hot water runs out upstairs - not downstairs


i have a 3 story little house with 2 bathrooms, one on the 2nd floor, one on the top floor. the hot water comes from a 40 gallon gas hot- water-heater in the basement. i cannot even get enough hot water to fill a bath on the 3rd floor. after about 6 inches the water goes cold. However, when i go downstairs to the kitchen on the first floor, there seems to be plenty of hot water. There doesn't seem to be any sediment at the bottom of the tub so i don' think the pipes are blocked but i guess i could be wrong. can anyone point me in the right direction?
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dwn_plt wrote:
Does this only happen in the tub, or is it true in the lav too?
If it is only in the tub, what type of tub faucet is it?
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happens in the lav as well - the faucet is a kohler, i've checked the scald protector and it's on max (no scald protection).
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dwn_plt wrote:

This will require some detective work. There's a cross-connection somewhere between the downstairs and upstairs that's drawing water from some cold pipe into the hot water pipes of the upstairs. What's unusual is that it's not happening downstairs, but stranger things have happened.
First of all, go to your washing machine, and shut off the cold water to the WM where the hoses connect. Then see if the problem is solved.
If it's not, then go to the upstairs sink, and shut off the cold water at the shutoff valve underneath. Then see if the prolem in the shower is solved.
Do the same test to each fixture you can (even the downstairs fixtures).
The problem might also be in the water heater, but if it were, you'd see it happening downstairs. Are you certain it's not happening downstairs too?
Also, did this problem suddenly appear?
Mike
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