Water boiling in the heating system

4 Sep 2004
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United Kingdom
I have a Glow-worm fuelsaver F boiler, Myson Aerjec auto-venting thingy and a Honeywell MPV which is stuck in the mid-position. There is also a manual bypass valve. With the boiler stat set on 2 (out of 6) and with the bypass valve almost shut the water boils up through the Aerjec vent. With the bypass open too far the H/W and rads don't get hot. Should I have to control the water temperature this way? I've just bought a new boiler stat (£40+ :cry: ) but haven't fitted it yet. Is that likely to cure it? I thought the boiler stat controlled the temp of the flow water, not the return?

Can anyone help please?
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Boiler stat range is something like 65 - 82, so of course the water shouldn't boil. Seems pretty clear you need a new thermostat.

Thanks for that - sorry if I sounded impatient in previous posts, these plumbing jobs get my nerves jangling, especially on a Sunday! Hopefully the thermostat replacement won't prove too challenging!

Cheers, John
It isn`t unusual for a correctly fitted myson aerjec to become blocked with black magnitite thus restricting the flow out of the boiler thus creating noise in the boiler and lack of heat to rads.Also if you have a mid position valve there is no need for a properly installed system to have a bypass valve as one port will always be open for the pump over run facilty etc.
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OK - I've just been inside the boiler ... the thermostat DOES sit in the return pipe which explains why the flow water can get so damn hot. If I didn't open the bypass the return wouldn't heat up quick enough to shut the boiler off, hence the "boiling" problem.
(I got the wrong part from the local supplier - they gave me an overheat stat not the control stat!)
The feed pipe into the tank gets really hot very quickly and that's after the Aerjec. However, I guess there must be restricted flow somewhere and if the Aerjec is the culprit how do I "clean" that?

I'm off to replace the immersion heater now - so thanks to all who contributed to some relevant threads about that.

Thanks for your help so far chaps!
Buy a new Aerjec if that's blocked. YOu get calcium/iron compounds in there which don't dissolve in anything much - slow in hydrochloric acid!

Check the feed pipe too if it isn't connected to the aerjec - likely to have crud at the bottom end. You could reconnect it from the underside of the flow pipe if it isn't already, which stops convection currents in the fresher water.

Magnetite (sludge/rust) is magnetic. If a small magnet sticks to to your copper aerjec, you know what's in there!

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