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Boiler pressure keeps dropping ? What to check

Boiler Pressure keeps dropping or continually low ? 

Why does my boiler pressure keeps dropping ? This is a common question , and following in this guide we’ll take you through the most common boiler problems, what to look for , and actionable things that you can do to fix the dropping boiler pressure. Most of the time these fixes are easy , that can be done by yourself, as you don’t need to be an expert in order to do a little troubleshooting on your own.

By following these simple steps , you should figure out what is causing your low boiler pressure , and why it continually keeps on dropping , in no time at all, before having to call a plumber out. Some of the fixes can be done by you , whilst others you may have to get a plumber or heating engineer out to look at your heating system as you may have a boiler leak causing it to lose pressure.

work through the following table of contents in an orderly manner

Things to check on the boiler

Firstly you need to check your boiler pressure gauge , and see what’s it’s reading. This gauge is most commonly built into the boiler, or underneath it and should look like this or similar –

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The Heating System Pressure Gauge – The fist thing to investigate as why your boiler pressure keeps dropping

In the picture above the red marker on the gauge is just an indicator of where the level should be, the thin black marker is the actual pressure of the heating system. A pressurised boiler system or a sealed central heating system needs a certain amount of pressure to work, normally about 0.5 Bar – 1.5 Bar, although some will continue to function with far less pressure. If this pressure reading is lower than the recommend pressure level, this means you heating system water pressure is low and that you have low boiler pressure which needs to be topped up so the level is above the red line on the pressure gauge normally via the filling loop, which will fix the low boiler pressure.

It is usually best to contact a plumber if you have any concerns about your boiler losing pressure or are unsure of how to re-pressurize the system yourself.

But before that is done, you really need to establish why the boiler keeps losing pressure as you may have pressure loss in the pipework i.e. copper pipe via a leak or even a leaking radiator valve, gland nut or heat exchanger etc.

Combi boiler pressure keeps dropping ?

It doesn’t matter whether your boiler is a heating or heat only boiler or a combi boiler, as we are talking about losing water pressure on the central heating system here .If your combi boiler is losing pressure on your domestic hot water side of the boiler ( this is you hot water that is provided to your hot water taps , showers or bath taps ) then this will be a mains water pressure problem, and you would need to consult your water supplier.

First thing to check –

Is there air in the heating system?

This is an easy thing to check yourself. You just might be able to diagnose your problem with just a few checks,

Before you do anything, try bleeding your radiators. But before you set off to do this, a top tip is to have a towel or rag to catch any drips from the radiator air vent, as the water form the heating system can be black in colour and stain anything that if falls onto. To bleed your radiators, use a radiator key. Find the radiator air vent and slowly turn the radiator key counter clockwise until water starts dripping out, this releases the trapped air and lets hot water fill that void or space in the radiator again, ensuring a nice hot radiator. Always bleed the top floor radiators first and make sure you bleed the radiators in sequence. Bleeding the air from the radiators will then cause the pressure to drop lower again, so check it again after bleeding all the radiators and adjust it accordingly.

If bleeding the radiators doesn’t work, then you may have a bigger issue such as an air lock in the heating system or you may even need to have the heating system itself flushed out. I wouldn’t recommend you doing this yourself , unless you feel completely comfortable, and have the tools and equipment to do it properly. So it’s time to call a plumber for most people.

Second thing to check –

Is there a leak in your central heating system?

Look for signs of a water leak such as a damp floor, leaky pipes, a small hole in a radiator somewhere or a stain on your ceiling, any or all of these signs point to you have a leak on your central heating system, which is causing your boiler to lose pressure. Finding a leak may sometimes feel like finding a needle in a haystack, but it’s worth trying to find it yourself, before calling out a plumber, as most heating engineers charge an hourly rate, which can become quite expensive, should be tasked with finding the leak on you central heating system.

Third Thing to check –

Is the boiler pressure relief valve leaking ?

Is the pipe coming off the pressure relief valve on the boiler continually dripping water ? If so at some point the valve has probably operated and there may be some internal debris that is stop it from properly shutting off again. This faulty pressure relief valve may need to be cleaned or replaced completely and is a fairly cheap repair or you may need to replace this faulty pressure relief valve. This job really should be left to a qualified plumber or heating engineer to do.

Fourth thing to check –

Is the Boiler Expansion Vessel Faulty?

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An external expansion vessel, used in central heating systems

All modern sealed system boilers or pressurised boiler systems use an expansion vessel with pressurised air within the unit, either placed within the boiler but can be located externally too , that accommodates any expansion that is created when water is heated up in the heating system. If the expansion vessel is faulty, it normally means that it will not be able to handle the expanding hot water resulting in the pressure in the heating system rising rapidly and this water will be ejected through the pressure relief valve, once it goes beyond 3 bar in pressure. The expansion vessel’s internal bladder, which is normally made from neoprene rubber (Neoprene rubber resists degradation more than natural or synthetic rubber), must be inflated by air or ideally nitrogen ( when the gas side is filled with nitrogen, and not with air, corrosion is prevented and the pressure loss is even more limited). This level of nitrogen or air, commonly known as it’s charge must be maintained through annual servicing of the boiler or eventually it will dissipate completely, eventually causing the boiler to have continual low-pressure problems and ultimately failure of the central heating system.

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Expansion Vessel internal picture, showing the “Bladder”

If that doesn’t seem to be the problem, then the next thing to do is find the Schrader valve on the expansion vessel, which looks like an air valve found on a car or bike tyre. Then depress the valve in the centre using a screwdriver or similar. If water comes out of the valve it means that the diaphragm has failed inside the expansion vessel, and you will need to replace the vessel or with some units you can just replace the internal neoprene bladder.

Dropping boiler pressure from Flexiheat UK; boiler pressure keeps dropping; boiler pressure keeps dropping to 0;
Expansion Vessel Schrader Valve Picture & How to Test

If air comes out of the Schrader valve you need to check the pressure of the charge in the vessel. To do this you will need a suitable measuring device, such as a car tyre pressure checker. The air or nitrogen pressure for an internal boiler expansion vessel should be around 0.8 bar ( but please consult your boiler’s operating manual and check exactly what pressure the vessel should be charged to) but this could be higher for externally mounted expansion vessels. To fix this call your local heating engineer and ask him to re-inflate the expansion vessel. This is done by draining off all the heating system pressure first, then then using a pump (like a bicycle type or similar) the expansion vessel is re-inflated.

So what if the boiler pressure keeps dropping after doing this ?

So after your expansion vessel has been re-inflated keep an eye on the boilers pressure gauge, to see that it’s still correctly holding the system pressure for the next couple of weeks, as sometimes the internal bladder that holds the air or nitrogen has a pinhole puncture and it leaks out over days or even weeks , which isn’t always obvious, until you see your boiler pressure dropping.

Pressurizing your expansion vessel should typically be done as part of an annual boiler service, by a qualified plumber / heating engineer. We highly recommend that any work that is required to be done, is not done by a yourself or a novice, as we are dealing with hot water and pressure here, and these two items can cause physical damage to people or the property

Boilers with low water pressure – other things to consider

One thing that people often overlook is that if your boiler has been under pressured for a long period and you’ve been constantly topping the heating system up via a filling loop, then it would have depleted the central heating inhibitor chemicals within the heating system, it is therefore very important to add more inhibitor to the heating system , otherwise your heating system will start to corrode which can lead to a failure of your boiler, in a relatively short period.

Refilling your low pressure boiler heating system

After diagnosing your problem, and getting it fixed, the final step in getting you low pressure boiler back up to the desired pressure is refilling the heating system with what is called a filling loop . On some boilers the filling loop is internal to the boiler but in most systems the filling loop is an external piece of kit, and should be what is termed as a part “L” of the building regulation compliant and looks like this –

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Part L Compliant Filling Loop

You should only refill your central heating system when the system has cooled down. You simply open the valves up, slowly and wait for the pressure gauge on the boiler to rise to 1.2 Bar to 1.5 Bar maximum (please consult you boiler manual for the recommended pressure that they stipulate).The process is then finished off by closing the valves , thus sealing the heating system from the mains water

Hopefully this guide “boiler pressure keeps dropping” has been helpful, and shown you some of most common reasons why this happens and how to remedy it.