Is Your Boiler Losing Pressure? Discover The Causes Of Pressure Loss

What checks you can do if your boiler is losing pressure? – DIY Pressure Loss Checks Before Calling The Experts

Is your boiler losing pressure? If so then you may be wondering where to start…

We at ADI have been finding hidden leaks on central heating systems, water pipework and swimming pools for over 10 years now.

The times we have been out to Leak detection’s when the leak should be picked up by a standard heating engineer, well let’s just say is quite a few.

Now I have been working in the field of plumbing and heating for 20 years and do understand that finding a leak on a heating system without some pretty complicated tools, can be like looking for a needle in a hay stack but when there is a boiler losing pressure we must get to the cause of the problem.

Although through our Trace and Access service, our aim is to save our customers money (sometimes thousands of pounds!!!) by locating their leak under the floor with accuracy, most of the time, we are called in as a last resort to carry out a thorough Leak Detection because the plumber or heating engineer exhausted their effort in finding the cause of the boiler pressure loss after being there several times.

Without rambling on, the fact is we can’t save our customers money if a totally obvious fault has not been picked up by the Plumber. So we have written a list of obvious faults and how to diagnose them. If you go through this list and still need help, just give us a call and we will be pleased to help.

Faults to look for if your Boiler & Heating System is Losing Pressure

It’s always worth remembering that when a boiler losing pressure, in the range of 1.5 bar to 0 Bar on a standard 3 or 4 bedroom house, the system is probably only be losing 300 – 400ml of water when losing all its pressure. That’s not a lot of water and such a leak may go unnoticed in the most obvious places.

Gravity Feed Systems & Boiler Pressure Loss

First of all, let me explain a little about what you’re looking at, without going into technical mode! There are two types of heating systems.

The first we are going to talk about is a gravity feed system. This system is filled by a small cistern in the loft called a Feed and Expansion Cistern.

I have made a special effort to call it a cistern, as I can always remember those dulcet tones of my tutor Mr Jones when I went through 5 years of training all those years ago, “Tanks are for fish, we have cisterns!!!!”

Anyway, moving on!

The cistern is filled by a Part 2 ball valve, which is set below the overflow level.

Its job has two parts. First is to make sure that water is always in the heating system. A pipe, normally on the side at the lowest point fills the system.

Then a second bigger pipe returns back from the system a different point of the heating circuit and loops over the Feed and Expansion Cistern.

The common name of this system is an open vented system, which takes us to its second job.

When water heats up from room temperature to boiling point, it expands by roughly 4%. So for example, say you have 100 litres of water in your system when it’s cold.

When you turn that thermostat up, the water may start heating to around 80 degrees and circulating around the system.

That temperature change expands the water by about 4%, giving you 104 litres of water in your system. So as the name implies it feeds the heating circuit and allows for expansion.

Pretty important if you would like stop your pipework blowing apart at high pressure!

Gravity Feed System Leaking Causing Boiler Pressure to Drop

“How do I tell if there is a leak on my open vented system?” I hear you cry!!

Well the simplest way is to isolate the ball valve feeding the cistern and see if the water drops in the cistern. Please note, if you’re using the system for your heating or water while the isolation test is in progress, its hot water and can evaporate so keep an eye on it.

You don’t want your heating system running dry. Circulation pumps can be expensive to replace! If you don’t fancy doing that then a visual inspection to see if the Part Two ball valve is constantly filling the cistern.

I recommend that this test is done while the system is not in use. It should give you a lot of information, unless the leak is thermally activated, and I think we would need a whole new blog for that topic!

If the Feed and Expansion cistern drains down or the ball valve continually drips, there is a leak somewhere on the heating system.

At this point the leak has to be on either the boiler, or pipework call us at ADI Leak Detection on 0800 731 3843 if you need help with finding the leak and solving your boiler losing pressure.

Sealed or Pressurised System Causing Boiler Pressure To Drop

A sealed or pressurised system is literally a heating system without a Feed and Expansion cistern. The way of filling the system is via a filling loop, normally a silver braided hose connecting from the category 1 water supply with an isolatable lever valve and double check valve.

This then connects to the braided hose, and in turn is then connected to another lever valve connecting to the heating circuit.

The reason for the two leaver valves is so that when they are both isolated, the silver braided hose can be disconnected to prevent water from your heating circuit pushing back in to your fresh water.

The Double Check Valve is also there to protect your water supply from water coming back from your heating circuit.

A pressure gauge is normally within close proximity, allowing you to fill the system to the correct pressure. Why is this protection necessary?

Let’s put it this way, would you like to have your cold drink or cuppa with a dash of old sludgy water that’s been going around your heating system for years? If you are of sound mind, I would probably say no!

Moving on back to the sealed system, remember there is no Feed and Expansion Cistern like the gravity feed, open vented system and the same expansion rules apply, water still expands 4% from cold to hot.

To take the expansion on the sealed system, an expansion vessel is fitted. On a heating system, these are usually red.

Central Heating System Expansion Vessel - cause of boiler losing pressure?

The vessel has an inflatable bladder inside it.

In most heating systems, the bladder is inflated to 1 bar with air. If your system is set at 1.5 bar cold and has 100 litre of water, there will be approximately 104 litters when hot.

The expansion on the water pushes against the air in the bladder. This controls the pressure from shooting up into the possible 3 bar plus area and dumping the water out of the safety Pressure Relief Valve (PRV).

If the Expansion Vessel is correctly charged with air and correctly sized, there should only be a small rise in pressure when the system is hot, and it should then drop back down to 1.5 bar when cold.

Sealed or Pressurised System Leaking

On your Sealed or Pressurised System, as mentioned in the previous paragraph, there is a filling loop or a way of pressurising the heating circuit at the boiler. Most pressurised systems have a boiler with a pressure gauge, if not there is usually a pressure gauge by the filling loop if not part of the boiler.

The best way of finding out if there is a leak on the heating circuit is to check the pressure gauge and see if there is any drop in pressure. If unsure set the gauge pressure to 1.5 bar by opening the filling loop and allowing water to enter the system to the correct pressure. Do this when the system is cold!

Let’s take for example a heating system with 8 radiators. Believe it or not, a leak of around 300 – 400ml will cause the pressure to drop all the way to 0 bar, it doesn’t take a lot of water loss for your boiler to lose all its pressure. If there is no pressure loss there is no leak.

As pointed out earlier, water expands by approximately 4%. With this in mind, there will be a slight fluctuation on the pressure gauge.

So the best time to check the pressure on the gauge is over a 24 hours and when the system is cold. Here you can see the black needle pointing to zero, in this case the boiler has lost all its pressure.

The red needle on this gauge can be manually adjusted, in this case it shows where the black needle should be – at 1.5 bar.

boiler losing pressure

If the gauge shows the system has lost pressure, then water is going somewhere! Obvious I know. The question now is what size leak am I looking for?

I will normally try to visualize the leak quantity. In a leak detection situation, the process helps me understand what the faults can be! Let me give you an example. If it takes exactly 24 hours to go from 1.5 bar to 0.1 bar, on a 12 rad system, let’s assume that the amount of water being lost is 500ml.

That’s a loss of water of 20ml per hour

That’s a loss of 0.35 ml per minute

That’s a loss of 0.005 ml per second

Let put things in to perspective. The scientific community have found that an average water droplet contains 0.05 ml, that’s 20 drops per ml

So if your system is losing the pressure and amount as above, that is a droplet every 10 seconds. Just to prove, it don’t take a massive leak to create a problem.

In fact, even if your boiler is losing all its pressure in 1 day, the leak is still pretty small! We generally ask our customer’s to wait until the boiler is losing its pressure at this rate. This indicates to us that the leak should be big enough to be found and we could solve the mystery of your boiler losing pressure for you.

Things to check if your boiler is losing pressure

Is Your PRV dripping Causing Pressure Loss?

On all sealed, pressurised systems, there are safety devices fitted. Normally in the form of a PRV, a Pressure Relief Valve.

The purpose of this valve is to relive the system of pressure if the system over pressurises. For example, we have talked about the expansion of water, so if we go back to the example of 100 litres of water expanding 4% to 104 Litres, remembering that 500ml can drop a pressure gauge from 1.5 bar to 0 bar, an extra 4 litres is going to dramatically increase the pressure in the heating system.

Now, if the expansion chamber is not working correctly there is nowhere for the expanding water to go. So the valve’s job is to open and release the pressure, otherwise the pipework or associated fittings or appliance will go pop. Not a good idea!! So it’s an important component of your heating system.

The PRV is attached to a piece of copper that terminates externally and should be pointing back to the wall, so should the worse happen, the hot water is pushed against the wall to cool the water down and disperse the water safely.

Heating PRV - boiler losing pressure

faulty heating system losing pressure










Now we know what this does, a good idea is to check this pipe externally, to see if it’s dripping. Remember it only takes a droplet 10 seconds or so to lose all pressure in a day. If any droplets are evident, this may be your problem. Picture above of pipework shows pipe coming down vertically from expansion chamber. Pipework in this case runs to a plastic viewing window commonly known as a tundish. These are commonly fitted in new builds and will give you a visual display if your pressure relief valve is letting by.

Is Your Expansion Vessel Losing Pressure?

Heating expansion vessel

We have touched on the expansion vessel in previous sections. But believe me that this piece of equipment and the problems it can cause is overlooked by most engineers and plumbers.

As mentioned, the vessel is normally red and is attached to the pressurised heating circuit. The size of the vessel should reflect the water volume of the heating system.

Undersized, the vessel will not be adequate to take the expansion of water, therefore pushing the pressure up on the pressure gauge to high. It could also be opening the Pressure Relief Valve when the expansion is too much as the sole job of the expansion vessel is to take the expansion of the water. The most obvious place for these to be are by the filling loop area or integrated into the boiler.

The purpose of this unit, is to take up the expansion of the water. The vessel is fitted with a bladder which is normally set at 1 bar, a manufacturers label should be attached for correct pressures. The bladder is filled with air via a Schrader valve, the same as to inflate a car tyre.

When checking the pressure in the vessel, it import to remove some of the water from the heating system, otherwise the pressure reading on the bladder in the vessel will be incorrect. The reading will include the pressure of water in the heating system, so it is most important to make sure that there is no water pressure pushing on the bladder.

The most common faults on the Expansion vessel are the Schrader valve leaking and deflating the Bladder inside the vessel or the bladder itself has perished and not able to hold air pressure.

Either way, the faults above result in the over pressurisation of the heating circuit, activating the PRV.

Are Leaking Radiator Valves Causing Your Boiler Pressure To Drop?

As worked out earlier, loss of water of 0.005ml per second is not a very big leak, so before spending your money on ADI’s Non Invasive leak detection, it may be a good idea to check radiator valves.

If several valves are leaking on the gland nut, the water may be evaporating before it is noticeable. Take the heads off the valves and just run a piece of tissue around the head of the radiator valve to see if it picks up any moisture.

leaking radiator valves causing the boiler losing pressure

Does You Heat Exchanger Crack or Leak Causing Boiler Pressure Loss?

This is probably a little tricky to check but there are some tell tail signs. Most boilers in operation today are condensing boilers, so if the heat exchanger in the boiler is leaking, the water will be dripping into the combustion chamber.

In simple terms, because of the design of condensing boiler, the heat escapes from the bottom of the combustion chamber, allowing the boiler to utilise the latent heat, hence making the boiler energy efficient. When the gas ignites in the combustion chamber, one of its by-products is water.

This water has roughly the same PH value as a tomato, which is very corrosive!! Hence the condensate trap in the boiler. The trap is connected to 20mm plastic pipework, which then terminates to a point that the water cannot do any damage, i.e. drainage system of lime trap.

So what happens if the heat exchanger is dripping into the combustion chamber?

Well one possible outcome, is incomplete combustion. The water drips on to one or several of those beautiful blue, clean burning flames crating flame impingement. In other words, creating a yellow flame.

A yellow flame will create soot and carbon monoxide. If incomplete combustion is occurring, I would like to think that your registered gas engineer would pick that up while servicing your boiler. Or one of the tell tail signs is carbon deposits in the Condensate trap.

Although is worth getting a gas engineer to check it out, remember, the leak may be so small that it is not visible to the eye without really looking for the leak.

Condensate dripping when boiler not firing Causing Boiler Pressure Loss?

If the system is leaking a little faster, it may be possible to disconnect the condensate pipe once the boiler has been switched of for a little while. The only water coming from the condensate pipe, is the condensation that is the by-product of the burning gas. If the boiler is not burning gas, there should not be any water vapour. In short if the condensate pipe is dripping while the boiler is not in operation, then it’s a probable leak on the heat exchanger.

Condensate dripping when boiler not firing

AAV not Sealing

Automatic Air Valves are used to purge air from the heating system automatically. They operate by a small float in the valve that drops when there is air and then floats back to the top of the valve when the air is expelled and the water is back in the valve, sealing the valve until air needs to escape. They are normally fitted to high points on pipework, where air can be trapped. Like all mechanical devices they can fail, letting water leak from the outlet. Again only a droplet now and again may cause a drop in pressure on the boiler pressure.

aav not sealing causing boiler losing pressure

Pipework Leaking above Ground Level

As a general rule, if pipework is leaking from above ground level, for example the first floor or above, the leak should be pretty obvious.

Water damage or damp should be visible. It may still need the services of a professional leak detection company to locate the exact location. However a regular plumber may be able to remove the floor above the visible leak area and gain access to the damaged pipework.

Pipework leaking below ground level – A prime cause of Boiler Pressure Loss

Leaks on the pipework below ground level. Under concrete, screed or even wooden raised floors is a completely different issue!!

The only way of finding a leak under these floors, is to expose all the pipework. As you can imagine, this can be very costly, very messy and if there are multiple rooms, very time consuming! That’s where we come in at ADI Leak Detection.

At ADI Leak Detection, we use the most advanced techniques to locate the leaks under your floor, saving mess, time and massive expense. our web site will give all the ways we find leaks and how we can solve your Leak location problems efficiently and most of time even repair on the same day.

Please note the content of this blog is for information purposes only. Please contract or seek advice from the relevant qualified trades. We hope that this blog is helpful in sourcing your leak. Please feel free to contact us at ADI Leak Detection if we can be of any further help

Adrian Morgan MCIPHE RP RHP

Head Engineer