http://www.straightdope.com/classics/a2_394.html
The same thing happens in my dorm hall (on the 6th floor), but with cold water as well. So it wouldn’t be expanding washers.
Actually, according to thermal expansion, when a washer is heated, the hole increases in size as well (i.e., it behaves the same as if there were metal there). So what gives?
Oh, and hi. 
Welcome to the boards.
Water flows around the outside, not through the inside hole, of the washer. Look at the diagram:
I don’t buy this explanation at all.
My guess is that there is a restriction somewhere in the line like a partially closed valve somewhere, build up in the pipe restricting the flow, or something like that.
When the system is at rest pressure has a chance to build up over time and you get an inital “blast” of water that looks like normal flow. As soon as that inital pressure is relieved you get the restricted/reduced flow you are experiencing.
Expanding washers…I vote no on that idea.
John F
Sounds like the house I lived in with a bunch of friends way back when. The problem there was that no one had used the water softener for years, allowing a lime build-up in the pipes.
Basic fluid dynamics.
In a static sytem, i.e., the water is not flowing, the pressure is the same at all points in the system (I’m ignoring gravity)
As soon as you open the faucet, the system is dynamic, not static and there is a pressure difference. it’s what makes the water flow.
It takes energy or work to move the water through the pipe and the smaller the pipe diameter, the harder it is to force the water through. Some of the energy (or pressure) put into the pipe at one end is lost to this work, hence there is less pressure and the water trickles out.
So before you open the faucet, the pressure is at it’s max. It gushes out and once the water starts flowing the pressure drops and it slows down.
This is true for all the faucets in your house. Try them, for some the pressure drop is not as significant because the pipes are bigger, closer to the pressure source or both.
If your on the 2nd floor, it’s even worse. The water has to be pushed up, and hence even more energy/pressure is lost.
Why? On what basis? Makes perfect sense to me. Your solution fails because I’ve seen this effect completely choke off the flow of water to nothing.
I have fixed my shower of the exact problem by replacing the washer on the hot water tap with a non expanding washer.
For starters could I buy a set of expanding washers and non expanding in the same size to test out the theory?
They do actually exist right?
Can they be purchased from one source or does somebody specialize in expanding / non expanding versions?
If the system has an element of elasticity in its components and includes non-return valves, the pressure (of parts of it) when at rest may actually be greater than the mains supply pressure.
Anyway… both the hot and cold taps (which are separate, not a mixer device) in my bathroom display this behaviour of slowing to a trickle; given that the ambient room temperature is somewhere above the cold water supply temperature and below the hot water supply temperature, how can it be that a thermal expansion effect causes the same phenomenon in both taps?
I suspect thermal expansion does have some kind of effect, in some cases, but it can’t be the whole story.