Header tank vent pipe constantly running

18 Mar 2005
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United Kingdom

I've read a number of posts but none quite seem to match my particular scenario so I'm doing the recommended thing and starting a new post.

I've had a couple of problems. The worst has been lots (10 to date) of radiators springing leaks. One went 15 months after being replaced the first time. Most of this has probably been caused by the system never being topped up with Fernox over the last 10 years and what with radiators being taken off for decorating and thus taking any protection that was in the system out over time it not so unexpected.

However, in the course of this I've found something else that I now know to be bad and probably adding signifcantly to the radiator corrosion problem.

The header tank vent pipe constantly runs hot water into the header tank, probably aerating it much like the little water feature does on my pond. While the fish like it, the radiators don't I'm sure.

The water in my central heating system is pure and clear. Its been flushed out a number of times due to having to drain down the heating system because of the failing radiators. The header tank is spotless. The heating system has in it (to date) 8 litres of Fernox MB-1.

The system is a Potterton Profile 80e, a Grundfos super selectric 15-60 (on setting 2 I think), and there are two Honeywell V4043H valves, one for the CH and one for the HW. Both are working. The expansion pipe from the header tank is not blocked. We get lovely hot radiators and lovely hot domestic hot water.

If the HW valve is open and the CH valve is closed, the vent does not run and the header tank stays cold.

If the CH valve is open, and the HW valve is open or closed, the vent runs and the header tank gets hot water in it.

The header tank overflow is not running, ie no evidence of the hot water cyclinder leaking water back into the central heating system.

A couple of radiators do need to be bled every now and again, but I'm assuming thats because of the aeration occuring in the header tank.

There are air type and kettling type noises that come from the boiler and pipework around the pump, but they're not constant, almost sounds like a big air bubble rushing around part of the system on a regular basis, although it sometime comes and goes.

So, I'm baffled by the running vent but I know I need to get it resolved or I'll have my new rads eaten away before I know it. If there was a blockage in the CH side that might explain it but all my rads get hot. And the vent doesn't run if just the HW is on. But it does run even when the HW is on when the CH is on.... which I can't get my head around.

So sorry for the long post, but anyone got any ideas as what I need to look at?

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were on the system does the vent pipe come off :?:

were on the system does the feed pipe come in :?:

in relation to the pump and the 3 port valve etc.
On a fully pumped system, feed and vent must join the system close together on the same piece of pipe with nothing between. If you ignore this rule the pressure in the vent can exceed that in the feed. Result - water pumps constantly out of the vent pipe. You can also have problems if the vent pressure is less than that in the feed because, in extreme cases, air gets sucked down the vent.

Your problem is unusual because the trouble starts only when a valve is open. Perhaps your vent and feed are on a pipe (or pipes) on the heating circuit so the pressure differential appears only when water flows in this circuit. You need to find them.

Sorry for not responding more promptly.

The expansion pipe feeds in just before (12 inches) the vent pipe branches up into the loft just before (12 inches) the pump. Hope that makes sense. Imagine it like a capital F ... with the vertical carrying on up as the vent and the pump on the end of the top horizontal of the F. The lower horizontal is the feed from the header tank. That exactly how it looks.

Now in my original text I got something wrong. I thought the pump was on speed 2. Infact it was on speed 3 (its installed in a particlarly bad angle and difficult to see or get at).

So I changed it to speed 2. And guess what, the vent doesn't run any more in any combination of open valve. Also very little boiler noise and almost no pipe noise at all.

So I guess (and I'd like to understand the reasons here rather than just letting it go as it might be of use to others) that the high pump speed was creating enough pressure to lift the water in the vent pipe high enough that it started to run into the tank. Additionally the speed of the water is causing drag where the expansion feed meets the main flow, draging water into the system, thus creating an almost independent loop of vent, header tank and expansion pipe. Is that right?

So what about the lower boiler noise with a lower pump speed. When the pump was running at speed 3 it sounded like I was getting a boiling type noise in the boiler. With a lower pump speed this seems to have gone away. With more water passing through the boiler on a higher speed shouldn't that stop any "boiling". I'd thus sort of expected to get more "boiling" at a lower water speed. I'm obviously missing the point.

Or is it all to do with there now being less aeration of the water because I've managed to stop the vent running?

I'd really like to understand this.

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I take it that the pump direction is TOWARDS the vent pipe. Nothing else would really make sense.

When water reaches the vertical pipe it sees the column of static water in the vent pipe above and another column of static water in the feed pipe further down. As long as nothing moves, the water levels in these two pipes will be equal as you would expect. The trouble is, it IS moving.

The water must flow down a length of pipe between the attachment points of vent and feed and this flow creates a pressure differential, raising the level in the vent. The greater the flow, the greater the differential - until the water goes over the top. Turning down the pump speed reduces the flow. Shutting off CH also reduces the flow. Neither option is ideal.

To cure this one, remove the feed pipe from its present position and reconnect it higher up the vertical. You can even put it just above the pump pipe if you like, ie tapped into the side of the vent itself.

And the boiler noise? You say air and Kevplumb says turbulence. Both are entirely possible. How about SLUDGE!

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