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I am planning to change the radiator in the bathroom for a heated towel rail, but before I do, I want to be sure of the air bleeding procedure as I am only used to central heating in 2 storey homes.

The process is normally easy to understand when you know how. As air rises in the system, in a 2 or more storey building, you work from the top floor to the bottom, bleeding the air out.

We moved a few years ago into a bungalow and the pipework leaves the boiler, goes into the loft, and drops down to a radiator, through the wall to another radiator in an adjoining room and then back up into the loft to go to another radiator in the same way.

I cannot find any bleed valves in the loft, so how do I bleed above the level of the radiators?

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    A word of warning: Radiators are designed to create air convection. Towel rails are not, they are much less efficient at heating a room. And when covered in towels they are even worse. A towel rail covered in towels does not replace a radiator. Think about whether you want a colder room and warmer towels.
    – jay613
    Apr 29, 2023 at 14:50

4 Answers 4

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Bleed at the highest point.

If there are no bleed valves at that point then fit them, which if you are planning the other changes should not be an issue.

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You can force air out of a system even without bleeders at the upper location.

Bleeders are good for small amounts of air.

The general way to get bulk air out of a system is to leave the pressure into the system open, close the return to boiler pathway and open the drain on the return. New water will be pushed into the system forcing old water and air to go out the return drain. No new air will be introduced to the system as your water in feed won't have any air in it.

You can still have small amounts of air in the system that take a while to work out so bleeders at the top locations are still a good idea.

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    After which, you need to re-treat the whole system with corrosion inhibitor. Apr 29, 2023 at 17:03
  • Depends what your components are made of... i have stainless steel manifolds / circulator, oxygen barrier crimppex and brass connectors so no corrosion stuff needed. Apr 29, 2023 at 21:29
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    I live in an area where the water comes from deep aquifers with limestone and chalk strata -- hardest water I ever saw. Inhibitors also neutralise hard water. May 1, 2023 at 21:22
  • my water is almost as soft as the rain water so I don't have those kind of issues but good points. what do you use as a corrosion inhibitor, do you rely on a swing type check valve to prevent backflow into potable or do you air gap / reduce zone valve your system? May 2, 2023 at 0:23
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    Standards in UK require an air gap, which comes preinstalled on the boiler fill point. Older version uses two quarter-turn valves and a flexible pipe with hand-tightened connectors. Newer type can use a key handle that bridges between primary and potable circuits, and interlocks with valve handles. May 2, 2023 at 6:58
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Without bleed valves any higher than the radiators themselves, you can't. While the system is being changed, it's best to fit one, or two bleed valve in the loft, at the pipework's highest point. Problem solved for a very long time!

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    I'd consider putting an automatic bleed valve there, so if air starts to accumulate in the future it can self-correct.
    – keshlam
    Apr 29, 2023 at 13:24
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You can usually get the air down into the radiators. Once there, it will rise to the top of each radiator, and get trapped where you can bleed it.

To assist the process, you can (a) turn the pump up the max, and (b) close off most of the radiators in rotation, to maximize the flow in other sections.

You are not working against the whole head of water in any pipe. If you can get some flow going, it will break up air pockets gradually and smaller bubbles will get dragged downwards into radiators quite happily. When it stops making gurgling noises, you have sorted it.

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  • You only get tiny amounts of air - so bleed any radiators which are cold near the top. You don't need a turbocharged pump or anything. Apr 29, 2023 at 20:21
  • @JeremyBoden I put about 16 radiators into a sailing clubhouse building (Scandi log cabin style, about 20 metres long). The primary circuits ran the whole length, in 28mm copper. First fill really didn't work out too well, and I resorted to auto bleed valves near both ends of each run. May 1, 2023 at 21:29

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