Odd Radiator Behaviour Featured

Sounds like air is getting in the system somehow. Do you have a leak anywhere?

I think you can also get hydrogen generation due to anaerobic corrosion. Is there inhibitor in the system? Has the water been changed recently? Is it an open (with header tank) or closed system?

Theo

You mention attic room ... if this is an open vented system (probably not the norm these days) is the feed/expansion tank at roughly the same height as where the rad gets warm to?

Not that I've noticed. I drained the system down a few months ago and this rad has always been troublesome. The system has a 15 yo combi, and the water was very black. Removed and flushed all rads except this one.

Closed system, newly dosed with inhibitor.

I'd guess there's still some air lurking in the system - or a leak. But why does the rad go from (pretty much) all or nothing so easily?

It's a closed system . . .

Mine does that on the top floor. Sometimes the pressure drop makes the boiler turn itself off and if you try to bleed the radiator air goes in not out. I am told there is a leak somewhere hot - in the boiler in fact so it evaporates and there's never any sign of actual water leaking, and the best thing is to hope it will block itself up.

TW

The highest radiator will generally be the one that accumulates the most air. I’ve lived in several houses that have all had radiators that need bleeding occasionally without any obvious leaks.

What’s worked very well for me is these automatic bleed valves for radiators.

Tim

Is the pressure in the system dropping? The radiator is likely to be the high point of the system and collecting any air in the whole system.

If you have always had problems it may be an airlock or partial airlock in the pipes somewhere. In my mother's house this kind of problem happened when a full or partial drain down was performed. The solution was to close all radiators down in the whole system apart from the one with the problem. This tended to force more flow of water to the problem area. However this was with a fixed speed pump and may not work with a modern proportionally controlled pump.

If it's a leak causing a substantial amount air to enter the system I would expect the pressure to drop over a period of weeks and a fairly constant need to use the filling loop to re-pressurise the system - this happening without bleeding radiator).

If it was an open system one cause would be pumping over - the pump speed to fast and the water circulation through the header tank.

I've found them to be very problematic after a full drain down if there is appreciable sludge in the system.

If you fit them after a drain down or on a new radiator they work fine for the first time because the pin hole is not blocked. When filling the radiator the sludge in the bottom of the radiator is stirred up and as the (now) contaminated water level reaches the top of the radiator it will enter the pin hole. The sludge in the radiator will tend to settle but any sludge in the bleed valve gets hot when the CH is on and hardens. I've found that subsequently a few then fail to auto bleed, especially when a system has been drained down a couple of times. It's easy to spot that these valve are not working on a complete refill but nor so obvious during normal CH operation when a small amount of air need to be released - the only way to check is to unscrew the whole valve to see if more air can be liberated :)

Suffice to say that these bleed valves are no long part of my system despite having a system power flush for a new boiler.

What is the system pressure? Where is the pressure gauge - boiler location, airing cupboard, etc.? What is the vertical distance between the pressure gauge location and the top of the troublesome radiator?

Have you tried bleeding with one radiator valve closed and then swapping over and doing the otther side? This will make sure that *both* the feed and return pipes are air-free.

I understand you recently drained and refilled the system, including adding inhibitor, and have 'bled' the problematic rad several times.

Given the sources of 'air' in a system are:

What is there when filled What gets in if there is a leak What is produced by chemical reaction (not truly air but ....)

You've added inhibitor, assuming you've added enough, that leaves not the other 2.

You can rule out a leak by pressurising the system an checking it 'holds'.

If air is 'stuck', I'd try closing off some rads while filling, then opening them, to try ensure water is forced down any 'branches' in the system.

A plumber came to the house for some reason that I don't remember some years ago, and when he saw the bleed valve he said he always took them out whenever he saw them because 'they don't work.' 'Well, I've not had to bleed a radiator since I installed it,' I told him, which rather surprised him.

I had extreme problems bleeding two radiators at the end of a long loop in the roof going down to an annex.

The twin pipes now feature T sections with bleed valves on the top of small stubs pointing upwards....

Automatic bleed valves are pointless in a sealed system since the released air cannot be replaced with water unlike an open vent system with a header tank. On sealed systems bleeding radiators means re-pressurising the system.

If the system stays pressurised but some radiators are not getting hot then it might be a case the system needs re-balancing particularly if the cool radiators are at the end of the system.

Richard

If you have a filling loop, re-pressurising the system is just opening the valve to the mains water supply until the gauge shows the required pressure.

That would be the case that the whole radiator is not getting hot, which is different from the case here where the top part of a radiator is not hot but the bottom is.

Theo

Don’t see why you would ditch them. They’re very easy to disassemble to clean the disks and any orifices. Once cleaned, they work just fine.

Tim

Depriving him of easy work! ;-)

Sorry, that’s just bollocks. The amount of air that needs to be bled is often very small. My bathroom radiator needed bleeding very regularly before I fitted the automatic bleed valves. 2 years after fitting I had to clean it out but now it’s working just fine again.

Of course one may have to repressurise occasionally but I’ve never had a system that didn’t need the very occasional top up so that’s no hardship.

Sounds like you’re speaking from a theoretical basis rather than firsthand experience.

Tim

There were two versions, one fixed and the other with a replaceable inner section and the latter were a lot more expensive.

The link you gave is for the former type and the text says: PLEASE NOTE THIS IS THE HV30 WITH A NON REMOVABLE CARTRIDGE.

From memory, the fixed versions were about 10 quid each MANY (possibly

It depends on how much air is released from the system after commissioning.

Yes, if a large amount of air is released then the system needs to be re-pressurised via the filling loop.