Hot and cold water sometimes switching

July 24, 2020 10:55 AM

I live in a relatively small (6 floors/36 units) apartment building in Manhattan. Every so often the hot and cold water taps switch so that the cold water tap serves hot water and vice-versa. This may last for a few hours and then it goes back to normal (perhaps with a tepid period in-between). Any reason this might be happening in a building of this size? As far as I know the boiler was replaced approximately a dozen years ago.
posted by slkinsey to Home & Garden (6 answers total)

In our place, the cold water pipes are on a side of the house that gets lots of sun, and in the summer, warm water does come out of a cold tap, especially if water hasn't been flowing for while. But it's not "hot" water coming from the water heater. On the other hand, water coming from the heater cools in its pipes when it's not flowing, so I feel that as "cold." If you're taking "hot" showers for 20 minutes with only the cold tap open, that would be weird, but I'd guess this is a weather thing.
posted by sageleaf at 12:03 PM on July 24, 2020

I remember seeing a show where this problem was addressed, but do not recall the exact cause or fix. I think it had to do with a faulty valve.
posted by tman99 at 12:18 PM on July 24, 2020

Best answer: Some buildings have recirculation systems where water is forced through thermal valves at distant fixtures to keep hot water ready "at the tap" - the not-yet-hot water returns to the heater/boiler via the cold supply lines. Normally the valve closes the loop when the water gets to a certain temp to keep the hot/cold separate, but a broken valve can send hot water through the cold lines at all times. These systems usually run on timers, so in the morning pumps will push hot water to all the lines in prep for heavy use, while turning off mid-morning with the expectation that use will be low. Are you home at times where in the past you haven't been home? You might be experiencing the cycling of the recirculation system. But you say you're in Manhattan - so you might have a roof storage tank for gravity fed cold water... so things might be different.
posted by niicholas at 12:31 PM on July 24, 2020 [3 favorites]

In my apartment (16th floor of a 17 floor building) the water from the cold tap is always warm early in the morning in the summer.
posted by srboisvert at 5:49 PM on July 24, 2020

Response by poster: nicholas, I suspect that may be what it is. It didn't start happening until after the new boiler (etc.) was installed.

Two additional items and then I'll shut up:

1. I have normally observed this in the middle of the night when getting up to have a drink of water. This is when I will notice that the taps are literally switched, which is to say that the hot water tap is drinkably cold while the cold water tap is warm to hot.

2. Today at noon the taps were both quite warm at noon and back to normal within 15 minutes.

3. Building does not have a roof tank. Building isn't tall enough to require one.

4. Typically the cold water here is quite cold after running the tap for a few minutes, even in the summer. (This may sound wasteful, but is recommended to minimize the possibility of bad things from old pipes.)
posted by slkinsey at 5:53 PM on July 24, 2020

Best answer: We had a similar thing happen, where the radiators and the hot water taps seemed to switch – I believe the problem was a faulty diverter valve on the boiler. It meant the radiators were getting hot when it tried to make hot water for the taps, and possibly vice versa.
posted by lucidium at 9:08 AM on July 25, 2020