Reply With QuoteHot water expands. It builds up pressure in the HWS tank. When you open the tap the pressure gradually decreases back to mains water pressure
Why do some hot water taps do this?
There's got to be some explanation of the physics behind this. In a mains pressure hot water system house, when you turn on the hot water tap, the pressure of the water coming out of the tap often decreases. Why is that?
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Hot water expands. It builds up pressure in the HWS tank. When you open the tap the pressure gradually decreases back to mains water pressure
Reality is an invention of my imagination.
ಠ_ಠ
osci (27-05-10)
as mentioned because the water is hot, when the water heats up this heats up all the irons and minerals in the water and as such creating more friction.
to see what's in your water lift the lid on your toilet system and you will see the deposit of the minerals and irons and slime
dont say linux if i wanted it id install it
Depends on the grime!
G'Day Cobbers,Originally Posted by Fernbay
Whilst I cannot disagree that the above is a factor, there are others to consider.
The first that springs to mind, is the tap washer, particularly the leather/ fibre type. They swell with heat.
The small increase in volume, 0.05%, from 0 to 100 degrees C, is lost on turn on before the hot water reaches the tap.
Since the change in temperature/volume between successive uses of the tap would be very much less than that, it stands to reason, that there are other contributing factors.
I too have to re-adjust the mix when having a shower, but again, that is empirical.
Kindest Regards, " The Druid ".
flicker (28-05-10)
Why does that girl limp ?
Last edited by watchdog; 27-05-10 at 09:43 PM.
Sorry, couldn't resist...don't you mean poo LOL
Democracy & Ignorance = A Winning Combination
Pussies always do a Cat Walk.
Water does expand with heat.
A simple demo with a test tube when heated would show that the water will rise a measurable amount.
if you put a cork on the end, heat the water below boiling point, no steam, it will dislodge the stopper.
The very reason why you dont not never fiddle with the safety valve of a HWS !!
It has been known to dislodge the whole side of a house.
I stand unequivicably behind everything I say, I just dont ever remember saying it !!
you have full pressure at the tap befor you turn it on
once it is on you loose pressure due to friction ect
cause it's not a girl, it's a tranny.....
Ahh , I remember trannies. I had a Toneace 6 trannie in high school. Used to chew through the batteries.
It's not actually the water that expands but the miniscule amount of air.
Thanks guys, all sorts of good ideas & theories there.
I like the one of the hot water cylinder releasing some of its pressure once the hot water tap is turned on... I'm going to run with that one as the most likely reason.
Frictional losses increasing as deposits in the pipes heats up? Not too sure on that one myself, but then my initial thought was that the deposits in water pipes might expand as the hot water flows through them and thus decreases the inside pipe diameter (or at least the available cross section of area for water to flow through) - but this would cause a greater pressure, given a constant flow rate, surely?
In a mains pressure hot water system, a pressure build-up due to liquid expansion with the application of heat, will occur when the heat source is doing its thing. Ultimately, this is constrained by the yield point of the container, or in most cases, the temperature/pressure release valve on the hot water cylinder. This explains why water can be seen dripping fron the overflow pipe during, or just after, the heating phase when the thermostat has been triggered by an influx of cold water.
It should be noted this pressure increase can only be released by failure of the containment vessel, the temperature/pressure release valve, or by the action of turning a hot water tap on. This also explains why hot water taps can start dripping as the heating cycle commences, particularly as tap washers and seats wear.
The pressure increase can not be reflected back to the cold water inlet, as there is a one way valve on the cold water supply, for obvious reasons.
Now, should a hot water outlet be turned on when the cylinder has a higher than normal pressure, then, yes, there will be a surge of water from this point until the pressure buildup reduces back to the incoming mains, which is one explanation for the effect noted.
However, this does not explain the observed reduction of the hot water flow when the hot water tank has already had the initial higher pressure reduced, ie, by turning a hot water tap on elsewhere to the one you are noting the effect on, to remove the initial stored higher pressure.
In this case, it can be explained by the expansion of the tap washer, and to a lesser extent, the brass body of the tap as the water flowing through these components raises their temperature, and also to the practice of only partially opening the tap, which exacerbates the effect.
If you have a tap body with ceramic elements controlling the flow, since the coefficient of expansion of ceramics when heated is low, the effect of flow reduction with a rise in temperature of water is rarely seen. You may only notice a change in pitch of the sound of the water through the ceramic elements, as the water temperature increases.
This effect is not usually oberved on hot water systems using a header tank, as the hot water cylinder is open vented, and the hot water tap is usually opened fully anyway, to get enough water flowing out due to the much lower than mains pressure.
The expansion effect can be noted with instantaneous hot water services, however, the magnitude of this will be related to the set point of the temperature of the hot water.
Reminds me of a situation many years ago, working on a building site, when we use to hit the tinnies, (when they really were steel cans), in the afternoon on a hot day.
A couple of months after finishing a house, my plumber mate got a call from the new owner, complaining of his toilet flushing with hot water. Turned out the obvious had happened, and required just a little bit of rework to hook the pipes up correctly.
The hot flushes could have been caused by the onset of menopause - or in these days of political brainwashing, "personopause".
My pipes always expand when I get hot.
@beer4life: Ummm...Aussies don't use the term "cobber", "shazwaggle" or "sheila" - it's just a media stereotype - cheers.
Last edited by Arbiter; 26-06-10 at 06:43 PM.
Godzilla (27-06-10)
cobber : Former Australian greeting/address which no self-respecting Aussie has used for at least 40 years!
i still use sheila often, ( have a look at that big fat sheila )
dont know what a shazwaggle is, never heard it in my 73 yrs.
Bloodie immigrants!
Hey you,
You haven't been around as long as I.
I've been told what mates do. Pal, Buddie, well enough said.
Bill Gates brain washing methinks.
If you don't like Mick Dundee, Strop, Wally, Donk, Nugget and a Host of others calling you Cobber, I'll gladly refund you your 10 Pound assisted passage and gladly assist with your deportation.
And with a sprinkling of Ausie politeness.
Kindest Regards, " The Druid ".
Last edited by beer4life; 26-06-10 at 11:11 PM.
quote : @beer4life: Ummm...Aussies don't use the term "cobber", "shazwaggle" or "sheila" - it's just a media stereotype - cheers.
we dont? omg , i guess you dont live in the bush then cobber.
but i dont know what a shazwaggle is though. sounds like when you're driving an old truck and the chassis is moving on top of the old leaf springs, you turn the steerin wheel and wait a minute then it starts to turn....
getting back to the hot water question : we have a tank on a 15 foot stand to give some water pressure to the house out here. to stop the old hot water tank from pressurising up when heated from the woodstove its got a 'shepherds crook' , its a bit of black inch polypipe sticking up out of the roof above the height of the water tank. this releives the pressure build-up.
mains systems dont have this
cheers from the bush , 73's beer4l
member. Austech dog squad: if it can be fixed the dogs will sniff it out
beer4life (28-06-10)
Mains systems all have a Safety Valve fitted to the top of the storage tank where any steam may collect and be released safely.
I have a hot water donkey similar to yours and it is at atmospheric pressure hence the 'Blow Off' pipe.
Generaly you put the 'Shepards Crook' into the Header tank to recover any water pushed up the pipe when the system boils.
This is partly to save water particularly when your on tank water.
When I bought the sytem it actualy had pressure valves on both the Cold IN and the 'Blow Off' but I could not make it work so stupid altered the valve settings and split the water tank.
That took about 4 hours to remove the copper tank and braze the 3 foot long split and reinstall it.
So I made up a Header tank from the 15 gallon electic hot water tank the donkey replaced.
In summer I can boil the 40 gallon tank in around 50 minutes, in winter about 90 minutes.
All on a 4 gallon, 20 l bucket of coal.
Its been in use some 25 years now and I woul love not to be lighting fires but not at the price of Gas and Electricity today.
Some may remember reading an article several years ago where a similar Donkey system blew up and killed several junior Football players as it demolished a concrete block wall as they showered after a Football match.
My guess is that the hotwater/'Blowoff' pipe got blocked and when it was lit, the pressure had nowhere to go so it violently exploded.
Electric/Gas HWS can be equaly dangerous if the pressure isnt safely vented off via the Safety Valve.
Last edited by gordon_s1942; 28-06-10 at 11:38 PM.
I stand unequivicably behind everything I say
[QUOTE=
Electric/Gas HWS can be equaly dangerous if the pressure isnt safely vented off via the Safety Valve.[/QUOTE]
Mythbusters had a good show where they stop the pressure vent working and sent a HWC through a roof and quite a few feet into the air somewhere between 30 to 50ft from memory
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