One for the plumbers [Archive]

View Full Version : One for the plumbers

Daddles

16th March 2006, 11:40 PM

Here's a question for you.

You turn on the hot water tap.
Cold water rushes out (coz it's cooled while waiting).
Then, when the hot water finally arrives, the pressure drops away dramatically.

Why?

Richard:confused:

Ashore

17th March 2006, 12:36 AM

You turn on the hot water tap.
Cold water rushes out (coz it's cooled while waiting).
Then, when the hot water finally arrives, the pressure drops away dramatically.
Why?

The hot water leaves the tank at temp X taking up volume Y
buy the time it reaches the tap the temp has dropped to X-m
thus the volume of the water has dropped to Y-m

Or the volume of the water is dependant on the temp and the pressure it is under ,if the temp drops as it travels through the pipe pushing cold water before it then its volume changes as does its pressure

Or when you turn the tap on the pressure that has built up in the hot water tank as the water heats up and expandes pushes the cold water out of the tap at a higher than mains pressure but as the tank refills with cold water at mains pressure the pressure in the tank drops back to mains pressure as it does at the tap

Or as water can't be compressed then air trapped in the system compresses causing a pressure drop

Or the hot water entering the system from the tank heats the cold water in the pipe causing it to heat and expand and temporally increase the pressure at the tap until all the cold is expelled and constant temp and pressure are achieved at the tap

OrThe first onrush of water is due to turbulant flow in the pipes and as the hot water comes through laminal flow is achieved and pressure drops but flow increases

Or as the hot water comes through it heats the pipe causing it to expand and as the pipe expands and allowes more water through the pressuer drops

Or.......:rolleyes:

journeyman Mick

17th March 2006, 01:03 AM

The hot water leaves the tank at temp X taking up volume Y
buy the time it reaches the tap the temp has dropped to X-m
thus the volume of the water has dropped to Y-m

Or the volume of the water is dependant on the temp and the pressure it is under ,if the temp drops as it travels through the pipe pushing cold water before it then its volume changes as does its pressure

Or when you turn the tap on the pressure that has built up in the hot water tank as the water heats up and expandes pushes the cold water out of the tap at a higher than mains pressure but as the tank refills with cold water at mains pressure the pressure in the tank drops back to mains pressure as it does at the tap

Or as water can't be compressed then air trapped in the system compresses causing a pressure drop

Or the hot water entering the system from the tank heats the cold water in the pipe causing it to heat and expand and temporally increase the pressure at the tap until all the cold is expelled and constant temp and pressure are achieved at the tap

OrThe first onrush of water is due to turbulant flow in the pipes and as the hot water comes through laminal flow is achieved and pressure drops but flow increases

Or as the hot water comes through it heats the pipe causing it to expand and as the pipe expands and allowes more water through the pressuer drops

Or.......:rolleyes:

Or the hot water system gremlins are just doing it to mess with your head :D

Mick

Schtoo

17th March 2006, 02:55 AM

Richard, it's because your sig line is a lie.

Yes, you are mad. Isn't he? :D

And no, I don't know why your hot water is playing games with you.

If it was a power point, maybe... ;)

Termite

17th March 2006, 06:35 AM

Simple explanation.
When you turn on a tap the valve actually only opens a suprisingly small amount. When the hot water comes through the heat expands the metal, and because the stem of the valve is only thin it expands very quickly and reduces the valve opening, so you get less flow. :D

Iain

17th March 2006, 07:44 AM

It's the contemplation valve, it detects when you have finished pondering on the meaning of life and chucks a new obstacle in your path.

silentC

17th March 2006, 08:13 AM

Because hot water is thicker than cold water. I thought everyone knew that.

Daddles

17th March 2006, 09:25 AM

Silly buggers. While I'm enjoying the humour, I would actually like some sort of explanation for this one:D

Richard

Felder

17th March 2006, 09:31 AM

Or as the hot water comes through it heats the pipe causing it to expand and as the pipe expands and allowes more water through the pressuer drops

I always thought it was this one.

The water that sits in the pipes is allowed to become cold, and hence the pipes also become cold and contract to the same diameter as a cold water pipe. When the hot water comes through, it expands the pipe and the pressure drops.....

But what would I know? I ain't a plumber!:D And if I was, I would charge you an exorbitant fee for my advice....:D

Iain

17th March 2006, 09:38 AM

My brother retired from plumbing at age 35 with chronic back problems, we always reckoned it was because he carried his wallet in his back pocket:D

doug the slug

17th March 2006, 03:11 PM

Silly buggers. While I'm enjoying the humour, I would actually like some sort of explanation for this one:D

Richard

then why didnt you post it on a plumbing forum, this is the silly buggars forumhttp://www.woodworkforums.ubeaut.com.au/images/icons/icon10.gifhttp://www.woodworkforums.ubeaut.com.au/images/icons/icon10.gifhttp://www.woodworkforums.ubeaut.com.au/images/icons/icon10.gifhttp://www.woodworkforums.ubeaut.com.au/images/icons/icon10.gifhttp://www.woodworkforums.ubeaut.com.au/images/icons/icon10.gif

Anyway, i thought termite's solution was the right one, at least thats what i've always put it down to

Wongo

17th March 2006, 03:36 PM

Wrong wrong wrong.

Hot water goes into the pipe and the cold water says "hot hot hot" and runs as fast as possible to get out.:D

chrisp

17th March 2006, 03:56 PM

Anyway, i thought termite's solution was the right one, at least thats what i've always put it down to

I'm not too sure about the thermal effect... I recall a calculus problem that asked the question - if you had a metal disk with a very small hole in the middle and you heat this disk does the hole get smaller or bigger? (i.e. do to walls of the "thick pipe" expand and close the hole?)

The answer is that the hole gets bigger.

I suspect the initial surge to due to the stored pressure in the hot water system (the water expands more than the water as it heats). When the pressure is released the gate valve on the inlet opens - but I've noticed the gate valve can stick a bit (due to the high pressure?) and you can sometimes notice a short pause (or slowing) in water flow before the normal flow begins.

If your hot water is very slow, you might want to consider replacing the pressure reducer on the inlet to the HWS. These are prone to clogging in hard water areas.

Chris (who isn't a plumber)

Skew ChiDAMN!!

17th March 2006, 08:03 PM

Also depends on the type of hot-water system.

We've one o' those gas "insta-heat" jobs that uses a thermostat-operated valve on the outlet to regulate the output temperature. When the tap is off, the water pressure equalises through the line until the tap's at mains pressure. Turn the unit on, cold water comes out quickly at first but pressure soon drops off as the thermostat partially closes the valve to "quick-heat" the cold water in the system.

The thermostat'll open slightly again once it's all up to operating temperature, so tap pressure improves slightly over time but it isn't readily noticeable unless, like us, you've got a really sensitive cold-water tap that you've gotta constantly fiddle with to find the sweet spot between scalding and chillblains... :rolleyes:

bsrlee

17th March 2006, 08:36 PM

You obviously have a mains pressure water heater - this does not occur with a gravity feed 'off peak' water heater - like I've got:D :D :D :D

Firstly, a mains heater pressurises to the same pressure as is in the mains supply pipe, then closes its inlet valve & heats the water inside to 170F or whatever. When you turn the tap on, the cold water standing in the pipe is replaced by hot water coming from the reserve & pressure in the heater starts to drop. If you are just doing a sink of washing up there is no problem, but if you are having a shower more water is required. The inlet valve opens when pressure in the reserve tank has dropped 'enough' - depends on the setting of the valve - and water enters the reserve tank and starts the cycle again. A key indicator to this type of water heater is that the temperature drops with time also.

There is a second type of mains heater - the 'flash' heater. In this type of heater there is either no or a small reserve tank. When you start the hot tap water flows thru' a small diameter pipe which goes thru' some type of fast heating element - electric or gas flame - that heats the water on the go - but flow is limited by the small diameter pipe needed to ensure fast heating - heat levels stay the same but volume rapidly drops. These are small units often used in flats/units and are commonly found as 'fast boil' water heaters in industrial kitchens/meal rooms.

taswegian

2nd July 2006, 09:22 PM

my totally non expert understanding is that it has to do with the type of washers - the older fibre type swell up with the warmer water and restrict the flow, and the newer ones made of ?? dont. I dont know if this is exactly true, but I can say that this phenomenon me off no end till I took myself off to the plumbing shop asked for top quality washers and replaced the buggers (in 6month old taps) and have not had that trouble since.

savage

3rd July 2006, 10:55 PM