Ed's excellent sealed system FAQ covers the principles of operation plus
what to do when something goes wrong, but it doesn't tell you how to design
a system from scratch or how to convert a vented system (apart from
mentioning a couple of pre-requisites).

Is there a FAQ or similar anywhere which goes into things like:
* How to determine the volume of a system
* How to decide on the size of pressure vessel required
* Any rules about the location of the pressure vessel relative to boiler,
pump, zone valves (if it matters)

According to my calculations, you need a pressure vessel whose capacity is
about 10% of the water volume. Does that sound reasonable? [I'll expand(!)
on that below]

So you need to be able to estimate the water volume. If you know how much
pipe there is of each size, that bit's not too difficult. Then there's the
radiators. Are there any rules of thumb for litres per square metre for
typical single-panel radiators? [I'm sure I could make a stab at it by
estimating the cross-sectional area of all the 'tubes' - but it would be
nice if I didn't have to]. That leaves the boiler and the heating coil in
the HW cylinder. Are there any typical values for these?

I'm sure there's scope for a FAQ here if anyone cares to write one (if there
isn't one already).

[Back to my calcs. How does this grab you? Assume you have a 10 litre
pressure vessel, and prime it with air at 0.7 bar (1.7 bar absolute). Then
you pressurise the system with water to 1 bar (2 bar absolute). ISTR from
the dim and distance past about the Law of either Boyle or Charles or
somebody along the lines of the volume of a gas at constant temperature
being inversely preportional to the absolute pressure. If this is correct,
our 10 litres of air at 1.7 bar will reduce to 8.5 litres at 2 bar. If we
now heat up the system so that the pressure rises to 2 bar (3 bar absolute)
the volume of air will reduce further to 5.67 litres. The difference between
8.5 and 5.67 (2.83 litres) represents the amount of expansion water which
has entered the vessel. So, for a pressure rise from 1 bar to 2 bar, a 10
litre vessel can accommodate about 2.8 litres of water.

My research on the volumetric expansion of (tap) water suggests that it
expands by about 2.5% between 4 decC and 80 degC. So a 100 litre system
would expand by about 2.5 litres - which could be nicely accommodated by a
10 litre vessel, giving a pressure rise of just under a bar.
(Actually, the temperature rise will be less than this since the system will
be a room temp rather than 4 decC when cold. But the air in the vessel
probably warms up up a bit, which I have ignored - so maybe the two tend to
cancel.) Are there any major flaws in my reasoning?]
--
Cheers,
Set Square
______
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Set Square wrote:

a system from scratch or how to convert a vented system (apart from
mentioning a couple of pre-requisites).

Here was my plan of action for that (going from conventional boiler to
combi, original tanks in the loft):

Boiler Replacement Procedure

Determine pipe arrangement
Determine gas pipe sizes
Calculate quantities required
Establish pipe routing for CH
Add desludging agent to existing system
Order boiler and enough fittings and pipe to make sure all eventualities
are covered!

Prep

Ensure hot water at full temp
Turn off boiler

Loft Work

Tie up ballcock in CH header
Drain system - ensure 3 port valve is in centre pos to ensure heating
loop is drained.
Cut CH feed and return in loft and loop back. Include gate valve on a
tee in loop
Cap pipe ends to prevent seepage

Kitchen

Take static gas pressure reading and ensure loss is zero
Turn off gas
Pull fuse from boiler FCU
Disconnect boiler electrics and remove programmer
Disconnect CH feed and return
Disconnect gas feed
Open boiler
Remove from wall
Remove old flue
fill old flue hole with bricks and mortar
Using template mark out positions
Core bore new flue exit
Fit boiler mounting frame (remove tiles as required)
Connect CH feed and return
Remove existing gas feed pipe to under worktop
Cut new hole through worktop for DHW feed and return, 22mm gas, condensate
Install 22mm gas feed from existing ¾" pipe
Turn off water
Cut in new DHW feed and return and connect with service valves
Install filling loop
Mount boiler on tray
Install new flue
Install condensate drain

Loft

Tie off cistern ballcock
Drain remains of hot water
Disconnect hot water output from cylinder and cap off
Disconnect input mains water from cistern and CH header tank
Disconnect cold shower feed from cistern and connect to rising main

Bathroom

Remove bath panel and identify CH stat wire - join to existing down wire
and disconnect from remainder. (Leave remainder available for pulling
through new wires later!)

Kitchen

Connect boiler electrics and stat
Carry out any pre checks as per install manual
Purge gas meter and pipework
Carry out gas soundness test
Commission boiler as per manual
Flush as required
Treat with corrosion inhibitor
Remove tanks etc. and cylinder from loft.

Job Done!


All in all getting on for four days work!

So you need to be able to estimate the water volume. If you know how much
pipe there is of each size, that bit's not too difficult. Then there's the

If in doubt, err on the side of too large for your expansion chamber,
since too small is a problem too big is not.


[Back to my calcs. How does this grab you? Assume you have a 10 litre
pressure vessel, and prime it with air at 0.7 bar (1.7 bar absolute). Then

Sounds ok.... here is a table from the install manual for my boiler:

Assuming pre charge pressure between 0.5 and 0.75 bar

System volume Expansion vessel
(litres) volume (litres)
25 1.8
50 3.7
75 5.5
100 7.4
125 9.2
150 11.0
175 12.9
190 14.0
200 14.7
250 18.4
300 22.1
For other system volumes
multiply by the factor across 0.074

Are there any major flaws in my reasoning?]

Not obviously.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In an earlier contribution to this discussion,
John Rumm wrote:

Set Square wrote:

a system from scratch or how to convert a vented system (apart from
mentioning a couple of pre-requisites).

Here was my plan of action for that (going from conventional boiler to
combi, original tanks in the loft):

Boiler Replacement Procedure

Determine pipe arrangement
Determine gas pipe sizes
Calculate quantities required
Establish pipe routing for CH
Add desludging agent to existing system
Order boiler and enough fittings and pipe to make sure all
eventualities are covered!

Prep

Ensure hot water at full temp
Turn off boiler

Loft Work

Tie up ballcock in CH header
Drain system - ensure 3 port valve is in centre pos to ensure heating
loop is drained.
Cut CH feed and return in loft and loop back. Include gate valve on a
tee in loop
Cap pipe ends to prevent seepage

Kitchen

Take static gas pressure reading and ensure loss is zero
Turn off gas
Pull fuse from boiler FCU
Disconnect boiler electrics and remove programmer
Disconnect CH feed and return
Disconnect gas feed
Open boiler
Remove from wall
Remove old flue
fill old flue hole with bricks and mortar
Using template mark out positions
Core bore new flue exit
Fit boiler mounting frame (remove tiles as required)
Connect CH feed and return
Remove existing gas feed pipe to under worktop
Cut new hole through worktop for DHW feed and return, 22mm gas,
condensate Install 22mm gas feed from existing ¾" pipe
Turn off water
Cut in new DHW feed and return and connect with service valves
Install filling loop
Mount boiler on tray
Install new flue
Install condensate drain

Loft

Tie off cistern ballcock
Drain remains of hot water
Disconnect hot water output from cylinder and cap off
Disconnect input mains water from cistern and CH header tank
Disconnect cold shower feed from cistern and connect to rising main

Bathroom

Remove bath panel and identify CH stat wire - join to existing down
wire and disconnect from remainder. (Leave remainder available for
pulling through new wires later!)

Kitchen

Connect boiler electrics and stat
Carry out any pre checks as per install manual
Purge gas meter and pipework
Carry out gas soundness test
Commission boiler as per manual
Flush as required
Treat with corrosion inhibitor
Remove tanks etc. and cylinder from loft.

Job Done!


All in all getting on for four days work!

So you need to be able to estimate the water volume. If you know how
much pipe there is of each size, that bit's not too difficult. Then
there's the

If in doubt, err on the side of too large for your expansion chamber,
since too small is a problem too big is not.


[Back to my calcs. How does this grab you? Assume you have a 10 litre
pressure vessel, and prime it with air at 0.7 bar (1.7 bar
absolute). Then

Sounds ok.... here is a table from the install manual for my boiler:

Assuming pre charge pressure between 0.5 and 0.75 bar

System volume Expansion vessel
(litres) volume (litres)
25 1.8
50 3.7
75 5.5
100 7.4
125 9.2
150 11.0
175 12.9
190 14.0
200 14.7
250 18.4
300 22.1
For other system volumes
multiply by the factor across 0.074

Are there any major flaws in my reasoning?]

Not obviously.

Thanks for that. It looks like 10% is a bit conservative and that 7.4% is
ok.

Your todo list looks pretty comprehensive - and goes further than I
envisaged. I plan to keep the existing boiler and stored hot water system,
simply replacing the F&E tank with a pressure vessel and associated bits.

Some rules of thumb for estimating radiator/boiler/cylinder coil capacities
would still be nice - as would guidance on acceptable locations for the
pressure vessel.

--
Cheers,
Set Square
______
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Some rules of thumb for estimating radiator/boiler/cylinder coil capacities
would still be nice - as would guidance on acceptable locations for the
pressure vessel.

Put the expansion vessel on the return to the boiler in a position where there are no controls between the vessel and the boiler like zone valves or gate valves, so that it is effectively always in open series with the boiler.

You can put the pressure guage filling loop and blow off valve all together in the same place if that is feasible. Check ebay components like that come up cheap once in a while.

Set Square wrote:

See;
http://www.rwc.co.uk/rwc/pdfs/training/julaug2000.pdf

There's at least one other document on the RWC website about sealed
systems, in the section about 'Training Guides'. These are Acrobat
document and, for some reason, I can only get them to open during
office hours.

In an earlier contribution to this discussion,
Aidan wrote:

Set Square wrote:

See;
http://www.rwc.co.uk/rwc/pdfs/training/julaug2000.pdf

There's at least one other document on the RWC website about sealed
systems, in the section about 'Training Guides'. These are Acrobat
document and, for some reason, I can only get them to open during
office hours.

Thanks. A useful reference!
--
Cheers,
Set Square
______
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In an earlier contribution to this discussion,
Paul Barker wrote:


Put the expansion vessel on the return to the boiler in a position
where there are no controls between the vessel and the boiler like
zone valves or gate valves, so that it is effectively always in open
series with the boiler.

Is that mandatory? It would be much easier to put it in the airing
cupboard - using the pipes which currently go to the F&E tank, which connect
into the flow pipe just before the pump and zone valves.

So there *would* be an open path to the boiler - but it would be on the flow
rather than return side. is there a problem with that?
--
Cheers,
Set Square
______
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On Wed, 02 Feb 2005 22:34:37 +0000, Set Square wrote:

Ed's excellent sealed system FAQ covers the principles of operation plus
what to do when something goes wrong, but it doesn't tell you how to design
a system from scratch or how to convert a vented system (apart from
mentioning a couple of pre-requisites).

Is there a FAQ or similar anywhere which goes into things like:
* How to determine the volume of a system
* How to decide on the size of pressure vessel required
* Any rules about the location of the pressure vessel relative to boiler,
pump, zone valves (if it matters)

According to my calculations, you need a pressure vessel whose capacity is
about 10% of the water volume. Does that sound reasonable? [I'll expand(!)
on that below]

So you need to be able to estimate the water volume. If you know how much
pipe there is of each size, that bit's not too difficult. Then there's the
radiators. Are there any rules of thumb for litres per square metre for
typical single-panel radiators? [I'm sure I could make a stab at it by
estimating the cross-sectional area of all the 'tubes' - but it would be
nice if I didn't have to]. That leaves the boiler and the heating coil in
the HW cylinder. Are there any typical values for these?

I'm sure there's scope for a FAQ here if anyone cares to write one (if there
isn't one already).

[Back to my calcs. How does this grab you? Assume you have a 10 litre
pressure vessel, and prime it with air at 0.7 bar (1.7 bar absolute). Then
you pressurise the system with water to 1 bar (2 bar absolute). ISTR from
the dim and distance past about the Law of either Boyle or Charles or
somebody along the lines of the volume of a gas at constant temperature
being inversely preportional to the absolute pressure. If this is correct,
our 10 litres of air at 1.7 bar will reduce to 8.5 litres at 2 bar. If we
now heat up the system so that the pressure rises to 2 bar (3 bar absolute)
the volume of air will reduce further to 5.67 litres. The difference between
8.5 and 5.67 (2.83 litres) represents the amount of expansion water which
has entered the vessel. So, for a pressure rise from 1 bar to 2 bar, a 10
litre vessel can accommodate about 2.8 litres of water.

My research on the volumetric expansion of (tap) water suggests that it
expands by about 2.5% between 4 decC and 80 degC. So a 100 litre system
would expand by about 2.5 litres - which could be nicely accommodated by a
10 litre vessel, giving a pressure rise of just under a bar.
(Actually, the temperature rise will be less than this since the system will
be a room temp rather than 4 decC when cold. But the air in the vessel
probably warms up up a bit, which I have ignored - so maybe the two tend to
cancel.) Are there any major flaws in my reasoning?]

I'd go along with your reasoning. If another poster were (able?) to argue
as well he might have more credibility.

Questions about converting to /designing a sealed system do come up
from time to time but question about sealed systems and their maintenance
are almost daily. Hence the syllabus of the FAQ. There is a
greater need IMHO for a 'choosing a heating system' FAQ, 'choosing a
boiler make' FAQ and 'diagnosing gas boiler faults' FAQ.

Those three questions must now be pushing 20% of the threads in one form
or another.

When converting expansion vessel can often go in the loft where the old
F&E tank was and connected to the vent pipe (but some boiler makers,
Keston and Potterton at least, want it on the return so watch for that).

Usually the most frequent hassle is that the most convenient place for the
filling loop (airing cupboard where rising main and circuit pipes are
close) is the least convenient for the relief pipe. The releif valve
usually has a place for a pressure gauge to screw in. The filling loop
does not. You end up having to add another pressure gauge near the filling
point.

A useful guesstimator for system volume is 10 x radiators = system litres.
My local plumbers' merchant basic tells people to buy nearly 1 litre /kW
of boiler - this is probably rather excessive vessel sizing - mainly
because modern boilers a modulating units and are often well over-sized.

--
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

In an earlier contribution to this discussion,
Ed Sirett wrote:


I'd go along with your reasoning. If another poster were (able?) to
argue as well he might have more credibility.

Many thanks for your reply. [Hopefully my degree in Physics - albeit 40
years ago - wasn't totally wasted!]


When converting expansion vessel can often go in the loft where the
old F&E tank was and connected to the vent pipe (but some boiler
makers, Keston and Potterton at least, want it on the return so watch
for that).

The installation manual for my boiler (Baxi Solo 70/4 PF - MkI) says that
the vessel should be connected close to the *inlet* side of the pump - which
is right where the F&E tank is connected at present. This gives me the
option of the airing cupboard, or the attic in place of the F&E tank. My
only concern with the latter is that it would be more difficult to get at to
check and re-charge the air pressure if needed.

The manual also says that the relief valve should be close to the boiler on
the flow pipe. I would rather put everything in the airing cupboard - still
on the flow pipe, but about 10 metres from the boiler (as the pipe runs). Do
you see a problem with this?

Usually the most frequent hassle is that the most convenient place
for the filling loop (airing cupboard where rising main and circuit
pipes are close) is the least convenient for the relief pipe. The
releif valve usually has a place for a pressure gauge to screw in.
The filling loop does not. You end up having to add another pressure
gauge near the filling point.

My airing cupboard would be the most convenient place for the filling loop.
There would be no problem getting the relief pipe to the outside world. Are
there any rules for locating the spout, relative to where people may be
standing? Incidentally, the boiler manual says that the filling loop must be
fitted at "low level" - but the diagram shows it between the pump and the
hot cylinder! I presume it means low level with respect to the floor, not in
absolute terms?

A useful guesstimator for system volume is 10 x radiators = system
litres. My local plumbers' merchant basic tells people to buy nearly
1 litre /kW of boiler - this is probably rather excessive vessel
sizing - mainly because modern boilers a modulating units and are
often well over-sized.

That gives rather conflicting results! I have 10 radiators (albeit including
a couple of large ones) so that gives a system volume of 100 litres. Using
my 10% rule, the expansion vessel would need to be 10 litres. [Someone
quoted another source at about 7.4% of system volume]. However my boiler has
a max output of 20kW (26kW input) which indicates about 20 litres. This
sounds a bit excessive to me.
--
Cheers,
Set Square
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