More plumbing help: hot water coming from cold-water pipes

Our house (built in 1994) has recently started exhibiting a weird symptom. Frequently when a faucet is opened to deliver cold water, it will deliver hot water for a period of time before cold water arrives at the faucet. Depending on which fixture is involved, this period of time may be up to fifteen seconds. My guess is that somewhere in the house, one of the faucets is leaking water from the hot side into the cold side; presumably this happens when a faucet somewhere else in the house draws cold water and lowers the pressure of the cold side.

Any suggestions on how to determine which faucet might be causing the problem?

Are you sure it’s not because the cold pipe runs next to a central heating pipe?

Weird problem. I’d just turn off the hot water supply under the sinks one at a time to test those. Showers will be more difficult.

Or the pipe is exposed to the hot open air (during the summer) like in the attic?

Another possibility you could could consider, which happens in my house, is that the cold water pipe runs alongside a heating duct. Since you say it recently started, this doesn’t seem likely, but maybe it’s because this year has been especially cold in some parts of the country. Or maybe the heating duct sagged recently to contact the pipe, where before there had been an air space.
ETA: Must. Post. Faster.

Do you have underfloor heating? I suspect rather than a crossflow of fluids (Water systems normally run at different pressures to ensure no contamination to the cold supply) that your cold water pipe is picking up some heat from an adjacent heat source.

cold water lines if not used recently are at room temperature. the pipes that go near some part of the heating system ducts/pipes/tubes gets warm and this will warm the water until the cold water reaches its source or your water softner’s temperature.

There are temp balancing faucets that perhaps can cause such a cross leak, they are usually showers.

Some other applications for a crossover valve to blend hot and cold water, and if you have solar hot water you may have a active pump to do this. There are also such devices that intentionally pump water from the hot line into the cold water line for the purpose of having hot water ready at the faucet.

As mentioned there is a possibility of your cold water line being heated by a close run to your heating system or gnomes having a campfire or something.

I asked about the same issue last year, and it was the pressure balancer in my shower. Never would have figured that out on my own without the Dope.

I have one of these systems. Last year the check valve in the pump assembly failed and hot water filled the cold water supply pipe. Had to run the cold for quite a while to actually get any.

Central heating/air conditioning, no solar, no underfloor heating. We’ve lived in this house since 2006, and I’ve only noticed this problem over the past few months.

I suspected it might be a pressure-balancing shower faucet, so I took off the access panel behind our master bath shower. The faucet body says “KOHLER TYPE P 1016.” I can’t sort out from online searches whether this is a pressure-balancing type or not.

Any shower valve made in the last ten years is going to be pressure compensating / anti scalding. Don’t know about 1994. There is usually a ring that adjusts the upper temperature limit that can be set with the trim off. The only deductive way to determine the culprit I can think is to isolate each valve if you can, which I am sure you would have tried already if there were shut-offs. You could try shutting off the hot water supply and see if this still happens. There are shark bite type shut-offs available now that can be quickly and easily installed, but that could get expensive fast.
I did have a problem with a shower valve cartridge the plumber installed backwards (not usually even possible). We were getting intermittent cold water out of a hot tap and it took a long time to chase down the problem.

The problem you describe is something that just seems to be a common part of winter here. Practically every house I have lived in has this problem on some cold tap that will get heated water in the line in winter. Somewhere it is near a heating run or hot line, once the line is drained the temperature is stable.

The pressure balancer in mine was a cartridge that plugs in behind the main valve assembly and looked similar to the part on the right in this picture. Isolating the valves as suggested by FluffyBob and Joey P in the thread I linked above is a good way to figure out the problem.

Your problem is hot water leaking into the cold line somewhere. It’s not something heating the cold-water pipes. Focus on any single-handle faucets.

I think I’ve nailed down the problem.

Early this morning I got up and went to feel the supply pipes to our master bathroom shower faucet. Both sides were warm. Neither of us had been using any water anywhere in the house for several hours prior, so it appears that hot water is indeed leaking through this particular faucet. The cold-water pipe was warm all the way from the faucet (several feet up the wall, nowhere near an HVAC duct) to halfway across the basement ceiling, again with most of that piping nowhere near an HVAC duct. I had thought leakage could only happen when pressure on the cold side dropped due to cold-water usage elsewhere in the house, but it appears that the leakage is so severe that the differing densities of hot and cold water is enough to drive a circulation loop all by itself: hot water travels up from the heater in the basement, through insulated hot-water pipes to the shower faucet, through the faucet to the cold side, and then cools off in the uninsulated cold water pipes on its way back down to the bottom of the water heater through the cold-water supply piping.

The only pipes that were warm were the ones on the path between the master bath shower faucet and the water heater; none of the other pipes elsewhere in the house (for cold or hot water) were warm at that time.

I’ve downloaded a couple of documents from Kohler’s website to help me sort out what replacement parts I need to buy; from their diagrams/descriptions I’m 90% sure what I need to get, but I’ll need to take another look when I get home tonight. It looks like I can just replace the pressure-balancing mechanism, which will save me from having to buy/install the entire valve assembly.