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I'm as environmentally conscious as the next American (probably not very) but here's an example: I like to rinse dishes that I've just washed in a stream of hot water as this works best for drying. I would like to run the faucet at a trickle, as that's all I need to rinse off soap and put onto the drying rack. But if I run the hot water at a trickle, it is COLD, like 65 degrees F. Opening up the valve fully results in hot water.

Conversely, the cold water tap gives lukewarm water (85-90 degrees F) unless it is opened fully. Both these behaviors persist over time, so it does not seem like it's just room temperature water in the lines. This particular dwelling is also equipped with hot water baseboard heat and recirculating pumps for the hot water lines, which I thought was to avoid this sort of thing.

Is there something wrong with the recirculation pump, where it's forcing backward flow when not overcome by a higher volume due to a fully opened tap? That's the only thing I can think of, I'm stumped. I should also note that this place is a rental, so I will need to get the landlord to fix this, but he's not very, er, mechanically inclined...so if I present the problem in a nutshell with proposed solution, hopefully it will get fixed. Also the fact that we actually have hot water and plenty of it mitigates against action. I just would like to stop wasting so much hot water!

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Assuming you have a double basin kitchen sink, you could fill one sink with wash water and the other with rinse water. That doesnt solve your problem, but could reduce the amount of water used and get you hot water. –  SwDevMan81 Jan 7 '13 at 18:05
You don't have a tankless (on-demand) water heater, do you? Tankless heaters only turn on when the flow is above a certain threshold. –  Tester101 Jan 7 '13 at 18:08
@Tester101, no, it's a boiler with storage tank. I've been toying with the idea of one of those 2-3 gallon point of use tank heaters under the sink though. –  atroon Jan 7 '13 at 22:07
@SwDevMan81 That's true, and probably what I should do. I just have this aversion to whatever might be living on the side of the sink...I'll just go take my notional OCD meds now. ;) –  atroon Jan 7 '13 at 22:09
Do you have a thermo-siphon hot water circulation device that uses the cold line as a return? One device recently advertised in the US has a name like lobster or something. Otherwise a warm cold supply line means something is wrong, you could be wasting a fair bit of energy. –  bcworkz Jan 8 '13 at 0:39
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Many new kitchen faucets use a single arm and a cartridge to control how much hot vs. cold water is released through the fixture. Sometimes, the cartridge can go bad as it wears out, causing the mixing process to not work correctly. Shower controls use a similar system. You may be able to get your landlord to replace the cartridge (a simple process for most major brands), solving your problem.

You might also try balancing the hot and cold water using the water shutoff valves under your sink. You can open the hot water valve all the way and then turn the cold water valve down some. This might help if you have significantly more cold water flow vs. hot for some reason.

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