Airlock in Combi Boiler - How to Diagnose and Fix

A fairly common occurrence that combi boiler heating systems face is that of airlocks.

But what are airlocks, how do you diagnose them and how do you fix them? 

Of course, when we say an airlock in a combi boiler, the airlock may be found anywhere in the system and not necessarily in the section of the pipework leading into and out of the actual boiler.

In this troubleshooting guide, we’ll answer these questions and explore additional frequently asked questions relevant to this subject.

Let’s begin!

PS - we fit new combi boilers with gas safe installation nationwide. Simply answer these questions, get your fixed price and you could get it installed as quick as tomorrow.

What is a Boiler Airlock?

Airlocks are simply the presence of trapped air somewhere in a heating system.

When trapped air appears, it acts as a block, limiting the flow of hot water travelling to your radiators or hot water outlets (e.g., taps or shower).

Unsurprisingly airlocks can, as a result, cause a number of issues with your heating system.

Symptoms of Airlock(s) In Your Heating System’s Pipework

Thankfully, there are many signs to look out for that are indicative of an airlock (or perhaps many airlocks) in a heating system.

These potential signs of the airlock(s) include:

• No hot water coming from your taps or showers.

• Hot water is emerging from taps, but it is spluttering out in an awkward way.

• You notice cold spots on at least one radiator in your property, if not on several radiators.

• Radiators start performing to a lesser degree than usual.

• There are tapping and banging sounds coming from your heating system.

If you notice or more of the above issues are present, there’s a strong chance that there are one or more airlocks present somewhere in your heating system.

Of course, that’s not to say that there aren’t other causes at hand (e.g., the accumulation of sludge in your heating system), but these signs do point toward airlock(s).

How to Remove an Airlock from Your Heating System

Either way, you should still primarily rely on the manufacturer’s instructions.

Moreover, if you have any doubts about any part of the process (or your DIY efforts do not fully succeed), hire a qualified Gas Safe Register engineer.

There are several primary approaches to removing an airlock from a central heating system.

Which approach will be best in your case will depend on where you suspect the airlocks are present.

Method #1:  Bleeding Radiators

If you think that the airlock(s) are present in the pipework leading to your radiators, this is the section for you.

However, if you believe the airlock(s) are affecting your hot water system instead (i.e., the pipes leading to your hot water outlets), skip to the next two methods. 

Of course, it’s possible that there are airlocks in the pipework leading to your central heating radiators and the pipes heading to your hot water outlets.

To bleed radiators you’ll need some equipment, but thankfully you should already have all of these items, namely:

• A bucket

• A radiator key

• Cloth or rag

• Safety gloves

Here are the steps involved:

1. Prepare the Area - Since bleeding radiators can result in plenty of water spillages, you’ll need to make certain preparations first. We suggest laying down towels or a rage just beneath the radiator you intend to bleed. Also, you’ll probably want to pop on some old clothes that you don’t mind getting messy.

2. Let Your Radiators Cool - It’s very important to give your radiators time to cool down. Simply turning off the heating will achieve this. There’s no need to turn off the power to the boiler entirely so long as the heating is off. Oh, and don’t forget to ensure your boiler timer/thermostat isn’t set to turn the heating on while you’re bleeding your radiators. All in all, you don’t want to risk hot water spilling out onto you or your floor. 

3. Open Up the Thermostatic Radiator Valve or Lockshields - If you intend on bleeding multiple radiators, you should start with the lowest point of your property (e.g., downstairs radiators). Once you’ve found a suitable starting radiator, open up the thermostatic radiator valve (TRV) fully. You can do this by simply turning the TRV to the highest number on its dial. Should your radiators lack TRVs, then open the lockshield valves fully instead.

4. Open the Bleed Vent - Next up you need to find the bleed vent (a.k.a., radiator valve). This will generally be found at the upper right-hand side of your radiator. As soon as you’ve found it, fit the bleed key and turn it counterclockwise. You should hear air escaping at this stage. You’ll probably want to place some tissue/cloth just beneath the bleed valve when you start opening it up. That way you can catch dirty water that might spill out. Afterwards, water will begin leaking from the valve. Allow a small quantity of water to exit from the radiator with a bucket ready to catch any water that comes out. 

5. Close the Vent and Repeat if Necessary - As soon as you’ve fully bled your radiator, close the vent. However, do not overtighten it since this could lead to damage. Once you are satisfied with your work on the first radiator, feel free to move on to others (if required). You should work your way up from the radiators at the lowest point of your home before moving on to those at the highest point (e.g., upstairs or the loft if you have a radiator there).

6. Inspect Your Boiler’s Pressure Gauge - If you have a pressurized boiler and central heating system then bleeding radiators will probably result in a drop in water pressure. You can check whether your boiler’s pressure has dropped to an unacceptable level by inspecting the pressure gauge of the unit itself. In general, the green range is between 1 and 2 bars. With that said, you should always verify with the manufacturer’s instructions to check what the ideal range is in the case of your specific boiler. Either way, if your boiler water pressure is too low (usually this is anything below 1 bar), you’ll have to repressurise the system. For more details on this, check out our boiler losing pressure guide. If not, then you can simply skip to the next step, turning your heating back on.

7. Feel Free to Turn Your Heating on Again - Your heating can now be turned back on to test it out or you can continue to allow your boiler timer/thermostat to turn the heating on at allocated times. If you switched off certain allocated times on the timer/thermostat to ensure it wouldn’t coincide with the process of bleeding your radiators, don’t forget to set those times back in place for the days ahead.

The steps described above should fix any problems you’re facing with your heating system, so long as there are no additional airlocks leading to your hot water outlets (with solutions provided below with methods two and three). 

If in doubt, always consult a qualified heating engineer.

Method #2: Hose Pipe + Taps

If you suspect there are one or more airlocks in the pipework leading to your hot water outlets then you’ll want to first consider this approach.

This method involves using the following equipment:

• Duct tape

• One metre of a garden hose pipe

• Protective gloves

Here are the steps involved in this particular method:

1. Preparation - First, find a suitable sink on your property and put on some protective gloves. Get yourself duct tape and a hose pipe. Connect each end of the hose pipe to both the cold and hot water taps at the points where the water exits. Wrap duct tape around the connection points to ensure the hose pipe is firmly in place. At this stage, your cold and hot water taps should be securely connected to the hose pipe.

2. Attempt to Remove the Airlock - Next, turn the hot water tap on before turning the cold water tap on for around 3 to 5 seconds before turning the cold water tap back off again. Now, you’ll need to run the hot water at the sink in another room. 

3. Repeat the Process - You’ll likely need to complete the above steps an additional 4-7 times before you can fully remove the airlock(s). Once this is done, turn the initially hot water tap off and carefully remove the hose pipe. Watch out for any water that may come splashing out on you, particularly if it’s hot. Hopefully, at this stage, your hot water outlets will work as intended and flow out normally. If not, consider the third method below.

Method #3: Alternative Method for Hot Water Outlet-related Airlocks

If the second method fails, try the following steps instead:

1. Turn Off Water Supply and Taps - First and foremost, you’ll need to turn off the water supply at the mains. Next, turn on every tap starting from the top floor to contain taps (presumably the upstairs and generally not the attic) before moving on to the taps downstairs. Wait until there is no water flowing out of your taps.

2. Flush Toilets - Flush every toilet in your home until they too contain no water.

3. Adjust Your Taps - Adjust every tap in your property such that you’ve almost closed them entirely. However, you want to keep the taps on just a bit so that (if water was flowing), a small quantity would come out.

4. Water Supply Back On - Turn the water supply back on at the mains. 

5. Turning Taps Back on Carefully - Turn on the first taps to the halfway open point before opening them fully. If it works, then turn it off. Repeat this step with each tap in your home.

If this method also fails, then you’d have to hire a qualified heating engineer. 

Cost of Hiring a Heating Engineer

Note: In this section, we’ve listed the costs of having work undertaken that is unrelated to removing airlocks.

This is because, as touched on earlier, there may be additional issues at play, and it's even possible that airlocks have nothing to do with the issues you are seeing with your heating system.

On average, qualified heating engineers charge around £30 to £60 per hour in terms of labour costs.

So, any job you hire a heating engineer for will include the labour fee (likely in or around the amount given above) + the supply cost.

The average costs of hiring an engineer to fix issues with your combi boiler heating system:

• Bleed 5-10 radiators - £80 to £100

• Bleed 10-15 radiators - £100 to £1500

• Removing air locks - £30 to £120

• Power flushing to remove sludge - £400 to £800

• Replace an entire boiler - £800 to £2,800 *

*While unlikely, it’s possible that you’ll discover that your boiler is simply worn down such that it is now beyond economical repair. In that case, it would be best to have it replaced.

Obviously, given the current financial climate, such a scenario is something households are eager to avoid.

However, if your boiler is simply too worn down to be maintained effectively, you may want to consider purchasing a boiler on finance.

This would essentially involve buying a boiler now and paying later, usually in the form of monthly instalments. While interest generally applies to purchases on finance, zero-interest options are also available. 

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