• Electrocution
  • Flood
  • Scalding
  • Burns
  • Tools needed

  • Large flat blade screwdriver
  • Large grips
  • PTFE tape
  • If in doubt call a professional!!

Getting air out of central heating system


The idea of this is so you get an idea of your system, so you know if your engineer is doing his job correctly, if you attempt a repair yourself,be careful

Getting air from a system is easy right? Nooooooooo! Bleeding rads is OK, but getting serious air from reluctant systems is not so simple.

This guide is for those that have discovered an air lock....

First off, look by the cylinder / boiler for these little beauties. Also locate your pump. Now source some towels. There are many types of bleed valve - I describe most, but cant picture all. All will be on the top of a stub of pipe as shown below

OK, The knurled top cap type, the cap contains a loose rubber washer, do not loose it. (in a pinch balled up PTFE will substitute) If you open it and nothing comes out, find a paperclip, straighten it out, remove the cap entirely and poke the end of the paperclip down the hole. Beware - hot water will shoot out as soon as its clear. practice screwing the lid back on WHILE its still blocked!
The lovely item with the screwdriver slot on top is not likely to be blocked, but its likely to refuse to seal up properly afterwards. unscrew it, usually with grips as its too tight, and have the ptfe tape ready. if it wont close down properly, grasp the pipe just below the vent, and pinch the end of the ptfe. hold the end and the tape in one hand and slacken off the vent. You will be surprised by how much water comes out! with the cap loose, wrap the ptfe around the thread/gap the water is coming from. then re tighten. the big looking item (first pic) is usually easier to handle. In theory, its an automatic air vent. Loosen the top cap, and air should escape. A sharp tap to the side sometimes helps (there's a float like a carb in there).

Heres the pump. This is you best tool for getting the air out of the system. You ever seen an off road truck stuck in the mud? see how the driver rocks it back and forward to get it going again? That's what we are going to do. Some notes - you can 'rock' the pump by switching the main system power on/off/on/off, but this will close the port valves too, negating most of the effect. So, turn the boiler to 'low' or, ideally 'off' - depends on whether your pump is powered via the boiler or by the programmer. In most modern boilers it will be the boiler that controls the pump. Anyway, on the side of the pump there is a selector, it usually has 3 settings. set it to '3' or 'max' , and bleed the center nut/screw on pump. You are now ready to start.
Now I expect you have reached this page after an unsuccessful attempt to refill your system, as such, you most likely have a red hot boiler, probably locked out on overheat, (See overheated boiler). Your pump will be very hot, probably making screeching or groaning noises. Bleeding the radiators isn't helping!
This section doesn't need pictures..... it needs lots of detail.

First thing is - how do you know you have no flow / air lock? - the boiler will get very hot, the pump will get very hot and, most crucially, the pipes after the pump wont get hot. The indicator is usually a 'woooooooooo' noise from the pump, not loud but audible.

Step one - bleed everything - rads, air pips, pump, auto air vents etc. turn the heating on and stat to max, turn the water on too.

Step two - turn on system. Set the boiler stat to minimum, and go to the pump. Feel the pipes - you should feel the heat coming to the pump, but probably not much past it. Rotate the pump selector to between 1 and 2, you will hear the pump stop and hear water falling back to the pump. Wait 1-2 seconds only and switch it over to setting 3. This 'rocks' the system water, listen for nasty noises from the boiler - clanks bangs etc - if you get those turn the boiler off but leave the rest of the system on. A quick note to those that are now bitching that we have just run the boiler with no flow - i assume your on this page because you already did that and have now looked for a solution. So you did it before reading this! Anyway - repeat the pump thing - with gaps in between of 10-15 seconds. Listen to the flow. You will start to hear longer and longer periods of 'clean flow' - where the water flows smoothly after you turn on the pump to 3 - before it reverts to 'wooooooo' again. This is good. If this isn't happening, turn off the heating and first do with the hot water only.

Once you have got some flow, turn up the boiler a bit - the heat will expand the air in the system, forcing it to move. Try to make sure that the 'woo' is to a minimum. If its not going well, stop and re bleed everything. This is a game of patience. Then repeat from the top.

Step three - once the hot water is heating and some radiators are heating, turn the boiler up higher and bleed again.

Step four - the radiators that don't work now need to be forced - with the system running - water and heating, go and turn off ALL radiators except one that doesn't work, and one that does. Wait 2-3 minutes - if the rad hasn't started to work, turn off the hot water, leaving the heating on. If still no joy, turn the boiler down and turn off the working radiator (at the valve) so your left with just the cold radiator on. You will hear a gurgle of blocked air at the rad before it starts to work. If it works (or even if it doesn't) move on to another radiator.

Tricks for serious problems
  • Go to the drain valve you drained it from and connect up the garden hose. Open the valve and let some water in. This is called backfilling and will often force air back up to the tank,l check the overflow from your feed tank is up to it however as if you overfill the water needs somewhere to go.
  • If your having trouble getting the radiators to fill - then don't worry - just don't fill them up - sounds stupid right? the place you need the water most is the primary pipe work, so get that running and then leave it run half full for a while, then bleed 5-10 seconds off each radiator once an hour - no more or you will bleed it off the primary pipework and be back to square one again.