Hot CH expansion tank!

[amudda]

Hi,

Hi all - hoping you can provide some guidance on a rather strange problem!

I have a open vented CH system, with a feed and expansion tank in the attic, the hot water cylinder and pump in the airing cupboard on the first floor, and a boiler on the ground floor. The pump is mounted vertically, and pumps downwards. There's a vent pipe from just above the pump. We've been in the house three years, and the only real playing around I've done was to move a radiator about a year ago, and I drained and refilled the system back then, but it was fine.

I've just drained the system again, to add some cleaner to the system, and refilled it. Now the expansion tank in the attic starts to get warm, and I can see water swirling slightly in the tank, which means it's coming back up the feed pipe. Also, when I turn the system on, I can feel air being pulled through the vent pipe - when I turn it off, water comes out the vent pipe.

Any ideas what this could be?

 

[croydoncorgi]

Follow up threads here talking about 'blocked cold feed'. Likelihood is, that's what you have, if the water out of the vent pipe has suddenly started happening.

 

[amudda]

I don't think it's that - I've just unblocked the cold feed (earlier problem!), and confirmed that it's working by draining some water out the bottom of the system, and watching the water level in the expansion tank drop, so water's flowing down the cold feed.

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Also, when I turn the CH off, the cold feed pipe is hot - as though hot waters being drawn up it - when I turn the CH back on again, it goes instantly cold - so when I turn my heating on, water is drawn down the cold feed pipe, and air is drawn down the vent pipe at the same time?

The pump is on 1 - the lowest setting.

 

[chrishutt]

We need to know where the feed pipe and the vent pipe connect to the circulation pipework, particularly in relation to the pump. I would expect that above the pump you would have first the feed pipe connection and then the vent pipe connection, the flow pipe then leading back to the boiler (that order is the reverse to the direction of flow). How much pipework is there between the vent and feed pipe connections?

 

[amudda]

Right - it goes :

Pump - pumping downwards.

From the pump, it goes up about 10cm of pipe to a T-junction.

The straight on the T goes straight up into the attic and is the cold feed. The right angle part of the T goes through about 5cm of pipe into a brass cylinder (air separator, I think), and out the top of that is the vent. Out the other side of the air separator is the pipe to the boiler.

So in order, it goes boiler, vent, feed, pump - pretty much what you said. There's about 5cm of pipe between the vent and the feed - plus the air separator.

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[chrishutt]

I would suspect a partial blockage between the deaerator (vent connection) and the feed pipe connection. This would explain air being sucked in on the vent and water being pushed up the feed pipe. Can you take the pipework apart (reassemble with compression fittings) to check and clean?

 

[amudda]

Not without needing to replace quite a bit of it - it's all soldered, and there's not much room for manouvre. Will give it a try over the weekend, though.

If it was a blockage between the vent and the cold feed, how would hot water still be getting up the cold feed? I think that the hot water going up the cold feed is due to air being drawn in on startup through the vent - the air gets trapped the other side of the pump, and prevents water from flowing properly through the system, so it backs up into the cold feed was my thoughts.

But if that's the case, I still don't understand why it's drawing air in on startup, through the vent...

 

[chrishutt]

You're right, I got it mixed up. A blockage where I suggested would mean water pumping over the vent and then being drawn back down the feed.

Just a thought - could the pump be fitted the wrong way round - pumping back towards the cold feed connection? That combined with a blockage as suggested would explain the symptoms.

 

[amudda]

Don't think so - the arrow on the pump points downwards, so everythings on the suction side.

Also, this has just started to happen since I drained and refilled the system - wonder if something's got clogged?

 

[chrishutt]

You haven't said what controls you have. Is there a motorised valve that diverts water to CH and/or HW? Also is there an air vent point on the branch into the cylinder? Have you vented it?

 

[amudda]

There is a motorised valve, but it's never worked since I moved in three years ago - it's always been stuck on for both CH and HW - we just turn the rads of individually when we don't want heating!

The radiators are getting warm at the moment, and the hot water is still hot - the pipe into the cylinder is hot, and so is the pipe coming out (not as hot, but still hot).

I haven't vented the cylinder - not sure how to do this? There's nothing between the cylinder and the pump besides the motorised valve, though.

Just felt the pipe - when I turn the system on, it draws in both cold water and air - I know it draws air, because I can feel it going in the vent pipe, and I know it draws water, because the cold water feed pipe gets colder very quickly.

 

[chrishutt]

when I turn the system on, it draws in both cold water and air - I know it draws air, because I can feel it going in the vent pipe, and I know it draws water, because the cold water feed pipe gets colder very quickly.

This can't be right. You can't have fluid entering both - where would it go? Can you check if the vent pipe is really sucking by holding a cup of water over the end. If it's sucking, the water level in the cup will drop as it's drawn up the vent pipe. Also can you check if water is being drawn down the feed pipe or not - close off the float valve and observe the water level in the tank - does it go up or down?

 

[amudda]

Right - air is entering the vent pipe when I turn the heating on, as the water in the cup disappears up the pipe.

Also, the level in the feed and expansion tank drops by about 1 cm when I turn the heating on, so it's drawing in water too.


I think what then happens is that the air gets trapped on the flow side of the pump - the bouyancy of the air counterbalances the push down from the pump - and stops the flow, forcing water from the boiler up the cold feed pipe - what I don't get is why it's drawing the air in the first place!

Thanks for all your help on this by the way - really appreciate it - I'm stuck!

 

[chrishutt]

I think what then happens is that the air gets trapped on the flow side of the pump

That makes sense. Then when the pump starts, it compresses the air trapped on the flow side thus creating the volume to accommodate the air and water drawn down the vent and feed pipes.

I suggest then that there must be a partial blockage (air or sludge) on the main circulation through the boiler (including flow and return pipes either side). Air is drawn in through the vent because the flow of water from the boiler is restricted. If the blockage is air, look for a high point (probably flow pipe from boiler). If it's sludge, look for low points on pump return.

Later edit - amudda, you're running two threads on this problem! That it not very helpful!

 

[amudda]

That makes sense.....I have no idea how to go about checking for that though!

Maybe it's time to call someone in, but that goes against everything I stand for!

Sorry about the two threads, but as the first one was for a different problem, I thought it would be better to start a new one for a new problem.