Where does all this air come from :?

[Jazzycat999]

We have a really old gravity fed system.

The problem is that the hot water keeps stopping due to air.

We have connected the mains cold to the hot and blown the air out, it will run one bath full of water and then stops again.

Been going on for ages....

Please any suggestions

 

[dextrous]

Water gets hot, gives off bubbles which rises to a high point in the pipework, and creates airlock. Or, water sucks air into pipework via vent pipe due to velocity of water flowing through it, which again gets stuck in high point of pipework.

Solution - identify where the "inverted" U section of pipework is and rearrange so water can only flow downhill and air uphill.

 

[kirkgas]

might not be air, but gas(ferrous oxide?) caused by corrosion, a common occurence in old systems, technically you only "air" rads on first fill up and subsequent refill after drain downs, but often we say we "air" the rads when we mean they are cool due to a build up of corrosion gas, usually when we bleed the rad there is a pungent smell which indicates corrosion, so yes pipework design can be a prob but you can test the gas coming out of the rad is caused by corrosion by trying to light it if it lights it isnt air (cant remember what it is but there is corrosion in the system)

 

[dextrous]

might not be air, but gas(ferrous oxide?) caused by corrosion, a common occurence in old systems, technically you only "air" rads on first fill up and subsequent refill after drain downs, but often we say we "air" the rads when we mean they are cool due to a build up of corrosion gas, usually when we bleed the rad there is a pungent smell which indicates corrosion, so yes pipework design can be a prob but you can test the gas coming out of the rad is caused by corrosion by trying to light it if it lights it isnt air (cant remember what it is but there is corrosion in the system)

All true, of course, but the op is getting the airlock on the DHW

Oh, and the gas is hydrogen, released from H2O via oxidisation of the iron element within the steel.

 

[Jazzycat999]

Wahoo, thank you so much for replying so quickly...

We live in a bungalow and the hot water tank is in the hallway, the water then goes up into the loft, across the loft floor and down the wall into the bathroom and kitchen.

Could this be the problem ???

Bit blonde when it comes to plumbing, I understand the bubble theory but don't get the air getting sucked in theory, but feel I have made more progress in 5 mins than 3 months...so thank you

 

[Razor900]

You need to check that the inlet valve to the tank in the loft is actually working properly and not partially blocked, run a bath and see how much water is left in the cold water cistern afterwards

 

[Jazzycat999]

Thanks
I have changed the restrictor on the ball valve for a larger one so after running a bath the header tank is still over half full, could it possibly be the fact that the water has to go up around 3ft then across loft about 5ft then back down to floor level and around walls to the taps

 

[dextrous]

Thanks
I have changed the restrictor on the ball valve for a larger one so after running a bath the header tank is still over half full, could it possibly be the fact that the water has to go up around 3ft then across loft about 5ft then back down to floor level and around walls to the taps

Yes

With regard to air being sucked in, it is down to the Bernoulli effect (which explains why aeroplanes fly) - if you hold a piece of paper up and blow over the top of it, the paper will rise due to the increase of speed of air above it compared to that below it. As water flows through a pipe, it creates a similar effect, and if the speed of the water is sufficient (as, for example, when you run a bath), then it will suck air in through the vent pipe.

Solutions include:

not to have this inverted U-section

don't run the bath tap so quickly,

or find some way to vent the high point of the pipework - an automatic air vent may do the trick, but it's not really what it's designed for.

 

[TicklyT]

Where is the open air vent (usually ending in a pipe suspended above the cold water storage tank) connected to the hot water outlet from the cylinder?

If the vent is teed from the hot outlet below the highest point on the hot water pipe runs, it could invitie an airlock.

After many years of use, it's not unknown for limescale and sediment to build up in the bottom of a hot cylinder, restricting the cold feed to the cylinder. The system will then try to draw air down the vent pipe if the demand exceeds the available flow.

 

[Jazzycat999]

Many Thanks

To Everyone helping me with this problem, I have now been told that as the header tank sits on the joists there may be a need to raise it up as high as possible to increase the header pressure while i am doing this i will check for possible sludge and blockages once again many THANKS to all

 

[dextrous]

Maybe, but I think TicklyT's onto a good thing when he (kind of) suggests relocating the tee for the vent to the highest point of pipework, which may well be where the airlock is occurring anyway. Cheaper than faffing around raising the tank.

 

[Jazzycat999]

Thanks looking at it the tee for the vent is just above the tank around 2ft 6 from the highest point of the pipework will try moveing this first thing tomorow

 

[Jazzycat999]

Hi All
Today I have moved the vent /overflow pipe to the highest point Drained hot water cylinder etc moved pipe refilled tank turned on tap got water & air (never happened before ) no air lock have filled 6 bath fulls still no air thanks to every one hopefully problem may be sorted

Many THANKS again for all your help

 

[dextrous]

Well done. Hoorah