bleeding daily

diy_dude · 23 Oct 2006

I have an open vented central heating system with 3 radiators downstairs and 4 upstairs. 6 don't give me any trouble what-so-ever, however the one in my sons bedroom does.

If I don't bleed it it gets about 1/4 full of air in a week. If I bleed it daily, I get air out every time. I can sometimes hear the air bubbling into the radiator when the pump is on.

I have only noticed this problem in the last 2 weeks as the heating has been off all summer. This radiator is probably the 2nd furthest from the boiler / pump upstairs.

My questions are:

Where is all the air coming from?
Do I need to worry about it?
If yes above, what do I need to do?

cross thread · 23 Oct 2006

yes it will cause corrosion in the system,but cant comment on cure as it may be system design causing it,you need a heating eng to look at it and offer advice to cure it

bathjobby · 23 Oct 2006

With an open vented system the first thing to check is for 'overpumping' into the feed and expansion (F/E) tank i.e. when the system is running is there water flowing though the expansion pipe into the F/E tank ? If so, this is constantly introducing oxygenated water into the system and the oxygen will separate out into a high point and will also cause corrosion in the process.....
Suggest you check this first.
A common cause of this is having the water level in the F/E tank too high. The level in this tank should be just covering the feed pipe in the bottom of the tank. Check this out and let us know.

diy_dude · 24 Oct 2006

will check the tank tonight and post back....

diy_dude · 26 Oct 2006

I went into the loft last night to have a look at the F/E tank. I took some photos so please have a look.

The tank has 15 cm of water in the bottom, thats about half full. While the pump was running there was no water coming out of the expansion pipe and it was not sucking air in (tested by putting my hand over the pipe and holding a full beaker of water over the end of the pipe). The water in the tank was luke warm (possibly 25 - 30C). However there was a lot of brown stuff in the bottom of the tank, presumably rust.

Does any of this help?

I should have mentioned in my first post that all radiator valves were replaced with TRVs during the summer when the heating was not in use. Inhibitor was added after draining the system for valve installation.

diy_dude · 26 Oct 2006

Forgot to mention:
The water level in the tank looks pretty constant. I guess that from the line on the side of the tank and the fact the ball valve opened when I removed a few cup fulls of water.

Softus · 26 Oct 2006

bathjobby said:
A common cause of this is having the water level in the F/E tank too high.

Hm. How does a high water level lead to air in the system?

Softus · 26 Oct 2006

diy_dude said:
Do I need to worry about it?

I don't know how you're measuring 1/4 of a radiator full of air each week, but lots of "air" can be the product of corrosion in full congress. The fact that there is rusty water in the F&E shows that there is oxygenated water in the system.

If it were my system I'd drain it down and observe the quality/colour of the water coming out, and consider refilling with a system cleaner to dissolve sludge and dislodge rust, the drain and flush out again. There are lots of previous posts on this subject.

diy_dude · 29 Oct 2006

I drained some water off the system yesterday - it came out lovely and clean...

Decided to tightened the TRV on the radiator that was collecting air. The compression joint to the pipe recieved about 1/4 turn. Since then I have heard no more air entering the radiator. Fingers crossed that fixed it...

Softus · 29 Oct 2006

diy_dude said:
I drained some water off the system yesterday - it came out lovely and clean...

Subject to knowing where you drained it, bear in mind that the first water drained off often does appear to be clean; it's the dreggs that reveal the true quality...

Decided to tightened the TRV on the radiator that was collecting air. The compression joint to the pipe recieved about 1/4 turn. Since then I have heard no more air entering the radiator. Fingers crossed that fixed it...

Lets hope you're right. If there was such an air leak then the system may well have corroded some anyway, and depending how bad things are you might not have an activity left in the inhibitor in your system.

diy_dude · 29 Oct 2006

I spoke to soon....

All was OK when the heating was on for an hour this morning: No air heard and none could be bled from the radiator.

This evening the good old bubbles came back and air came out of the bleed valve when I opened it.

I always wait about an hour after the heating has gone off to ensure its cool and the pump is off.

I can only hear air bubbles right near the TRV. Cannot hear any in the pipes under the floor.

Am I working along the right tracks??

The drain valve nearest the front door is leaking very slightly. Changing it is a good excuse to fully drain the system, check the colour of the water and add new inhibitor. Will stop the leak too.....

Softus · 29 Oct 2006

Before you drain, work out whether the TRV is on the flow or the return, and if the return confirm that it's a bi-directional TRV.

The rest of your plan sounds correct, but bear in mind that if you find out that the system is sludged then you'd need to flush it out, possibly with sludge cleaner, and drain again before finally filling with a chemical inhibitor.

diy_dude · 30 Oct 2006

The TRV is a Danfoss RAS c2 (bi-directional). Its installed on the flow pipe in the correct direction.

Will drain once I have the new drain valve and post back.

Thanks Softus

sooey · 30 Oct 2006

softus asked how does a high water level in the f&e cistern lead to air in the system.
I believe it's because if the level is set too high with the system cold (ballvalve adjustment), then when the system heats up, the water will expand into the already full tank and out of the overflow, causing the ballvalve to open. As the water contracts again on cooling, this then introduces freshly oxygenated water into the system.

Softus · 30 Oct 2006

sooey said:
softus asked how does a high water level in the f&e cistern lead to air in the system.
I believe it's because if the level is set too high with the system cold (ballvalve adjustment), then when the system heats up, the water will expand into the already full tank and out of the overflow, causing the ballvalve to open. As the water contracts again on cooling, this then introduces freshly oxygenated water into the system.

Interesting theory - not one I'd heard before. I can see a few flaws in your reasoning, viz:

1. The float valve, in your scenario, wouldn't open until the system cools, so any fresh water in the cistern won't be drawn into the system until the cooling cycle after one in which it was admitted to the cistern.

2. Given point 1 above, I expect that any fresh water drawn into the cistern will contain no more oxygen (whether free or otherwise) than water that had been sitting in the cistern a long time (e.g. months).

3. Notwithstanding point 2 above, if any more-than-averagely-oxygenated water did enter the system, it wouldn't immediately appear as bubbles in a radiator - it would be rather more likely to accelerate the corrosion in the radiators, releasing Hydrogen as one of the usual suspects in source of the corrosive reaction. If this has happened, then the action you need to take is the same as if the system were sludged up.

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