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The building just installed some new pipes and since then I believe that the hot showers cannot last as long as they once did.

Is there a sub-optimal way to hook up a new cold water intake that would mean that cold water would mix prematurely with the hot water running to the bath or anything that might cause a lowering of the available hot water?

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possible duplicate of Did the plumber hook up my water heater backwards? –  longneck Jan 17 at 15:39
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Installing a smaller tank, or hooking it up backwards. –  Tester101 Jan 17 at 15:40
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@longneck This is not a dup because the OP's asking specifically about cold water mixing. –  Edwin Jan 17 at 19:22
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2 Answers

There is a way that cold water would mix prematurely with the hot water. This could happen if there is something wrong with the cold water dip tube. The cold water dip tube takes the cold water from the connection at the top of the tank to the bottom of the tank where it is heated. If the dip tube becomes faulty or is removed, the hot water from the tap will run out quickly.

Failure of dip tubes is common, especially with water heaters from the mid 90's. There is also the outside unlikely chance that it was removed when replacing the pipes.

Of course, you get the same effect if the hot and cold connections are reversed.

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Yes. There is definitely a right, and wrong way to connect the plumbing to a water heater. On most modern water heaters, the cold inlet is marked with a blue marker, while the hot outlet is marked with a red marker.

Side Notes:
Not sure why blue means cold and red means hot... Possibly because metal tends to glow red when it's hot, and glacial ice is often blue?

Some people get angry when you call a water heater a "hot water heater". The truth is, sometimes the water heater does indeed heat hot water. So to call it a "hot water heater", is not completely incorrect. This is especially true, given the fact that hot is a relative term. As long as the water is hotter than the "cold" water coming in, it can be thought of as hot in comparison to the cold water.

The term "hot water heater" probably came from combining "hot water tank", and "water heater". Which at one point in time, may have been two different parts of the same system. The water heater would heat the water, and then store it in the hot water tank. At some point the heater was combine with the tank itself, and the names naturally combine as well.

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