Hot Water Loft Tank Dip Tube Siphoning?

[diychicken]

Hi,

In our loft, we have two tanks - a small header tank for the central heating and a larger tank for the hot water - cold water is mains fed.

The other night, we noticed that the overflow pipe from the hot water tank was dripping outside. I was looking out of the window to see how much was coming out and, suddenly, the water flow turned from a minor stream to a constant flow of water that projected about 4m from the back of our house.

I ran upstairs to the loft and lifted the lid on the tank. The water level was below the overflow exit hole on the tank, but above the bottom of the dip tube so that it was still submerged. However, the water level was lower than what would shut off the ball valve so water was being discharged into the tank.

Despite the water level being below the overflow exit hole, the water was still running through the overflow pipe - I could hear it and could feel it when I put my hand onto the elbow on the overflow pipe on the outside of the tank.

I'm guessing that the water was being siphoned out but I'm not sure how to combat that. It's a big concern.

I isolated the inlet to prevent it from filling further and the overflow stopped at some point (can't remember when - it was quite stressful). I turned the water back on and, since then, we've had no issues.

Has anyone had any experiences like this and could anyone offer any alternative theories or solutions?

Many thanks,

Dave

 

[Nige F]

2 things 1the dip tube shouldn't dip into the water- it's a vent pipe and should be just above the overflow outlet level. 2the rest I can't figure fully without a picture - can you post pics of the setup

 

[diychicken]

Hi,

The picture below shows what it looks like currently. I've searched the web for examples of how it should look and have come across a number of sites talking about Byelaw 30 and images such as the following links.

https://www.directwatertanks.co.uk/image/data/polytank/Cold Water Loft Tanks/Tank installation.jpg
http://www.diyplumbing.co.uk/feed-and-expansion-tank/

In further research since my first post, I came across http://community.screwfix.com/threads/cistern-overflow-dip-tube.147323/ so it looks like someone else has considered this.

BTW, the house was constructed in 1999 if that helps regarding what regs may have been applied at the time.

Hot Water Header Tank

• diychicken
• 8 Jan 2017
• 1

Lid lifted to take photo.

 

[picasso]

Hate to disagree with nige but the diptube should be under the waterline, apprantely it stops cold draughts in the water tank, probably defeating the object of having the diptube but either cut the diptube, drop the water level or drill a hole in the top of the diptube.

 

[Madrab]

Nice and clean in there, looks like the bylaw kit is doing its job. As long as the water level stays below the highest level of the warning pipe then it should be ok. The dip tube is just that, it dips into the water and avoids cold air entering the cistern and freezing it up.
I did come across a situation once where outflow was occurring with a bylaw30 cistern due to the breather in the top of the cover being blocked with crud after they cleaned out their attic, the air pressure had built up just enough in the tank to push water out of the overflow but this is rare. I guess in another rare scenario would be where if it was windy enough that wavering out could cause a pull on the pipe

You have done what would normally fix it, isolate the inlet and let the water drop below the dip tube and then let it fill back up

 

[bernardgreen]

diptube should be under the waterline,

That is OK provided the over flow system can carry water from the cistern faster than the rate that the supply can feed into the tank when the water level is above the bottom of the dip tube and syphonic discharge has started.

If more water is coming in than going out the water level will rise lifting the ball cock and reducing the in-flow rate until at some point the system stabilises with out-flow equal to in-flow.

The question is what caused the rise in water level in the cistern that started the over flow and subsequent syphonic flow.

 

[Nige F]

Hate to disagree with nige but the diptube should be under the waterline, apprantely it stops cold draughts in the water tank, probably defeating the object of having the diptube but either cut the diptube, drop the water level or drill a hole in the top of the diptube.

I thought the OP was talking about the vent dipping in Overflow probably did set up a syphon and chuck that water out until the level dropped - good idea to drill a hole in the top

 

[diychicken]

Many thanks for all your replies. Is "symphonic discharge" a designed thing to purposefully discharge as fast as possible? I'd be concerned about drilling a hole in the top of it to prevent it. If it's supposed to siphon, then I wouldn't want to prevent that. I guess the only real way to prevent it from happening would be to bend the arm of the ball valve so the water level is lower than the bottom of the dip pipe - but I'm still unsure whether that is sensible or not.

It certainly seemed like the discharge was keeping up with the inlet flow - it was really gushing out and the water level wasn't encroaching on the top of the overflow pipe. Glad it seems like the configuration that we have is standard - just glad we were in when it happened otherwise we could have had a huge water bill! At the end of the day, though, I'd rather have a larger water bill than a flooded house.



Yes, there's still the outstanding question about why the initial overflow started - will need to keep an eye on that. We had a couple of instances of overflowing a couple of years ago and I replaced the valve and ball with a new high pressure valve.

 

[bernardgreen]

Yes, there's still the outstanding question about why the initial overflow started

Possibly the ball cock stuck open ( can the ball float touch the sides of the cistern ? )

If the normal water level is very close to the over flow level then when the hot water cylinder heats up form the expanding water going back into the cistern may be enought to trigger the syphon. In that case adjust the ball cock to have a lower water level to allow more space for the expansion