If you are running hot water through a pipe, it will heat things up.
That's how they do radiant heating.
I know where the hot water line runs under my slab and along the entire line I can feel the carpet/tile warm. The warm "spot" is not exactly a spot. It is a straight line about 3ft in width that runs exactly above the hot water line, which is about a foot under the slab. I had a professional plumber come in and check the situation. We shut off all the water and the water meter did not move. He also used a listening device along the warm line and he assures me that there is no leak. The hot water line is copper and is under a 3 inch concrete slab and a foot of dirt. Is it possible that the heat radiating from the copper pipe is heating my carpet? I noticed this 3 weeks ago and it has been warm in the exact same spots (not expanding), but that doesn't mean it hasn't been like this for longer.
I see your point. I just though that with 1 foot of dirt and 3 inch of concrete would be enough to insulated. Also, I have no idea how hot the copper pipe actually gets. Does it get hot enough to do what I've been seeing?
Do you have a recirculation system? If not, then the pipe will cool off between uses. Only when you are running water a lot might you notice it heating things up much. The much more common thing is if it is leaking, but if the water meter isn't turning, that sort of lets that off the hook.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014
Back in my fraternity days we had a very warm spot appear during midwinter on the floor in the dining hall. It got worse and the water heating bill went through the roof and water appeared from under the slab...so out came the jack hammer and the shovels. We had a lovely hot spring down there. As we were shoveling out sand, up popped a plastic sail boat--the last brothers who had this leak had a sense of humor. Leak and floor were repaired, the sail boat was buried in the hole again and all was good...although looking back on it, we should have changed the piping configuration since the leak was at an elbow at the end of a very long run. Should have gone to an enlarged diameter and heavy fitting there so that it couldn't erode or fatigue so easily in the future.
If the pipe is bare in the ground or in the slab it will conduct heat quite well. Concrete is a very poor insulator and earth isn't great either. Bet it takes awhile to get hot water out of the other end of the line...
An easy test for this would be to turn off the hot water tank shut off and leave it for 5 mins then turn it back on while listening for any water flow as you open it. If it does make the sound of running water you have a leak. Also make sure no taps are dripping.
The only times I have seen a concrete floor heated by a domestic hot water line were when it was either embedded in the concrete or was leaking. IT would have to be VERY hot, and continuously hot, to heat the floor with that much dirt and concrete over it.
Licensed residential and commercial plumber
I have a customer with an irrigation system leak right now.....it leaks somtimes and somtimes it doesn't. I suspect a hairline crack in the pvc and leaks when the pipe expands and stops when the pipe contracts or gets alittle crud in it. Now thats based off the meter.....sometimes a very very small leak will not show on the meter. I verified that from my own experience and also with the water departement.....older meters especially so.