Water discharging from the loft vent pipe as CH turns off?

[Markw996]

Hi,
I have just moved into my first home, a 2 bed semi-detatched and as the title says I have a very noisy central heating system.

As it comes on I get lots of bubbling / gurgling noises that last about 10 seconds and seem to originate from either the upstairs radiators or the airing cupboard.

Also as the central heating clicks off it sounds like someone emptying a bucket of water up in the loft right above my bedroom (very worrying the first time it happened!).
I traced the loft noise to the expansion tank in the loft where, as the heating clicks off, I am getting about half a litre of water discharging from the vent pipe back into the tank.

The system is open vented circa 1992 and I take the vent pipe to be the one that rises about a foot above the tank, then bends in a U shape back down into the tank.

I've had breif discussions with a plumber friend and he says it could be the boiler thermostat?!? but I suspect it is more likely to be air in the system somewhere (although I am certainly not clued up on central heating systems) as I did have to bleed a lot of air from by bathroom radiator when I first moved into the house due to it being very cold at the top.

Just to summarise:
I have 2 tanks in my loft and the problem is relating to the smaller tank which does the central heating.
My boiler is located in a downstairs cupboard.
My airing cupboard is upstairs and contains an immersion tank, pump, and a motorised valve of some sort.

I hope someone can help as the noises wake me up in the mornings and annoy the hell out of me in the evenings! (

Many Thanks,
Mark.

 

[SafeGasInstall]

good point apple -plumbing but this will be continues overflowing in the f&e ,not just a high level of water

 

[Horse]

good point apple -plumbing but this will be continues overflowing in the f&e ,not just a high level of water

Thanks for the input. The fill is only a 'spurt' when the pump stops, in CH or Hot Water, not a continuous flow. I wondered if it been like it a long time, and has only shown due to the tank ballvalve refilling after the radiator was put back, slightly raising the level to near the overflow. But I will do the tests above and report back as soon as I can. Cheers

 

[SafeGasInstall]

and put some pictures here to make it easy for all of us

 

[Horse]

and put some pictures here to make it easy for all of us

Will do

 

[tamz]

The fill is only a 'spurt' when the pump stops, in CH or Hot Water, not a continuous flow.

It is caused by a bounce in system pressure as the pump stops. It will be cured if you lift the vent higher.

 

[jase158]

The basic principles of system design are being lost due to combi's and sealed systems being more popular bit it should be important to know the basics.
There are dozens of ways to pipe a system all based on a few basic layouts.
Understanding how the position of the feed vent and pump effects the water circulation helps.
Here are a couple of drawings. (this could have been a far longer post but i have simplified a lot of things)

I'll start with this one. Basic common layout for a gravity HW, pumped heating on an old cast iron boiler.

View attachment 2125

The position of the feed connection to the system is called the neutral point. Everything from the pump to the feed is under positive pressure (from the pump) and everything after the feed is negative.
In this drawing the only part of the system under positive pressure is from the pump to the boiler (as this is effectively the point the feed connects). The rest of the circuit is under negative pressure (the water is being pulled).
Because the cast iron boiler holds so much water it also acts as a neutraliser and the water flow from the pumped side slows right down so has very little effect on the gravity side.
It used to be though having the pump on the return like above would help the gravity circuit. In reality it didn't have any real effect.
A system laid out like this will not draw air or pump over (unless a blockage is introduced).

View attachment 2126
Same system with the pump on the flow. The heating circuit is now under positive pressure (being pushed). This is the better pump position as pumps can push better than they pull.

Move on a few years and motorised valves are introduced and systems become fully pumped. Still using high water content cast boilers, so we get something like this.
View attachment 2127
Feed and expansion still connected to boiler so position of the pump does not cause it to pump over or draw air. Pump on the flow so the system is under positive pressure.

View attachment 2128
Same thing with pump on the return. System under negative pressure.


If you use a low water content boiler or the feed and expansion are taken from the pipework, where or how they are connected starts creating problems.
View attachment 2129
Pipe it as above and a suction effect is introduced to the expansion pipe and if the pump is set too high it will draw air. Fitted like this to a low water content boiler where the pump speed usually had to be set high this is why a lot of systems fitted in the 80's/90's are full of sludge.
Easy quick way to over come this was to combine the feed with the expansion like this
View attachment 2130
This stopped the problem as the pump had to pull water from the F&E tank rather than air. It was not ideal though.

So they developed this system
View attachment 2131
Provided the feed and expansion tees were kept within 150mm of each other, the pump would have very little effect on the expansion pipe. Go much beyond the 150mm spacing and it will pump over.

There are dozens of variations of these layouts. Just think how the position of the feed and expansion will effect the system or introduce blockages and think what would happen.

A bit quickly explained and i have missed out a lot but the basic principals are shown. Hope this is of some use to someone.

Wow thank you for this, I never got taught any of the other systems in college. The first system was the only one we got taught. I paid 600 pound at college and did not get shown any of that, the only part we did get taught was that pumps must be after the vent and cold feed. But nothing to that extent, thank you

 

[apple-plumbing]

In the early stages of a breech in the coil in the HWSC, the symptoms can worsen when the Primary Circuit is hot, as it can open allowing water to back fill from the Secondary Circuit. When the Primary cools down it could close. Combine this with a 4% drop in water level plus bailing the F&E, this may give the impression the overflow is not continuous. I agree, I would expect the Vent Pipe to vent due to a blockage when expanded water takes the shortest route to escape from the Primary Circuit. If there is a blockage in the Cold Feed this will prevent the water from re-entering the Primary Circuit trapping it in the F&E causing the level to rise every time the system vents until it overflows. But if the volume of water continues to rise intermittantly and the shortest route is the one open to atmosphere (as it has least resistance), this maybe the early signs of a breech too. Replacing the CF section of pipe as it joins the Flow would confirm if there was a blockage at this point. Partial blockages around the system can also cause these symptoms, Power Flush on Thursday and let us know the outcome

 

[tamz]

Wow thank you for this, I never got taught any of the other systems in college. The first system was the only one we got taught. I paid 600 pound at college and did not get shown any of that

Stick a pound in the charity box of your choice for the lesson

 

[apple-plumbing]

One final test for a coil breach inside the HWSC is to isolate the taps & toilets in the house and electricity to the boiler. Isolate the Float Valves to the CWSC & F&E too and mark the water levels on the inside of the tanks. If after a set period of time the water level in the CWSC has dropped and risen in the F&E there is a breach. The longer the period of testing, the more conclusive the results will be

To test for Pump Over, cut a 2 litre soft drink plastic bottle in half and securely & safely hang this from the top of the Vent Pipe using string (may have to insert a fitting to have something to hang off) so that the open end is under the end of the Vent Pipe where it would discharge into the F&E, to catch any water pumped over. It's important to test that should it fill the plastic container it will still overflow into the F&E. Ensure the plastic container is large enough to hold a fair amount of water without putting too much strain on the Vent Pipe. If it's full after a normal cycle of demands for HW and/or CH (and these can be tested for separately) there may be a blockage (or partial blockage) somewhere in the pipe work. This is normally where the Cold Feed from the F&E tees into the Primary Circuit. Cut out & replace. Pump Over normally introduces oxygenated water into the CH too, so more air than normal being collected in one of the Radiators and black when bled are signs of Pump Over

Pump Over can also be attributed to the CH Pump being installed the wrong way round and bad design; where the Vent Pipe & Cold Feed (in that order) are not connected correctly (normally within 150mm of each other) at the neutral part of the circuit

 

[Horse]

Sorry taking so long to post a response. I extended the vent pipe the extra amount as suggested to give a better arc. I also passed a small 6mm od tube down the outlet of the expansion tank all the way down and past the joint that we thought may be blocked. There were no discernible blockages. I bent the ballcock arm to roughly leave the tank 33% full. Everything now appears ok and when the pump stops I can see a little warm water (thermal blur) coming back up the Expansion tank outlet, like it should do, with nothing spurting out the vent pipe. Thanks to all the people who contributed advice, it was most appreciated and proved to be 100% correct.