Hot & Cold Water Lines Crossed?

While replacing the hot water heater I turned off the cold water inlet valve and noticed water was still surging in through the hot water outlet pipe. All of the hot water taps in the house were also still fully pressurized, albeit with cold water. After turning off the water main the tank completely drained, only to fill back up when the main was turned back on (with the cold inlet valve still closed). I suspect the builder might have accidentally crossed a hot & cold water pipe somewhere in the house.

Is there any legitimate reason for the hot and cold lines to be crossed like this? The house does have a hot water recirculating system, but there's a 1/2" pipe feeding that pump with a valve that was turned off during the swap, and later completely disconnected during the tank swap.

This also reminded me when we first moved in the downstairs toilet was being feed with hot water (steam billowing up) and the kitchen sink never gets hot water... only luke warm at best. They came out and installed a 1-way valve on the cold water inlet to the hot water tank, which appears to have fixed the toilet heating problem, but the kitchen sink still doesnt heat properly. In retrospect, the cold water throughout the house has always been a bit on the warm side. That plus the hot water tank refilling has me wondering if something else is going on.

Any thoughts?

It sounds as though you might have a crossover between the hot and cold piping systems. Single handle faucets can allow a crossover if the cartridge malfunctions, hot water piped to the air handler for heat (hydronic systems) can do the same, I have seen hot and cold piping looped on "future fixture" roughins cause this.

Phish, thanks for the quick reply!

Most of the faucets are single handle, so I'll try turning off all of the sink valves to see if that resolves it. There's only 1 future stub that I'm aware of (laundry room sink) with two capped pipes sticking out of the wall. Any other "future fixture" scenarios that I should be aware of?

P.S. There's no hydronic heating or visible mixing valves. Only extras are a water softener and the RO, fridge, and dishwasher connections under the kitchen sink (all in use).

P.P.S. No luck turning off every faucet / valve in the house including the washer, dishwasher, and toilets. I even disconnected both of the hoses feeding the water heater and cracked the main to confirm the hot / cold stubs weren't reversed. Water started flowing equally from both. The only fixtures still connected during the test were 2 showers (both single handled). The house has flexible tubing in lieu of rigid pipes (neither insulated nor marked) so I could imagine how easy it would be to accidentally cross connect.

Any suggestions for troubleshooting further?

Can you remove the trim plates on the shower valves and check to see if there are supply stops on those? Many shower valves have a valve that can be operated with a flat screwdriver on both the hot and cold supplies. Shutting those off would eliminate two more possibilities.

If you have a Pressure balanced shower valve then you could be getting a cross due to designm do as phish here has said. and otherwise try putting everything back as it was and simply check all the fixtures, if hot and cold are where they are supposed to be then there is no cross, and further investigation will discover the problem. If they did not DO anything about your steamy toilet other then put a valve on the hot water tank then They didnt fix anything. Also, it is common with pex pipe ( plasic waterlines ) installed by Rookies to have then group both the hot and the cold lines together or to secure cold lines above the hot lines, either of Which would cause you to have Tepid water on opening of a cold Valve.... though only for a short while.

I'm betting the showers on this one.

This sounds intriguing. I wonder how it ended.
Hey Vegas - you still out there? Did you solve the problem?
I'm guessing since you're in Vegas that you don't have a basement to trace the lines easily.

Yes, it turns out the builder did screw up (hot & cold water lines crossed in the wall).

I ended up replacing the mixing cartridges for all 3 showers/baths - no luck. Then I completely disconnected the pex feeds to each shower/bath - still nothing. Finally I started from scratch by cutting into the drywall at the water main and tracing each line.

I temporarily disconnected each line one at a time until I found the problem... the kitchen sink! It was a royal pain isolating that line because it didn't split off until it was below the staircase - no access panel for that area (so yet another drywall cut) and only 8" to play with before it hit the foundation.

That's where I stopped (no cold water to the kitchen sink), but at least the rest of the house has hot water!

Anyone want to guess at how the lines got crossed?

I should have time in the next week or two to expose the pipes under the sink and finish the repair.

Builder was looking at kitchen layout from wrong perspective ( read blueprint wrong ? ) and so the hot always being on the left when was now the right cold R.I. ? couple that with a T feeding it and violla, crossed line?

...it was a fun guess.

Sounds really frustrating. It's fun to think about how I might have tried to troubleshoot the problem. Not so much fun if it's your problem to solve.

I'm gonna guess that the cold water got tied into one of the recirculating lines. It was a 2nd line next to the Hot, so it must be Cold!