Apartment shower temperature changing abruptly

Here's the story: Pre-war 14 story NYC apartment building Apartment on 3rd floor
Shower has two knobs (hot & cold), two similar knobs below for the bath. Pl umbing in the apartment is original (this apt, can't speak for the folks up stairs or downstairs).
Turn on shower (mostly hot, a bit of cold water) and it's fine for a minute or so then becomes COLD. Not cooling off, but COLD. Eventually it gets bac k to normal but sometimes repeats itself. Not sure if this happens only if the cold is on or if would happen with only the hot water on (that'd be too hot to shower in and it's not my apartment so I haven't had time to experi ment).
The sink does not have this problem.
Is it possible that somebody upstairs redid their plumbing and some mixing valve is sending cold water down the hot water pipe?
Any other ideas?
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Here's the story: Pre-war 14 story NYC apartment building Apartment on 3rd floor
Shower has two knobs (hot & cold), two similar knobs below for the bath. Plumbing in the apartment is original (this apt, can't speak for the folks upstairs or downstairs).
Turn on shower (mostly hot, a bit of cold water) and it's fine for a minute or so then becomes COLD. Not cooling off, but COLD. Eventually it gets back to normal but sometimes repeats itself. Not sure if this happens only if the cold is on or if would happen with only the hot water on (that'd be too hot to shower in and it's not my apartment so I haven't had time to experiment).
The sink does not have this problem.
Is it possible that somebody upstairs redid their plumbing and some mixing valve is sending cold water down the hot water pipe?
Any other ideas?
== Does this piping arrangement look reasonable?
http://imageshack.com/a/img585/9852/rz5i.gif
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On Wed, 15 Jan 2014 17:55:53 -0800 (PST), snipped-for-privacy@brainchampagne.com wrote:

What street, between what two avenues?
None of the discussion of NY water that I've read has ever brought up specific location within the city, but I don't see how it could not be very relevant.
I lived for one year on the 4th floor of 420 Clinton Ave. in Brooklyn, and for 10 years on the 5th floor of the same six-story building.
I had no water problems on the fourth floor, but did on the fifth, not exacctly like yours, but might be related. Well, don't get your hopes up. As I write, my expectation gets lower, so just consider this a novel, and not as help.
Before you get to the novel, what happens when you flush the toilet while the shower is running? The building uses flushometers, right? Not tank toilets? Does it change the temp of the water?
The novel starts here:
After about 4 years, the widowed owner, who had probably helped run the building when her husband was alive, didn't want the burden anymore, or wanted to move to Florida, and she sold it to a Greek immigrant, a plumber!
I thought that meant one of two things. We'd get any plumbing problems solved quickly and well because he was a plumber. OR We'd get plumbing problems solved last, because he always wanted to work for paying customers. But as I've said, there is always a 3rd possibility.
The 3rd was that he thought he was a plumber but he was no good at it, at least he didn't understand plumbing over 4 stories. (nor could he fix the oil pipe that went to the oil furnace, and it dripped for years, I think.)
Clinton Ave. goes to the top of Clinton Hill, and it's really a hill so it's that much higher than a building only a few feet higher than the river or harbor.
City water pressure is supposed to be able to push the water up to the 5th floor, and if the building is 6 floors, like ours was, there is an water/air tank in the basement. A water pump forces water into the bottom of the tank, compressing the air inside, until the pressure is enough to turn off the water pump. Then the air forces the water to even to the 5th floor, until the air pressure drops and the water pump goes on again.
Eventually air in the top of the tank is absorbed by the water in the bottom, and some other gauge turns on the air pump which runs fof a while.
Probably because the new owner was confused by all this, he turned off the air pump completely and let the water pump run constantly. Using I believe a lot more electricity than needed, and I think respsonsible for my shower problems. That was, if the mixed water was not TOO hot to begin with, when someone in the same line and perhaps the same bathroom within that line flushed the toilet the water then became too cold to stand under. If we had had tank toilets, refilling the tank would have taken only a little water at a time, but since it was a flushometer, it used a lot to flush, and he ddin't know how to maintain pressure. . The tub was big enough to move, but I was cold already. Eventually I switched to baths, in my great big 1930 luxury building bathtub, so big I could float with only 1 square inch of my skin touching the tub.
I found a book in the libary that devoted one page to explaining how it was supposed to work, complete with a diagram, and I gave him a copy, but it made no difference.
He wasnt' really a plumber it turned out. He only knew how to install trash compactors using water pressure to power them. IOW, all he knew was how to screw pipes together.

Doesnt' seem likely. Are these co-ops? If so, anything is possible. Someone with a co-op even hired me as an electrician, if you can believe that.

Flushing toilets. Maybe someone goes to the toilet a lot, maybe every 10 minutes, (some medical problems can cause this) and that's why he always does so a minute after you start a shower. Maybe the sound of running water in your shower makes him want to go to the bathroom. (Although in my apt. I could never hear water running upstairs, from anything. )
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On Wednesday, January 15, 2014 10:53:39 PM UTC-5, micky wrote:

Plumbing in the apartment is original (this apt, can't speak for the folks upstairs or downstairs).

ute or so then becomes COLD. Not cooling off, but COLD. Eventually it gets back to normal but sometimes repeats itself. Not sure if this happens only if the cold is on or if would happen with only the hot water on (that'd be too hot to shower in and it's not my apartment so I haven't had time to exp eriment).

ng valve is sending cold water down the hot water pipe?

One place I would start is to determine if this problem happens all the time or not. If it's related to what someone else has turned on, etc then you would think it would happen frequently at say 6 -8AM, probably less frequently during the day, and happen far less of the time at say 2 - 4AM. If it happens about the same without regard to time, then it's probably not related to other water usage. Next I'd ask the neighbors if they have the same problem. And then if it affects others, I'd make a joint complaint to the landlord, manager if it's a co-op, etc.
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On 1/15/2014 8:55 PM, snipped-for-privacy@brainchampagne.com wrote:

I live in a trailer, in a park. I have some what the same problem, after a couple seconds. Have to set the hot faucet, and a couple second later, turn the faucet open a bit more. I've always thought it was just the faucet washer expanding when the hot water hits.
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wrote:

bath. Plumbing in the apartment is original (this apt, can't speak for the folks upstairs or downstairs).

minute or so then becomes COLD. Not cooling off, but COLD. Eventually it gets back to normal but sometimes repeats itself. Not sure if this happens only if the cold is on or if would happen with only the hot water on (that'd be too hot to shower in and it's not my apartment so I haven't had time to experiment).

mixing valve is sending cold water down the hot water pipe?

I've always wondered about that myself. Well, not always. Just now I wondered if that girl with a wrench in the picture was really a girl.... I'm hoping so anyway.
Anyway, long story short... this feller has the same idea as you...
http://www.straightdope.com/columns/read/583/why-does-the-hot-water-slow- to-a-trickle-after-initially-blasting-out
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