Boiler Overrun? Here's What That Means

Boilers keep your home nice and toasty thanks to a complicated series of devices that help monitor and control heat flow in your home. If a single bit goes out, then the whole thing can come to a halt. If you're noticing that your boiler pump is still running or displaying the "boiler overrun" code, don't panic quite yet. This is a totally normal part of your boiler's operation and is likely no cause for concern.

If you see the "boiler overrun" code for only 5-10 minutes, all is well. If it's lasting for hours, it's a sign of a defective thermostat or timer.

Let's talk a bit more about what a boiler overrun is and what it means, shall we?

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What Does Boiler Overrun Mean?

This is the first piece of information you need if you don't already know. First things first - what does an overrun code even mean?

Generally, pump overrun occurs after heating or hot water has been used. It's designed to get rid of residual heat and avoid any potential damage to your boiler. When pump overrun is active, it keeps the pump running for an extra 5-10 minutes. This is generally controlled by a thermostat or timer, depending on the model.

What happens during an overrun is hot water is pumped through a bypass valve and recirculates until it's cooled to an acceptable temperature. This stops (in theory) water from boiling inside the boiler. Depending on whether or not you have an S or Y-plan heating system, there could be an additional bypass circuit included in the piping. Other larger boilers (generally high-end residential ones, or commercial boilers) have special equipment designed to do this job, rendering the pump overrun code unneeded.

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My Pump Overrun Lasts for Hours - Why?

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Like we said above, an overrun should only last for roughly ~10 minutes. It will continue if the heat fails to dissipate, or if you have a faulty timer or thermostat in the mix. You could also have a damaged pump circuit board, all of which are repairs for a professional to make.

Remember how I said that boilers are complicated machines? That means that no matter how simple the fix may seem, it's often best left to a professional to do. You don't want to risk damageing your boiler further with a shoddy installation or repair. So if you think there's an issue, call a Gas Safe engineer for repairs - trust me on this.

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Causes

Now we get to move onto the actual causes for an abnormally long overrun cycle. The general two most common issues are a defective thermostat or timer in your boiler (and no, we're not talking about the thermostat on your wall).

Timer Issues

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A broken timer is generally pretty easy to notice. It'll go from making regular changes to having the timing drastically off from where it used to be. If you notice that certain functions are taking far longer (hours compared to minutes) to complete, this is likely the cause.

Thermostat Issues

Also known as Thermostatic Radiator Valves (TRVs), thermostats control the temperature of your radiator and also manage their on/off cycles. If you notice one of the following, it's a sign you need a new thermostat:

• Temperature gauge is incredibly high. This is a sign that your boiler engine is overheating. You’ll likely see irregular changes in temperature as well if this is the case.
  • This is caused by a coolant being unable to be pumped into the boiler, usually due to a thermostat being stuck closed.
• Sound coming from your pump. If your pump doesn’t normally make noise, this is a clear sign something is wrong. Whether it’s a sucking sound, banging, or something else – it doesn’t matter. Your pump should be making minimal (if any) noise when functioning properly.
• Software Issues. These are harder to spot, but also a sign that there’s an issue with the thermostat. If you notice drastic changes in temperature or readings, or the thermostat seems as if it’s acting of its own accord – you likely need it replaced.

Pump Circuit Board (PCB) Issues

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This is the electric circuit board that controls most modern boilers' water pumps and their water pressure. Their function is to prevent any damage to the boiler and maintain a safe device. If they stop working, you'll notice negative feedback loops (or more accurately, your engineer will).

This is an electronic circuit board in a device filled with water - do not try to replace this yourself. If you even suspect that this is an issue, you need a Gas Safe engineer to check it out as soon as possible, as it could lead to much more serious issues if left unaddressed.

Wiring Issues

Again, this is not a problem for you to fix - hire a professional. If your pump and boiler won't turn off, it's a sign that there's frayed, damaged, or improperly installed wiring present in your boiler. The most likely place for this is in the low-temperature switch, or in the PCB. The low-temperature switch is designed to keep your boiler running if it detects water that's too cold. If there's a fault in the wiring (or thermostat), it'll keep running constantly, thinking there's an issue where there may well not be.

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Final Thoughts

If your boiler is displaying a "boiler overrun" or some variation on that code - don't panic. Most modern boilers have this as a specific safety precaution to keep the water inside from reaching a boiling point. While this is a normal function, it's only designed to last for 5-10 minutes - not hours. If your boiler overrun code persists for hours, it's a sign that something has gone wrong inside the boiler.

Whether it's a fault in the thermostat, PCB, timer, or wiring, none of these is an issue you should attempt to remedy alone. You're dealing with electric circuits and wires in a metal device filled with water. That is a recipe for electrocution and even fire if addressed poorly, and that's not something anyone wants in their home. If you think there may be a problem with your boiler, call a Gas Safe certified engineer and have them come take a look. Not only will they pinpoint the problem much faster than you, but they'll do it safely, which is the most important part. We like our readers alive and well, so please - ask for help when it's needed.