On occasion, when I open the cold water tap on the kitchen sink, the flow from the faucet briefly becomes warm: it starts cool, becomes warm for about 30 seconds, and then returns to the desired cold temperature.
I have only observed this to happen early in the morning, when the plumbing has not been in use all night. It seems to happen no more than once in a day. What could be the reason for this?
Some hot water recirculation systems use the cold water as a return. In these systems, warm water is returned in the cold water pipe until hot water reaches the the temperature valve that controls the recirculation. Or, perhaps the one-way valve near the pump is failing.
Hot water pipes could be touching a cold water pipe (probably near the water heater). The two pipes touching would heat up the cold water pipe.
Cold water pipes could be touching a ventilation duct. In the winter, the duct would be warm and would heat up the cold water.
No heat trap in the water heater inlet. Hot water might flow up out of the water heater into the cold water pipes.
In the morning, try to feel which parts of the cold water pipe are hot, and that'll be your answer.
Another thing to check, but is likely unrelated, is the condition of a water expansion tank (generally mounted near the water heater) if you have one, or contemplate installing one if you do not (they're needed if you have a pressure reducing valve or a one-way valve in your water meter).
Mixing valves can cause this, a typical culprit can often be found in a washing machine or also a single handle plumbing fixture, they malfunction and allow the hot to pressure into the cold line. That is if you get hot water constantly out of the cold side, the question of why is it warm at first is pretty normal stuff, basic physics.
Hot water in lines causes the lines (especially copper lines) to heat up. In some faucets the hot water supply gets warm enough to heat the whole body of the faucet, including the cold water lines in the immediate vicinity of the faucet. This warms the cold water nearest the faucet, causing it to be warm for a short while. Once some cold water flows through the line, it eliminates this warming effect.
I have used recirculating couplers, without a pump, in two of my houses; relying on convection only to keep the water hot at the tap, which has worked quite well. They have the same effect as what you have stated. I would say that if you get almost instant hot water out of the hot tap and warm water out of the cold tap, which lasts a short while, then you may have a recirculating coupler somewhere in the system.
I have reviewed about a dozen different websites and nothing matched up...the solution, found by accident, was a malfunctioned heat tape that had locked on and fried the thermostat and was heating the cold water line from the well all the way to the cold water taps in the house...in fact we had hot water going INTO the hot water tank in the cold line...which was part of the confusion since that matched some of the possible causes I read about online and checked out, such as routing etc.
The problem appeared suddenly, as in the days following the first freezing temperatures at night.