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How to Solve a Cold Boiler Return Pipe

If your boiler return pipe is cold, then you could be having problems with your boiler flow that need to be addressed

Resolving your cold boiler return pipe doesn’t need to be an impossible or expensive task when you use our handy guide! We have shared the common reasons for a cold return pipe and the quick fixes that you can try – take a look now!

What’s a Boiler Return Pipe?

Boilers come with flow and return pipes that work to firstly send the hot water around the system via the flow pipes and then return the water via the return pipes. They are an essential component of any boiler system that has central heating attached to it, and pipes tend to measure between 22mm and 28mm depending on the size of the property.

The boiler flow pipe and boiler return pipes are often different temperatures, with the return pipe feeling cooler than the flow. The simple explanation for this is that the boiler pumps the water out via the flow pipe at the hottest temperature (around 75o C), and it then makes its way around a cold system, heating it up. This means that when it is eventually returned to the boiler, the water is much cooler than it was at the start – but your return pipe should never be completely cold when your system is up and running. The longer you have your central heating turned on, the warmer the return pipe will get as the entire system gets hotter. 

Common Boiler Return Pipe Problems and Solutions

Having a cold return pipe is one of the most common issues that people have with their boilers, and there are a few different reasons for this. We’ve shared the most common problems below, along with the fixes you can use to help get your system running properly again.

Faulty Boiler Pump

Your boiler’s job is to first heat the hot water you want to use and then move that water around your central heating to warm up your property. If your pump is not working properly, then this can slow the water flow speed and leave your water to get cold before it is returned to the boiler. There are a few reasons why your pump may not be working, including:

  • Having a shaft stuck
  • Sludge build-up that blocks the pump
  • PCB issues that stop the right messages from being sent to the pump
  • The wrong pump speed setting is being used

Fixing this issue depends on the reason for the pump not working properly in the first place but can include power flushing the system, replacing the PCB or even installing a new pump altogether. 


Another common problem that can stop your return pipe from working properly is airlocks in the central heating system. Airlocks are usually located in the radiator but can also be found in the boiler pump and special radiators such as towel rails. 

Fixing airlocks requires you to bleed each radiator to let the water escape, and you will need to use a bleeding key to do this. Remember that you only need to turn the lock slightly to let the air out, and you will get some leaking water when all the air has gone.

If bleeding your radiators doesn’t work, then you may have an airlock in your pump that often results in a banging noise when the central heating is turned on. To fix this issue, you will need to call a Gas Safe registered engineer who can remove the cover and release the airlock for you.


Many people believe that once their central heating has been installed, they have a sealed system that will work with no issues. In reality, radiators can experience build-ups of sludge that can stop the heat from circulating properly, causing your return pipe to feel cold. The blockage can be made up of sludge, slime, limescale or even rust from the radiators, but it has to be removed to keep your system running effectively.

The quickest way to resolve this issue is to invest in a Power Flush service that will push a concoction of chemicals through your heating system at speed, removing all the debris and cleaning each radiator. Once you have done this, then it is also wise to have a limescale reducer and a filter installed to slow down any further build-ups. 

Microbore Piping

When it comes to pipes, if you have 8mm to 10mm microbore piping that feeds into your towel rails, then this could be causing the problem. Smaller pipes slow the flow and can seize the system when they get blocked, stopping your return pipe from warming up.

This issue needs to be addressed by a Gas Safe engineer who can look at your pipework and recommend how to resolve the issue. There may be a case to change the pipework to resolve the issue, but if this has already been done, then the engineer will be able to check if it has been fitted properly and solve the problem if it hasn’t.

New Additions to the System

When you have your boiler installed, the engineer will set your system to work with the number of radiators and heated rails that are in your property at the time of their visit. If you add more radiators or heated rails after your boiler installation, then this could be the issue with your cold return pipe, especially if the new additions are large!

The only way to check if this is the issue is to get a Gas Safe engineer to survey your system and check that your system is able to handle the additional pressure. They will be able to advise you on how to resolve this if it is a problem.

Finding the Right Help

If you are keen to find the right fix and need a qualified Gas Safe engineer, then the Boilers Prices team is here to help. Simply share the details of your job via our handy online form, and let us help you find the right engineer in your local area. We offer an impartial service and provide you with a number of results to choose from, meaning that you are more likely to find the right fix for your cold boiler return pipe when you let us help!