I am living in an apartment in Belgium. My bedroom radiator is basically useless; it's about 1m long but only the left 10cm ever gets warm.

I have been told to bleed the radiator by opening the valve and letting the pressurized system fill the radiator with water, pushing air out until the water starts leaking out of the valve. This, however, is not working for me. I have tried leaving the valve open for weeks and nothing ever changes. I have also tried opening and closing the main radiator valve. When I close it, I hear a hiss from the bleed valve. When I open it... I also hear a hiss from the bleed valve.

I suspect that I have somehow actually worsened the problem as when I first moved here the radiator was maybe 50% operational; now it's 10%.

Is it possible that I am allowing air INTO the radiator rather than letting it escape? Is there some magic combination of "open bleed valve, open radiator, close bleed valve, close radiator" that I need to try?

I have read other DIY sites on the net but basically they say what I've already tried, or talk about turning off the central heating system which is not possible in my apartment building.

I have tried contacting the landlord but it's a monumental task to get her to take any action. That is a separate issue I'm dealing with; right now I just want to feel comfortable.

My fingers are freezing making it hard to type, so I'll wrap this up :D


I managed to get it working, but I think it's more magic than anything. A combination of turning the radiator on / off/ leaving the valve open / closed and it started working again. For the moment I simply vow not to touch it anymore!

  • are there people living higher up than you in that building?
    – Posipiet
    Feb 26, 2011 at 20:50

4 Answers 4


To get your landlady interested, point out that the radiators are filled with moist, warm air, and that the system is corroding from the inside, shortening the life of the radiators and clogging the boiler with rust. That means the boiler's life is being drastically shortened and may soon cause a massive expense. Fixing it now would be much cheaper than fixing it later. The question is, is she greedier than she is lazy?

As to what's actually the problem?

It seems to me that your apartment's system wasn't being replenished with water. Opening the radiator bleed valve wasn't doing anything because there was no additional water available to replace the air.

Central heating systems come in two flavours -- open vented and sealed -- and each refills with water in a different way. In an open-vented system, there is a tank at the highest point in the system, that uses a ballcock to fill up with water: see this diagram at diydata.com. The ballcock is in the feed and expansion tank. If the ballcock isn't operating correctly, the feed and expansion tank doesn't fill with water, and so no extra water will be available to fill your radiator. The ballcock could be stuck, or it could have been tied off so that some work could be done, and never untied. If you've got that kind of system, the fix would be to get the ballcock moving freely again. Is there some kind of maintenance area you could check out?

Alternatively, it may be a sealed system; there's another diagram showing that. These systems are filled up with a tap -- see the 'filling stop cock' in the other diagram. Normally, you'd check the pressure in the system every few months and use the filling stop cock to make sure the pressure is high enough -- say, 2 to 3 bar. Again, this will probably be very close to the boiler itself and so would need access to your building's maintenance area.

My guess is that it's a sealed system, and that the internal pressure has dropped right down to atmospheric pressure on the fifth floor. Lower apartments won't be suffering because gravity gives them more than their fair share of water, so most tenants won't suffer. However, any rust generated in your radiator will sink down into the system and clog it for everyone.


From my experience with German systems:

The whole system should be filled with water, with no air at all.

The system pressure should be 2 bar or so.

If it is not all filled with water, and the pressure is low, the air will collect at one point.

When a healthy pressurized system has air, you will hear the bubbles moving about. When you bleed a radiator, you either get water immediately (no air), or within a very short timespan of a few seconds, and you notice pressured air dissipating from the valve before the water comes.

If you can leave the bleeding valve open for a long timespan, either the whole system is unpressurized, or your valve is blocked.

There should be a system to pressurize the heating system from the water system, but water pressure is usually higher than the heating pressure. You cant have a direct connection between water and heating system. You may have to ask the landlord about the situation.

A direct connection between heating water and drinking water is a kind of a problem: the stale rusty water might leech back into the drinking water supply, which is a circuit. No idea what is growing in your heating water after maybe 30 years of darwinism at 45 C.


Typically when you bleed the radiator you will hear the hissing sound as the air is released from your radiator. You normally just leave the hissing going until you get a small amount of water coming out of the bleed valve. (recommend an old towel held underneath the bleed value while this is done as the water is usually pretty mucky). The bleed values should be at the top of the radiator and on each side (at least they are in the UK). If necessary you should open one of the sides first, wait for the hissing to stop, close it again and then open the other side, ie left and then right etc. If you have a double leafed radiator (a front and rear panel) you may need to repeat the action on both sides.

This should all be done when the system is on obviously as the boiler needs to be pumping the water through the system to force out the air.

If you do not get any hissing or the hissing stops before the water comes out of the bleed valve then it is likely that the pressure in the system is not enough and needs to be primed. Most boilers have this capability, but understand that you might not be able to get to the system yourself. As @Posipet says, if you can leave the bleed value open for a long period and no water comes out the pressure is not enough.

  • 2
    Thanks. I did just manage to get it working somehow. Basically I left the radiator off for a day, opened the valve, turned on the radiator. Heard no hissing and it was still only working 10%, but I left it like this for about a day, and it filled up when I checked it next. I closed the valve, and it's been working now for a couple days.
    – tenfour
    Mar 5, 2011 at 11:38
  • 1
    @tenflour : you should post that as an answer, and accept it, so people who come later can easily find what worked.
    – Joe
    Mar 5, 2011 at 13:42

I'm going to assume that you have a water filled radiator, and not a steam radiator.

The only thing I can think of is that for the radiator to fill, there needs to be sufficient water pressure in the system ... if you have extremely low pressure, and this is on an upper floor, then yes, it's possible that opening the bleed valve might make things worse.

But, I'd also check to make sure that there's a flow of water into the system ... it's possible there might be a valve where the drinking water system connects to the radiator circulation system ... make sure it's fully open, and without that, there won't be any water pressure (or way for new water to enter the system)

(and this is based on what I know of US radiator systems; I've heard that Eurpoean systems use a higher pressure system, so there might be some other parts involved that I don't know about).

  • It is water filled, and yea I'm on the 5th floor (no elevator; fear my legs!) I'll look for more valves.
    – tenfour
    Feb 24, 2011 at 13:31

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