Hi Marshall,

In a sense this is your lucky day, since I've just been through this, and it took a while, with the help of others here.

It sounds as if you're experiencing thermal expansion...does your system there have a water meter, pressure reducing/regulating valve or a check valve or backflow preventer? These usually have to be present to allow this to happen.

What I surmise is happening is the offending sink is closest to your hot water tank. As the water in the tank heats, it expands, and backflows into your cold system...now this may happen in a system with no meter or backflow measures, which means that the warm water pushes some of the town water back down the line towards the city supply as it expands.

This is an undesirable situation, as backflow can often be contaminated, especially if there's a garden hose or other situation where dirty water can be siphoned back...for that matter, the water from the hot water tank isn't the best thing to drink as bugs can grow in the warm environment, and copper compounds tend to build up in it too...

So this is why we have backflow valves, to prevent this undesirable situation. Unfortunately, without provision for expansion protection, this results in the expanding water having nowhere to go, so it produces undesirable side-effects like leaking taps, seals breaking down, the softening (by the warm water) and subsequent leaking of fixtures like humidifiers and water filters that are supplied by plastic tubing.

Of course the temperature and pressure relief valve is there on the side of the hw tank for safety but it doesn't release pressure until it's at quite a high level, also, it's not considered a 'continuous duty' device and will probably fail if deployed too often (although you should test and clear any buildup of salts etc once a month by lifting the little metal lever on it til water flows...

So your solution is to instal/have installed an expansion device, the best is probably an expansion tank - this tank looks like an air compressor bottle and has a plastic bladder inside, you actually pressurise the bladder with air from a tire pump or compressor (it has a tire valve on it) when it's installed and it takes any overflow from the tank's expansion and lets the overflow back into the hot water system when a hot tap is opened.

There are other solutions including special toilet cistern fillers which deal with the pressure or expansion/shutoff combination valves that allow the excess water to flow to waste, the disadvantage is, you waste a lot of water and you need a drain for the water to flow to, and you will also end up heating a lot more water...water and energy are precious so I favour the expansion tank. They are well under $100 and whilst the installation looks a little complicated, it's not that hard (or you could get it put in for you) Whilst a check valve (a one-way valve) in the cold supply to the hw tank would stop this situation it wouldn't take care of the dangerous day to day pressure increase, which incidentally can cause the hw tank to fail prematurely.

From what I know, having an expansion tank or additional overflow/pressure device is mandatory anyway...and it'll fix the annoying warm water thing, I first discovered it in my house when I was brushing my teeth....

I did have someone here suggest my house was plumbed incorrectly (well it is since there's no expansion device, and I have a pressure reducing valve) or there was 'crossover' from a mixer, but here's a foolproof way of checking.

Go to your hot water tank and find the inlet where the COLD water enters the hw tank - it'll be marked. If no tap's been run for a while, the cold inlet pipe will feel warm or hot near the tank, and gradually feel colder as it goes back towards the cold supply. Have a helper in the bathroom turn on that 'cold' tap that runs warm....you should feel the warm water run back up that pipe

Hope this was helpful.

Here is a link that might be useful: What is thermal expansion?

Marshall,

I note that you're on well water which may have a high mineral content, so that makes it more imperative that you keep an eye on that temp/pressure valve on your hot water tank and test it with the lever regularly, as those minerals can block the valve. Tanks have been known to explode and/or become impromptu rockets in these situations.

It is physically impossible for this problem to be a result of thermal expansion. Even if this faucett is in the near proximity of the water heater the line is contantly full of water and water is not compressible therefore, in order for thermally expanding water to enter that line it would need to first displace the volume of cold water already in the line and there simply is nowhere for the cold water to go.

It is possible during long static periods that heat from a water heater can migrate into the cold water line without actually moving any water but generally this would be limited to about 6' vertical or 4' horizontal from the tank. Heat migration can be eliminated by installing "Heat Trap dielectric nipples" on the water heater.

One common cause of warm water on a cold water line results from the installation of a hot water instant on recircutor that uses the cold water line as the return path rather than having a dedicated return line. These type of units are commonly sold in the big box stores as a simple DIY installation for recirculating hot water.

It is also now a common prsctice to install a small tempering valve on the watercloset supply line to keep the supply water to the watercloset tank at room temp to prevent condensation on the tank walls. If the lav in question shares a common line with the watercloset fill valve and if the tempering valve is improperly installed before the lavatory connection it would account for this problem.

It is also possible that the cold water line is run in parallel to a heating duct that is not adequately insulated and the water is being heated from heat radiating from the duct.

I was just about to post the exact same problem. The reply by pjb999 makes sense. The thing I dont understand is why did this problem of hot water coming down the cold water line, slowly get worse over the last few months? In the last 2 weeks, it has become awful. It's happening on all our cold water lines and some are 30 feet from the HW tank. What is causing our HW tank to produce so much excess pressure now? Is it about to blow?

Another possibility is discussed here:

http://clippings.gardenweb.com/clippings/ihart
or
http://ths.gardenweb.com/forums/load/plumbing/msg081412093903.html

Basically, the problem could be a failed SINGLE handle faucet on a sink or shower. It operates normally but when you shut it off, the hot water expands across to the cold water at that location. If it expands enough, all your cold lines will run hot for a while. I'll check for this first tonight on our single handle sinks by feeling for a hot, cold water line. The showers will be tough though so I hope it's a sink.

OK.. I just wanted to post my troubleshooting of the "hot water coming out multiple cold water faucets" problem I've been having. I think the problem is in a Kohler single handle shower faucet mixing valve in our master bathroom. When I removed the outer "bell" handle portion on this unit, the inner stem feels warm to the touch. It had been 10 hours since anyone used the shower. The other 2 identicle shower faucets we have in our home feel cold. I called Kohler and they are sending me out a "rebuild kit" for this faucet. Hopefully this will solve the problem. The technician there said that it should. I will post again when I install the parts they send me. They are backordered for some reason...

I received the parts (free!) for the Kohler shower mixing valve. It took about 10 minutes to replace. Problem is solved!!! No more hot water coming out of the cold water line!

As a side note, I dissasembled the old Kohler mixing valve. After 8 years, the rubber diaphram part that separated the hot and cold side had basically melted away. It just crumbled in my hand. Soundsl like they used bad materials for this part.

I am having this problem right this moment! Searching all over and finally realized that my hot water line inside the slab was broke and all that leaked hot water heated up my MBRoom and my cold water line.
If your copper plumbing lines are inside the concrete slab then this is what you eventually gonna face with it. Bad plumbing design and poor building code. This thing should never have allowed to happen. This happen in CA.
Solution is to re-pipe the whole house through attic space and come down from the ceiling. Prepared to spend $7000.

I live in a 26 unit condo and recently have been experiencing warm water in my cold water side Kohler faucet in the Kitchen. We have a central water tank for all units including the hot water which is part of the common area. Recently this has gotten worse and I'm concerned at how to proceed. should it be an Association issue OR is it inside my unit to begin correcting this. I also have noticed it in the guest bath but not as often as in the kitchen which is the next room. We recently (last year) got a new washer installed but I don't know how that would be the cause. Sometimes the cold water comes out just as it should so it's not a constant issue. We live in Los Angeles and it's been a hot summer. Thanks so much!!! Concerned this has bacteria in it after reading some of the responses on this site.

My cold water faucet, shower and even the toilet tank lines are all having warm water coming out in the first 30 second or so, but only in one bathroom, and it is upstairs the most far away on......

violiu, warm water from a cold water faucet is not normal...can you contact a licensed plumber?