System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in
pressure blow off exit pipe outside house.

Someone enlighten me as to whether the safety valve(s) operate on the
DHW circuit, the CH/DHW primary pressure, or both...and what pressure
they operate at?

Mains pressure is a stonking 2 bar plus...and I normally run the
primaries at 1.5-1.8 bar...

I assume the boiler has an expansion vessel in it, but should there be
one in the DHW circuit? I ocasionally get water hammer on one hot tap...

"The Natural Philosopher" wrote in message
...
System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in
pressure blow off exit pipe outside house.

Check the DHW unvented cylinder expansion vessel. If a silly Mergaflow,
they have an air bubble, which has to be re-instated. Pressure is building
up and opening the relief valve. It should not blow off any pipes, check
the pipe fixings.

Someone enlighten me as to whether the safety valve(s) operate on the
DHW circuit, the CH/DHW primary pressure, or both...and what pressure
they operate at?

Safety valves on the DHW. The boiler primary side has its won safety
blow-off valve. The boiler primary and cylinder secondary sections are
separate.

Mains pressure is a stonking 2 bar plus...and I normally run the
primaries at 1.5-1.8 bar...

2 bar is not much. Unvented cyldiners are pressure reduced to about 3-3.5
bar.

The boiler pressure should be charged to around 1 bar NOT, 1.8bar.

I assume the boiler has an expansion vessel in it,

Should be, check for a large red thing at the back of it.

but should there be
one in the DHW circuit?

Yep, or if a Megaflow they don't have one using an air bubble that
dissolves all too easily.

I ocasionally get water hammer on one hot tap... The air cushion has
gone.

On Tue, 18 Oct 2005 11:03:08 +0100, The Natural Philosopher wrote:

System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in
pressure blow off exit pipe outside house.

Someone enlighten me as to whether the safety valve(s) operate on the
DHW circuit, the CH/DHW primary pressure, or both...and what pressure
they operate at?

Mains pressure is a stonking 2 bar plus...and I normally run the
primaries at 1.5-1.8 bar...

I assume the boiler has an expansion vessel in it, but should there be
one in the DHW circuit? I ocasionally get water hammer on one hot tap...

Most things you may need to know about the primary are in the sealedCH FAQ.
However the tundish is fitted to the cylinder not the primary.

The commonest reason for discharge from an unvented cylinder is failure of
ITS air bubble or expansion vessel. Not all unvented cylinders are fitted
with a gauge - it is however a key tool for diagnosing problems with the
cylinder.

PS the blow off pipe (for the cylinder) should have been installed so as
to discharge safely to ground level not on the house.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

Ed Sirett wrote:
On Tue, 18 Oct 2005 11:03:08 +0100, The Natural Philosopher wrote:


System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in
pressure blow off exit pipe outside house.

Someone enlighten me as to whether the safety valve(s) operate on the
DHW circuit, the CH/DHW primary pressure, or both...and what pressure
they operate at?

Mains pressure is a stonking 2 bar plus...and I normally run the
primaries at 1.5-1.8 bar...

I assume the boiler has an expansion vessel in it, but should there be
one in the DHW circuit? I ocasionally get water hammer on one hot tap...


Most things you may need to know about the primary are in the sealedCH FAQ.
However the tundish is fitted to the cylinder not the primary.

Thanks.

The commonest reason for discharge from an unvented cylinder is failure of
ITS air bubble or expansion vessel. Not all unvented cylinders are fitted
with a gauge - it is however a key tool for diagnosing problems with the
cylinder.

Ok...see what you mean

PS the blow off pipe (for the cylinder) should have been installed so as
to discharge safely to ground level not on the house.


It does...but there is spray over inside

Looks like some setting/testing of the HW circuit pressure is in order -
thanks.

"The Natural Philosopher" wrote in message
...
Ed Sirett wrote:
On Tue, 18 Oct 2005 11:03:08 +0100, The Natural Philosopher wrote:


System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in
pressure blow off exit pipe outside house.

Someone enlighten me as to whether the safety valve(s) operate on the
DHW circuit, the CH/DHW primary pressure, or both...and what pressure
they operate at?

Mains pressure is a stonking 2 bar plus...and I normally run the
primaries at 1.5-1.8 bar...

I assume the boiler has an expansion vessel in it, but should there be
one in the DHW circuit? I ocasionally get water hammer on one hot tap...


Most things you may need to know about the primary are in the sealedCH
FAQ.
However the tundish is fitted to the cylinder not the primary.

Thanks.

The commonest reason for discharge from an unvented cylinder is failure
of
ITS air bubble or expansion vessel. Not all unvented cylinders are
fitted
with a gauge - it is however a key tool for diagnosing problems with the
cylinder.

Ok...see what you mean

PS the blow off pipe (for the cylinder) should have been installed so as
to discharge safely to ground level not on the house.


It does...but there is spray over inside

Looks like it is undersized.


Looks like some setting/testing of the HW circuit pressure is in order -
thanks.

On Thu, 20 Oct 2005 09:25:57 +0100, Doctor Drivel wrote:


"The Natural Philosopher" wrote in message
...
Ed Sirett wrote:
On Tue, 18 Oct 2005 11:03:08 +0100, The Natural Philosopher wrote:


System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in
pressure blow off exit pipe outside house.

Someone enlighten me as to whether the safety valve(s) operate on the
DHW circuit, the CH/DHW primary pressure, or both...and what pressure
they operate at?

Mains pressure is a stonking 2 bar plus...and I normally run the
primaries at 1.5-1.8 bar...

I assume the boiler has an expansion vessel in it, but should there be
one in the DHW circuit? I ocasionally get water hammer on one hot tap...


Most things you may need to know about the primary are in the sealedCH
FAQ.
However the tundish is fitted to the cylinder not the primary.

Thanks.

The commonest reason for discharge from an unvented cylinder is failure
of
ITS air bubble or expansion vessel. Not all unvented cylinders are
fitted
with a gauge - it is however a key tool for diagnosing problems with the
cylinder.

Ok...see what you mean

PS the blow off pipe (for the cylinder) should have been installed so as
to discharge safely to ground level not on the house.


It does...but there is spray over inside

Looks like it is undersized.


Looks like some setting/testing of the HW circuit pressure is in order -
thanks.


Assuming the tundish is 15mm in and 22m out the maximum length is 9m for
the 22mm discharge pipe. Each bend takes 0.8m away from the limit. The
discharge pipe must always go downwards.
There must be a direct drop of 200mm after the tundish before any bend.
Sometimes when a lot of lime-crud has built up on the tundish it then
fails to work right.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

Ed Sirett wrote:



Assuming the tundish is 15mm in and 22m out the maximum length is 9m for
the 22mm discharge pipe. Each bend takes 0.8m away from the limit. The
discharge pipe must always go downwards.
There must be a direct drop of 200mm after the tundish before any bend.
Sometimes when a lot of lime-crud has built up on the tundish it then
fails to work right.



Its almost new install, and its always blown off a little...never great
clouds, just like a little phart now and again, so its not an issue of
it failing to cope with the volume..

I can't see why it goes at all though..

On Tue, 25 Oct 2005 11:37:03 +0100, The Natural Philosopher wrote:

Ed Sirett wrote:



Assuming the tundish is 15mm in and 22m out the maximum length is 9m for
the 22mm discharge pipe. Each bend takes 0.8m away from the limit. The
discharge pipe must always go downwards.
There must be a direct drop of 200mm after the tundish before any bend.
Sometimes when a lot of lime-crud has built up on the tundish it then
fails to work right.



Its almost new install, and its always blown off a little...never great
clouds, just like a little phart now and again, so its not an issue of
it failing to cope with the volume..

I can't see why it goes at all though..

I doubt very much that either blow off valve is defective.
I strongly suggest that you obtain a pressure gauge (0-10 bar or
thereabouts) less than £10 and with a 1/4" BSP connector.
You will find that the there are very likely some tapping(s) on the
main pressure reducing valve. The valve screws into one of those.


If the type you have is an internal air pocket the I suggest you renew the
air pocket in line with the instructions.
If the type you have is the other sort with an expansion vessel then check
that vessel (as you would for a sealed CH system - see FAQ).

When you have the gauge you will be able to see if
a) The blow off occurs at the wrong pressure (duff blow off valve)
b) The pressure is always too high (duff inlet PRV).
c) The pressure rises on heating are excessive (duff air pocket /expansion
vessel).

HTH
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

Ed Sirett wrote:

On Tue, 25 Oct 2005 11:37:03 +0100, The Natural Philosopher wrote:


Ed Sirett wrote:



Assuming the tundish is 15mm in and 22m out the maximum length is 9m for
the 22mm discharge pipe. Each bend takes 0.8m away from the limit. The
discharge pipe must always go downwards.
There must be a direct drop of 200mm after the tundish before any bend.
Sometimes when a lot of lime-crud has built up on the tundish it then
fails to work right.



Its almost new install, and its always blown off a little...never great
clouds, just like a little phart now and again, so its not an issue of
it failing to cope with the volume..

I can't see why it goes at all though..


I doubt very much that either blow off valve is defective.
I strongly suggest that you obtain a pressure gauge (0-10 bar or
thereabouts) less than £10 and with a 1/4" BSP connector.
You will find that the there are very likely some tapping(s) on the
main pressure reducing valve. The valve screws into one of those.


If the type you have is an internal air pocket the I suggest you renew the
air pocket in line with the instructions.
If the type you have is the other sort with an expansion vessel then check
that vessel (as you would for a sealed CH system - see FAQ).

When you have the gauge you will be able to see if
a) The blow off occurs at the wrong pressure (duff blow off valve)
b) The pressure is always too high (duff inlet PRV).
c) The pressure rises on heating are excessive (duff air pocket /expansion
vessel).

HTH

Thanks Ed..its in the round tuit file..will do as suggested..