If you’ve tried to do the washing up only to find there’s no hot water again, it could be a sign your boiler’s cut out.

A combi boiler requires pressure to run. The pressure in the system helps the warm water to circulate around your heating system – while also helping to protect the delicate internal components within the boiler itself. If the boiler pressure keeps dropping, the boiler shuts down.

Since your boiler is used to power your central heating system and provide hot water, losing pressure in your central heating pipes also means you’ll lose the ability to heat your water – since it’s the same heating mechanism within the boiler that warms both.

Time to top the pressure up?

You might find that well-meaning people suggest you simply “top the pressure” up on the boiler and try again to wash the dishes – but this can be a bad idea.

Your boiler has lost pressure for a reason – and topping the boiler pressure back up with the filling loop isn’t the solution to any of those problems. In fact, continually topping the pressure up can cause even bigger problems.

Why?

Well, your central heating system is filled with inhibitor fluid – a blend of chemicals that are mixed with the water in the system to prevent corrosion and sludge building up. Boilers that are losing pressure could be leaking, so topping the water up means you might be diluting this fluid; potentially causing long-term damage inside the pipes – and serious leaks.

This kind of damage is exactly what insurance assessors will look for if you make a claim for a water leak further down the line – so don’t be tempted to keep topping up – it could be an expensive mistake!

You might like: How to Drain a Pressurised Central Heating System

What’s the problem likely to be?

There are generally 3 reasons your boiler is losing pressure – so let’s take a look at each one in a little detail – and think about what you can do to solve each problem.

Reason 1: You’ve got air in the system

Air that’s trapped in your heating system is the easiest loss of pressure problem to fix – so it’s a good place to start your checks. 

Air is much easier to compress than water – so, as your heating temperature increases and the pressure naturally goes up, the air in the system compresses instead of the water, causing the system pressure to drop off again. The more air that’s in the system, the more pressure you’re going to lose.

How to fix trapped air

If you’ve got air in the system, you simply need to let it out by bleeding the radiators. You’ll need a radiator bleed key, a towel, and you might want to put a pair of rubber gloves on, just to make sure you don’t scold your hands.

Turn your heating on and let it warm up. When it’s close to being warm, you should carefully open the bleed valve on the radiator. If there’s trapped air in the radiator, you’ll hear a hissing noise, similar to letting down a bike tyre. Don’t take the bleed key off the valve – instead, be ready to tighten it back up again when the hissing stops and the air has fully released. 

It’s good to have the towel to hand just in case you accidentally let a bit of water out too – as it could be discoloured and stain your carpet or clothes. 

Work your way around the full house and repeat the process on each radiator and towel rail. When you’ve finished, you’ll need to let the system cool and top the pressure up slightly. If air was the problem, your system will now hold its pressure – but if it doesn’t, you might need to explore further. 

Reason 2: The boiler’s expansion vessel is faulty

Some combi boilers have what’s known as an ‘expansion vessel’ nearby. An expansion vessel is a small additional tank that’s used to take up any expansion caused when the water in your system heats up. If this tank’s got a fault, it could mean that it’s either letting air into the system, or it’s letting water out – neither of which is good news for your pressure.

How to check for a problem

Your expansion vessel will have a pressure release valve – and, it may spring a leak from here if it’s damaged and can’t handle the additional pressure. Although each vessel is slightly different – you should be able to find the release valve easily enough – and check if it’s got water coming out of it. 

If this doesn’t seem to be the case, you might find that the diaphragm inside the vessel is ruptured. To test this, you should take the dust cap off the valve on the vessel (it’ll look like the valve from a mountain bike or car tyre). Carefully press the pin that’s inside the open valve – and if water comes out instead of air, then you’ll almost certainly need to talk to a heating engineer about replacing the vessel.

If you get to this stage and you still can’t find an issue with the vessel – it might need pressurising. This will be a job you need a professional to do for you, since it will need to be isolated from the system, before being drained and pressure tested.

Reason 3: There’s a leak 

If you’re sure there’s no air in the system and your expansion vessel is operating as it should – the next logical step is to assume there’s a leak somewhere in your central heating pipes. 

Now, leaking water in your house can be very difficult to find – and even if you can see evidence of a leak (stained ceilings, damp patches, etc) it doesn’t mean the leak is actually where moisture’s appearing. Water has an annoying habit of appearing a long way from the actual leak – so it’s time to get a professional leak detection firm to help.

Fortunately, almost all buildings insurance policies cover you for ‘trace and access’ – the process involved with locating and getting to the source of any leak – so it’s unlikely that you’ll need to dip into your savings.

How do leak detectors work?

Don’t worry. Leak detection might sound like a process that involves lifting your floors and taking plaster off the walls – but that couldn’t be any further from the truth. Now, professional leak detection companies use specialist cameras and tracing equipment to test your home without even needing to lift a rug or move your furniture – and they can work through a huge range of materials – even concrete and brickwork.

If you’ve ruled out everything else and think you’ve got a leak, you should act sooner rather than later. Water can be extremely destructive if left – and it’s only going to get worse if you top that pressure up again…