How Big is your Hidden Leak?

Is your Central Heating System Leak big enough to be found Non-Invasively??

Our dream is to be able to guarantee to find every single central heating leak we receive an inquiry about.

But the reality is that some leaks are simply too small to be found non-invasively.

Make no mistake, our aim is help each and every one of our customers save money that would otherwise be spent rectifying damage caused to your floors in the process of finding and accessing your leak.

The last thing we ever want to do is book in a Leak Detection which we know is going to be unachievable, just for the sake of getting extra bookings in our calendar.

Some Leak Detection companies will book you in with no questions asked, but we believe in being fair and honest with our customers, however we much prefer to ensure that our Trace and Access service is perfectly suited to your problem before taking your booking.

We wholeheartedly believe that we provide a value for money service, but only if we can first establish that your leak is big enough to stand a good chance of being found.

We’ll ask you a series of questions to find out about this, so please bear with us if it feels like you’ve ended up on a quiz show when you make your first phone call to us!

Read on to find out some of the questions we will ask you over the phone before taking your booking. By the end of this article, you might even be able to work out the answer yourself!

We go into quite a lot of detail here, so if you find any of the information going over your head, just give us a call and together we will get some clarity on your situation.

You may also wish to read about How a central heating system works to increase your understanding.

Is your Combi boiler losing pressure and if so, how often?

(Disclaimer: Please remember that we are not boiler engineers. Boilers may lose pressure for reasons other than a leak. If in doubt please feel free to give us a call or contact your boiler engineer for advice specific to your boiler’s age, make and model.)

For many people a boiler losing pressure is the first sign of a leak. We can roughly work out the size of your leak by considering how often your boiler loses pressure and how many radiators there are altogether on your heating system.

If your boiler loses pressure slowly of a period over weeks or months, that’s a sign that your leak is minuscule! A very slow weep or a leak that may not be visible even if the pipe was exposed. While this rate of pressure loss can be annoying, as it does mean you’ll have to keep repressurising the boiler until the leak gets worse, we would not recommend booking a Leak Detection at this stage because your leak will be virtually untraceable.

However if your boiler loses all its pressure within a day or several times a day, this is a sign of a bigger leak that we will stand a much better chance of finding.

Side note: It might surprise you that such a fast rate of pressure loss is required for successful leak detection. But remember, it doesn’t take very much water at all to completely depressurise your boiler, actually much less than people tend to imagine. In an average 3 or 4 bedroom house with about 12 radiators, you’re probably talking about three to four hundred millilitres or half a pint of water as depicted in the photograph below:



It might feel like a lot when you re-pressurised the boiler because the water being added to the system is working against the pressure contained in the heating system.

But now visualise that much water (roughly equivalent to a small can of fizzy drink) coming out of your pipe gradually over a day. Now the leak probably doesn’t seem so big?

If your central heating system is ‘gravity fed’, i.e. fed by a header tank in the loft, rather than a Combi boiler, we might ask you to tell us how quickly the tank is refilling but it is a bit harder to get an exact measurement of the size of the leak.

What are your floors and sub floors made of on the downstairs of the property?

If your leak is on the first or second floor of your property, you will probably have a rather undesirable water feature coming through your ceiling! And that will give you a very good idea of the location of your leak. So the vast majority of leaks we investigate are on the ground floor. The material of your floors and sub floors will have a huge impact on the methods we use to detect your leak.

Sub floors

Are your sub floors solid, with the central heating pipes buried in a material which surrounds them completely (such as screed)? Or are they in a void underneath suspended timber beams?

If they’re solid, when the water leaks out of the pipe, it will soak into the surrounding material. Providing you haven’t got a non-porous material over the top of the sub floor, we should be able to pick up damp traces and thermal readings from the leak area, as well as gas traces and acoustics.

This is great news because the more evidence we can obtain from different leak detection methods, the more confident we can be that we have pinpointed the leak location correctly.

However if your pipes are underneath a suspended floor, when the water leaks out of the pipe it will drip down into the void. This renders the thermal imaging camera and damp meter more or less ineffective, forcing us to rely more heavily on our gas sensors and acoustic devices. Even then, our tracer gases will dissipate into the void underneath your floor before they rise up and can be detected by the gas tracer.

For these reasons we say that if you have concrete or screed floors, we recommend booking your Leak Detection only when your boiler is losing 1.5 bar of pressure consistently once every 24 hours or less.

However If you have suspended floors we recommend booking your Leak Detection only when your boiler is losing 1.5 bar of pressure consistently twice every 24 hours (or once every 12 hours) or less.

If you want to book your Leak Detection even though your boiler is losing pressure less frequently than the above recommended time periods, that’s fine! We understand that you could have a responsibility to others to try and resolve the leak, or maybe you’re trying to sell the property. Perhaps it’s just been going on for ages and it’s driving you crazy, or you’re worried about damage.

We will ALWAYS do our best to help you but please understand that it will be much harder for us to find a leak that falls short of the above recommended pressure loss rates.

By the way, if you don’t know whether you have solid or suspended sub floors, we can usually make an educated guess depending on the age of the property. Newer properties are almost always constructed with concrete floors.

Or if you bounce up and down gently on your floor and it sounds hollow underneath, chances are, it’s a raised floor! If it feels solid, it’s probably concrete/screed or block and beam. Please don’t jump too hard because we would not want you or your property to incur any unnecessary damage 😉


The porosity of your floors will also affect the ‘findeability’ of your leak. Porous floors allow moisture and tracer gases to penetrate, while non-porous floors do not.

If you have materials like celotex, insulation, a damp proof membrane, or vinyl in your floors, or even Denzo tape around the pipes, these will prevent moisture, heat and tracer gases from penetrating through. This means the majority of our Leak Detection methods will be rendered ineffective and we will pretty much be relying on our ground microphones to pick up the sound of tracer gases escaping from the leak. 

If your floors have non porous materials in them, it’s even more important that your leak meets the size requirements described in the above section!

Is there any Leak Sealant in your Central Heating System?

Leak sealant is a chemical that you can add to your heating system that is sometimes capable of sealing very small leaks. We have found it is rarely a long term solution on larger leaks. From our professional experience, it is something that we never recommend even if your leak is really small.

There are two main reasons for this.

  • Firstly, it may invalidate the warranty on your boiler, so check this with your boiler manufacturer if you do choose to use it.
  • Secondly, if the sealant doesn’t work and you later want us to try and locate your leak non-invasively, the presence of leak sealant in your central heating system can significantly impede the leak detection process.

Without going into too much detail, what we have found is that when we fill the system with tracer gases, this causes a reaction which activates any residual sealant, sealing the very leak which we are trying to find! This may not sound too bad until we tell you that again, this is not a permanent fix!

There are currently no non-invasive ways of actually fixing a leak on underground heating system pipework, and there is no substitute for locating the leak, exposing it and replacing the section of pipe or fitting that’s leaking.

If there is leak sealant in your heating system, we recommend appointing a plumber to try and clean out the sealant, either with a chemical cleaner or by power flushing. Even if the sealant was put in a long time ago and you think it should have leaked out by the time you book your appointment, it probably hasn’t, as it is a very stubborn chemical.

It might be worthwhile talking to the suppliers of your leak sealant to get some advice on how best to remove the chemical.

You might be asking yourself, why don’t we just stick to thermal imaging and damp meter testing? We can try that of course, but now you know the potential limitations of these methods with certain floor types.

How many radiators are there altogether on your heating system?

As well as helping us establish the findeability of your leak, this will also tell us how many engineers will be required to perform the leak detection. Draining down and re-filling a central heating system, along with all other aspects of the investigation, is quite a process and we find that there is a maximum upper limit for one Engineer to work on alone.

So we’ll ask you to count up your radiators (including towel rails and under floor heating circuits if applicable), and we’ll need you to answer as honestly and accurately as you can!

So there you have it. A bit of insight which you can use either to find out if you’re ready to call us or to prepare your answers to our ‘Leak Quiz’ which you’ll encounter when you phone us! We may not need to ask you all those questions, or we might pick up clues which cause us to ask you even more.

We like to think that it’s worth it, and most of our ‘could be’ customers are grateful not to have been charged for a Central Heating Leak Detection investigation, only to find out their leak is too small, when really all it would have taken was a free conversation with one of us at ADI Leak Detection.

So whether you have more questions, or you’re ready to book your Leak Detection, give us a call on 0800 731 3843. We will be more than happy to take your booking or simply give you some free advice. 

Adrian Morgan

Adrain Morgan CIPHE RP RHP is the founder & Managing Director of ADI Pro Leak Ltd. Adrian is also the Head Plumbing, Heating & Leak Detection engineer at ADI Leak Detection.

Adrian CIPHE RP RHP is proud to be registered with the Chartered Institute of Plumbing & Heating Engineering, a Registered Plumber and a Registered Heating Professional.