Pumps, head and flow rate?

28 views
Skip to first unread message

Dave Liquorice

unread,
Jan 29, 2012, 8:49:35 AM1/29/12
to
Domestic CH circulator type pumps. The graphs giving flow rate use
"head" as the other axis. What head is this? Highest to lowest point
in the loop? Pump to lowest, pump to highest? Something completely
differnt like flow resistance of the entire loop?

I'd like to have a rough guesstimate as to the flow rate the load
controller on the woodburner is producing so I can get an idea of how
much energy is being transfered from the woodburners boiler to the
thermal store.

--
Cheers
Dave.



Bob Minchin

unread,
Jan 29, 2012, 9:31:30 AM1/29/12
to
Hmm! I'd be interested in what the experts think. Lets hope it does not
deteriorate to sniping though!

At first thought I would expect it to be the highest point that water
has to be pumped to with respect to the lowest point (rather than
anything to do with the pressure in an older vented system- second later
thought!)

Unless you have very small bore pipe and looong horizontal runs, I'd
expect the work needed to pump top to bottom to be greater than pipe
friction.

Does the chart give any other fixed conditions such as pipe bore?

Bob

Onetap

unread,
Jan 29, 2012, 10:02:54 AM1/29/12
to
On Jan 29, 1:49 pm, "Dave Liquorice"
<allsortsnotthis...@howhill.co.uk> wrote:
> Domestic CH circulator type pumps. The graphs giving flow rate use
> "head" as the other axis. What head is this? Highest to lowest point
> in the loop? Pump to lowest, pump to highest? Something completely
> differnt like flow resistance of the entire loop?

It is the hydraulic resistance of the system, expressed in metres head
of water.

If you have the pump running in a test rig, you'd measure the water
flow rate and the water pressure at the pump inlet and outlet. The
pressure difference is equal to density x g x h, where h is the
equivalent head of water.

A pipe circuit and radiators say, piped in 28mm tube would have one
resistance at a certain flow rate. If you pipe the same circuit in
22mm tube, it will have a higher resistance at the same flow rate
requiring a bigger pressure difference to shove the water around the
loop. You'd typically need a bigger pump or the same pump running at a
higher speed.

Stephen Mawson

unread,
Jan 29, 2012, 10:14:24 AM1/29/12
to

"Bob Minchin" <bob.minc...@YOURHATntlworld.com> wrote in message
news:jg3l96$12t$1...@dont-email.me...
>
> At first thought I would expect it to be the highest point that water has
> to be pumped to with respect to the lowest point (rather than anything to
> do with the pressure in an older vented system- second later thought!)
>
> Unless you have very small bore pipe and looong horizontal runs, I'd
> expect the work needed to pump top to bottom to be greater than pipe
> friction.
>
> Does the chart give any other fixed conditions such as pipe bore?
>
> Bob
>

In a full-of-water circulating system, whether closed or open with an
overflow pipe into the header tank, I would expect all the head (that is,
the pressure produced by the pump) to be used in overcoming the resistance
of the pipework. The height of the system makes no difference, the water on
the downward leg exactly balancing that on the upward one.

The pressure required to pump water at a given rate through a pipe is
proportional to the length of the pipe (not surprisingly) and inversely
proportional to the fourth power of the pipe's diameter so long runs of 15mm
pipe produce a lot of resistance, nearly five times that of 22mm if my
arithmetic is correct.

Stephen


dennis@home

unread,
Jan 29, 2012, 10:28:20 AM1/29/12
to


"Dave Liquorice" <allsortsn...@howhill.co.uk> wrote in message
news:nyyfbegfubjuvyypb...@srv1.howhill.co.uk...
> Domestic CH circulator type pumps. The graphs giving flow rate use
> "head" as the other axis. What head is this? Highest to lowest point
> in the loop? Pump to lowest, pump to highest? Something completely
> differnt like flow resistance of the entire loop?

Flow resistance related.

There is also a static head figure which will be the minimum head needed to
avoid cavitation.
There is also a maximum head before it breaks, but I don't recall seeing it
quoted on any domestic circulator.

>
> I'd like to have a rough guesstimate as to the flow rate the load
> controller on the woodburner is producing so I can get an idea of how
> much energy is being transfered from the woodburners boiler to the
> thermal store.

You need to workout the flow resistance of all the pipes and joints and then
lookup the flow in the table.



Bob Minchin

unread,
Jan 29, 2012, 10:48:54 AM1/29/12
to
Stephen Mawson wrote:
> "Bob Minchin"<bob.minc...@YOURHATntlworld.com> wrote in message
> news:jg3l96$12t$1...@dont-email.me...
>>
>> At first thought I would expect it to be the highest point that water has
>> to be pumped to with respect to the lowest point (rather than anything to
>> do with the pressure in an older vented system- second later thought!)
>>
>> Unless you have very small bore pipe and looong horizontal runs, I'd
>> expect the work needed to pump top to bottom to be greater than pipe
>> friction.
>>
>> Does the chart give any other fixed conditions such as pipe bore?
>>
>> Bob
>>
>
> In a full-of-water circulating system, whether closed or open with an
> overflow pipe into the header tank, I would expect all the head (that is,
> the pressure produced by the pump) to be used in overcoming the resistance
> of the pipework. The height of the system makes no difference, the water on
> the downward leg exactly balancing that on the upward one.

Thanks - makes sense
>
> The pressure required to pump water at a given rate through a pipe is
> proportional to the length of the pipe (not surprisingly) and inversely
> proportional to the fourth power of the pipe's diameter so long runs of 15mm
> pipe produce a lot of resistance, nearly five times that of 22mm if my
> arithmetic is correct.
>
Quite an eye-opener!

Bob


harry

unread,
Jan 29, 2012, 10:58:19 AM1/29/12
to
On Jan 29, 1:49 pm, "Dave Liquorice"
<allsortsnotthis...@howhill.co.uk> wrote:
The head of a pump is the maximum pressure it can generate.
Usually expressed in feet or meters.
One foot head + 0.4336 psi.

The flow rate is the maximum flow it can put through.
Usuallty in gallons per minute or liters per second.

However, it can't do both at the same time, these are the limits of
it's performance, the two ends of the graph. In practice,you wouldn't
run it close to either end.

So, It can do max flow at zero pressure and max.pressure at zero
flow..

So, for practical purposes, it operates at less then maximum of
either.

There is a performance graph, the figures just outline the extreme
parameters on the graph.

So if you had a pump that was say 10m head and 20 litre/second it
would probably move about 10 liters.sec at 5m head.

This not exact because the performance curve is non-linear, you need
to look at the manufacturers graph.

Onetap

unread,
Jan 29, 2012, 11:31:52 AM1/29/12
to
On Jan 29, 1:49 pm, "Dave Liquorice"
<allsortsnotthis...@howhill.co.uk> wrote:

> I'd like to have a rough guesstimate as to the flow rate the load
> controller on the woodburner is producing so I can get an idea of how
> much energy is being transfered from the woodburners boiler to the
> thermal store.

Most solid fuel boilers are, or should be, on a gravity circulation
loop in which the differential pressure is produced by the different
densities of the flow and return water columns.

Those that are connected to a central heating system are usually
connected through an injection tee, so that the circulation route
through the boiler is not restricted by the pumps and valves. The pump
can become blocked, the valves can be closed.

Roger Mills

unread,
Jan 29, 2012, 12:04:22 PM1/29/12
to
On 29/01/2012 13:49, Dave Liquorice wrote:
> Domestic CH circulator type pumps. The graphs giving flow rate use
> "head" as the other axis. What head is this? Highest to lowest point
> in the loop? Pump to lowest, pump to highest? Something completely
> differnt like flow resistance of the entire loop?
>
Yes, indeed - something different as already explained by several people.

> I'd like to have a rough guesstimate as to the flow rate the load
> controller on the woodburner is producing so I can get an idea of how
> much energy is being transfered from the woodburners boiler to the
> thermal store.
>

If you can somehow measure the differential pressure across the pump and
convert that to head of water, you can then look at the graph to
determine flow rate at the appropriate pump speed. You might be able to
do that with a differential pressure gauge, or two separate gauges - or
even a U tube containing mercury (it would have to be quite big if it
only had water in it!).

Alternatively, if it's flow you're interested in, why not install a flow
meter in the pipe?
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

Onetap

unread,
Jan 29, 2012, 12:36:18 PM1/29/12
to
On Jan 29, 5:04 pm, Roger Mills <watt.ty...@gmail.com> wrote:

> If you can somehow measure the differential pressure across the pump and
> convert that to head of water, you can then look at the graph to
> determine flow rate at the appropriate pump speed. You might be able to
> do that with a differential pressure gauge, or two separate gauges - or
> even a U tube containing mercury (it would have to be quite big if it
> only had water in it!).

Pressure tappings should be some way downstream (10 D) from the pump.
The water discharged is swirling and the velocities cause an apparent
drop in the pressure as per Bernouilli. The swirling dies away
downstream and the pressure recovers. Pressure gauges fitted directly
to the pump's inlet & outlet (or flanges on big pumps) won't give you
an accurate reading, but are slightly better than nothing.

Dave Liquorice

unread,
Jan 29, 2012, 7:04:25 PM1/29/12
to
On Sun, 29 Jan 2012 08:31:52 -0800 (PST), Onetap wrote:

>> I'd like to have a rough guesstimate as to the flow rate the load
>> controller on the woodburner is producing so I can get an idea of
how
>> much energy is being transfered from the woodburners boiler to the
>> thermal store.
>
> Most solid fuel boilers are, or should be, on a gravity circulation
> loop in which the differential pressure is produced by the different
> densities of the flow and return water columns.

There is a gravity loop.

This pump is in a load controller that circulates water through it
and the boiler until the water gets up to 60C it then has
thermostatic valve that closes forcing the circulation via the
thermal store. The idea is that a) the cold water in the boiler and
loop doesn't reach the store thus cooling it b) once up to
temperature the water in the boiler is never below 60C which
hopefully reduces condensation (water and tar) on the boiler.

--
Cheers
Dave.



Dave Liquorice

unread,
Jan 29, 2012, 7:10:22 PM1/29/12
to
On Sun, 29 Jan 2012 15:14:24 -0000, Stephen Mawson wrote:

> In a full-of-water circulating system, whether closed or open with an
> overflow pipe into the header tank, I would expect all the head (that
> is, the pressure produced by the pump) to be used in overcoming the
> resistance of the pipework. The height of the system makes no
> difference, the water on the downward leg exactly balancing that on the
> upward one.

Ta, I figured that in a closed system all the pump has to do is
overcome the resistance but not being familiar with pump
specifications got confused by "head" not really being a vertical
distance just another way of expressing a pressure.

--
Cheers
Dave.



Dave Liquorice

unread,
Jan 29, 2012, 7:07:40 PM1/29/12
to
On Sun, 29 Jan 2012 15:28:20 -0000, dennis@home wrote:

> You need to workout the flow resistance of all the pipes and joints and
> then lookup the flow in the table.

Is there a nice handy look up table for 28mm copper 45 and 90 bends a
resistance per unit length of tube?

--
Cheers
Dave.



Onetap

unread,
Jan 29, 2012, 7:25:29 PM1/29/12
to
On Jan 30, 12:07 am, "Dave Liquorice"
<allsortsnotthis...@howhill.co.uk> wrote:

> Is there a nice handy look up table for 28mm copper 45 and 90 bends a
> resistance per unit length of tube?
.

http://www.copperinfo.co.uk/plumbing-heating-and-sprinklers/downloads/pub-150-copper-tubes-in-domestic-heating-systems.pdf

harry

unread,
Jan 30, 2012, 3:40:30 AM1/30/12
to
This is complete drivel.

harry

unread,
Jan 30, 2012, 3:39:44 AM1/30/12
to
On Jan 30, 12:04 am, "Dave Liquorice"
The purpose of a gravity lop is in case of pump/electricity failure.
It provides somewhere for the heat to go.

Onetap

unread,
Jan 30, 2012, 4:11:46 AM1/30/12
to
On Jan 30, 8:40 am, harry <haroldhr...@aol.com> wrote:
> On Jan 29, 4:31 pm, Onetap <One...@talk21.com> wrote:

> This is complete drivel.

No Harry, it is not. You contradict yourself in your very next post,
in which you say that "The purpose of a gravity lop is in case of pump/
electricity failure".
It seems that you do agree, in that most solid fuel boilers are, or
should be, on a gravity circulation loop.

It is mostly taken from the HETAS recommendations and handbooks. I
have got the HETAS handbooks, having done their training course some
years ago.

Do you have any particular solid fuel training or experience to
support your "drivel" comment?

Will you be retracting your "drivel" comment and apologising for the
same?

Dave Liquorice

unread,
Jan 30, 2012, 4:19:01 AM1/30/12
to
On Mon, 30 Jan 2012 00:39:44 -0800 (PST), harry wrote:

>> There is a gravity loop.
>
> The purpose of a gravity lop is in case of pump/electricity failure.
> It provides somewhere for the heat to go.

What don't you understand about "There is a gravity loop."?

--
Cheers
Dave.



robgraham

unread,
Jan 30, 2012, 3:02:36 PM1/30/12
to
On Jan 30, 12:04 am, "Dave Liquorice"
<allsortsnotthis...@howhill.co.uk> wrote:
I'm still not convinced I understand your set-up. You say there is 'a
gravity loop' - is this just a dump into a small radiator, or is this
all the heat extraction from the stove ?

I'm interested in that long before we all got together as Usenet -
actually before the web and PC's existed in any numbers, I added an
oil burner to my wood burner CH system and incorporated a Dunsley
Neutraliser to meld the output of the two sources. My understanding
is that a thermal store is at the system neutral point and does much
the same thing.

What I don't recognise is the term 'load controller' - the concept of
a thermostatic valve in a gravity feed circuit seems an anathema too.

Rob

Dave Liquorice

unread,
Jan 30, 2012, 5:55:18 PM1/30/12
to
On Mon, 30 Jan 2012 12:02:36 -0800 (PST), robgraham wrote:

> I'm still not convinced I understand your set-up. You say there is 'a
> gravity loop' - is this just a dump into a small radiator, or is this
> all the heat extraction from the stove ?

The load controller sits across the flow and return pipes from the
woodburner. Feed from bottom of thermal store passes through a one
way flap valve in the load controller and then down to the wood
burner. Flow back up from woodburner goes to the top of the thermal
store but there is a T into the top of the load controller. There is
a thermostatic mixer valve that starts to move at 60C (fully moved by
70C), this valve selects where the pump takes its feed water from.
Either from the flow from the woodburner (cold state) or pre the one
way valve in the feed from the thermal store (hot state). The output
from the pump is feed into the return to the woodburner after the one
way valve. Without power the one way flap valve is biased open so
there is a "normal" open loop between the woodburner and the store.
With power the +ve pressure from the pump shuts the flap valve and
circulates water to maintain the feed to the woodburner at 60C (or
above if the store bottom is hotter than 60C).

Simple enough as far as the water side is concerned. The hard bit is
the electrical control side. Don't particulary want the thing running
when the stove isn't lit but don't want to have to keep remembering
to switch it on/off either. Automagic switch on is easy, pipe stat on
the flow from woodburner set at 50C. The switch off when the stove
has died down and is no longer providing any heat is the important
one. By then the bottom of the store is probably above 60C so the
load controller circulates from the store to the woodburner and back
until the store temperature drops below 60C. And I do mean the whole
store as the woodburner goes in at the very top and out at the bottom
so store stratification goes out the window. Trouble is the backup
oil boiler cuts in when the store gets down to 65C at about 1/3 up.
This has given the installers something to think about... it should
be getting some form of differential control at some point.

> I added an oil burner to my wood burner CH system and incorporated a
> Dunsley Neutraliser to meld the output of the two sources. My
> understanding is that a thermal store is at the system neutral point and
> does much the same thing.

Yes all the flows/returns (woodburner, oil boiler, CH) end up in the
thermal store which is the neutral point. Tapping points are
different, wood burner is very top and very bottom, oil boiler hot
half way and bottom, CH flow just below the oil boiler and bottom. CW
feed for DHW goes in just above the oil boiler hot input. There is a
solar coil in the bottom as well can't remember the relationship of
the bottom tappings and that.

> What I don't recognise is the term 'load controller' - the concept of
> a thermostatic valve in a gravity feed circuit seems an anathema too.

See above its not in the gravity loop as in it doesn't control the
gravity flow.

--
Cheers
Dave.



Dave Liquorice

unread,
Jan 30, 2012, 6:47:36 PM1/30/12
to
On Sun, 29 Jan 2012 16:25:29 -0800 (PST), Onetap wrote:

>> Is there a nice handy look up table for 28mm copper 45 and 90
bends a
>> resistance per unit length of tube?
>
> http://www.copperinfo.co.uk/plumbing-heating-and-sprinklers/downloads/
> pub-150-copper-tubes-in-domestic-heating-systems.pdf

Thanks for that will have to inwardly digest the do a bit of number
crunching.

--
Cheers
Dave.



Message has been deleted

Onetap

unread,
Feb 1, 2012, 4:37:37 AM2/1/12
to
On Jan 30, 8:40 am, harry <haroldhr...@aol.com> wrote:

> >   Those that are connected to a central heating system are usually
> > connected through an injection tee, so that the circulation route
> > through the boiler is not restricted by the pumps and valves. The pump
> > can become blocked, the valves can be closed.
>
> This is complete drivel.

Harry has gone very quiet.

I suspect 'Harry' might be my old mate Alhoa/Rhondo.

I said Rhondo was a clueless half-wit bodger on the DIYnot forum
about a year ago and has been stalking me ever since. He thought he'd
been clever taking a drill and hacksaw to a 2-port Honeywell normally
closed zone valve when he wanted a normally open valve for his solid
fuel boiler. Rhondo/Alhoa/Harry has a limited vocabulary, 'drivel'
being one of the few words in it. I could find the link if anyone
wants a laugh.

Having my own personal DIY internet stalker really makes me feel
like I've achieved something. :-)
Reply all
Reply to author
Forward
0 new messages