Hi Folks,

Can anyone suggest a reason why when I open my hot tap fully I get air
bubbles in the hot pipe, yet if it is run slowly there are none.

I've already tried looking at the hot output from the top of the hot
water cyclinder (which is in the loft beside the cold water tank) to
see if there was any air getting trapped in a malformed pipe, but it
looks like it should in most diagrams. I've also tried opening the
cold water source valve fully, but it is already fully opened.

I'm hoping to install a power shower, so I need to get rid of the air.

Any ideas?

Cheers!
Colin.

Colin Docherty wrote:

Can anyone suggest a reason why when I open my hot tap fully I get
air
bubbles in the hot pipe, yet if it is run slowly there are none.


Yes. I had this problem previously in a 4 storey house ( hot water
cylinder on 3rd floor, bath on 1st floor).

Causes?
Assume the cold water storage tank isn't running dry.
With no water flow, the water pressure (static pressure) at each point
in the system is proportional to the height of water above it. P =
density x G x H. Hydro-statics, easy stuff.

When you open the hot tap, water flows down the cold feed, up through
the hot water storage cylinder and then off through the hot
distribution pipes, passing the open vent pipe connection. The moving
water possesses a kinetic energy and there is a hydraulic resistance to
the flow of water, which absorbs some more energy. All this energy is
taken out of the water pressure, so that when the water is flowing the
pressure at every point along the pipe will drop. The pressure at any
point now depends on the water height as previously, minus the water
velocity pressure and minus pressure due to the upstream hydraulic
resistance. The pressure at the branch where the open vent pipe
connects will drop, so the water level in the open vent pipe would
normally drop when there is a draw-off.

So what has this got to do with your problem?

In your system, the flow rate and/or the upstream hydraulic resistance
is so great that the pressure at the open vent branch has dropped below
atmospheric. The water in the open vent pipe drains out and air is
sucked down the open vent pipe into the flowing water. You can confirm
this by putting your hand over the open vent and getting someone to
turn on the affected bath taps. You should feel your hand being sucked
onto the pipe. Air won't get entrained in the water until you remove
your hand. You'll hear the air hissing in if you restrict the opening.
Alternatively, immerse the end of the open vent in a glass of water.
The water should get sucked out of the glass.

Solutions? Don't put a non-return valve in the open vent, as someone
is bound to suggest.

You have to reduce the hydraulic resistance of the pipe from the
storage tank to the open vent branch, AND/OR reduce the flow rate at
the affected taps.

Check the cold feed pipe for restrictions ( partially open valve,
blockages, etc). Check the cold-feed pipe size is adequate. Check there
are two separate cold feed pipes, one to the hot cylinder and one
serving the cold water down service. Consider fitting flow limiters to
the affected taps. In my case, I got the storage cylinder moved to the
second floor.

Beyond this you'd need to start doing some calculations for which I
don't have time.

On Tue, 25 Jan 2005 09:45:08 +0000, Colin Docherty
wrote:

Hi Folks,

Can anyone suggest a reason why when I open my hot tap fully I get air
bubbles in the hot pipe, yet if it is run slowly there are none.

I've already tried looking at the hot output from the top of the hot
water cyclinder (which is in the loft beside the cold water tank) to
see if there was any air getting trapped in a malformed pipe, but it
looks like it should in most diagrams. I've also tried opening the
cold water source valve fully, but it is already fully opened.

I'm hoping to install a power shower, so I need to get rid of the air.

Any ideas?

Cheers!
Colin.

It's usually as a result of an inadequate flow between the roof tank
and hot water cylinder.

Since both are on the same floor, there is very little head between
them and wneh you turn the tap on full, the water level in the vent
pipe drops and air is sucked into the flow.

The only improvement that you can make without radical movement of the
tank and cylinder is to improve the flow between them.
This could be done by upgrading the pipe from 22mm to 28mm, or
possibly even higher. 28mm pipe needs a larger bending machine
normally and you need to have slowish bends rather than sharp elbows.
I did this on my system by using pairs of 135 degree bends with a
straight length of pipe in between so that the flow is not disturbed
as much. It works well.


Other than this, the only other real option is to convert to some sort
of mains pressure system like a heatbank as long as the mains flow is
adequate.




--

..andy

To email, substitute .nospam with .gl

Other than this, the only other real option is to convert to some sort
of mains pressure system like a heatbank as long as the mains flow is
adequate.

Especially as a pump would likely make the situation much worse. The
situation is pretty bad if a gravity tap can suck the vent dry already.

Christian.

"Aidan" writes:

When you open the hot tap, water flows down the cold feed, up through
the hot water storage cylinder and then off through the hot
distribution pipes, passing the open vent pipe connection. The moving
water possesses a kinetic energy and there is a hydraulic resistance to
the flow of water, which absorbs some more energy. All this energy is
taken out of the water pressure, so that when the water is flowing the
pressure at every point along the pipe will drop. The pressure at any
point now depends on the water height as previously, minus the water
velocity pressure and minus pressure due to the upstream hydraulic
resistance. The pressure at the branch where the open vent pipe
connects will drop, so the water level in the open vent pipe would
normally drop when there is a draw-off.

In your system, the flow rate and/or the upstream hydraulic resistance
is so great that the pressure at the open vent branch has dropped below
atmospheric. The water in the open vent pipe drains out and air is
sucked down the open vent pipe into the flowing water. You can confirm
this by putting your hand over the open vent and getting someone to
turn on the affected bath taps. You should feel your hand being sucked
onto the pipe. Air won't get entrained in the water until you remove
your hand. You'll hear the air hissing in if you restrict the opening.
Alternatively, immerse the end of the open vent in a glass of water.
The water should get sucked out of the glass.

Solutions? Don't put a non-return valve in the open vent, as someone
is bound to suggest.

Hi,

As far as I remember, and I'll check tonight, there is NO non-return
valve on the vent pipe, so maybe this is the solution.

So, if I fit the non-return valve, does this mean that when running
the tap hard the vacuume created by the downward water to my tap will
effectively provide a suction force on my hot water cyclinder, thus
providing sufficient flow to the tap.

Thanks for the help.

Cheers!
Colin.

"Christian McArdle" writes:

So, if I fit the non-return valve, does this mean that when running
the tap hard the vacuume created by the downward water to my tap will
effectively provide a suction force on my hot water cyclinder, thus
providing sufficient flow to the tap.

Yes. It may also explode, killing you, your family and your neighbours.

Sorry Chritian,

Did I miss something here? So its not a good idea to fit a non-return
valve on the vent? When you said explode, did you mean implode? I don't
like the bit about killing people and stuff.

The input pipe from cold tank to water cyclinder is already 28mm,
but the cold water tank is only slightly above (couple of foot)
above cylinder. But it's not possible to move the cylinder down
a floor (no room!).

Cheers!
Colin.

On Tue, 25 Jan 2005 11:16:42 +0000, Colin Docherty
wrote:

"Aidan" writes:

When you open the hot tap, water flows down the cold feed, up through
the hot water storage cylinder and then off through the hot
distribution pipes, passing the open vent pipe connection. The moving
water possesses a kinetic energy and there is a hydraulic resistance to
the flow of water, which absorbs some more energy. All this energy is
taken out of the water pressure, so that when the water is flowing the
pressure at every point along the pipe will drop. The pressure at any
point now depends on the water height as previously, minus the water
velocity pressure and minus pressure due to the upstream hydraulic
resistance. The pressure at the branch where the open vent pipe
connects will drop, so the water level in the open vent pipe would
normally drop when there is a draw-off.

In your system, the flow rate and/or the upstream hydraulic resistance
is so great that the pressure at the open vent branch has dropped below
atmospheric. The water in the open vent pipe drains out and air is
sucked down the open vent pipe into the flowing water. You can confirm
this by putting your hand over the open vent and getting someone to
turn on the affected bath taps. You should feel your hand being sucked
onto the pipe. Air won't get entrained in the water until you remove
your hand. You'll hear the air hissing in if you restrict the opening.
Alternatively, immerse the end of the open vent in a glass of water.
The water should get sucked out of the glass.

Solutions? Don't put a non-return valve in the open vent, as someone
is bound to suggest.

Hi,

As far as I remember, and I'll check tonight, there is NO non-return
valve on the vent pipe, so maybe this is the solution.

NO!!!!!!!!!!!!!

You will make the system very dangerous.

Read what Aidan says again.

The solutions a

1) Increase the size of pipe between tank and cylinder. Will improve
but still may not be enough

2) Move the cylinder down one floor.

3) Change to an alternative system involving mains fed hot water.
Either a heatbank, a pressurised cylinder or even a combi boiler.
Check that the cold water mains is adequate before contemplating this.




So, if I fit the non-return valve, does this mean that when running
the tap hard the vacuume created by the downward water to my tap will
effectively provide a suction force on my hot water cyclinder, thus
providing sufficient flow to the tap.


It might do, but shouldn't be done because you are risking an
explosion and a shower of boiling water coming through your ceiling.



Thanks for the help.

Cheers!
Colin.


--

..andy

To email, substitute .nospam with .gl

Did I miss something here? So its not a good idea to fit a non-return
valve on the vent? When you said explode, did you mean implode? I don't
like the bit about killing people and stuff.

No. I meant explode. With maximum prejudice. Enough to entirely demolish
sizable portions of a house. Easily enough to blow through solid 9" masonry
walls. Don't even think about it. It might work until there was a fault on
the system, possibly even for a few years. Then, boom. The purpose of the
vent is to allow steam to escape when a fault leads to overheating. If it
can't get out because some nutter has put a valve on the vent, the amount of
energy in the resultant explosion is immense.

Of course, it might implode as well during that time, but that is less
likely to kill anyone, although someone was killed when a hot water cylinder
ruptured and dumped 120l of scalding water on top of their bed through the
ceiling.

The input pipe from cold tank to water cyclinder is already 28mm,
but the cold water tank is only slightly above (couple of foot)
above cylinder. But it's not possible to move the cylinder down
a floor (no room!).

Rather than get a power shower, consider upgrading to a heat bank. This will
provide excellent power shower performance without the air, or the debris.
You can buy new ones, or convert your old cylinder as a DIY project.

Christian.

Colin Docherty wrote:

Sorry Chritian,

Did I miss something here?

Yes. You misread my previous post. It said;

"Don't put a non-return valve in the open vent, as someone
is bound to suggest."

Don't, do not, bad idea, etc.. No valves in the open vent pipe. None.

"Christian McArdle" writes:


Rather than get a power shower, consider upgrading to a heat bank. This will
provide excellent power shower performance without the air, or the debris.
You can buy new ones, or convert your old cylinder as a DIY project.

Thanks for the sound advice Christian, I'll investigate the heat bank option.

Cheers!
Colin.

Andy Hall writes:

NO!!!!!!!!!!!!!

You will make the system very dangerous.

Read what Aidan says again.

Hi Andy,

I've carefully read this again, and also Christian's comments. Thanks for
the advice, and

I *** WON'T *** fit a non-return valve :-)

I'm going to investigate the heat bank option, and also check that there
are no flow restrictions on the cold water input to the cylinder.

Thanks again...
Colin.

Andy Hall wrote:

1) Increase the size of pipe between tank and cylinder. Will improve
but still may not be enough

2) Move the cylinder down one floor.

3) Change to an alternative system involving mains fed hot water.
Either a heatbank, a pressurised cylinder or even a combi boiler.
Check that the cold water mains is adequate before contemplating
this.

Yes &/or 2a) move the tank up.

I'd misread your post and missed the bit about the cylinder in the
loft, which aggravates the problem. If the flow out of the tank exceeds
the flow rate in (e.g., if you put in a shower pump, the water
suppliers reduce the pressure, etc.) the problem will return as the
water level in the tank drops, unless you put the cylinder down a
floor.

The high level space in the stairwell, over the stairs at ground floor
level, is usually unusable. I've seen cupboards constructed in this
space, to house a cylinder. It avoids putting it in the (unheated) loft
& the associated risk of freezing.

If you do nothing else, you need to ensure that the existing
installation cannot freeze, particularly the cold feed and open vent
pipes. Ensure the pipes are very well insulated.

Colin Docherty wrote:
"Aidan" writes:


When you open the hot tap, water flows down the cold feed, up through
the hot water storage cylinder and then off through the hot
distribution pipes, passing the open vent pipe connection. The moving
water possesses a kinetic energy and there is a hydraulic resistance to
the flow of water, which absorbs some more energy. All this energy is
taken out of the water pressure, so that when the water is flowing the
pressure at every point along the pipe will drop. The pressure at any
point now depends on the water height as previously, minus the water
velocity pressure and minus pressure due to the upstream hydraulic
resistance. The pressure at the branch where the open vent pipe
connects will drop, so the water level in the open vent pipe would
normally drop when there is a draw-off.

In your system, the flow rate and/or the upstream hydraulic resistance
is so great that the pressure at the open vent branch has dropped below
atmospheric. The water in the open vent pipe drains out and air is
sucked down the open vent pipe into the flowing water. You can confirm
this by putting your hand over the open vent and getting someone to
turn on the affected bath taps. You should feel your hand being sucked
onto the pipe. Air won't get entrained in the water until you remove
your hand. You'll hear the air hissing in if you restrict the opening.
Alternatively, immerse the end of the open vent in a glass of water.
The water should get sucked out of the glass.

Solutions? Don't put a non-return valve in the open vent, as someone
is bound to suggest.


Hi,

As far as I remember, and I'll check tonight, there is NO non-return
valve on the vent pipe, so maybe this is the solution.

So, if I fit the non-return valve, does this mean that when running
the tap hard the vacuume created by the downward water to my tap will
effectively provide a suction force on my hot water cyclinder, thus
providing sufficient flow to the tap.

If the steam generated by a small coal fire in a steam locomotive
contains enough energy to move the pistons on said locomotive and drive
said locomotive and many ton(ne)s of train when used in a controlled
way, what do you think said steam energy will do to your house when it's
been trapped by force unable to expand and then explodes violently in an
uncontrolled way?

Colin Docherty wrote in
:

Sorry Chritian,

Did I miss something here? So its not a good idea to fit a non-return
valve on the vent? When you said explode, did you mean implode? I don't
like the bit about killing people and stuff.

A bit of irony, like "not enough pressure - tie down the safety valve"

And with the serious point that some folks might actually do it.

It's what you don't know you don't know that kills you

mike