We've been in the house for 9 years. It is a townhouse, and is about
20-25 years old. Last night, I noticed a good deal of heat radiating
from a small area of the on-grade concrete slab. As far as I know, I do
not have radiant heat (but I suppose anything is possible, especially
considering the way things were done in these houses). The area is
about 8-10 feet square, and extends from a tiny powder room to the
adjacent laundry room. The powder room has tile on the concrete, the
laundry room is bare concrete. Overall, the slab is roughly 15 x 50.

The hottest spot seems to be in the area close to where the powder room
sink drain goes into the floor. The drain pipe itself is not hot, just
ever so slightly warm from conducting the heat up from the floor. It's
barely noticeable, actually. The floor, however, is VERY warm to the
touch- I would estimate somewhere between 110-120 degrees. I felt the
heat even through a bathroom rug (and the tile)! I put a thermometer on
the floor at what felt to be the warmest spot, and the air temperature
at the floor was about 15 degrees higher than the rest of the house (86
vs 71).

The further you get from this area, the less warm the floor. I do not
see any pipes (other than the drain) going to the floor. There is a
drain in the laundry room for the washing machine. I do not know what
path the drain takes once it is under the slab. The main drain stack is
on the opposite wall. The main stack pipe did not feel warm.

I thought I heard the sound of water running in a pipe. This sound was
coming from the wall nearest to the warm spot (wall is shared with my
next-door neighbor). I shut off everything in the house that made
noise, and put my ear to the wall. The sound was defintely there, sort
of a hiss. I next looked at my water meter, and saw that the tiny white
spinner on top was turning. There was no water running in the house
that I know of.

I checked with the neighbor, and there is nothing odd going on there.

We have heat pump heat, and the coil is located on the wall opposite
from the hot spot. No smoke or odd smells.

I am stumped, but I have one possible theory, which is that I must have
a small section of radiant heat under the powder room that I have no
control over. This wouldn't totally surprise me, as for example, I have
no control over my porch light. The porch light is on a light sensor,
and there is no switch- just the breaker. So, anything is possible in
this place. They did some really odd things.

Any ideas of what is going on here? Thanks.