Hi,
We live in south coast British Columbia, Canada (moderate climate) in an older (1955) one level, flat roof 900 sq. Foot house on a concrete pad. All of our water pipes have been installed under (or in) the concrete.. . And we have a leak, somewhere in the SUPPLy lines. We hear running water - a "hissing" sound - somewhere and think it's in the conrete/under the house as there is no visible water running anywhere else where it should not be.
To stop the flow - and waste of water - we have been turning the water off at the main incoming supply when not needing it - the hissing (leak/running water sound) then stops.
The curious thing is this: when we do turn the main water supply back on to have a shower, wash dishes etc. We only have about one tank (40 gal.) of hot water, then it all goes cold. Pressure is good all around. We then have to turn the main supply line OFF again for a few hours to get hot water again. (the only way we have hot water is to turn the main supply off!)
So my question is: where is the leak? If the leak is somewhere between the water tank and the faucets would the hot water not run out of the tank when the main supply is turned OFF? Is there some kind of pressure valve that keeps the water IN the tank until a faucet - shower etc - is opened?

Sounds like you have several issues.
Let's address the leak. It sounds like you have a leak under the slab. You have options hers.
1. You can call in a leak detection service , such as Slueth, to pinpoint the location and then a plumber to repair it. N **OR**
2. You can do what vI did when faced with the same problem. You can cut out the entire hot or cold line under the slab and repipe with plastic. This can be installed in the attic and dropped down in the walls to pick up your fixtures.
Is your water heater gas or electric? Before I get into the water heater I have to know what line's leaking. The hot or cold. Let me know, Tom

Do you have a water meter, does it have a small diamond shaped pointer in the center. That pointer will spin if you have a leak and the main is shut off.

Hi ballengerb1,

No water meter. .

Thanks speedball1,

Pretty sure hot line is leaking because we have hot spots under the floor.. .
Electric heater
Thinking about re-piping the whole house with plastic pipe
But.. . still befuddled why we only have hot water when the main is shut off?
Could the fix be something a lot less costly than an entire re-pipe

You only have hot water when the water is turned ON.

You only get hot water out of the water heater when you have cold water coming in to push it out. Obviously because you have a leak and are turning the main valve off, you are concentrating you hot water usage to a very narrow period. You are using all of the hot water in the tank. It simply takes time to heat 40 gallons of water from what ever temperature it comes in to 120 degrees. You could have a defective heating element that is extending that time. If you don't have hot water when the main valve is left on, it is because the hot water is being pushed out as fast as it is being heated up. Just as if you left a hot water faucet open all the time.

If you feel warm spots in the floor the leak is probably in a hot water line. You can turn off the stop valve at the cold water input to the water heater. If the hissing stops the leak is in the hot water line. As a temporary measure you could open and close the input valve to the water heater as you need hot water. You would have cold water all the time.

If the leak is in the hot water line, you may or may not be able to re-pipe the hot water lines through the walls of the house.

Thanks hkstroud,

Defective heating element makes sense, because now only one person can get a decent shower whereas before the leak we could get two showers plus a load of dishes in before the hot water ran out.. .

What I still don't get though is why, when the main supply is turned off, if there is a leak, a hole somewhere in the hot water line, why does the hot water not leak out of the water tank and out through the leak, therefore leaving no water in the tank at all? Something is keeping water in the tank despite a leak in the line. Is it some kind of pressure valve that only opens when a faucet is opened?

Because the hot water comes out of the top of the tank. If can only come out if there is cold water coming in to push it out. Assuming you have a leak in the hot water pipe under the floor, some of the water in the pipe will leak out. But only some. Nature abhors a vacuum. Most of the water in the pipe will not leak out, even if it is down hill. It can't leak out unless air can get in to replace it.

Therefore, turning off the stop valve on the cold water input to the water heater will stop the leaking and that should be a lot more convenient than turning off all the water. At least you can flush the toilet.

Thanks harold
Now it makes sense

Some graphics for you.
(I had nothing else to do)