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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Plumbing Questions >> Moving my hot water cylinder https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1243712531 Message started by Phill in Skeg on May 30^th, 2009, 8:42pm

Title: Moving my hot water cylinder
Post by Phill in Skeg on May 30^th, 2009, 8:42pm

I have a hot water cylinder just below my header tank in the roof of my bungelow. The how water pressure is pathetic and can take ages to fill a sink. I have had 2 suggestions: (1) put in a pump - the cold for the taps comes from a different header tank, also in the roof - can I boost just the hot water? (2) Move the cylinder downstairs to increase the distance between the header tank and the cylinder - will this help? Which is the better solution?

Title: Re: Moving my hot water cylinder
Post by thescruff on May 30^th, 2009, 11:51pm

I assume you mean storage tank and not header tank, e.g. the tank that feeds the cylinder.

The pressure is the distance between the storage tank and the highest part of the system, so if you lower the cylinder you would increase the pressure to the next highest e.g the top of the cylinder or the pipework if they are higher.

Yes lowering the cylinder should increase the pressure, larger pipes would also increase the volume at the taps.

You can buy single pumps, but you need a minimum 600mm head for them to work, it would also be noisy if it was in the loft.

Need a bit more information on what you are trying to achieve.

Title: Re: Moving my hot water cylinder
Post by Phill in Skeg on Jun 1^st, 2009, 12:06pm

Thanks for the info. Yes the cold water tank is the one I mean. It is just above the hot water cylinder. This is the highest point in the system. If I move the hot water cylinder, then the highest point would be the pipes that carry the water from the cylinder to the showers and sinks. But these would be lower than the current position of the cylinder.

When you say 600mm is needed for a pump to work, what is this distance please?

I am trying to get better flow out of the hot water to my showers and bathroom sinks and bath. The flow to the bath is so slow the water cools down before the bath fills!

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 1^st, 2009, 12:42pm

You need a minimum 600mm from the top of the pump to the base of the storage tank, normally it's not a problem if the pump is stood at the base of the cylinder.

With low heads, it's also better to get a negative head pump that works on pressure and not flow.

Salamandar make a couple of pumps that would be suitable.

Title: Re: Moving my hot water cylinder
Post by Headrush on Jun 3^rd, 2009, 6:11pm

The height of the water in the cold storage tank is what gives you pressure at the outlets.

To raise the head of pressure in a gravity feed system, raise the cold water storage tank. Build a platform in the attic and get it up as high as you can.

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 3^rd, 2009, 8:40pm

What he said. It's the height of the cold storage or expansion tank that matters. lowering the cylinder won't make any difference. The head in these two cases is the same...

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 3^rd, 2009, 8:46pm

wrote on May 30^th, 2009, 8:42pm:

Do you have two header tanks in the loft? If the one supplying the cold taps provides sufficient head/pressure the simplest thing might be to use that one to supply the hot as well.

If the cold pressure is also very low perhaps consider switching to a mains pressure system. Before you do that someone must measure the mains pressure AND flow rate to ensure it's adequate. It's common for the cold tap on the kitchen sink to be connected directly to the incoming cold main. Is that noticibly better?

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 4^th, 2009, 12:53am

Oh dear, the blind leading the blind ;D

Title: Re: Moving my hot water cylinder
Post by runninwata on Jun 4^th, 2009, 8:55pm

::) lol ::)

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 5^th, 2009, 7:34am

So only two of us passed their Physics O-Level?.... ;D

http://books.google.co.uk/books?id=97k3-VfkyiwC&pg=PA81&lpg=PA81&dq=physics+water+pressure+independant+of+shape+of+vessel&source=bl&ots=_bBen1DfiV&sig=QSLDV4JmQ2HuztuRA0ZZsUHsNbo&hl=en&ei=d7ooSpvLK9CZjAf1v5DmCg&sa=X&oi=book_result&ct=result&resnum=6

Quote:

The pressure due to a liquid is equal to the height of the liquid (h) times it's density (d) and g.
.
.
Liquid pressure is independant of the size or shape of the container, it depends only on the depth and density of the liquid

See below. The pressure at the bottom of all four columns is the same. If shape mattered the "extra" pressure in one would cause the water level in another to rise. It doesn't.

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 5^th, 2009, 10:15am

No good posting fancy drawing which are wrong CW,
Under stand the question and my answer

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 5^th, 2009, 11:57am

Note the increase head.

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 5^th, 2009, 6:03pm

Ok so I oversimplified it. Try this one... note the constant head.

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 5^th, 2009, 6:30pm

It appears we differ as to the definition of "head".

I assert that the pressure at the tap depends only on the distance from the tap to the surface of the water in the header tank.

See also..
http://www.qpsltd.co.nz/pdf/hot_water_cylinder.pdf

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 5^th, 2009, 8:31pm

The head is measured to/from the highest points in the installation, in the first example the pipe from the cylinder, in the second the same pipe on the loft floor, the head is increased by that distance.

For calculation purposes you never use the water level, as it drops as soon as you open and outlet, if you insist on boosting the figures then you should use the center line of the storage tank.

Time how long it takes to fill a small bucket stood in the bath from a shower hand set, then do the same experiment with the hand set at say 1m higher.

Do you want to tell qpsltd they are wrong or shall I ;D

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 5^th, 2009, 8:40pm

Think about it in imaginary terms.

We have a storage tank 10m in the air, therefor at ground zero we have 1bar.

If I connect a hose to the end of the pipe, and walk it up the stairs, the higher I get the lower the pressure will be, until I get to the water level whereas the water will stop running.

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 5^th, 2009, 10:31pm

thescruff wrote on Jun 5^th, 2009, 8:40pm:

You make my point for me. Looking at your diagram of the existing system... The dimension arrows suggest that the head doesn't depend on the height of the tap but clearly it does.

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 5^th, 2009, 10:40pm

Examples:

Header tank at 10 meters. Tap at 8 meters. Pressure = 1.0 - 0.8 = 0.2 = 2 meters of water.

Header tank at 20 meters. Tap at 18 meters. Pressure = 2.0 - 1.8 = 0.2 = 2 meters of water

Header tank at 1000 meters. Tap at 998 meters. Pressure = 100 - 99.8 = 0.2 = 2 meters of water.

Ok so far?

Note how the diameter of the pipe doesn't matter. You could put a large bulge in the pipe the size of a hot water cylinder anywhere you like. Makes no difference to the static pressure.

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 5^th, 2009, 11:17pm

You are still missing the point.

Draw a straight line from the tank to the ground floor and call it one bar because its 10m long, now calibrate it into 1m drawoffs.

10m =1bar
9m=0.9bar
8m=0.8bar
7m=0.7bar
6m=0.6bar
5m=0.5bar
4m=0.4bar
3m=0.3bar
2m=0.2bar
1.5m=0.15bar call this the bottom of the cylinder.
1m=0.1bar,

Now notice the connection from the top of the cylinder=500mm 0.05bar.

The highest point in the system is the top of the cylinder, or the pipe from it, and not the floor it is standing on.

The static pressure (head) is as my drawing, ok give or take a bit for the depth of the water.

Note;- the diameter of the pipe doesn't matter, true, however a bigger pipe would deliver more water

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 5^th, 2009, 11:22pm

CWatters wrote on Jun 5^th, 2009, 10:31pm:

thescruff wrote on Jun 5^th, 2009, 8:40pm:

You make my point for me. Looking at your diagram of the existing system... The dimension arrows suggest that the head doesn't depend on the height of the tap but clearly it does.

The head depends on the highest point, be it a tap/shower rose or the pipework.

If you lower the rose in a shower the pressure increases, by 0.1bar per/m.

Title: Re: Moving my hot water cylinder
Post by thescruff on Jun 6^th, 2009, 1:07am

Is this any clearer.

Title: Re: Moving my hot water cylinder
Post by CWatters on Jun 6^th, 2009, 7:44pm

I know and understand all of that and a lot more. However your diagram still doesn't quite show how to calculate the pressure at the tap itself. See if you agree with this one..

Edit: For some reason it's hard to read the text. Click on it to make it readable.

Title: Re: Moving my hot water cylinder
Post by greg on Jul 8^th, 2009, 8:02pm

With Scruff on this one, in plumbing systems, the head is measured from base of cistern to highest point.

Title: Re: Moving my hot water cylinder
Post by londonman on Jul 21^st, 2009, 7:03am

It's quite clearly explained here http://en.wikipedia.org/wiki/Hydraulic_head ;D

Title: Re: Moving my hot water cylinder
Post by CWatters on Jul 29^th, 2009, 5:33pm

wrote on Jul 8^th, 2009, 8:02pm:

With Scruff on this one, in plumbing systems, the head is measured from base of cistern to highest point.

Ok I can buy that this is the definition of the "system head" but what matters is the pressure at the tap not at the bottom of the cistern.

Lowering the cistern increases the pressure at the bottom of the cistern but does nothing to increase pressure at the tap. The only way to do that is to raise the expansion tank or lower the tap.

http://www.google.co.uk/search?hl=en&q=water+pressure+head++%22above+the+tap%22&btnG=Search&meta=

Title: Re: Moving my hot water cylinder
Post by thescruff on Jul 30^th, 2009, 12:54am

Wrong CW.

Lowering the outlet at the top of the cylinder will increase the head in this case, to the next highest point, the pipes in the loft.

You could get a secondary head, if the top of the cylinder to the outlet is greater than the storage tank to the highest point.

Title: Re: Moving my hot water cylinder
Post by Headrush on Jul 30^th, 2009, 3:28am

Quote:

Lowering the cistern increases the pressure at the bottom of the cistern but does nothing to increase pressure at the tap. The only way to do that is to raise the expansion tank or lower the tap.

Yep ;D

[/quote]

Title: Re: Moving my hot water cylinder
Post by bill577 on Jul 30^th, 2009, 6:32pm

I would have to agree with.... cw , if the head is measured from the bottom of the cold water tank to the highest tap, it doesnt matter where the hot water cylinder is.

Title: Re: Moving my hot water cylinder
Post by thescruff on Jul 30^th, 2009, 11:20pm

wrote on Jul 30^th, 2009, 6:32pm:

I would have to agree with.... cw , if the head is measured from the bottom of the cold water tank to the highest tap, it doesn't matter where the hot water cylinder is.

And you would also be wrong bill :o

Title: Re: Moving my hot water cylinder
Post by big_all on Jul 31^st, 2009, 2:12am

no expert here but you need the mass to be as high as possible otherwise your hydro electric dams would be 20 foot high ;D ;D

you need volume[mass] x height to give weight[ pressure]

a bit off pipe 200 ft tall above a tank will give you high pressure/flow for a very short time because the body off the water is low down

height is weight volume is weight so volume at height weighs more ::)

Title: Re: Moving my hot water cylinder
Post by CWatters on Jul 31^st, 2009, 7:20pm

Ok my last attempt to convince you all....

I contend that the pressure at the tap is the same in all of the following. If the drawings aren't clear clicking on them should improve.

Original "high" cylinder...

Lowered cylinder..

Lowered cylinder with lowered take off...

Obviously I can't vouch for the flow rate on that last one :) but the static pressure is the same..

Title: Re: Moving my hot water cylinder
Post by thescruff on Jul 31^st, 2009, 9:07pm

Now do another one with the connection out the cylinder lowered to the floor level.

Or are you deliberately leaving it stuck up in the air a 1mtr.

Title: Re: Moving my hot water cylinder
Post by bill577 on Jul 31^st, 2009, 9:18pm

if the connection of the hot water cylinder is taken to the floor, it will still have the same head once it has climbed back up to the tap, sorry I still cant see how your theory holds water scruff [smiley=lolk.gif]

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 1^st, 2009, 12:09am

You ain't trying Bill, and it ain't theory. :o The head is the distance to the highest part of the system, in this case the pipe from the cylinder is higher than the outlet, so by lowering the pipe you increase the head.

Head is not the higher the better, although it works out that way most of the time, but the greater the distance betwen the storage tank and the highest part, not necessarily the outlet.

Read this and see if it's any clearer, they are talking of fill head, which is slightly more than static head by the distance to the outlet.

Also notice the negative head, then under stand how the head increases as you lower the outlet.

http://www.stuart-turner.co.uk/media/5273-Installation-Monsoon-Universal-U1.5-2.0-bar-max-Single.pdf

In the next pdf look at fig5 and you should be getting warm ::)

http://www.stuart-turner.co.uk/media/5199-Installation-Monsoon-Standard-1.5-2.0-bar-max-Single.pdf

Title: Re: Moving my hot water cylinder
Post by CWatters on Aug 1^st, 2009, 9:56am

thescruff wrote on Jul 31^st, 2009, 9:07pm:

Now do another one with the connection out the cylinder lowered to the floor level.

Or are you deliberately leaving it stuck up in the air a 1mtr.

Not at all, like this you mean...

Title: Re: Moving my hot water cylinder
Post by CWatters on Aug 1^st, 2009, 10:02am

wrote on Jul 31^st, 2009, 9:18pm:

Exactly.

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 1^st, 2009, 12:37pm

That example has a bigger head than the one before it ::)

Title: Re: Moving my hot water cylinder
Post by bill577 on Aug 1^st, 2009, 5:55pm

believe me scruff I am trying, [smiley=headbang.gif] so we have two fixed points, the tap and the cold water tank and one variable, the hot water cylinder, if the measurement of head is taken at the tap ,which is the distance between the tap and the tank, I cant see how moving the hot water cylinder will affect the head, if the measurement was taken at the cylinder take off I could understand the increase, but I am going to be very embarrased if you turn out to be right.

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 1^st, 2009, 7:33pm

Not if Bill. ;D

Draw a storage tank.

Drop a cold pipe say 3mtrs to a bath tap, so the distance between the two points is 3m, now to increase the head I have to increase the distance between A, the tank and B, the outlet, I can do that by raising the tank, or lowering the outlet. Think of the top of the cylinder as a big shower rose.

See my first drawing on page one, and how The distance has increased between A, tank and B, highest point.

Now go back to the cylinder, the head is the pipe coming out of the cylinder, because it is the highest part of the system, if I could increase that distance I will increase the head.

The next highest point is the pipes on the roof joists, so if I could lower the top of the cylinder to the joist level, I have increased the head.

Look at the Monsoon pdf it explains it quite well.

Title: Re: Moving my hot water cylinder
Post by bill577 on Aug 1^st, 2009, 8:38pm

think we,ll have to agree to disagree scruff, hopefully it will be sorted out one way or the other ;)

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 1^st, 2009, 10:43pm

wrote on Aug 1^st, 2009, 8:38pm:

think we,ll have to agree to disagree scruff, hopefully it will be sorted out one way or the other ;)

Never in a million years ;D

What bit don't you understand.

Title: Re: Moving my hot water cylinder
Post by Headrush on Aug 2^nd, 2009, 1:59am

A million lemmings can't be wrong, can they? :-?

Title: Re: Moving my hot water cylinder
Post by bill577 on Aug 2^nd, 2009, 5:22pm

ok where am I going wrong,

house 1
cws tank in loft
hot water cylinder on first floor

head at bath tap 0.2 bar
head at kitchen sink 0.5 bar
head at basement sink 0.8 bar

house 2
cws tank in loft
hot water cylinder on ground floor

head at bath tap 0.2 bar
head at kitchen sink 0.5 bar
head at basement sink 0.8 bar

house 3
cws tank in loft
hot water cylinder in basement

head at bath tap 0.2 bar
head at kitchen sink 0.5 bar
head in basement 0.8

the head is determined by the distance between the tap
and the cws tank, I still cant see how moving the hot water cylinder
makes a difference to this ,

lemmings dont commit suicide, its an old wives tale ;)

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 2^nd, 2009, 11:14pm

Because the outlet of the cylinder is the highest point.

It's above the taps.

It's above the pipes.

It's above everything accept the storage tank.

The pipe out of the cylinder is the head, if you lower it the new head would be the pipes in the loft.

Take your tap and stick it on the top of the cylinder in my first drawing, what head have you got.

Now lower the cylinder so the top is level with the pipes in the loft, now what head have you got

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 2^nd, 2009, 11:20pm

Quote:

house 3
cws tank in loft
hot water cylinder in basement

head at bath tap 0.2 bar
head at kitchen sink 0.5 bar
head in basement 0.8

As you can see by this example (I think) ;D.

The higher the outlet the lower the head.

Consider the next bit very carefully, the hot water pipe from the cylinder is an outlet the same as a tap.

Keep thinking, if you could lower that outlet a 1mtr ::)

Title: Re: Moving my hot water cylinder
Post by bill577 on Aug 3^rd, 2009, 9:39am

I cant understand why your measuring to the top of the hot water cylinder [smiley=duh.gif] .if the head is determined by the height between the cold tank and the outlet, the hot water cylinder is nothing more than up and over in the pipework.

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 3^rd, 2009, 2:42pm

Because the top of the cylinder is the maximum available head, it's where the hot water comes from.

If you connect a hose to the bath tap, 0.2 bar and say 10 Ltrs/min.

What do you think will happen in the kitchen, would you expect 0.5 bar and more than 10 Ltrs per min, you can't get more than the maximum.

The pressure for the whole house is determined by the length and size of the cold feed in the bottom of the cylinder, less the height of the cylinder.

Better add, if the top of the cylinder was below the pipes in the loft, then the pipes would be the head and not the top of the cylinder.

Title: Re: Moving my hot water cylinder
Post by bill577 on Aug 4^th, 2009, 9:00am

strangely enough I did expect more at the kitchen sink, I suppose the static pressure would be higher but the dynamic is limited by the head from the tank to the cylinder, always learning , thanks for your time and patience scruff
bill

Title: Re: Moving my hot water cylinder
Post by thescruff on Aug 4^th, 2009, 1:37pm

That's how it work, if all you get out of the top of the cylinder is 10 Ltrs/min, then that is the maximum anywhere in the system, whether its in the basement or the bathroom. Try to get more out and you suck in air, which would be likely if you exceed the head, your basement for example.

The original question, if you can think back that far, gave you the opportunity, to either lower the top of the cylinder to below the highest service pipes, raise the storage tank, or both.