I have an older house (1940s). Some of the plumbing was updated ~10 years ago before I bought it. I don’t know the details of what was updated, but what I’ve seen behind the walls is PEX.
Up until a few months ago, the upstairs sink (farthest from the water heater) worked like you’d expect - turn on the hot water and it starts cold then gradually warms up. But now it is reversed - the hot faucet is hot immediately and the cold faucet starts hot and then cools down. I didn’t install a recirculator in my sleep, so what could be causing this?
If there has been no change in the plumbing, I’d suspect a usage factor. Do you use more hot water in winter? Or more often? Presuming the pipes lie closely parallel at some point, maybe lately there’s been more hot water in the pipes than usual, which may be warming up the cold water supply lines, leading to the situation you describe.
I’ve seen cases where a leaking shower valve caused hot water to be introduced into the cold water supply. This usually shows up as the cold water not being particularly cold but might be more pronounced in your case.
Sounds like the hot side is bleeding over to the cold side. Sounds like you have two faucet handles instead of one handle though. If it was one, it could be leaking internally. Do you have a shower, bathtub, or other bathroom close by with just one faucet handle? It could be there.
Do you have the type of faucet where both hot and cold water lines feed into the same tap outlet, or separate lines and taps?
Because it sounds like your hot water is bleeding off somewhere, so the water comes out ‘instant hot’ without a warm up period. Where the hot water is bleeding appears to be into your cold water line.
So if you have the type of faucet where both hot and cold come out the same tap, your faucet is probably the culprit.
And your hot water is running, at least a little bit all the time to come out hot right away, so that is an energy waste issue too.
Sorry, I wasn’t clear, it’s one tap, two handles. Hot water bleeding into the cold sounds like the only reasonable explanation. I’m surprised it’s possible - I’d think the pressure in the cold water line would prevent that.
Consider if you flush a toilet. The pressure on the cold water side would drop allowing water to leak over. Also heat expands. If you have a check valve to keep hot water from backing up from the heater, the hot side could have slightly more pressure in it.
No matter what, the valve on the single handle faucets isn’t too hard to change. Often on an older house, it’s getting the crusty valves under the sink to shut off the water so that you can chance the valve. I once had to shut off the water to the entire house as the local shutoff valve wouldn’t.
Just make sure you figure out which mixing valve is even causing the issue. The two handled bathroom faucet is almost certainly not the culprit - each handle manually shuts off the water on its own side and the only place they could mix would be in the faucet, where it would come out into the sink (assuming the faucet is not blocked), not back into the other line.
An alternate explanation is that part of your heating system is routed near the piping for the faucet, and heats up both pipes. Farfetched perhaps, but if you have a steam pipe running right next to your water pipes, I could see them getting pretty warm.
I considered that, but the heating system hasn’t changed since I moved in and this hot water from the cold tap is new this year. And the water isn’t just warm, it’s pretty hot at first.
I’ll almost guarantee the pressure balancer in the shower that’s in that bathroom is broken. It’s a small rubber diaphragm that moves back and forth so that one someone turns on the water somewhere in the house it will slow down the other source as well. That is, if I someone flushes the toilet, it’ll throttle down the hot as well.
Anyways, if it wears out and rips or tears it’ll allow water to flow between it. You’ve effectively connected your hot and cold water system. That sink is the closest so it’s most noticeable, there, but left on long enough the symptoms would show up at other places.
It may be possible to replace just the device, but usually you have to replace the entire valve and it’s located at the back.
If you have access to the shower valves, try turning off the water to JUST the shower, then use the sink again and see if the problem clears up.
If it doesn’t, the next probable cause is a bad/cracked valve in a single handle faucet somewhere (causing the same basic problem). Is there a bathroom below that bathroom? Try shutting off the water to that sink and try the problem sink again. If that fixes it, the innards of that (now turned off sink) need to be replaced.
I finally got around to fixing this, and Joey P wins the prize. I replaced the pressure balancer in the shower and all is well again. Thanks for the tip, you saved me a lot of time and money.
If you have access to the back of the shower, where all the plumbing is, a quick and dirty check for this is to put your hand on the pipes when it hasn’t been used for a few hours (but the faucet has). If you say to yourself “Boy, the shower hasn’t been turned for 3 hours, why is the pipe still hot” or “Boy, the shower hasn’t been turned on all day, why is the cold water pipe warm”. Then this is a symptom of your problem.
When someone else tells you that this is happening at their house, that’s a good thing to check (and you’ll be a plumbing genius for figuring it out in 2 minutes) It’s easier then shutting off the water to the different sources and then having them turn on the sink and saying “Well, I think it’s cold, maybe it’s a little warmer then it should be, I’m not sure”.
Also, now that you understand the problem, hopefully you’ll understand the concept behind shutting off the water to the shower and testing the faucet. By shutting off the hot/cold valves entering the shower, water can’t travel across the broken part and flow to the sink. Any faucet with a single handle lever (like a kitchen sink) can do this if the cartridge cracks. This method is how you isolate the problem.