the system, don’t panic. You may need to bleed your radiators to bring the pressure back down.

You should keep an eye on the pressure over the following days and weeks to make sure the pressure doesn’t drop again. If it does, there’s a chance you may have a leak.

If you get stuck while trying to re-pressurise your Viessmann boiler, you can always contact us for support.

How long does it take to re-pressurise a boiler?  

Depending on how low the pressure of your boiler is, you should allow water in through the valves for around 10 seconds. This should be enough water to get your combi boiler back up to pressure again. If it takes longer than this, you could have a leak in the system.

The whole process, including attaching the filling loop, opening and closing the valves and removing the filling loop should only take a couple of minutes. It might take a bit longer if you’ve never done it before.

How to re-pressurise a boiler without a filling loop

A combi boiler system should be installed as a pressurised system. However, if you have a conventional boiler system, you may have a self-filling water tank (usually located in the loft) which controls the pressure for you. The system works by heating up cold water in the hot water storage cylinder which feeds into your taps. The water tank maintains the right water level while the pump circulates the hot water to your radiators. You should never have to re-pressurise this kind of boiler.

Combi boilers need to be connected to the mains in order to increase the pressure. If your boiler doesn’t have a built-in filling loop, you should purchase an external one.

An older combi boiler might have a filling key rather than a filling loop. The key can be inserted into the keyhole and put into the unlock position. Use a spanner or wrench to turn the valve next to the filling key until you can hear water flowing through it. When the pressure is at 1.5 bar, close the valve with the spanner and remove the key by turning it to the lock position. This prevents the valve from being turned on accidentally.

Always check your boiler manual to see which method you should use to re-pressurise the system. If in doubt, you can visit the manufacturer’s website and get in touch with them directly.

Why won’t my boiler re-pressurise?

You might have noticed the pressure of your boiler drops as soon as you re-pressurise it. The most common reasons for lack of pressure include a leak somewhere in the system or a damaged pressure relief valve.

If you’ve tried to re-pressurise the system once and it hasn’t worked, don’t keep trying it. There’s clearly a problem and it’s best to get an engineer to have a look at what’s going on.

Re-pressurising your boiler isn’t a complicated procedure, and you should regularly check the pressure to make sure the system is running as it should. If you’ve spotted that your pressure is always dropping, you could try to find a leak yourself, but if you’re unsure, call an engineer to help you determine what the problem is. You don’t want to be left without hot water or heating.

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