Homeowners Professionals

How to fix a frozen boiler condensate pipe

There are few things more enjoyable than breakfast with the family on a cold, frosty morning. Seeing the clear blue sky, bright sunlight and beautiful white frost outside while you are nice and warm indoors makes such mornings worth all the effort - unless you wake up freezing to discover that your combi boiler has unexpectedly stopped working. If you have found that your gas boiler is suddenly not working when the weather goes cold, it is highly possible that you have a frozen boiler condensate pipe. Read on or watch the video to learn more about how to diagnose, solve and prevent this very common problem.

Understanding your boiler condensate pipe

What is a condensate pipe?

SInce 2005, it has been a UK requirement that all gas combination boilers use condensing technology to increase their efficiency.  

Condensing technology allows heat existing in the flue gasses to be captured and ‘recycled’ back into the heating system to heat the water inside the boiler. During the condensing process, the temperature of the flue gas is reduced from around 130℃ to 50℃.

This rapid fall in temperature causes condensation, which needs to be drained - as much as 2 litres per hour for an efficient boiler.

The boiler condensate pipe is the pipe that allows this water to be  drained  from the heating system and disposed of with your household waste water. Although it is advised that this pipe is connected to your waste water system internally, there are many situations where this is not possible and instead the pipe will be fitted externally, leaving it vulnerable to becoming frozen in extremely low  temperatures  or prolonged cold spells.  

Condensate pipes that run through wall cavities or unheated outbuildings such as garages are also at a higher risk of freezing.

How to find your boiler condensate pipe

Because metal piping cannot be used, the condensate pipe is relatively easy to identify. It will be the only plastic pipe connected to your boiler.  

Due to UK legislation the pipe will typically be 22mm in diameter if the drainage system is connected internally, and may be 32mm where the pipe is drained externally. The increased size helps to prevent internal freezing, but will not completely eliminate the risk.

Outside, the condensate pipe should be a relatively short length of pipe that connects directly to your drainage system, and should also be steeply angled to ensure that other waste water, for example from appliance outflows, is not able to travel up it.

It will often emerge at around the same height as your window sills. Ideally, it will be well clad but this may not always be the case, especially in a country like the UK where we are not used to extreme cold.

Fixing and preventing a frozen condensate pipe

How do I know if my boiler condensate pipe is frozen?

The first indication that your boiler condensate pipe is frozen will usually be that you have no  hot water  and heating, even when you are sure your boiler has been working absolutely fine. You may also be able to hear an unfamiliar gurgling  sound  coming from your boiler.

On closer inspection of the boiler control panel, you will find that the boiler has stopped operating. If the condensate pipe has become frozen, the boiler’s sensors will detect that it has become blocked and prevent the boiler from operating. This is a normal safety precaution to prevent damage to the boiler.

Depending on the make and model of your boiler, different fault codes may be displayed. If you have a Viessmann  Vitodens gas boiler  it is likely you will see an ‘F4’ error on the display, full details can be found in our  fault code tool.  

For safety, you will not be able to reset and relight your boiler until the obstruction in the pipe has been cleared.

In many cases, it is the exit that has frozen, often due to the  water  trickling out gradually turning into a build up of ice or becoming immersed in frozen water. This can be easily confirmed with a visual inspection.

What to do if your boiler is working but you have no hot water

How to clean a boiler condensate trap


How to fix a frozen boiler condensate pipe

Once you have located the boiler condensate pipe, the next step - if you are confident to do so - is to try and unblock the frozen section.

NEVER attempt to cut or remove the boiler condensate pipe. It is generally considered to be part of your gas boiler’s flue system, which means that only a Gas Safe engineer is legally authorised to perform physical work on the pipe itself.

  1. Prepare some warm to hot water using a kettle or microwave. Transfer the hot water to a suitable container such as a watering can or a jug. Do not use boiling water as this is an accident risk in cold conditions and can cause damage to the pipe.
  2. Pour the water over the section(s) of pipe that you believe are frozen. If the pipe’s outlet is fully or partially frozen, start there as it will allow any water that has gathered elsewhere in the pipe to escape once it is able to. After this, it is recommended to pour hot water on any flat sections or elbows as these typically freeze faster.
  3. If this is not sufficient to melt the ice inside, you may try using a hot water bottle. This will allow heat to be applied to the frozen section for a longer period of time. Ideally, you will see water begin to leave the pipe or hear a trickling sound.
  4. Once you believe the blockage has melted, you can attempt to reset the boiler. If you were successful, any fault codes will be cleared and the boiler should restart.

Be sure to  reset your boiler  in accordance with the manufacturer’s instructions, as provided in the manual. If in doubt, consult a trained engineer.

How can I prevent a frozen boiler condensate pipe?

It is said that prevention is always better than cure, and this is certainly the case for your boiler condensate pipe. There are a few ways to help prevent problems occurring in the event of freezing temperatures, although there are limitations to what you are able to do yourself. Here are the most common solutions:

Do it yourself:

1. Insulate your condensate pipe. This is something that you can easily do yourself. Be sure to use waterproof lagging, otherwise you will increase the risk of your condensate pipe freezing rather than preventing it.

2. Talk to your installer/engineer about moving the condensate pipe internally, so that it never falls below room temperature.

Gas Safe engineers only:

3. Increase the fall angle of the condensate pipe. This will help to improve the flow and reduce the likelihood of internal freezing.

4. Upscale the pipe from 22mm to 32mm to improve flow and reduce the risk of internal freezing.

5. Re-route the pipe to an internal drainage location to avoid exposure to lower temperatures.

Final thoughts

Hopefully with the aid of this handy guide you will have been able to fix your problem with a frozen boiler condensate pipe and are now able to enjoy your heating and hot water once again. If you have followed these instructions and are still experiencing problems with your boiler, we recommend that you seek advice from a  qualified professional.

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