Boiler Return Pipe Cold – How to Fix Boiler Flow?

Is your boiler return pipe cold? You’re in the right place — our 5-minute guide covers everything you should know about boiler flow and return pipes, possible problems, and fixes. 

We we also explain why temperatures differ between the flow and return pipes and talk about signs of trouble. Read on to find out more!

What Are Boiler Flow and Return Pipes

Boiler flow and return pipes function the same way whether you have a vented or sealed system. As your boiler heats up water, it’s pumped out and circulated around your central heating system, i.e. all the rads and towel rails. The hot water (not boiling hot — closer to 75°C) comes out of the flow pipe. This water flows around the whole system and returns to the boiler via the return pipe.

Why Temperatures Differ Between the Flow and Return Water

You’ll notice that the flow heats up quickly, and is usually hotter than the return pipe.

This is because all pipe work and radiators are cold when you fire up a boiler. The pipes and rads absorb some of the heat, so the return water coming back is cooler.

As your central heating slowly warms up, the differential in temperature between the two pipes will become closer. Despite the difference, the return pipe should never be stone cold when the boiler is operating.

Flow and Return Pipe Sizing

To circulate hot water quickly, boilers should be fitted with a minimum of 22mm sized copper flow and return pipes. This requirement can rise to 28mm and beyond for large properties and commercial boilers.

Boiler Return Pipe Problems

Below, we’ll discuss some of the common problems with boiler return pipes.

The Flow Pipe Is Hot, but Return Is Cold

When dealing with problems related to flow and return pipe temperatures, this one is the most common; return pipe is cold, even though the flow pipe is hot.

Naturally, the flow pipe will heat up quicker than the return. However, if the return pipe isn’t getting at least warm, there are a few potential culprits.

#1 – Faulty Pump, Or Incorrect Speed Setting

Once your boiler has heated the water, it’s the central heating pump’s job to circulate this water around the system.

If this motorised pump is faulty, there’s a good chance it’s not circulating this water quickly enough. By the time the water gets back to the boiler via the return, it will have cooled more than it should have. This could be due to:

  • Dirty heating water blocking the pump
  • Incorrect pump speed setting
  • Stuck shaft on the pump
  • PCB is not communicating with the pump correctly.
The Fix

Depending on the root cause, the solution could entail a power flush, getting a new pump, or replacing the PCB. We’ve created a detailed guide on heating pump problems and the fixes here.

#2 – Airlocked System

Airlocks of any kind can cause a heating system to work intermittently, and can be found in:

  • Pumps
  • Towel Rails
  • Radiators
The Fix

You’ll need to bleed all air from the system. Radiators and towel rails have a bleed valve (not the rad valves used for balancing) that will let air escape. They can be bled with a bleed key.

If the return pipe is still cold, the issue could lie in an airlocked pump. And if this is the case, there’s a good chance you’ve heard knocking and banging noises as the pump malfunctions.

As bleeding the pump requires removal of the outer casing, you’ll need to call a Gas Safe engineer.

#3 – Blockage In The Heating System

When return pipes are cold but the flow to the boiler is hot, your CH system could have a partial blockage.

Blockages usually come from:

Limescale comes from minerals in water, and will attach itself, well, just about anywhere.

Heating sludge, on the other hand, comes from internal rust from radiators and pipework. When this breaks off, your radiators, and even the return pipe on your boiler may get partially blocked.

This blockage restricts the flow of hot water, so radiators don’t get hot, and the return water temperature is lukewarm at best.

The Fix

First, you’ll need to power flush the system using cleaning chemicals. This will get rid of the majority of sludge and limescale.

Then, you’ll need to fit:

  • A scale reducer to catch limescale
  • A boiler filter to catch heating sludge

It’s worth noting, both of the above devices will need to be cleaned out at each service. If not, they will fill with debris, and won’t be able to catch anything else circulating in the system.

#4 – Microbore Pipework

Next, we have microbore pipework.

If you have 8mm or 10mm pipe feeding radiators and towel rails, there’s a chance it’s restricting the flow or is partially blocked (see #3).

The Fix

When you have a gas engineer come and diagnose the problem, make sure you show them any microbore pipework. Depending on the layout of your pipework, it might need to be replaced. And if you’ve had it recently replaced, there’s a chance it’s been fitted incorrectly.

#5 – You’ve Added Additional Towel Rails Or Radiators

Heating systems are sized based on BTU — a measurement of heat.

Pipework, radiators, your boiler, and even the boiler’s pump will have been spec’d to the BTU of your property.

By adding radiators or towel rails, you’re raising the BTU requirement, so the boiler starts to heat additional water, whilst the pump must circulate a higher water volume than it’s meant to.

It’s not very common, but if you have added towel rails or radiators (especially large ones, like a 1600mm+), there’s a chance your addition is starving the heating system and causing the return pipe on the boiler to be cold.

The Fix

Without surveying the size of the property, the boiler, the boiler’s pump, and the number of towel rails and radiators, it’s hard to know if this is indeed the problem. Have a qualified heating engineer come and check out your heating system.

What’s Next?

Thanks for reading our 5-minute guide to central heating and boiler flow and return pipe problems. Hopefully, we’ve explained why your boiler return pipe is cold, and what you can do to fix the issue. If you have any questions, please leave us a comment below and we’ll get back to you as soon as we can.

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  1. WOW THANK YOU SO MUCH COULD NOT FIND THE ANSWER ANYWHERE . THIS IS EXACTLY WHAT HAPPENED BOILER WAS OFF FOR 11 MONTHS AND WHEN BLED ,IT WAS LOADED WITH SLUDGE AND RUST . EVEN THOUGH I FLUSHED IT OUT IT STILL MAY BE CLOGGED SOMEWHERE.

    1. No problem Ray.

      For a system like that, focus on any TRVs, lockshields and anything connected to mircrobore pipe.

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