Noise-Air bubbles around pump

14 Dec 2003
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United Kingdom

I appear to have a noise coming from the pump area in the airing cupboard.It sounds like tiny air bubbles, a sort of 'rushing' noise, similar to rising main pressure noise.
It only happens when the system is hot, and the mid position valve has moved to CH only, from CH and HW.
I can't attribute it to any particular event, although I noticed it probably 12 months ago after fitting a new pump,as the other died after many months of knocking.The system did initially have FERNOX in it, but since draining I have not replenished it.I have also since fitted thermostatic valves to all rads, bar the bathroom one.When I recently drained the system, I noticed the water was black.Perhaps adding fernox again may help, as it appears as if bubbles are being created when the circulation is at its shortest. IE when only flowing through the rads.When the noise is present,I can cure it by turning up the cylinder stat,which moves the mid position valve across to HW and CH again, and the 'bubbles' disperse.
Direcly above the pump is a device I believe to be an air separator(MYSON AERJEC II).Could this be the problem ?
Someone else suggested filling the system with distilled water as this couldn't oxygenate and cause bubbles.
Could anyone cast any light on this problem, further info if required.

many thanks
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Blck water means corrosion which means hydrogen is being created. Ideally get a powerflush, else try just addng inhibitor.
ChrisR is right about the corrosion reaction. You don't need absorbed or free oxygen for this to happen.

Adding distilled water would be pointless, as you have an AERJEC, you have an open vented system, and even if the water had no oxygen in when you put it in the tank, it would absorb oxygen. This is how ponds support fish. Distilled water is demineralised rather than de-oxygenated, and the action of pouring it from one container will entrain oxygen. Though it makes little difference to the corrosion

You really need an acid flush before you add inhibitor, as the inhibitor needs to get to the metal to protect it. It would be protected by the magnetite (black sludge) but it'ds brittle, and breaks away as the heating system expands and contracts.

It can be costly to clean the system, but is cheaper than replacing the system, and could be made cheaper by ensuring the inhibitor is added and replenished every two to thre years.
Many thanks for your swift reply.

I have a container of FERNOX-MB1 that's been in the garage for 12 months that I have not yet used.Is this suitable? I think the boiler is cast iron.
I've read that 'potable' chemicals only should be used .Does this mean non-toxic,in case it leaks into the hot water cylinder ?

Presumably this is added via the header tank after draining the system
and tying up the ballcock.
I ususally bleed the system before initially firing it up, and then again when it is hot.I do however,usually have to open the draincock downstairs
to help pull the hot water through,otherwise the downstairs never heats up.some of the inhibitor may therefore get drained away and diluted as the header fills up.

PS, I think you're right about the hydrogen, as I keep trying to bleed the bathroom radiator,which is the newest,and always manage to get some air out every 5 days or so.

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PPS -can I buy an appropriate flush substance from B & Q -I've noticed
they sell quite a few different heating chemicals and could you recommend a brand.?

Sorry about all the questions, once I get started, I can't stop !
Most if not all of the brands will do the job. You will not be able to do it by draining to the same extent that power flushing will do.
Bought FERNOX system flush(Crystals) and added to system.followed instructions carefully and circulated for around 5 days,with the system bubbling away,and venting every day.
Then flushed out with clean water and added FERNOX inhibitor MB-1(funny colour,looks like a urine sample) as per instructions .
System now runs virtually silently.Pump seems happy,no air bubbles,
everything fine again.


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