There have been a number of posting recently about gravity HW systems which
I have read with interest, having put off the job of upgrading the current
layout year after year. After doing a good search on google I think I have
all the concepts, and think that the way to go is just to stick a valve on
the cylinder.

Can anyone help me out, and let me know if there is a
simpler/easier/better/more efficient etc etc option before I buy the bits.

I have stuck details on my system on a webpage, so you can see how it is all
plumbed together, any thoughts, comments, recommendations would be
gratefully recieved. I rather be 110% clear on what I am doing, and get it
right first time, than fiddle and have no heating at all!

Details at:

http://www.freewebs.com/jase12345/Fu...%20Heating.doc

In an earlier contribution to this discussion,
Jason wrote:

There have been a number of posting recently about gravity HW systems
which I have read with interest, having put off the job of upgrading
the current layout year after year. After doing a good search on
google I think I have all the concepts, and think that the way to go
is just to stick a valve on the cylinder.

Can anyone help me out, and let me know if there is a
simpler/easier/better/more efficient etc etc option before I buy the
bits.

I have stuck details on my system on a webpage, so you can see how it
is all plumbed together, any thoughts, comments, recommendations
would be gratefully recieved. I rather be 110% clear on what I am
doing, and get it right first time, than fiddle and have no heating
at all!

Details at:

http://www.freewebs.com/jase12345/Fu...%20Heating.doc

If this were my system, my first priority would be to add boiler/pump
interlocks to prevent the boiler from cycling when neither CH nor HW require
it.

I would thus be looking at either a C-Plan system or an S-Plan system. I
would *not* go for a Y-Plan because it would greatly complicate the venting,
and 3-port valves are best avoided anyway.

The choice really hinges on whether or not the HW performance is currently
adequate. If it is, go for a C-Plan - it's a lot easier. You will need a
2-port valve in the HW circuit right next to the cylinder, *after* the vent
pipe tee - so as to maintain an un-interrupted path from the boiler to the
vent pipe. You will, of course also need a cylinder stat and a room stat.
The radiator nearest to the room stat *shouldn't* have a TRV on it. I
presume that you don't get any unwanted gravity circulation in the CH
circuit when the pump is not running? If you *do*, you will need an
anti-gravity valve - which only opens when the pump generates a bit of urge.

If the hot water takes too long to heat with gravity, you need a fully
pumped system - so I would go for an S-Plan. Put a 2-port valve in the HW
circuit next to the cyliner, as for C-Plan. Put another one in the CH
circuit. The current pump location may be a convenient position. Move the
pump to a position in the common bit of pipe before the circuits split. In
order to prevent pumping over, move the fill pipe connection into the HW
flow pipe - very close to where the vent pipe connects.

I would go for a programmable room stat for several reasons:
* It gives you completely independent control over the timing of HW and CH -
which most ordinary programmers don't. [Set your main programmer to HW timed
and CH constant - so that the room stat conrols the CH]
* It adapts to the characteristics of your system/house - and maintains the
set temperature with less overshoot
* With 'Optimum Start' it decides when to turn the heating on in order to
achieve the set temperature by the required time
* In the 'Off' position, it acts as an automatic frost stat - and still
turns on the heating if needed to prevent freezing

I doubt whether you need a by-pass circuit - even if you go for an S-Plan
system. These are needed when you have a boiler which only holds a small
quantity of water and which will overheat due to the residual heat in the
metal bits if the water isn't carried away quickly enough. Such boilers are
only used in fully pumped setups - and have pump over-run stats which keep
the pump running after the boiler stops firing until it has cooled down a
bit. The bypass circuit provides somewhere for the water to go if all the
valves are closed.

If a boiler works ok on gravity without the pump being on at all (when
operating on HW only), it can pretty certainly take care of itself.

Not sure whether I have answered *all* of the questions. HTH, anyway!

--
Cheers,
Set Square
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Thanks for the comprehensive reply, you have given me a couple of things to
think about.

The choice really hinges on whether or not the HW performance is currently
adequate. If it is, go for a C-Plan - it's a lot easier. You will need a
2-port valve in the HW circuit right next to the cylinder, *after* the
vent
pipe tee - so as to maintain an un-interrupted path from the boiler to the
vent pipe. You will, of course also need a cylinder stat and a room stat.
The radiator nearest to the room stat *shouldn't* have a TRV on it. I
presume that you don't get any unwanted gravity circulation in the CH
circuit when the pump is not running?

Not exactly sure on how long it takes to heat HW from cold at the moment- my
guess is that we have a a resonable amount of hot water within about 40 mins
from cold - certainly within an hour. Up to now we have not had a problem
with having hot water (if we have a bath we might stick on the immersion to
"help" the heating) if someone wants a bath straight after. This makes me
inclined to stick with c-plan as you suggest. See below re stats.

.. In order to prevent pumping over, move the fill pipe connection into the
HW
flow pipe - very close to where the vent pipe connects.

Am I right in thinking that if the fill pipe is moved to the HW flow pipe
close to where the vent pipe connects that the system would then effectivly
be a "combined vent and feed"? i.e both vent and feed sharing a pipe back to
the boiler.

I would go for a programmable room stat for several reasons:
* It gives you completely independent control over the timing of HW and
CH -
which most ordinary programmers don't. [Set your main programmer to HW
timed
and CH constant - so that the room stat conrols the CH]
* It adapts to the characteristics of your system/house - and maintains
the
set temperature with less overshoot
* With 'Optimum Start' it decides when to turn the heating on in order to
achieve the set temperature by the required time
* In the 'Off' position, it acts as an automatic frost stat - and still
turns on the heating if needed to prevent freezing

Was going to stick in a programmable room stat in any case - Not sure if a
programmable cylinder stat (AFAIK only Danfoss make one - the WP75) would
be of any use in saving gas? We have hot water all day (house always
occupied), at the moment heating in the morning (no stat), topping up for an
hour at about 2pm, and then again in the evening say 5pm - 10pm. by using a
prog cylinder stat could use cooler water in day (only use it for washing
hands etc) - and hotter in the evening for a bath?

Using a regular cylinder stat we could heat the water in the morning to the
higher temp, similar to before, and then leave it to cool during the day as
it used up, before reheating the water again to the evening tempreture? The
only disadvantage with this is that if the water was used up in the day it
would not be replaced to the evening - a prog cylinder stat would realise
the water usage and reheat the replaced water to the lower temp.

A prog cylinder stat would realise that the temp of the water had fallen,
and then switch on to "top up"


I doubt whether you need a by-pass circuit - even if you go for an S-Plan
system. These are needed when you have a boiler which only holds a small
quantity of water and which will overheat due to the residual heat in the
metal bits if the water isn't carried away quickly enough. Such boilers
are
only used in fully pumped setups - and have pump over-run stats which keep
the pump running after the boiler stops firing until it has cooled down a
bit. The bypass circuit provides somewhere for the water to go if all the
valves are closed.

AFAIK the boiler does have a switch for "fully pumped" - therefore the
valves could be closed and the pump run. Suggesting again to keep to C-Plan

If a boiler works ok on gravity without the pump being on at all (when
operating on HW only), it can pretty certainly take care of itself.

I see your point. Looks like the boiler would be ok left on gravity (i.e no
overrun) - even on a fully pumped syste,

Not sure whether I have answered *all* of the questions. HTH, anyway!


Great. Thanks a lot for your help. I am pretty sure that C-Plan is the way
to go, unless someone come up with something else.

Now I only need to decide between the Honeywell CM67RF or Danfoss TP7000 -
and if to use a regular cylinder stat or programmable cylinder stat!!

Am temped to stick to danfoss with the TP7000 (optimum start/programmable
etc) and a HW cylinder programmeable therm WP75.

http://www.danfoss-randall.co.uk/site/downloads/560.pdf

http://www.danfoss-randall.co.uk/sit...loads/4457.PDF


Cheers,
Set Square

"Christian McArdle" wrote in message
...
Can anyone help me out, and let me know if there is a
simpler/easier/better/more efficient etc etc option before I buy the
bits.

The really important question is whether the boiler you have requires a
fully open gravity circuit as a safety measure. If it does (as many
ancient
or solid fuel boilers so require) then your options are far more limited.

What make, model and fuel is your boiler?

Thanks for your post.

Boiler is Potterton Netaheat running nat gas. According to the potterton
website the boiler can be be gravity or pumped.

Can anyone help me out, and let me know if there is a
simpler/easier/better/more efficient etc etc option before I buy the bits.

The really important question is whether the boiler you have requires a
fully open gravity circuit as a safety measure. If it does (as many ancient
or solid fuel boilers so require) then your options are far more limited.

What make, model and fuel is your boiler?

Christian.

Boiler is Potterton Netaheat running nat gas. According to the potterton
website the boiler can be be gravity or pumped.

You're laughing then. Run in whatever way the plumbing works best. Ensure
there is a direct unvalved path from boiler output to vent and a direct
unvalved path from water inlet to boiler return. Then place pumps and valves
where appropriate. You can share valves or use separate as you wish. You can
run each of the 28mm or 22mm system as your desire. Pumps don't count as
valves if in the water inlet path. A fixed (non automatic i.e. gate) bypass
valve counts as a path for the water inlet. If the boiler has an additional
manual reset overheat cutout, then the water inlet restrictions can be
ignored. The water inlet could even be combined with the vent under these
circumstances, which eliminates pumping over.

Christian.

In an earlier contribution to this discussion,
Jason wrote:


. In order to prevent pumping over, move the fill pipe connection
into the HW
flow pipe - very close to where the vent pipe connects.

Am I right in thinking that if the fill pipe is moved to the HW flow
pipe close to where the vent pipe connects that the system would then
effectivly be a "combined vent and feed"? i.e both vent and feed
sharing a pipe back to the boiler.

Sort of. It's probably best to keep them separate, but connect then both
into the same main pipe fairly close together. Mine are connected in a few
inches apart - the idea being that there should't be an appreciable pressure
drop between the connection points. [This only applies to fully pumped
anyway - and you can ignore it if you go C-Plan]

Was going to stick in a programmable room stat in any case - Not sure
if a programmable cylinder stat (AFAIK only Danfoss make one - the
WP75) would be of any use in saving gas? We have hot water all day
(house always occupied), at the moment heating in the morning (no
stat), topping up for an hour at about 2pm, and then again in the
evening say 5pm - 10pm. by using a prog cylinder stat could use
cooler water in day (only use it for washing hands etc) - and hotter
in the evening for a bath?

Using a regular cylinder stat we could heat the water in the morning
to the higher temp, similar to before, and then leave it to cool
during the day as it used up, before reheating the water again to the
evening tempreture? The only disadvantage with this is that if the
water was used up in the day it would not be replaced to the evening
- a prog cylinder stat would realise the water usage and reheat the
replaced water to the lower temp.

A prog cylinder stat would realise that the temp of the water had
fallen, and then switch on to "top up"


I can't see a lot of point in programmable cylinder stats. I suppose if you
really want different temperatures at different times of day, and want more
on/off cycles than are provided by the ordinary programmer, you could make a
case for one. However, if your tank is well lagged, the heat losses should
be very small anyway - and it does little harm keeping the water slightly
hotter than you actually need for some of the time. Features like optimum
start and the ability to adapt to the system are less likely to be useful on
a water heating system.


Now I only need to decide between the Honeywell CM67RF or Danfoss
TP7000 - and if to use a regular cylinder stat or programmable
cylinder stat!!

Use whatever you're comfortable with. I've got a (non-RF) CM67 with optimum
start (they don't all have it) - which I fitted a few months ago to replace
the original conventional room stat. It seems to meet my needs fairly well.
I've also got a conventional cylinder stat, which I don't intend to change.
[FWIW, I've got a Y-plan system - which was in the house when I bought it -
which works adequately but isn't what I would have chosen].

--
Cheers,
Set Square
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You're laughing then. Run in whatever way the plumbing works best. Ensure
there is a direct unvalved path from boiler output to vent and a direct
unvalved path from water inlet to boiler return.

I still reckon gravity with control is the way to go. as we do not have a
problem with HW recovery time and it is a lot simpler to fit.

Then place pumps and valves where appropriate. You can share valves or use
separate as you wish. You can run each of the 28mm or 22mm system as your
desire. Pumps don't count as valves if in the water inlet path. A fixed
(non automatic i.e. gate) bypass valve counts as a path for the water inlet
If the boiler has an additional manual reset overheat cutout, then the
water inlet restrictions can be ignored. The water inlet could even be
combined with the vent under these circumstances, which eliminates pumping
over.

The boiler is fitted with a manual reset overheat cutout, does this mean
that the 2port could be fitted in the HW circuit close to the boiler? The
vent would still have an open path - and the feed have the 2 port valve
inline but would still be in the HW circuit and stop circulation.. This
contradicts what set square recommened which was to fit a "2-port valve in
the HW circuit right next to the cylinder, *after* the vent pipe tee - so as
to maintain an un-interrupted path from the boiler to the vent pipe" whilst
this has the benefit of leaving the feed and vent open at all time - the
ability to stick the valve in the feed/HW circuit close to the boiler would
mean that I could use a RF tank stat
(http://www.screwfix.com/app/sfd/cat/...30214&ts=48069) and save
running a cable to the loft making fitting extremly simple.

I still reckon gravity with control is the way to go. as we do not have a
problem with HW recovery time and it is a lot simpler to fit.

Not much in it. Just put the pump right next to the zone valve and wired off
it, set on lowest speed. Check for pumping over, both continuous and at pump
start/stop and increase the length of the vent loop over if it does.

The boiler is fitted with a manual reset overheat cutout, does this mean
that the 2port could be fitted in the HW circuit close to the boiler?

Now you're really laughing. This allows you to close off the old feed pipe
and take the F&E cistern feed to the vent pipe with a T. Guaranteed no
pumping over. Only safe with a manual reset overheat boiler. These don't
require the quenching water flow for safety, so can live with having the
pipework blocked by escaping steam until the thermostat blows.

Yes, with such a boiler, you can place the valve anywhere that doesn't
interfere with the vent path.

____
/ \ vent
F&E | | |
|----| |
+--+-+ |
| |
feed +----+
| /---\
| | |
+------+ |
| | / |
| | \ |
| +-----+ |
| | | |
| | +-----+
+--------+ | |
| +--+ |
| Boiler | |
| w.c/o +---+
+--------+

= pump + zone valve (alternative locations)

Hope the picture makes sense!

Christian.

"Christian McArdle" wrote in message
et...
I still reckon gravity with control is the way to go. as we do not have
a
problem with HW recovery time and it is a lot simpler to fit.

Not much in it. Just put the pump right next to the zone valve and wired
off
it, set on lowest speed. Check for pumping over, both continuous and at
pump
start/stop and increase the length of the vent loop over if it does.

Maybe an option to go PF then. Will have a climb up tomorrow and have a look
how tight the pipework is to move the pump?


The boiler is fitted with a manual reset overheat cutout, does this mean
that the 2port could be fitted in the HW circuit close to the boiler?

Now you're really laughing. This allows you to close off the old feed pipe
and take the F&E cistern feed to the vent pipe with a T. Guaranteed no
pumping over. Only safe with a manual reset overheat boiler. These don't
require the quenching water flow for safety, so can live with having the
pipework blocked by escaping steam until the thermostat blows.

but....(brain ticking over)

1) is the bathroom radiator currently acting as a bypass?
2) if I place the 2port by the boiler would I cause any issues with not
having a bypass in the system? would the pump not overun - and need a
bypass? or is the system ok being fully pumped - and set to "gravity" mode?
3) If I go to Fully Pumped I assume a second 2port will be required after
the pump for the CH circuit.

"Jason" wrote in message
...
You're laughing then. Run in whatever way the plumbing works best.
Ensure
there is a direct unvalved path from boiler output to vent and a direct
unvalved path from water inlet to boiler return.

I still reckon gravity with control is the way to go.

It isn't. You already have a pump. You need zone valves on both CH and DHW,
so pump both. Far more efficient. It is silly not to.

In an earlier contribution to this discussion,
Jason wrote:


1) is the bathroom radiator currently acting as a bypass?

No. By-passes are only used in fully pumped systems where there may
otherwise be no water path if all the valves are closed. That ain't what
you've got!

2) if I place the 2port by the boiler would I cause any issues with
not having a bypass in the system? would the pump not overun - and
need a bypass? or is the system ok being fully pumped - and set to
"gravity" mode?

I'm sure that Christian is technically right - but I would feel uneasy if I
didn't have a clear path from the boiler to the open vent - even though the
boiler has a manual cutout. Belt and braces, maybe?

What you probably *could* do is put the 2-port valve for the HW in the
*return* before the CH and HW circuits re-merge - and move the cold feed so
that both vent and feed pipe are tee'd into the HW *flow* pipe. That way,
you'd still have your open vent path, and the 2-port valve would be in a
position to make the wiring easier - and you could have your RF cylinder
stat. You could do all this regardless of whether you end up with a C-Plan
(semi-gravity) or S-Plan (fully pumped).


3) If I go to Fully Pumped I assume a second 2port
will be required after the pump for the CH circuit.

If you go fully pumped (and assuming only *one* pump), the pump will no
longer be in the CH circuit - you'll need to move it to the common bit,
otherwise it won't pump the HW. I would put the CH 2-port valve where the
pump *currently* is - because you'll have to modify that bit of pipework
anyway.

I still don't think you need a by-pass circuit. The question is this: When
the boiler burner turns off - either as a result of the boiler's own stat
turning off, or the room stat turning the boiler off - does the water in the
boiler continue to get hotter due to residual heat to the extent that it
will overheat and trip its manual cutout *unless* water continues to
circulate through it? The fact that your boiler is quite happy with a
gravity system with no forced flow through it in HW-only mode indicates that
the answer is "no". If the answer were "yes", the boiler would have to
control the pump - by means of a pump over-run stat - and would have to keep
the pump running long enough for the boiler to cool down. Such boilers will
only work in a fully-pumped scenario - so yours isn't like that.

--
Cheers,
Set Square
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In an earlier contribution to this discussion,
IMM wrote:

"Jason" wrote in message
...
You're laughing then. Run in whatever way the plumbing works best.
Ensure there is a direct unvalved path from boiler output to vent
and a direct unvalved path from water inlet to boiler return.

I still reckon gravity with control is the way to go.

It isn't. You already have a pump. You need zone valves on both CH
and DHW, so pump both. Far more efficient. It is silly not to.


WHY? If he goes for a C-Plan, it's virtually ** as efficient as an S-Plan -
and only needs one zone valve!

** the only possible inefficiency is that the boiler is keeping itself hot
for a bit longer when heating the water

--
Cheers,
Set Square
______
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"Set Square" wrote in message
...
In an earlier contribution to this discussion,
IMM wrote:

"Jason" wrote in message
...
You're laughing then. Run in whatever way the plumbing works best.
Ensure there is a direct unvalved path from boiler output to vent
and a direct unvalved path from water inlet to boiler return.

I still reckon gravity with control is the way to go.

It isn't. You already have a pump. You need zone valves on both CH
and DHW, so pump both. Far more efficient. It is silly not to.


WHY? If he goes for a C-Plan, it's virtually ** as efficient as an
S-Plan -
and only needs one zone valve!

** the only possible inefficiency is that the boiler is keeping itself hot
for a bit longer when heating the water

Fully pumped is far more efficient than a gravity.

2) if I place the 2port by the boiler would I cause any issues with
not having a bypass in the system? would the pump not overun - and
need a bypass? or is the system ok being fully pumped - and set to
"gravity" mode?

I'm sure that Christian is technically right - but I would feel uneasy if
I
didn't have a clear path from the boiler to the open vent - even though
the
boiler has a manual cutout. Belt and braces, maybe?

You may have misunderstood. There still must be a clear path from the boiler
to the vent. The manual cutout removes the need for a clear independent path
from the cistern feed outlet to the boiler. This path is then allowed to
either be valved or shared with the vent path (but not both, obviously as
the vent path would then become valved).

The solution I suggested actually involved an additional pump, rather than
using the CH one. The CH circuit can stay just the same. The new pump just
pumps the hot water circuit. The new valve just controls the hot water
circuit.

You are correct in saying that you may now need an additional bypass,
although the effect of not having one may not be predictable and it might
not be required. With this type of system with two independent circuits,
there should be a bypass on each circuit. The CH circuit probably has a
bathroom radiator doing this task, whilst the hot water circuit should have
an automatic bypass valve that is simply plumbed in parallel with the zone
valve set to around 0.3 bar.

Christian.

** the only possible inefficiency is that the boiler is keeping itself hot
for a bit longer when heating the water

Not true, actually. The gravity circuit fails efficiency tests on several
counts. Firstly, the long reheat time keeps the pipework and boiler hot for
unnecessarily long periods, wasting heat. Secondly, the reduced heat
requirement from the poor circulation coupled with a gravity boiler's
typical lack of modulation ability leads the boiler to cycle excessively,
which wastes energy.

Christian.

In an earlier contribution to this discussion,
Christian McArdle wrote:

I'm sure that Christian is technically right - but I would feel
uneasy if I didn't have a clear path from the boiler to the open
vent - even though the boiler has a manual cutout. Belt and braces,
maybe?

You may have misunderstood. There still must be a clear path from the
boiler to the vent. The manual cutout removes the need for a clear
independent path from the cistern feed outlet to the boiler. This
path is then allowed to either be valved or shared with the vent path
(but not both, obviously as the vent path would then become valved).

The solution I suggested actually involved an additional pump, rather
than using the CH one. The CH circuit can stay just the same. The new
pump just pumps the hot water circuit. The new valve just controls
the hot water circuit.

You are correct in saying that you may now need an additional bypass,
although the effect of not having one may not be predictable and it
might not be required. With this type of system with two independent
circuits, there should be a bypass on each circuit. The CH circuit
probably has a bathroom radiator doing this task, whilst the hot
water circuit should have an automatic bypass valve that is simply
plumbed in parallel with the zone valve set to around 0.3 bar.

Christian.

Thanks for the clarification. In an earlier post, you appeared to be saying
that if the boiler had an over-heat cutout which had to be reset manually,
it was no longer necessary to have a clear path to the vent.

Using 2 pumps sounds a bit complicated to me. In a conventional S-Plan (with
a single pump) each thermostat switches the appropriate zone valve - and the
volt-free contacts on the zone valves switch the boiler and pump on whenever
one or more circuits are calling for heat. In cases where the boiler has a
pump over-run stat, the pump is controlled by the boiler rather than
directly by the zone valves.

If you have two pumps, I assume that each stat would have to switch one pump
and one zone valve - with the volt-free contacts switching just the boiler.
This would mean:
A) the pumps would start before the zone valves were open
B) the pumps could not be controlled by the boiler, so there could be no
pump over-run

My feeling is that the OP has sufficient scope to be able to convert either
to a C-Plan or an S-Plan, without the added complication of two pumps.

--
Cheers,
Set Square
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In an earlier contribution to this discussion,
IMM wrote:


Fully pumped is far more efficient than a gravity.

This sounds like another of your unsubstantiated assertions - because you
haven't explained WHY.

Is it because you haven't heard of C-Plan [See
http://content.honeywell.com/uk/homes/systems.htm ] and associate gravity
circulation with the boiler being on all the time, whether required or not,
and with no control over the HW temperature, so that it rises to near boiler
temp?

If so, gravity systems don't *have* to be like that!

--
Cheers,
Set Square
______
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"Christian McArdle" wrote in message
et...
2) if I place the 2port by the boiler would I cause any issues with
not having a bypass in the system? would the pump not overun - and
need a bypass? or is the system ok being fully pumped - and set to
"gravity" mode?

I'm sure that Christian is technically right - but I would feel uneasy
if
I
didn't have a clear path from the boiler to the open vent - even though
the
boiler has a manual cutout. Belt and braces, maybe?

You may have misunderstood. There still must be a clear path from the
boiler
to the vent. The manual cutout removes the need for a clear independent
path
from the cistern feed outlet to the boiler. This path is then allowed to
either be valved or shared with the vent path (but not both, obviously as
the vent path would then become valved).

The solution I suggested actually involved an additional pump, rather than
using the CH one. The CH circuit can stay just the same. The new pump just
pumps the hot water circuit. The new valve just controls the hot water
circuit.

In a two pump setup there is no need for a zone valve, just a non-return
valve after each pump.

In an earlier contribution to this discussion,
Christian McArdle wrote:

** the only possible inefficiency is that the boiler is keeping
itself hot for a bit longer when heating the water

Not true, actually. The gravity circuit fails efficiency tests on
several counts. Firstly, the long reheat time keeps the pipework and
boiler hot for unnecessarily long periods, wasting heat. Secondly,
the reduced heat requirement from the poor circulation coupled with a
gravity boiler's typical lack of modulation ability leads the boiler
to cycle excessively, which wastes energy.

Christian.

OK, I accept the modulation bit - but the comment was made in the context of
someone working with his existing boiler - so changing to a modulating type
didn't really apply. The other points are covered by my statement if you
include the associated pipework with the boiler.

[I guess there is a danger that comments can be made about specific cases in
such a way that they appear to be generalisations, and may not be valid as
such. I accept that I am sometimes guilty of this.]
--
Cheers,
Set Square
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Thanks for the clarification. In an earlier post, you appeared to be
saying
that if the boiler had an over-heat cutout which had to be reset manually,
it was no longer necessary to have a clear path to the vent.

I'm not sure quite how you read that, but there you go!

A) the pumps would start before the zone valves were open

Yes. One reason to have the bypass! Alternatively, you could use a couple of
relays and run the pumps and relays from the valve switches to give an OR
for the boiler interlock.

B) the pumps could not be controlled by the boiler, so there could be no
pump over-run

An ancient gravity circulated boiler is unlikely to have pump overrun.

My feeling is that the OP has sufficient scope to be able to convert
either
to a C-Plan or an S-Plan, without the added complication of two pumps.

Perhaps, although a twin pump solution would be an easier fix, as it leaves
the CH circuit untouched.

Christian.

"Set Square" wrote in message
...
In an earlier contribution to this discussion,
IMM wrote:


Fully pumped is far more efficient than a gravity.

This sounds like another of your unsubstantiated assertions - because you
haven't explained WHY.

Boiler cycling, slugglish performace in re-heat, etc, etc.

If so, gravity systems don't *have* to be like that!

That is so, then you fit another zone valve and cyl stat, so may as well
have the lot fully pumped and increase efficiency.

In an earlier contribution to this discussion,
Christian McArdle wrote:

Thanks for the clarification. In an earlier post, you appeared to be
saying that if the boiler had an over-heat cutout which had to be
reset manually, it was no longer necessary to have a clear path to
the vent.

I'm not sure quite how you read that, but there you go!

Sorry, I've just re-read it - and I obviously read something into it which
wasn't there!



My feeling is that the OP has sufficient scope to be able to convert
either to a C-Plan or an S-Plan, without the added complication of
two pumps.

Perhaps, although a twin pump solution would be an easier fix, as it
leaves the CH circuit untouched.

But he would surely need to insert a zone valve into the CH circuit - or
would he?

The thought has just occurred to me that with 2 pumps, and the boiler
interlock logic implemented using relays, he may not need *any* zone
valves - and almost certainly wouldn't need any by-passes. He'd just have to
make sure that the HW circuit didn't continue to circulate by gravity when
CH-only was demanding heat. This might require a null-flow valve on the
cylinder side of the vent/feed connection.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

The thought has just occurred to me that with 2 pumps, and the boiler
interlock logic implemented using relays, he may not need *any* zone
valves - and almost certainly wouldn't need any by-passes.

Indeed. That's another method that can be used.

Christian.

"Christian McArdle" wrote in message
et...
The thought has just occurred to me that with 2 pumps, and the boiler
interlock logic implemented using relays, he may not need *any* zone
valves - and almost certainly wouldn't need any by-passes. He'd just have
to
make sure that the HW circuit didn't continue to circulate by gravity when
CH-only was demanding heat. This might require a null-flow valve on the
cylinder side of the vent/feed connection.

Christian.


I read the postings with interest, and gain a lot of knowledge on the
way.....

I assume that the two pump option is similar in operation to the the
grundfos pump plan which uses a dual output pump.

What puzzles me on this is if, say the HW circuit was being pumped by its
own pump and CH was off, why there would be no circulation on the CH circuit
without the a zone valve? i.e wouldnt the HW pump push water through the
pump and around the CH circuit? (on the current gravity system, isnt the HW
assisted to a degree by the pump when CH is on, as we are pushing pumped
water into the boiler return)

What puzzles me on this is if, say the HW circuit was being pumped by its
own pump and CH was off, why there would be no circulation on the CH
circuit
without the a zone valve?

There won't be a substantial pressure difference over the two ends of the
unpumped circuit, so flow would be non-existent or very slight, easily
stiffled by a flap type non-return valve (or the 2 port zone valve, of
course!).

Christian.

"Christian McArdle" wrote in message
et...
What puzzles me on this is if, say the HW circuit was being pumped by
its
own pump and CH was off, why there would be no circulation on the CH
circuit
without the a zone valve?

There won't be a substantial pressure difference over the two ends of the
unpumped circuit, so flow would be non-existent or very slight, easily
stiffled by a flap type non-return valve (or the 2 port zone valve, of
course!).

After all this help I am temped to put in a second pump and go to a fully
pumped system (second option for me would to go to C-Plan)

Benefit of second pump is that it leaves the CH circuit - as is, and should
be cheaper than S-Plan as it only needs a pump (and I might be able to get
my hands on one!!), which should be cheaper and easier than having to move
the pump AND need to fit 2x 2ports AND a bypass.

The only thing that is puzzeling me is the "anti gravity valve" - what does
one look like, and where can you buy one from. I have looked at screwfix,
discountheating, BES etc and cannot fine what I need. - can anyone point me
in the right direction. I would need a 28mm fitting.

thanks