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Default Unvented system confusion.

OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the pressure
in the system will drop as I bleed it. Having completed the bleed I now
need to top up the system using the filling loop until a set pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up. Having reached the required
cold pressure I isolate the system at the filling loop. Now as the
radiators heat up the water in the system expands out into the expansion
vessel and the pressure rises to the working pressure. All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised. Do I simply turn off the rising main at the
cold water inlet point and let the pressure out of the hot tap? If so
how to I "repressurize" the system when the job is done?

Any help appreciated.





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Default Unvented system confusion.

Chris B wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the pressure
in the system will drop as I bleed it. Having completed the bleed I now
need to top up the system using the filling loop until a set pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up. Having reached the required
cold pressure I isolate the system at the filling loop. Now as the
radiators heat up the water in the system expands out into the expansion
vessel and the pressure rises to the working pressure. All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised. Do I simply turn off the rising main at the
cold water inlet point and let the pressure out of the hot tap? If so
how to I "repressurize" the system when the job is done?

Any help appreciated.


Youre only pressurising and depressurising the heating circuit when you
top up with the fillip loop or bleed a radiator.

The hot water system is just a €śthrough the boiler/heat exchanger€ť system
so just turn off cold inlet to boiler, change washer, job done.

Tim

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Default Unvented system confusion.

On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť system
so just turn off cold inlet to boiler, change washer, job done.


No, its the 185 lt of hot water in the pressurised cylinder I'm worried
about......


Tim



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Default Unvented system confusion.



"Chris B" wrote in message
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On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť system
so just turn off cold inlet to boiler, change washer, job done.


No, its the 185 lt of hot water in the pressurised cylinder I'm worried
about......


Surely there is a valve on the output of the pressurised cylinder
that stop hot water coming out of that that can be used to change
a hot water tap washer ?

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Default Unvented system confusion.

On Thursday, 16 August 2018 20:36:57 UTC+1, Chris B wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the pressure
in the system will drop as I bleed it. Having completed the bleed I now
need to top up the system using the filling loop until a set pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up. Having reached the required
cold pressure I isolate the system at the filling loop. Now as the
radiators heat up the water in the system expands out into the expansion
vessel and the pressure rises to the working pressure. All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised. Do I simply turn off the rising main at the
cold water inlet point and let the pressure out of the hot tap? If so
how to I "repressurize" the system when the job is done?

Any help appreciated.


You don't need to do anyting, the incoming mains water will repressurise it. The 2 pressured circuits are different in nature.


NT


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Default Unvented system confusion.

On 16/08/2018 20:48, Chris B wrote:
On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť system
so just turn off cold inlet to boiler, change washer, job done.


No, its the 185 lt of hot water in the pressurised cylinder I'm worried
about......


If you turn off the mains, does any water come out of the hot tap?

It maybe that to get water out of the tank you have to let air in by
loosening a compression joint at the highest point in the pipe work
between the cold water feed to the house and the hot water tank. Turning
on a cold tap might also let the hot water out.

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Default Unvented system confusion.

John Angus wrote:

Surely there is a valve on the output of the pressurised cylinder
that stop hot water coming out of that that can be used to change
a hot water tap washer ?


If you turn off the mains pressure cold going into the cylinder, there
will be no hot water coming out of the cylinder ...

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Default Unvented system confusion.

On 16/08/2018 20:36, Chris B wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the pressure
in the system will drop as I bleed it.Â* Having completed the bleed I now
need to top up the system using the filling loop until a set pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up.Â* Having reached the required
cold pressure I isolate the system at the filling loop.Â* Now as the
radiators heat up the water in the system expands out into the expansion
vessel and the pressure rises to the working pressure.Â* All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).


So far, that is all spot on.

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised.Â* Do I simply turn off the rising main at the
cold water inlet point and let the pressure out of the hot tap?Â* If so
how to I "repressurize" the system when the job is done?

Any help appreciated.


ok, first thing to realise is that the term "unvented" can be used for
two separate and unrelated systems... it can be used to describe a
sealed primary CH system - which it what you alluded to above. However
that is more commonly simply called a "sealed system".

It can also be used to described a mains pressure hot water *storage*
system. Which has the cold main feed directly into a hot water cylinder.
The cylinder needs to be rated to take that pressure and have a number
of other safety systems included with it. Including its own expansion
vessel. (the cylinder could be heated by a vented or unvented CH system,
or an immersion heater).

However many people will have mains pressure hot water provided via a
combination boiler - that heats water on demand - there is not stored
hot water.

In the case of a combi, shutting off the cold main supply to the boiler
(or indeed the whole house), will also stop the supply of hot water.

With an unvented cylinder, shutting off its cold main supply will also
shut off the hot water - but it will still continue to supply a few
litres of hot which is expelled by the pressure from the expansion
vessel that belongs to the cylinder (not the one in the boiler).



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John.

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Default Unvented system confusion.

On 16/08/2018 22:19, Michael Chare wrote:
On 16/08/2018 20:48, Chris B wrote:
On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť
system
so just turn off cold inlet to boiler, change washer, job done.


No, its the 185 lt of hot water in the pressurised cylinder I'm
worried about......


If you turn off the mains, does any water come out of the hot tap?


With an unvented cylinder there is another pressure vessel that copes
with the expansion of the DHW as it heats. Shown as item 10 he

http://wiki.diyfaq.org.uk/index.php/...ted_cylinde r

Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will reduce
to nothing as the pressure vessel de-pressurises.

It maybe that to get water out of the tank you have to let air in by
loosening a compression joint at the highest point in the pipe work
between the cold water feed to the house and the hot water tank. Turning
on a cold tap might also let the hot water out.


There should be no need to drain the cylinder itself. Once there is no
mains pressure to push the hot water out of the top of it, no hot water
will flow.




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Cheers,

John.

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Default Unvented system confusion.

On 16/08/2018 21:47, wrote:
On Thursday, 16 August 2018 20:36:57 UTC+1, Chris B wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the pressure
in the system will drop as I bleed it. Having completed the bleed I now
need to top up the system using the filling loop until a set pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up. Having reached the required
cold pressure I isolate the system at the filling loop. Now as the
radiators heat up the water in the system expands out into the expansion
vessel and the pressure rises to the working pressure. All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised. Do I simply turn off the rising main at the
cold water inlet point and let the pressure out of the hot tap? If so
how to I "repressurize" the system when the job is done?

Any help appreciated.


You don't need to do anyting, the incoming mains water will repressurise it. The 2 pressured circuits are different in nature.


+1, yup what he said - I left that off my earlier explanation.


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Cheers,

John.

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\================================================= ================/


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Default Unvented system confusion.

On Thursday, 16 August 2018 20:36:57 UTC+1, Chris B wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the pressure
in the system will drop as I bleed it. Having completed the bleed I now
need to top up the system using the filling loop until a set pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up. Having reached the required
cold pressure I isolate the system at the filling loop. Now as the
radiators heat up the water in the system expands out into the expansion
vessel and the pressure rises to the working pressure. All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised. Do I simply turn off the rising main at the
cold water inlet point and let the pressure out of the hot tap? If so
how to I "repressurize" the system when the job is done?

Any help appreciated.



The pressurised heating system and the water in your hot water tap are different unlinked systems.
Turn off the incoming cold water supply (stoptap) and then you can work on your taps.(Hot and cold)

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Default Unvented system confusion.

On 17/08/18 00:28, John Rumm wrote:
On 16/08/2018 22:19, Michael Chare wrote:
On 16/08/2018 20:48, Chris B wrote:
On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť
system
so just turn off cold inlet to boiler, change washer, job done.

No, its the 185 lt of hot water in the pressurised cylinder I'm
worried about......


If you turn off the mains, does any water come out of the hot tap?


With an unvented cylinder there is another pressure vessel that copes
with the expansion of the DHW as it heats. Shown as item 10 he

http://wiki.diyfaq.org.uk/index.php/...ted_cylinde r


Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will reduce
to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...

then the loo will allow air into the cold side of the system and the
hot water will all come out.


It maybe that to get water out of the tank you have to let air in by
loosening a compression joint at the highest point in the pipe work
between the cold water feed to the house and the hot water tank.
Turning on a cold tap might also let the hot water out.


There should be no need to drain the cylinder itself. Once there is no
mains pressure to push the hot water out of the top of it, no hot water
will flow.






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On 16/08/18 22:21, Andy Burns wrote:
John Angus wrote:

Surely there is a valve on the output of the pressurised cylinder
that stop hot water coming out of that that can be used to change
a hot water tap washer ?


If you turn off the mains pressure cold going into the cylinder, there
will be no hot water coming out of the cylinder ...

That depends on where you turn it off.

Critically, on whether there is a possibility of unsealing it downstream
of where you have switched it off.



--
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puts mah heel on um jess the same if'n I catches him around mah chillun".

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On 17/08/2018 00:29, John Rumm wrote:
On 16/08/2018 21:47, wrote:
On Thursday, 16 August 2018 20:36:57 UTC+1, Chris BÂ* wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the pressure
in the system will drop as I bleed it.Â* Having completed the bleed I now
need to top up the system using the filling loop until a set pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up.Â* Having reached the required
cold pressure I isolate the system at the filling loop.Â* Now as the
radiators heat up the water in the system expands out into the expansion
vessel and the pressure rises to the working pressure.Â* All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised.Â* Do I simply turn off the rising main at the
cold water inlet point and let the pressure out of the hot tap?Â* If so
how to I "repressurize" the system when the job is done?

Any help appreciated.


You don't need to do anyting, the incoming mains water will
repressurise it. The 2 pressured circuits are different in nature.


+1, yup what he said - I left that off my earlier explanation.


Thanks to all for your help, just one last thing: when "repressurising"
the HW cylinder there is no gauge I need to look at? I just turn the
rising main back on and the cylinder design just takes care of the
pressure?

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Default Unvented system confusion.

The Natural Philosopher wrote:

Andy Burns wrote:

If you turn off the mains pressure cold going into the cylinder, there
will be no hot water coming out of the cylinder ...


That depends on where you turn it off.

Critically, on whether there is a possibility of unsealing it downstream
of where you have switched it off.


Such as the tap with the leaky washer ...


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On 17/08/2018 07:50, The Natural Philosopher wrote:

Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will
reduce to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...

then the loo will allow air into the cold side of the systemÂ* and the
hot water will all come out.


Not in my experience, but in theory the head of water within the pipes
could push a small amount of hot water out of the cylinder. There's
nothing wrong with TNP's theory, so I guess it's stymied by anti-syphon
valves in the toilet cistern.

Just to be on the safe side and to avoid an argument with TNP, leave the
hot tap open and flush all the toilets in the house before changing the
washer.

My 2p-worth: turn the boiler off today and change the washer tomorrow!
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Default Unvented system confusion.

On 17/08/2018 08:12, Chris B wrote:
On 17/08/2018 00:29, John Rumm wrote:
On 16/08/2018 21:47, wrote:
On Thursday, 16 August 2018 20:36:57 UTC+1, Chris BÂ* wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the
pressure
in the system will drop as I bleed it.Â* Having completed the bleed I
now
need to top up the system using the filling loop until a set
pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up.Â* Having reached the
required
cold pressure I isolate the system at the filling loop.Â* Now as the
radiators heat up the water in the system expands out into the
expansion
vessel and the pressure rises to the working pressure.Â* All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised.Â* Do I simply turn off the rising main at
the
cold water inlet point and let the pressure out of the hot tap?Â* If so
how to I "repressurize" the system when the job is done?

Any help appreciated.

You don't need to do anyting, the incoming mains water will
repressurise it. The 2 pressured circuits are different in nature.


+1, yup what he said - I left that off my earlier explanation.


Thanks to all for your help, just one last thing: when "repressurising"
the HW cylinder there is no gauge I need to look at? I just turn the
rising main back on and the cylinder design just takes care of the
pressure?


Yes, that will do.

You might want to download the manual for your particular DHW cylinder
and follow the procedure for recharging the inbuilt pressure vessel.
That's supposed to be done annually but I know there are systems that
have been installed for years without it ever being done - mine for
instance! You are also supposed to test the pressure relief valve, etc.

Yours, living dangerously,
GB
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On 17/08/2018 08:38, GB wrote:
On 17/08/2018 08:12, Chris B wrote:


Thanks to all for your help, just one last thing: when
"repressurising" the HW cylinder there is no gauge I need to look at?
I just turn the rising main back on and the cylinder design just takes
care of the pressure?


Yes, that will do.

You might want to download the manual for your particular DHW cylinder
and follow the procedure for recharging the inbuilt pressure vessel.
That's supposed to be done annually but I know there are systems that
have been installed for years without it ever being done - mine for
instance! You are also supposed to test the pressure relief valve, etc.

Yours, living dangerously,
GB


Right. One brave pill just consumed.......


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On 17/08/18 08:12, Chris B wrote:
Thanks to all for your help, just one last thing: when "repressurising"
the HW cylinder there is no gauge I need to look at? I just turn the
rising main back on and the cylinder design just takes care of the
pressure?


correct


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On 17/08/18 08:32, GB wrote:
On 17/08/2018 07:50, The Natural Philosopher wrote:

Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will
reduce to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...

then the loo will allow air into the cold side of the systemÂ* and the
hot water will all come out.


Not in my experience, but in theory the head of water within the pipes
could push a small amount of hot water out of the cylinder. There's
nothing wrong with TNP's theory, so I guess it's stymied by anti-syphon
valves in the toilet cistern.

Just to be on the safe side and to avoid an argument with TNP, leave the
hot tap open and flush all the toilets in the house before changing the
washer.


it critically depends where yoy turn the cold feed off. If it is at te
tank, all well and good.

I turned off the whole house supply. My tank is in the loft.

I had drained the hot tap and was working on it when my ex wife decided
to have a pee.

And turned on the cold tap as well.

The tank emptied all over me, the floor, and the room below


My 2p-worth: turn the boiler off today and change the washer tomorrow!



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Josef Stalin



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"Chris B" wrote in message
news
On 17/08/2018 00:29, John Rumm wrote:
On 16/08/2018 21:47, wrote:
On Thursday, 16 August 2018 20:36:57 UTC+1, Chris B wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the
pressure
in the system will drop as I bleed it. Having completed the bleed I
now
need to top up the system using the filling loop until a set pressure
on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up. Having reached the
required
cold pressure I isolate the system at the filling loop. Now as the
radiators heat up the water in the system expands out into the
expansion
vessel and the pressure rises to the working pressure. All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised. Do I simply turn off the rising main at
the
cold water inlet point and let the pressure out of the hot tap? If so
how to I "repressurize" the system when the job is done?

Any help appreciated.

You don't need to do anyting, the incoming mains water will repressurise
it. The 2 pressured circuits are different in nature.


+1, yup what he said - I left that off my earlier explanation.


Thanks to all for your help, just one last thing: when "repressurising"
the HW cylinder there is no gauge I need to look at? I just turn the
rising main back on and the cylinder design just takes care of the
pressure?


Yep.

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"Chris B" wrote in message
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On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť system
so just turn off cold inlet to boiler, change washer, job done.


No, its the 185 lt of hot water in the pressurised cylinder I'm worried
about......


Turn off inlet rising main somewhere before it feeds into the cylinder. Run
hot water until no more comes out of the tap that you want to change. There
is now no nett pressure in the cylinder. Since the outlet from a cylinder is
always at the highest point of it, there won't even be a tendency for water
to siphon out of it - even supposing that air could enter to replace the
water which was lost by siphoning.

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Default Unvented system confusion.

"The Natural Philosopher" wrote in message
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On 16/08/18 22:21, Andy Burns wrote:
John Angus wrote:

Surely there is a valve on the output of the pressurised cylinder
that stop hot water coming out of that that can be used to change
a hot water tap washer ?


If you turn off the mains pressure cold going into the cylinder, there
will be no hot water coming out of the cylinder ...

That depends on where you turn it off.

Critically, on whether there is a possibility of unsealing it downstream
of where you have switched it off.


No. Once you have turned off the cold water supply somewhere, the only water
that will come out is in the expansion vessel and in the pipes above. Once
the pipes/expansion vessel are empty, there is no way that any more water
can come out of the cylinder because the outlet is at the highest point.

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Default Unvented system confusion.

On 17/08/2018 09:34, The Natural Philosopher wrote:
On 17/08/18 08:32, GB wrote:
On 17/08/2018 07:50, The Natural Philosopher wrote:

Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will
reduce to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...

then the loo will allow air into the cold side of the systemÂ* and the
hot water will all come out.


Not in my experience, but in theory the head of water within the pipes
could push a small amount of hot water out of the cylinder. There's
nothing wrong with TNP's theory, so I guess it's stymied by
anti-syphon valves in the toilet cistern.

Just to be on the safe side and to avoid an argument with TNP, leave
the hot tap open and flush all the toilets in the house before
changing the washer.


it critically depends where yoy turn the cold feed off. If itÂ* is at te
tank, all well and good.

I turned off the whole house supply. My tank is in the loft.


I thought most unvented DHW is direct from the mains? Our entire house
is supplied direct from the mains, and we don't have a tank.



I had drained the hot tap and was working on it when my ex wife decided
to have a pee.


I'm not sure I get that. You would have had to turn the mains off then
run all the water out of the cold water tank. If you didn't then I don't
quite understand what you meant by 'drained the hot tap'.




And turned on the cold tap as well.

The tank emptied all over me, the floor, and the room below

How annoying.



My 2p-worth: turn the boiler off today and change the washer tomorrow!




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Default Unvented system confusion.

On 17/08/18 11:17, NY wrote:
"The Natural Philosopher" wrote in message
news
On 16/08/18 22:21, Andy Burns wrote:
John Angus wrote:

Surely there is a valve on the output of the pressurised cylinder
that stop hot water coming out of that that can be used to change
a hot water tap washer ?

If you turn off the mains pressure cold going into the cylinder,
there will be no hot water coming out of the cylinder ...

That depends on where you turn it off.

Critically, on whether there is a possibility of unsealing it
downstream of where you have switched it off.


No. Once you have turned off the cold water supply somewhere, the only
water that will come out is in the expansion vessel and in the pipes
above. Once the pipes/expansion vessel are empty, there is no way that
any more water can come out of the cylinder because the outlet is at the
highest point.


Oh dear.


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into what the world ought to be, whilst we fail utterly to deal with
what it actually is.



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Default Unvented system confusion.

On 17/08/18 11:18, GB wrote:
On 17/08/2018 09:34, The Natural Philosopher wrote:
On 17/08/18 08:32, GB wrote:
On 17/08/2018 07:50, The Natural Philosopher wrote:

Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will
reduce to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...

then the loo will allow air into the cold side of the systemÂ* and
the hot water will all come out.

Not in my experience, but in theory the head of water within the
pipes could push a small amount of hot water out of the cylinder.
There's nothing wrong with TNP's theory, so I guess it's stymied by
anti-syphon valves in the toilet cistern.

Just to be on the safe side and to avoid an argument with TNP, leave
the hot tap open and flush all the toilets in the house before
changing the washer.


it critically depends where yoy turn the cold feed off. If itÂ* is at
te tank, all well and good.

I turned off the whole house supply. My tank is in the loft.


I thought most unvented DHW is direct from the mains? Our entire house
is supplied direct from the mains, and we don't have a tank.


Most may be, if they use a combi.

I have a pressurised HW tank and its in te loft.



I had drained the hot tap and was working on it when my ex wife
decided to have a pee.


I'm not sure I get that. You would have had to turn the mains off then
run all the water out of the cold water tank. If you didn't then I don't
quite understand what you meant by 'drained the hot tap'.

I dont have a cold water tank

What stopped the hot water running out of the hot water tank was the
partial vaccuum formed by shutting off inconing mains.

When that seal was broken, more hot water ran out.





And turned on the cold tap as well.

The tank emptied all over me, the floor, and the room below

How annoying.



My 2p-worth: turn the boiler off today and change the washer tomorrow!






--
The biggest threat to humanity comes from socialism, which has utterly
diverted our attention away from what really matters to our existential
survival, to indulging in navel gazing and faux moral investigations
into what the world ought to be, whilst we fail utterly to deal with
what it actually is.

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Default Unvented system confusion.

On 17/08/2018 08:58, Chris B wrote:
On 17/08/2018 08:38, GB wrote:
On 17/08/2018 08:12, Chris B wrote:


Thanks to all for your help, just one last thing: when
"repressurising" the HW cylinder there is no gauge I need to look at?
I just turn the rising main back on and the cylinder design just
takes care of the pressure?


Yes, that will do.

You might want to download the manual for your particular DHW cylinder
and follow the procedure for recharging the inbuilt pressure vessel.
That's supposed to be done annually but I know there are systems that
have been installed for years without it ever being done - mine for
instance! You are also supposed to test the pressure relief valve, etc.

Yours, living dangerously,
GB


Right.Â* One brave pill just consumed.......


Well that was easier than expected. (provided it doesn't blow up in the
next 24 hours.........)

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Default Unvented system confusion.

replying to Chris B, RaadhikaBhatia wrote:
Good Discussion. Need more answers to get the informative notes. The number of
tips and techniques will help us.
RaadhikaBhatia |

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Default Unvented system confusion.

On 17/08/2018 07:50, The Natural Philosopher wrote:
On 17/08/18 00:28, John Rumm wrote:
On 16/08/2018 22:19, Michael Chare wrote:
On 16/08/2018 20:48, Chris B wrote:
On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť
system
so just turn off cold inlet to boiler, change washer, job done.

No, its the 185 lt of hot water in the pressurised cylinder I'm
worried about......


If you turn off the mains, does any water come out of the hot tap?


With an unvented cylinder there is another pressure vessel that copes
with the expansion of the DHW as it heats. Shown as item 10 he

http://wiki.diyfaq.org.uk/index.php/...ted_cylinde r


Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will
reduce to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...


That does rather depend on a number of factors. Normally the cold shut
off to the cylinder is situated right before its inlet control valve. So
once that is shut, its isolated from the rest of the cold water system.

However if you were relying on the main cold stop tap to the property,
then there may be an issue.

then the loo will allow air into the cold side of the systemÂ* and the
hot water will all come out.


Depends a bit on where the cylinder is. On the ground floor, there would
not be an issue since you would need a head of water above it to get
more hot out. So at most you could get a "pipe full". If its in the loft
then potentially you might get a bit more - but the moment air is drawn
into the cylinder, it will break any siphon action and stem the flow.

--
Cheers,

John.

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Default Unvented system confusion.

On 17/08/2018 08:12, Chris B wrote:
On 17/08/2018 00:29, John Rumm wrote:
On 16/08/2018 21:47, wrote:
On Thursday, 16 August 2018 20:36:57 UTC+1, Chris BÂ* wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the
pressure
in the system will drop as I bleed it.Â* Having completed the bleed I
now
need to top up the system using the filling loop until a set
pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up.Â* Having reached the
required
cold pressure I isolate the system at the filling loop.Â* Now as the
radiators heat up the water in the system expands out into the
expansion
vessel and the pressure rises to the working pressure.Â* All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised.Â* Do I simply turn off the rising main at
the
cold water inlet point and let the pressure out of the hot tap?Â* If so
how to I "repressurize" the system when the job is done?

Any help appreciated.

You don't need to do anyting, the incoming mains water will
repressurise it. The 2 pressured circuits are different in nature.


+1, yup what he said - I left that off my earlier explanation.


Thanks to all for your help, just one last thing: when "repressurising"
the HW cylinder there is no gauge I need to look at? I just turn the
rising main back on and the cylinder design just takes care of the
pressure?


Yup, they run at (near to) mains pressure. In reality there is usually a
pressure reduction valve on the inlet to limit the pressure to somewhere
around 3 to 4 bar.



--
Cheers,

John.

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http://www.internode.co.uk |
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Default Unvented system confusion.

On 17/08/2018 11:17, NY wrote:
"The Natural Philosopher" wrote in message
news
On 16/08/18 22:21, Andy Burns wrote:
John Angus wrote:

Surely there is a valve on the output of the pressurised cylinder
that stop hot water coming out of that that can be used to change
a hot water tap washer ?

If you turn off the mains pressure cold going into the cylinder,
there will be no hot water coming out of the cylinder ...

That depends on where you turn it off.

Critically, on whether there is a possibility of unsealing it
downstream of where you have switched it off.


No. Once you have turned off the cold water supply somewhere, the only
water that will come out is in the expansion vessel and in the pipes
above. Once the pipes/expansion vessel are empty, there is no way that
any more water can come out of the cylinder because the outlet is at the
highest point.


Hmm

There are 3 forms of unvented system where both the primary and
secondary systems don't have a header tank.

1) Combi boiler.

2) Unvented/pressurised cylinder.

3) Thermal store.

With all 3, simply turning off the cold water feed stops any flow
through the hot water tap. [1]

There may be an expansion vessel to stop back return onto the cold
supply, that will then need a pressure relief valve, which of course is
mandatory for the Unvented/pressurised cylinder.

If the OP has another system then all bets are off! Unvented can mean
other types of system too.

[1] I have seen a standard tank collapse where the cold feed and
expansion were frozen, causing a partial vacuum in the tank to collapse
it. For the more informed, is there any mitigation of this for 2 and 3?
Are tanks so designed to not 'implode'?
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Default Unvented system confusion.

On 17/08/2018 11:22, The Natural Philosopher wrote:
On 17/08/18 11:18, GB wrote:
On 17/08/2018 09:34, The Natural Philosopher wrote:
On 17/08/18 08:32, GB wrote:
On 17/08/2018 07:50, The Natural Philosopher wrote:

Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will
reduce to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...

then the loo will allow air into the cold side of the systemÂ* and
the hot water will all come out.

Not in my experience, but in theory the head of water within the
pipes could push a small amount of hot water out of the cylinder.
There's nothing wrong with TNP's theory, so I guess it's stymied by
anti-syphon valves in the toilet cistern.

Just to be on the safe side and to avoid an argument with TNP, leave
the hot tap open and flush all the toilets in the house before
changing the washer.

it critically depends where yoy turn the cold feed off. If itÂ* is at
te tank, all well and good.

I turned off the whole house supply. My tank is in the loft.


I thought most unvented DHW is direct from the mains? Our entire house
is supplied direct from the mains, and we don't have a tank.


Most may be, ifÂ* they use a combi.

I have a pressurised HW tank andÂ* its in te loft.



I had drained the hot tap and was working on it when my ex wife
decided to have a pee.


I'm not sure I get that. You would have had to turn the mains off then
run all the water out of the cold water tank. If you didn't then I
don't quite understand what you meant by 'drained the hot tap'.

I dont have a cold water tank

What stopped the hot water running out of the hot water tank was the
partial vaccuum formed by shutting off inconing mains.


Still don't get it

When that sealÂ* was broken, more hot water ran out.


So not the whole tank, just the water stored in the pipes.

And turned on the cold tap as well.

The tank emptied all over me, the floor, and the room below

How annoying.


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On 17/08/2018 08:12, Chris B wrote:

Thanks to all for your help, just one last thing: when "repressurising"
the HW cylinder there is no gauge I need to look at? I just turn the
rising main back on and the cylinder design just takes care of the
pressure?


A pressure cylinder is only called such because it withstands water at
mains pressure unlike the traditional thin(ish) open DHW tank that was
topped up from a low pressure header tank in the loft etc.

Therefore your hot water is supplied at the same pressure as your cold
water rather than at a much lower pressure of an older vented tank where
the pressure is dictated purely by the difference in height between the
highest hot water outlet/tap and the header tank more commonly referred
to as water "head"

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On 17/08/18 15:25, Fredxx wrote:
There are 3 forms of unvented system where both the primary and
secondary systems don't have a header tank.

1) Combi boiler.

2) Unvented/pressurised cylinder.

3) Thermal store.

Correct.

With all 3, simply turning off the cold water feed stops any flow
through the hot water tap. [1]


Wrong


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Default Unvented system confusion.

On 16/08/2018 20:48, Chris B wrote:
On 16/08/2018 20:44, Tim+ wrote:


The hot water system is just a €śthrough the boiler/heat exchanger€ť system
so just turn off cold inlet to boiler, change washer, job done.


No, its the 185 lt of hot water in the pressurised cylinder I'm worried
about......


Tim



The great thing about it being a *tap* is that you can just open it to
check that you have managed to isolate it without causing a flood.

Exactly what you have to turn off depends on whether the cylinder is
pressurised by the rising main, or by a header tank. But you
*definitely* should have some some way of isolating it, without having
to drain down all that hot water. If not, then this is the time to add a
service valve. Often the easiest way is with something like this:

https://www.screwfix.com/p/flexible-...-x-300mm/3558g

since you then don't have to do any "precision" plumbing.
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