How To Repressurise Your Boiler, Even Without A Filling Loop


We all love to stay cosy and warm at home no matter the season, which is why having your boiler in perfect condition is essential. It provides a constant stream of hot water, heats your radiators and generally makes the place feel like home. So, what do you do when you face one of the most common boiler faults: an unexplained drop in pressure? Let’s take a look at why it matters, what might cause it and some simple ways to rectify it. Just what you need to keep the heating on.

Why might you lose pressure?

Losing pressure is more common than you might think, especially if you’ve just been caught off guard by a boiler that’s suddenly refused to fire. It happens when you have a leak in the system (water out of your taps does not count) and typically occurs at joints and valves. Because of the high degree of pressure in the system, you only need to have a very small hole to lose pressure gradually. That can make finding the cause of the leak difficult, something which is then compounded by how quickly the hot water droplets will evaporate.

Why does losing pressure matter?

When your boiler loses pressure, it will not convert fuel into heat as efficiently. That will then put a greater strain on your boiler’s internal components, as well as potentially increasing your heating bill. An added complication is that when the pressure drops significantly, the boiler will stop firing. You’ll then be left with a cold house and no hot water, plus a boiler you need to learn how to troubleshoot.

Can you repressurise too often?

Yes, you certainly can. You might be thinking that as long as you learn how to repressurise, everything else will take care of itself. Unfortunately, it doesn’t work like that. Repeated repressurisation will cause a buildup of debris, namely sludge-like accumulations, inside your boiler. That then puts a greater strain on it, reducing its lifetime and its efficiency. Two things you don’t want to happen if you want to reduce the likelihood of a costly repair job.

Does repressuring the boiler always work

Unfortunately not, but it is always worth a try. Over time it’s common to get a small leak on a valve or joint in the boiler, in which case isolating it from the rest of the system and repressuring will prove that this is the case.

If however, you repressurise the system as a whole and still see repeated drops in pressure, the chances are you have a leaking valve, pipe or radiator that needs attention. Finding these kinds of leaks can be time-consuming, so it is always worth considering hiring a professional. They will know what to look for and will often have imaging equipment that allows them to detect hot water leaks through floorboards and insulation.

What happens if you over pressurise?

Over pressuring a boiler is not advisable because it has been designed to work within a set pressure range. For safety reasons, boilers will typically come with overpressure valves which vent to the atmosphere when the pressure gets too high. That is designed to prevent large buildups of pressure from causing ruptures inside the boiler, and the explosion that would likely come with it.

How to prepare for repressurisation

Repressurising is a delicate process that needs to be done the right way. To ensure that you are set up to take care of it, make sure to work through the following hints and tips.

Check all bleed valves and fixtures on your radiators to make sure they’re closed. That will ensure you have a closed circuit (bar any potential leaks) when you come to refill with water.

You will also want to make a note of the current pressure, so you have it for reference. Additionally, check the optimal running pressure in the manufacturer’s guidelines. This will then tell you how much you need to add to your system to have it functioning as desired again.

Now that you know everything to get ready, you’re now prepared to get started. But from the title of this article, there’s one more question we need to answer…

What’s a filling loop?

The filling loop is a small braided hose with a valve attached. It can be found inside your boiler, and its purpose is to allow you to add additional water to the heating system. Because the volume of your system is fixed, adding more water increases the pressure. By opening the filling loop valve and monitoring the pressure as it begins to climb, you can repressurise in just a few seconds. Just make sure the valve is firmly shut before firing the boiler.

There is one more issue you may encounter at this stage, however, and that’s if you can’t find the filling loop. A number of older style boilers do not come equipped with them, which can make knowing what to do rather difficult…

You need to find the hot water cylinder

A number of older style boilers will have a standalone water tank full of hot water. If you notice you don’t have a filling loop, but you do have a large metal cylinder that’s warm to the touch, this is precisely where you need to start looking.

What you’re searching for is a braided hose with a small valve attached to it. Opening this valve while monitoring the pressure on the boiler will add extra water to your hot water tank. The tank forms part of your overall heating system, and so topping it up will increase the pressure in the system, and therefore repressurise your boiler.

Put too much pressure into your system?

Don’t worry, it happens. If you’ve exceeded the recommended operating pressure by approximately 0.5 bar, you can remove it from the system by bleeding a radiator with a radiator key.

What to do if you continually lose pressure?

Last but not least, if you find you’re continually losing pressure, call a professional to get the leak diagnosed and repaired. It will always save you money in the long run. 


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