Category - Boilers
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Will Scholfield, Engineer

9 Jun : Updated 27 Feb ● 5 min read

What does ‘boiler pump overrun’ mean?

After a long day of running your boiler, your home should be warm and toasty enough to switch it off. But when you double-check your boiler pump before heading off to bed, it's displaying a 'pump overrun' code or still running and making a slight noise

Before you start to worry that you'll have to replace your boiler, there are a few things you'll need to understand. You need to know what a pump overrun means, why it happens, and how to fix it if it's overrun for hours on end.

Luckily, that's where we come in. Our complete guide will provide you with everything you need to know about a pump overrun and what to do if you find a problem with your boiler.

What does ‘pump overrun’ mean?

Pump overrun means that your boiler pump is cooling down and getting rid of residual heat. This is a normal function on most modern Standard boilers and will take around five to ten minutes to complete.

When your boiler is displaying a pump overrun code, it's more than likely because you have just turned your central heating system off. The pump overrun occurs when you have used hot water or when you have used your heating.

It's perfectly normal for your boiler to stay running for around ten minutes after switching it off.

In bigger (and older) boilers, the pump overrun feature is unnecessary. Instead, these models are built to handle the residual heat until it has dissipated.

A newer model will have a smaller heat exchanger, which cannot regulate residual heat as well as its counterparts.

Why does central heating pump overrun happen?

When combi boilers are in pump overrun, they circulate the leftover hot water until it begins to cool down. It does this by circulating the hot water through a bypass valve within the boiler itself (not to be confused with a bypass radiator).

In short, a pump overrun is used to prevent any water from boiling inside the heat exchanger in the unit after use. When there is water boiling inside the heat exchanger, it can cause costly damage. If you have a standard boiler with S or Y-plan heating system usually in the airing cupboard, the pipework may include a bypass valve, this is usually located near the pump itself.

My pump overrun is lasting a long time - is this normal?

Usually, the process for a pump overrun will last for around ten minutes. But if you notice your boiler is still running for hours after you've turned it off, there may be another problem at hand.

These problems are easily diagnosed and only require a few steps to find the root of the issue. The most common causes of prolonged pump overruns are a defective thermostat or timer. However, it could also be a result of a faulty pump or a wiring issue (typically in a new pump).

What’s causing your boiler to be in constant overrun?

1. The central heating pump has a defective thermostat or timer

When your boiler is constantly in overrun, the first thing to do is check the timer. A broken timer is fairly easy to spot and will usually result in standard functions taking much longer to complete. The timer will also not switch on or off at set intervals and can typically make a fair amount of noise.

If the timer isn't the issue, the other major cause of a continuous pump overrun is a broken or defective thermostat.

They have a network of zone valves, which are motorised valves that control the flow of the water. When the thermostat is changed, it opens specific valves to allow the water to flow through and heat up.

The easiest way to spot a faulty thermostat is to look out for high readings on the temperature gauge, excessive noise, or changes in temperature readings.

2. The pump circuit board is broken

Once you have eliminated broken thermostats or timers as the primary issue, you may want to check the pump circuit board (PCB). This board helps to control water pressure and water pumps. When a PCB stops working, there will be negative feedback loops that point to the source of the problem.

If you suspect that this is an issue, it's best not to try and take care of the problem yourself. Without proper knowledge of the boiler's components, trying to fix a damaged PCB could cause injury and other harm to you or your home.

3. The pump has a wiring issue

If you switch your pump off and you notice both the pump and the boiler are still running, it's a surefire sign that your boiler is experiencing a wiring issue. This usually happens when a new pump is installed incorrectly. It causes the pump to keep running because it doesn't detect the presence of hot water in the heat exchanger.

How to fix a central heating pump overrun

When it comes to repairing a boiler that constantly has a pump overrun being executed, it's best to consult a professional. Most of these fixes require a good knowledge of the components and can possibly be dangerous if done incorrectly. So, you'll need to contact a Gas Safe registered engineer to conduct the boiler repair.

When they are repairing the central heating system, they can tell you what the problem is and the possible causes of the issue.

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Now that you know what a pump overrun means, there's no need to worry when it happens! After around five to ten minutes, the code should clear, and the pump should turn off.

But on the off-chance that the overrun code doesn't disappear, it's time to call a Gas Safe registered engineer. Remember not to try and fix the problem yourself, as this could potentially be dangerous.