I’ll try my best to put some pretty technical details into layman’s terms so bear with me…

As you might have read on the main heating page a central heating leak, it is not something to ignore! However, there are several reasons why boiler pressure can drop. Before destructive works are carried out, it is vital to undertake a proper diagnosis of exactly “why does my boiler pressure drop?”.

So…Why does my boiler pressure drop?

First, we need to understand how fast is the boiler pressure dropping and understand the sort of quantity of water it takes to drop that pressure from 1.5-bar cold to 0-bar. It’s pretty much about 5 litres on average, often we are told “But I open the valve for ages!” but this is different to filling the sink…

The water passes through a small valve and then has to push against the water within the central heating system, as the pressure rises there is more and more pressure pushing back against the water supply, normally about 3-bar. Now we know the rough amount of water, we can calculate the loss across time, and this will often be a surprisingly small leak.

A common customer call will be “My boiler pressure is dropping every day or so” or “I have to top up my boiler every day or so”, an average leak is about 2 days, so 5 litres divided by 48 hours is about 100ml per hour.

That 100ml is a small glass of wine and (not the generous slosh you pour yourself after realising you have a leak!) but a small wine glass filled to 2 thirds…

In leak terms, 100ml is 200 drips or a drip about every 20 seconds, and that is not a big leak… Now think that the water will often move and not be a consistent drip and is heated, evaporating and flash drying off the pipework… This is why accurate leak detection is an art that requires skill, experience and not only the right but also the best equipment available!

This is why we are straight when we talk to our customers, a leak reported as 0.5-bar in a week is going to be a tiny amount of water, our competitors will book “Mission Impossible” knowing it is doubtful they will find the leak, but you are getting a bill regardless! UK Leak detection would rather tell you it’s going to be a problematic leak and recommend ways to make our visit productive. Often this is just adding a simple and cost-effective bottle of system cleaner before our visit.

Several issues can be quickly checked before confirming a leak on the pipework, and about 10% of our leak detection finds issues with components rather than pipework.

Any-hoo, enough, maths!

Reasons for boiler pressure loss

Other than a leak in the pipework that we can locate with thermal imaging, acoustic location and tracer gas detection, there are common faults that can go unnoticed or are ignored as too small to cause the pressure loss…if it’s dripping, it is a problem!

Radiator gate valves, air bleeds and thermostatic valves

These are a common fault and will often leak when hot but flash dry in the heat, a system with radiators of a similar age will often have the same leak on several valves (particularly thermostatic), and rather than a single leak it is several tiny leaks that ultimately cause the boiler pressure to drop.

Expansion vessels

These act like a shock absorber on a car, if you remember your physics water is uncompress-able and it’s volume expands as it is heated.

In a gravity system, there is an open tank in the loft or at a high level, and this rises and falls as required, it also has a valve to auto-fill the system so often a leak is not noticed until it is huge.

In a pressurised heating system this expanding water needs to have somewhere to go, this is a pressure vessel (often part of the boiler) a chamber with a diaphragm or balloon filled with air. As the uncompress-able water expands, it compresses the air in the balloon/diaphragm and as the water cools the air expands, returning the pressure to its original level.

If however there is a puncture in the balloon/diaphragm or the pressure vessel has not been maintained (there is a car tyre valve to blow it up) then the water expands into space normally filled by the balloon/diaphragm, and this looks like a pressure loss in the boiler. The system is filled, and the next time the water has nowhere to expand and causes the pressure relief valve or PRV to open and discharge excess pressure to waste, the system cools and triggers a boiler low-pressure fault as there is no compressed air to stabilise the pressure level.

Pressure relief valve or PRV

These act to protect the system from excess pressure and are normally set to 3-bar, were the pressure to rise too high there could be damage to the system, and this could cause a burst component or even cause injury were scalding water to escape from a burst pipe or radiator. These are often triggered by poor maintenance of the pressure vessel or overfilling when ‘topping up’, but can fail, causing a loss of pressure.

Heat exchangers

Every boiler must have heat exchanger, and it’s no more technical than boiling a sealed saucepan on the hob a fire/water heat exchange. Water is pumped around and gets a little hotter every time it passes through the heat exchanger (or saucepan) until it reaches the temperature we set on the boiler (if it has an option) or the room thermostat tells the boiler that the area is hot enough.

What can happen is that the heat exchanger leaks into the combustion chamber (our hob) on an older boiler this is pretty obvious as the water has nowhere to go and like our hob would overflow onto the floor. However, with a modern condensing boiler (pretty much anything newer than 2005) there is a waste pipe that takes this away. This waste pipe normally takes the vapour you might notice on a cold day from the boiler flue very much like a car exhaust when cold.

Diagnosis is tricky unless the condensation trap gives the game away with brown rusty magnetic partials and brown rust staining in general; our trace gas will often confirm these leaks as boiler faults.

 Immersion Tanks and secondary heat exchangers

For hot water, a combination (or ‘Combi’) boiler will have another heat exchanger, this time a water/water version. Cold tap water runs through a matrix of small pipes and is heated by water from the heating system running in a similar set of small pipes, the 2 systems never mix, but the proximity makes the tap water warm. These can fail, and then the heating water ends up in your hot tap water, these leaks are often rapid.

An Immersion tank is a much simpler system, much like melting chocolate using a bowl in a pan of water, a coil of pipe in the tank warms the water around it, cold water enters the bottom of the tank, and hot water is drawn from the top. On a gravity hot water system (big cold tank in the loft, not to be confused with the small heating tank on gravity heating) a fault will see this water leak into the tank. The pressure drops, on a pressurised system (normally a taller white cylinder) a fault causes the pressure to rise, not drop. Faults on pressurised systems are rare, but they do have similar PRV’s that can fail.

There is no need to suffer and be cold! As you can see, several things need an investigation to locate your leak, and our engineers will test these as part of our leak detection along with other parts of the system as required. Often the fault is a simple fix, and there is no need for destructive works or lifting flooring etc…

Call us now on 0800 228 9000 for your leak detection