Common Boiler Faults

1. Venting Heating Systems

Venting heating systems is straightforward, just follow the steps below:

Use these necessary steps

Before bleeding, you first need to turn the heating on so that all the radiators warm up. This builds pressure in your radiator that will push the unwanted air out.

Go through your whole house checking each radiator for cold spots.

What are the signs that the radiator needs bleeding? If you can hear gurgling sounds, the radiator takes a long time to heat up or there are cold spots then it is likely there is trapped air. This is preventing the hot water from filling the radiator; you will need to bleed that radiator.

Remember: The radiators will be hot, so take extra care with this step. We advise wearing a pair of thin gloves so you do not burn yourself.

Do not forget: You need to switch off your central heating before you bleed a radiator. If your heating is on you’ll risk scalding yourself and covering the floor with water.

Water may be discoloured when bleeding an old radiator, by putting old towels down you can save a cleaning job later, especially if you have light carpets!

Take the radiator bleed key, you’ll need to insert this into the bleed valve. Often the bleed valve (or nipple) is found at the top of the radiator, to the side. It looks like a round hole with a square inside. When you insert the radiator key into the bleed valve, you will feel them lock together.

Carefully turn the valve anti-clockwise – as the air begins to escape you’ll hear a hissing sound. Be careful, the escaping air could be hot, keep sufficient distance.

Tip: If you do not have a radiator vent key it is sometimes possible to use a flat-headed screwdriver on modern radiators.

A quarter to half a turn will be enough, never open the valve fully because once you bleed air from the radiator water will come rushing out.

Continue this process, holding the radiator bleed key until the air stops coming out. When only water is dripping from your radiator, then you have completed the bleeding process. Turning the bleed valve clockwise will seal the radiator; take care not to over tighten.

Modern bleed valves may release water as a ‘jet’. Turn your bleed valve key with care, and be prepared to quickly close the bleed valve.

You will need to bleed all the radiators in your property. We suggest starting on the ground floor and working your way up your property because the air rises through the system.

Once you have completed the task of bleeding all your radiators, you will need to re-pressurise your heating system. When you bleed a boiler heating system you always lose some water. If it’s a large amount then your system may have difficulty heating the top floors of your property or the central heating system can fail entirely.

If the water pressure in your system is correct, the needle gauge on your boiler will be facing green. If it’s low then you will need to re-pressurise the system. To do this you’ll need to locate the central filling loop connected to your boiler. It looks like a tap and is connected to your main water supply; for reference, the pressure in a typical family home is usually between 1.0 and 1.5 bar.

Always turn the tap and slowly adjust the pressure. In the unlikely event that you add too much pressure and the needle faces the red, there is also a bleed tap.

You can find more information on that topic in our article about boilers losing pressure.

If you’re looking to bleed air from radiator systems you’ll need the following tools:

  • A radiator bleed key – used to open up the radiator vent valve (available from most hardware stores)
  • A cloth or towel – to catch drips

Recap: How to bleed a radiator system

  1. Turn your heating on
  2. Identify which radiators need bleeding
  3. Turn off your central heating
  4. Prepare the area
  5. Open up the radiator bleed valve
  6. Bleed the radiator
  7. Repeat the process on all radiators
  8. Check the pressure of your heating system
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2. Defrosting your Condensate pipe

If your boiler has stopped working in a cold snap, check to see whether its condensate pipe has frozen before you call out a heating engineer.

Your boiler’s condensate pipe will run directly into a drain, in most cases externally to your property. If this pipe gets blocked with ice, or anything else, then your boiler will automatically shut down as a safety measure.

In cold weather, one of the most common reported faults that will stop a boiler working is a frozen condensate pipe.  The boiler shutting down is its way of keeping you safe, if not warm.

Use our quick guide to defrosting your condensate pipe

1. Locate your condensate pipe. This will be a white pipe that comes out of the wall behind your boiler and runs directly into an outside drain.

2. Boil a kettle and leave it to cool for 10-15 minutes so that it’s warm rather than boiling.

3. Starting from the top and working your way down, pour the water over the pipe until the ice within has melted.

4. Reset your boiler and it should work as normal

How to stop your condensing pipe freezing again

Prevention is always better than cure.

 Follow our tips to stop your pipes from freezing in the first place. A large pipe Make the condensate waste pipe as large as possible. Check your manufacturer’s instructions – some will recommend that it should be at least 32mm or up to 40mm. However, this may not prevent freezing in extreme conditions. A heating engineer will be able to fit a fatter pipe, should that be necessary.


Consider insulating the pipe to protect it, especially if it’s high up and therefore difficult to unfreeze using hot water. You can insulate a pipe yourself, but if it’s tricky to access you should call in a professional.  Reduce the amount of outside pipe If possible, the condensate pipe should run internally as far as is possible. The less pipe that sits outside, the better.


The waste should fall as directly and steeply as possible so that gravity can help it along. The fewer bends in the pipe, the better. A lot of waste pipes are not installed according to the manufacturer’s instructions, with some running almost horizontally. If your pipe isn’t at a steep angle, the water spends longer in the pipe and is more likely to freeze. Again, if you need to change the angle of the condensate pipe, you will need a heating engineer.

Siphon Trap

If you’re buying a new boiler, then consider one with a ‘siphon trap’ type of condensate (water) release – it’s only available on certain models. This releases the water in one amount, reducing the risk of freezing. Most boilers release the water as a continual drip, which makes freezing more likely.

Reduce Exposure

Think about the pipe’s placement and the weather. Is the condensate waste pipe positioned on a wall that will get any sun? Is it particularly exposed to the weather? If the wall gets sun and the pipe is black, it can often absorb enough heat to prevent freezing.

Our tips for efficiently heating your home

When is the best time to bleed my radiator system?

We advise bleeding your radiators at the beginning of the heating season, before you really need it. Making sure your boiler and heating system is running at its best without any trapped air before you need it

Call Cambs Heating Limited who can help you prevent this problem from happening again.  Installing a thicker pipe and attaching insulation to it should help the pipe remain ice-free, even in the coldest of winters