Have you contacted the plumber who did the renovation?

Our GC says he hasn't seen this before. He called the plumber, who also didn't know. Plumber is coming on Monday; we were hoping to have some ideas for him. Thanks.

My guess is that the hw pipes were not insulated, and they have effectively created a heat exchanger inside the walls capable of making the cold into hot. The pipes should have been a) insulated, and/or b) isolated to prevent heat exchange. Sounds like it's so severe that a do-over will be the only solution.

Do you have any stop valves on your pipes in various places?

I'll bet your hot and cold supply pipes are connected together, and exchanging fluids, in a current that goes from the water heater (convection current) out to the pipes where your kitchen taps are and then back to the water heater on another pipe. It's a closed circuit. A loop.

Your second paragraph starts off with this:
"We had a recirculator at our hot water heater, which I turned off and unplugged, thinking that was the problem. No change. W..."

This means (but you can verify this) that the hot water recirculating line used the cold line as the return line (this is one way to route it), and that this current has not been cut.

The idea of the pipes exchanging heat just from proximity is not likely. It's impossible to exchange that much energy as to produce scalding hot water in the cold pipe.

tress21 i'm having a hard time believing your post because you leave out so much information. For example, what the bathroom faucets are like. There is a lot of other missing information along this "For example,.." kind of thinking. Like, what the GC did, which plumber he hired, or didn't hire, what routing he days he used for the recirculating circuit, or who built the recirculating circuit if it wasn't the GC, and how the "symptoms" of the problem appeared when the recirculating circuit worked, what changed, etc, a whole lot more.... I really mean it. Seriously. Also, you wrote that it takes 20 minutes to get cold water. Is this what you intended to write?

Here is a link that might be useful: same post in the kitchen forum

With a constant input of energy it's very possible to raise the cold water to heated water; that's exactly how heat exchangers work. The circulator (I'm curious as to why this is an actual pump; most of the recirc. HW systems are planned with only monoflow tee's) delivers fresh hot water at the maximum temp constantly 24/7. The sealed chamber of the wall/ceiling framing becomes a hotbox. Any pipe/pipe contact adds more efficiency to the heat transfer. If it's not insulated you must be spending tons on hot water.
But yeah, the OP sounds a little fishy. Could just be a lack of terminology/jargon gap.
Casey

"The circulator (I'm curious as to why this is an actual pump; most of the recirc. HW systems are planned with only monoflow tee's) delivers fresh hot water at the maximum temp constantly 24/7. "

Residential systems are not set up the same as commercial systems.
Most use a circulation pump on a timer (or other systems like motion detectors or even the bathroom light to turn on the pump to reduce energy costs).

Monoflow tees are not really required on potable hot water system, they are more commonly used on hydronic heating systems.

Did not know about the light switch trick! The monoflow tee is only to set up a passive recirc for hot tap water.
Casey
just trying to disambiguate all the statements.

"My guess is that the hw pipes were not insulated, and they have effectively created a heat exchanger inside the walls capable of making the cold into hot."

Seriously? Even copper piping is a pretty poor heat exchanger at 1/2" There is no way it could warm a cold pipe enough and then transmit that heat to cold, running water for 10 minutes. Just think of the reverse. There is always cold water in a wall. It certainly doesn't cool the wall enough so that you need to run the hot for 10 minutes to warm it up.

The cold and hot are clearly mixing someplace.

"The cold and hot are clearly mixing someplace. "

A likely occurrence.

Some shower valves cross over if the cartridge is not installed correctly.

The fix is usually simple though.
Remove the cartridge, rotate 180 degrees, insert it back in.

Ah, thank you all for the responses. Sorry if my lack of knowledge and terminology confused or irritated anyone. The hot water recirculator had been installed by previous owners and I did not know how it had been plumbed. Your explanations helped me understand what to look for.

So we discovered a mixing valve that had gone unnoticed in the kids' bathroom. I guess there were two mixing valves installed; we had removed one earlier from the master bath due to a leak. We removed the second mixing valve and now the problem seems to be solved. So I'm assuming that the hot water was recirculating using the cold as a return, as davidro1 explained. That makes sense although it took an awful long time to flush the hot water through--about 9 minutes. I still don't understand why very hot water was still circulating in the cold water return even after the recirculator was turned off and unplugged; it wasn't just warm, it was so hot you couldn't wash your hands. Regardless, the circulation loop seems to be broken and I'm very happy to have instantaneous cold water in the kitchen.
Thanks again GWers.

Great! Good for you to have fixed this.

We have a recirculator on our hot water tank because the distance to the kitchen made it necessary to run the hot water for at least 5 minutes before it heated up. We got around the cold water return by having a return tube installed under the kitchen sink. It was some kind of plastic tube that was usable for hot water, and was pretty flexable. The plumber ran it back to the inlet of the hot water tank so the loop didn't affect our cold water. That solution would let you use your recirculation pump.