Plumbing

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Leaking Combination Boiler Overflow Pipe: What You Need to Know

29th April 2022

If your overflow pipe is leaking, not only are you losing water unnecessarily, but it may be an indication that you have a problem with your boiler. Your overflow pipe prevents flooding in your home by removing excess water from your storage tank or cistern. Many items of basic plumbing in a home will have an overflow pipe, including toilet tanks, cold water feeds, copper cylinders, baths and sinks, and boilers. First, we will take a look at combination boilers, the most common cause for constantly dripping or running overflows, and what you can do about it.

Combination boilers

Most modern homes have a combination or "combi" boiler. A combi boiler is efficient, providing both heating and hot water without requiring large amounts of space. Unlike traditional boilers, which heat up water and store it in a storage tank until needed, a combi boiler heats water as it is required, providing hot water whenever needed.

Combi boilers are fitted with a pressure release valve, which enables water to be released when the pressure in the boiler gets too high. The overflow pipe is made of copper and goes from your boiler to the outside of your house. The overflow pipe will not release any water unless there is a problem.

If you notice a pipe leaking

If you notice that a pipe from your boiler is dripping outside, double check to see what type of pipe it is first. If the pipe is plastic, it is most likely to be your condenser pipe; the pressure release pipe will always be copper. You may not even be able to see your condenser pipe as often that runs directly to your waste.

Why is your overflow pipe leaking?

There are several reasons why your boiler overflow pipe might be leaking, but most commonly it will be because either the pressure release valve is incorrectly fitted, causing it to let water pass through, or because the pressure within the boiler has got too high as the result of a fault with the expansion vessel.

If you do notice that your overflow pipe is leaking it is best to get in touch with a gas safe engineer who will be able to come and assess the situation. It is important that any engineer you work with is gas registered or that they work in partnership with a gas safe engineer who will certify their work.

You can avoid, or at least anticipate, potential issues with your boiler by getting it serviced regularly. All of the traders registered in Trust A Trader’s online directory have been verified by us. To find a reliable, vetted and rated gas safe engineer near you, take a look at our directory. For more tips and advice, join our friendly community of reliable traders and happy customers on Facebook or Twitter.

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Find clear, practical answers to common plumbing questions, helping homeowners understand everyday issues, know what checks they can carry out safely, and when it is best to contact a qualified professional.

What should I do if I have a blocked drain or sink?

Start with a plunger - create a good seal and use firm, steady pressure. For a kitchen sink blocked with grease, bicarbonate of soda followed by vinegar can help. If that's not shifting it, a drain snake reaches further down the pipe. Go easy on chemical drain cleaners as they can damage older pipes and rarely sort the root cause. If the blockage keeps coming back, affects more than one outlet, or you're getting unpleasant smells, call a plumber rather than trying to patch up the problem.
How can I tell if I have a hidden water leak?

The most obvious clue is a water bill that's gone up without any change in your usage. Beyond that, listen out for running water when everything's turned off, and watch for damp patches on walls or ceilings nowhere near an obvious water source. A quick self-test is to turn everything off and watch your water meter - if it's still moving, there's a leak somewhere. Tracking down exactly where it is usually needs a specialist detection kit, which a plumber can bring in without having to rip everything up.
What should I do if I have a burst pipe?

Turn off your stopcock straight away - you'll usually find it under the kitchen sink or where the mains water enters the property. Once it's off, open your taps to drain what's left in the system, switch off the boiler if you can do so safely, and mop up any standing water before it reaches electrics or flooring. Then call a plumber. With burst pipes, the faster you act, the less damage you'll be faced with.
What is a gas safety certificate and do I need one?

A Gas Safety Record is issued by a Gas Safe registered engineer after checking that all your gas appliances, flues, and pipework are safe. Landlords in England, Scotland, and Wales are legally required to get one every year and give tenants a copy within 28 days. For homeowners it's not a legal requirement, but getting a check done alongside your annual boiler service is a smart idea, particularly in older properties. Only a Gas Safe registered engineer can carry out the work or issue the certificate - you can check anyone's registration on the Gas Safe Register website.
Why is my water pressure low and can a plumber fix it?

Low pressure usually comes down to one of a handful of things: a stop valve that's not fully open, limescale build-up in older pipes, a failing pressure-reducing valve, or an issue with your water provider's mains supply. In most cases a plumber can sort it - though if the problem is with the mains itself, you'll need to contact your water company first. Either way, don't just live with it - ongoing low pressure can sometimes point to something more serious.
How do I know if my boiler needs replacing or just repairing?

If it's under ten years old and it's one specific fault, a repair usually makes sense. If it's over fifteen, has needed multiple fixes recently, is making strange noises, or your energy bills have crept up for no obvious reason - replacement is probably the better investment. A useful rule of thumb: if the repair would cost more than half the price of a new boiler, it's probably time to replace it.
How often should I get my plumbing inspected?

There's no legal requirement for homeowners, but every two to three years is a sensible habit - especially in older properties or hard water areas where limescale builds up over time. Your annual boiler service covers some of the associated pipework too. And if anything seems off between inspections - odd noises, discoloured water, unexplained damp - don't wait. Get it looked at sooner.
Why do my pipes make a banging or knocking noise?

That banging sound (sometimes called water hammer) usually happens when water flow is suddenly stopped or redirected, sending a pressure wave through the pipes. It's most common after flushing the toilet, shutting a tap sharply, or when a washing machine valve closes. Pipes that aren't properly secured can also knock against walls or joists. It sounds alarming but it's often an easy fix - a plumber can figure out whether it's a pressure issue, a faulty valve, or just some loose pipework.

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