How to measure Hot Water Head
- Thread starter pooksahib
- Start date
Doitall you wrote:
I know this because I have a powerful shower in a my bathroom which is great and, surprisingly for your side of the argument, the hot water tank is in the loft (of all places) directly under the CW tank. Not more than 6 vertical inches between the top of the HW cylinder and the bottom of the CW tank.
Yes, if any water was drawn off at the top of the cylinder. If the draw off point is lowered the pressure will increase at that point and so will flow, if the diameter of the outlet is constant.
I know this because I have a powerful shower in a my bathroom which is great and, surprisingly for your side of the argument, the hot water tank is in the loft (of all places) directly under the CW tank. Not more than 6 vertical inches between the top of the HW cylinder and the bottom of the CW tank.
So you keep saying, and yet offer absolutely no science to back up your claims. What is your reasoning behind that?
Yes, he DID reduce the head by raising the shower outlet, but that doesn't prove that the head is the distance between the CWS and HW tank. All it proves is that the head, and hence pressure and flow rate, were reduced by raising the outlet height - it doesn't prove whether head is the distance between CWS and HW tank, or CWS and outlet, the latter being correct.
No, I still disagree and will continue to do so, and ask again... why do you think that the mere fact the water has to reach the top of the cylinder will somehow mean the head must be measured between that point and the HW tank?
Following your logic, I could take a piece of 22mm pipe from my CWS tank, increase it to 28mm after a 2 metre vertical run downwards, and then somehow this will 'magically' mean that the head must be measured from this point. The change in pipe diameter is no different to the change in diameter caused by the cylinder. It sounds stupid, but no more stupid than your claim above. I ask again, why do you consider the HW tank to be the point where head is measured to?
I think your mistake in in assuming that because water goes into the BOTTOM of the HW tank, and exits the TOP, this somehow negates the effects of gravity.
Nobody disputes that, but it does not prove your logic.
Thank the lord, yet another person with a similar setup to my old house (i.e. little/no distance between CWS and HW tank), and yet somehow it works! Do you really need any more than this to disprove your shaky theory?
How many people is it going to take, some of them plumbers, for you to finally see the error in your argument? You keep using the "The higher you raise the pipe from the outlet the lower the head will be", somehow in the belief that this proves your logic, but it would apply regardless of where we measure the head from, so proves nothing.
Ignorance is curable but stupidity is a permanent condition so stick to playing with sparky things Matthew.
I think that answers my questions, then
thanks for the laugh though.
Believe me, the feeling is mutual.
Answer the two basin example then Matthew.
Two basins side by side, you connect one straight from the top of the cylinder, and you run the pipe into the loft and drop down to the other.
Which has the highest pressure.
Two basins side by side, you connect one straight from the top of the cylinder, and you run the pipe into the loft and drop down to the other.
Which has the highest pressure.
I'm not sure what this will prove as you don't mention whether both basins are on the same floor level, but assuming they are, my thinking is thus:
The basin connected straight to the top of the cylinder will have the highest flow rate, assuming equally sized pipe runs to both basins, as there will be less restriction to flow on account of a shorter, more direct pipe run. The second basin will have a lower flow rate for the above reasons.
Now, the part you're probably looking to trip me up on, the HEAD on both basins will be exactly the same (assuming the pipework in the loft stays BELOW the level of water in the CWS tank), and the static pressure will also be the same at both basins.
Regardless of how the pipework is run, as long as the level in the CWS tank is always above the level of the highest pipework, the head will not change, as the height between the CWS tank level and the basin outlet is still EXACTLY the same.
If, however, the second basin was on the next floor level down, the distance between the CWS and basin outlet would be greater, hence a larger head. The greater mass of water in the pipes leading to the basin due to the increased vertical drop will also mean greater static pressure. Flow rate will be higher, assuming any increase is not cancelled out by resistance in the pipe run, as pressure is higher than the basin in the room above.
this is going to run longer than word association
this is going to run longer than word association
Hey, it's not my fault that sparkies are slow learners.
Ok why does the pressure get less the higher you raise the shower rose.
How long are you going to run with this same argument? You ask me to answer the 'two basin example', I answer it and make a valid point, although you don't like what you hear and completely ignore it. I might as well be banging my head against a brick wall, because every time I come up with a valid counter-argument, you just cite the above statement regarding pressure drop as the head becomes smaller.
Again, NOBODY disputes that less head (ooh-er) = less pressure, because it's true and we can clearly demonstrate that fact. What you seem to be doing is detracting attention away from your original claim that head is ONLY the distance between CWS and HW cylinder, and anything after that can (in your opinion) make no difference to the system head. You still haven't provided any evidence to prove this.
Ok why does the pressure get less the higher you raise the shower rose.
How long are you going to run with this same argument? You ask me to answer the 'two basin example', I answer it and make a valid point, although you don't like what you hear and completely ignore it. I might as well be banging my head against a brick wall, because every time I come up with a valid counter-argument, you just cite the above statement regarding pressure drop as the head becomes smaller.
Again, NOBODY disputes that less head (ooh-er) = less pressure, because it's true and we can clearly demonstrate that fact. What you seem to be doing is detracting attention away from your original claim that head is ONLY the distance between CWS and HW cylinder, and anything after that can (in your opinion) make no difference to the system head. You still haven't provided any evidence to prove this.
You agree then if you raise the shower rose in the air you get less pressure, Pressure = head so you are say you have less head the higher you raise the rose.
Whats the difference between raising the rose and running a pipe up the wall, NONE
By the same token the pressure in the bottom of the cylinder will be higher than the top.
And if you bother to read my post, instead of talking gibberish, you will see, I said the head is from the storage tank to the highest part of the system, in the basin example it would be the pipe in the loft.
You agree that raising the rose to the ceiling will reduce the pressure, yet you say if you hook the pipe over the joist and run the rose back to the bath level, you get the pressure back again.
What you can't understand is that once the pressure is lost by raising the pipe, it's gone for good.
Yes, I agree with this because the rose is an OUTLET.
A world of difference, because the pipe is NOT an outlet, merely a means of carrying water, at a certain pressure, from A to B. If you a simple way to test this for yourself, get two buckets, a table, and a length of hose. Fill one buckup up and place it on the table, and the other on the floor, then start a siphon to drain the top bucket into the bottom one. You can raise the hose as high as you like, even above the top bucket, as long as the OUTLET is below the water level in the top bucket. As long as the length of hose is constant, flow rate at the outlet will not change. Granted that this is not exactly the same scenario as we have with a hot water system, but it's close enough to validate my point.
Where have I disputed that? But it's irrelevant, as it's the height of the outlet that we're discussion here, which isn't necessarily at the same level as the HW cylinder.
So, if you were correct, running a pipe up into the loft to say, within 20cm of the bottom of the CWS tank, would reduce the head to 20cm. I disagree. If the outlet position remained the same, say 3 metres below bottom of the CWS tank, then the head is three metres, not 20cm. I stand by my statement that head is the distance between the level of water in the CWS tank and the outlet.
You're right, I can't understand that, because it's a closed system! If the water in the CWS tank is still 3 metres above the outlet, there's still a 3 metre head, regardless of the highest point in the pipe run, as long as it doesn't go above the water line in the CWS tank. In the same way as a siphon will pull water through a closed pipe as long as the outlet is lower than the inlet, water will still flow through the pipework at the same rate as if the run did not rise and go through the loft, negating the effect of pipework losses.
I'm not sure if you're being deliberately thick or just don't understand
Ok you agree the rose is an outlet, A copper pipe with a tap on the end is an outlet, a cylinder with a pipe in the top is an outlet, the copper pipe is no different to the flexible hose in a shower.
Your hose, bucket, siphon example is pathetic from a desperate man, you cannot siphon water from an open vent cylinder.
The pressure in the bottom will be higher than the top by the height on the cylinder, and of course it's relevant, because the head is less at the top.
So if I run the shower hose up to within 20cm of the tank and drop back down you would expect to regain full pressure
It is not a closed system, it's open vented and a siphon will not work.
Once again you loose pressure by raising the rose higher, exactly the same as you loose pressure by running a pipe up to the loft, you cannot mysteriously get back what is lost, by dropping it down again.
Ok you agree the rose is an outlet, A copper pipe with a tap on the end is an outlet, a cylinder with a pipe in the top is an outlet, the copper pipe is no different to the flexible hose in a shower.
Your hose, bucket, siphon example is pathetic from a desperate man, you cannot siphon water from an open vent cylinder.
The pressure in the bottom will be higher than the top by the height on the cylinder, and of course it's relevant, because the head is less at the top.
So if I run the shower hose up to within 20cm of the tank and drop back down you would expect to regain full pressure
It is not a closed system, it's open vented and a siphon will not work.
Once again you loose pressure by raising the rose higher, exactly the same as you loose pressure by running a pipe up to the loft, you cannot mysteriously get back what is lost, by dropping it down again.
I'm not sure if you're being deliberately thick or just don't understand
Pot calling kettle black springs to mind.
No, a cylinder with a copper pipe coming out the top is not an outlet. The outlet is the open end of the copper pipe attached to that cylinder, wherever it might be. The cylinder itself is merely a pressure vessel, and once again, is NO DIFFERENT to a length of large diameter pipe, for the sake of this argument.
The only reason you cannot siphon water is because if you were to try and remove the top of the cylinder to place a pipe in there to siphon from, water would be pouring out due to the CWS being higher than the hole you'd just made in the tank. However, the fundamentals of how water travels through a pipe are still the same. The siphon is able to work because the water molecules are cohesive, as water exits the siphon at the end of the pipe, more water is drawn along behind it. The same applies if you raise the pipe exiting the HW cylinder into the loft, and then back down again.
And if you drop the pipe exiting the top of the cylinder back down to the same level as the bottom of the cylinder, the pressure in the bottom of the cylinder and the bottom of that pipe will be equal, as long as the CWS tank water level is greater in height than the draw off point on the HW tank.
So if I run the shower hose up to within 20cm of the tank and drop back down you would expect to regain full pressure
Yes, it will, and I will prove this to you shortly.
My closed system comment referred to the pipework, not the system as a whole, although admittedly in hindsight this probably did appear confusing to you.
So, I've just conducted my own (very crude) test, the results of which are pretty incontrovertible. For the sake of simplicity I have omitted the HW cylinder, and am concentrating purely on your basin example, in the hope that this way, you will not accuse me of any bias.
The setup consists of a funnel, taking the place of the CWS tank and being continuously filled to a set level by a tap, acting as the filler valve. A length fixed of hose represents your pipe run, and the end of the hose is an outlet.
The two pictures below show the control test, with the pipe running downwards only from the CWS tank, and the resulting flow at the outlet:
View media item 14026
View media item 14027
The next picture shows the setup with the pipe looping back to a height just below the water level in the CWS tank (which, though it's hard to see, is at the rim approximately in the middle of the funnel). Also shown is the resulting flow from the outlet.
View media item 14028
View media item 14029
Well low and behold, despite the head being approximately 5cm (by your method of determining it, anyway), the flow rate is the same as when the pipe dropped straight down!
Just to prove I'm not fooling you, here is the flow rate when the OUTLET is placed approximately 5cm below the level in the CWS tank, giving an ACTUAL 5cm head:
View media item 14030
As you can see, this time, the flow rate (and hence, dynamic pressure, by assumption) are significantly lower!
Now, what utter rubbish are you going to come up with to prove me wrong this time?
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