Seeing as how we have a forum of experts on the kinds of issues that come
up in older buildings, let me describe the really annoying problem we're
having with our shower.  I don't even know how I would describe it to a
plumber without getting as long-winded as this.

The setting: we live in the lower flat of a two-flat house built in 1964.
Each flat has its own water heater, a kitchen with double sink and
built-in dishwasher, and in real estate parlance, one-and-a-half baths,
which is to say, one small room with a toilet and sink, and another larger
room with a toilet, sink, and bathtub/shower.

The bathtub is surrounded by ceramic tile walls on three sides (we use a
shower curtain for the remaining side).  The wall at the head
of the tub has the two valves for hot and cold water, and the single, um,
spout between them where the water comes out.  Near the tip of the spout
is a knob which can be pulled up to redirect the water flow to the shower
head, which is mounted much higher on the same wall.  A very ordinary,
typical installation.

Right behind that wall, by the way, though not directly accessible, is the
alcove that contains the hot water heater.

Normally, the person desiring to take a shower turns on and adjusts the
hot and cold valves, until the water flowing from the spout approximates
the desired temperature.  Then, the handle on the spout is pulled up, and
water begins to spray from the shower head.  The person then steps into
the bathtub/shower stall and engages in normal self-cleaning activity.

We have lived in this place for over six years now, and until recently had
no problem with the shower.

However, just in the last few months or so, the following happens:

At some point, anywhere from 30 seconds to several minutes from when the
water is turned on, a "thump" is heard from inside the wall at the head of
the shower.  The sound is not very loud, but it reverberates a little.
Simultaneously, the cold water pressure drops dramatically, and the
remaining water coming through the shower head is, by default, very hot.

At this point, you have to try to avoid the spray from the shower head
while rapidly turning up the cold water.  Once a good temperature is again
achieved, there is no further problem for the rest of the shower.

The "thump" and drop in cold water pressure happens exactly once with
EVERY shower.  All that varies is the timing.  When you're lucky, it
happens before you step into it, so you can adjust the temperature then
and shower without fear.  Turning up the cold water very high at the
outset seems to delay but not prevent the "thump".

Any ideas?  What is happening here?

(Additional notes: Janice thinks she may remember a time when the "thump"
happened more than once in the same shower.  Also, it seems possible that
the problem started happening right after the ceramic tile on one side of
the tub/shower was torn out and replaced, though the head wall with the
faucets and shower head was not disturbed in that project.)

---
Lawrence Kestenbaum, [log in to unmask]