Why is boiling hot water coming out of every faucet, and even the toilet?

Question

Help! I live in a 80 unit condo (8 floors), with hot water tanks on the top floor (4). They flow down 13 different lines to different units and hot water is recirculated through recirc pipes. Around April several units, around 6, started experiencing boiling hot water that would come out of the cold water faucets (we have levers which you turn for cold/hot). This is happening in the kitchen, bathroom, toilet, bathtub, washing machine, and you have to turn it on for 5-10 minutes and then cold water would finally come out. You have to do this EVERY single time as the boiling hot water would return the minute you turn the fixture off.

The 6 units are scattered through different lines, and throughout the 1st-5th floors of the building with the EXACT same problem, which started at the exact same time. We've had 3 different plumbers come on 4 different occasions. I've had someone even take a look at the drawings and look at the tanks, the pipes, with no answers. No one that we know has made any alterations to their plumbing in their units.

We thought we were getting close when we shut off the hot water valve in unit 102, and the problem was alleviated in that unit and the units above, but no solution was found. All the faucets seemed fine, and cartridges were replaced in shower fixtures that seemed like they were leaking. Turning off the hot/cold water lines to the washing machine in a couple units seemed to help in a couple units, but not for others.

The only work that was done in the building was section was repiped on the ground floor a couple weeks prior to al this (water was shut off, a section of leaking pipe replaced), but the plumber was adamant that did not cause the problem.

One plumber suggested we install individual shut off valves to each unit at a coast of $300 x 80 units (we cannot do it for just the 6), but some say this is pointless as it is expensive and may not even help.

We are at a loss as to what to do. Please help!

Answer 1

May I ask you to temper or clarify one adjective? Is this water literally BOILING hot (hotter than the tanks) or only SCALDING hot (as hot as the tanks, about 120*F)? If the tanks are also used for heating, then perhaps 180*F?

If the tanks are used both for heating and domestic hot water, then you have several large tempering valves, correct? These tempering valves mix hot and cold water together to achieve the industry-standard maximum 120*F for domestic hot water.

You're on city water, surely? High pressure, enough to push water to the top of the building, with pressure regulators on each floor?

This is the sort of thing that might happen if a tempering valve fails or is misadjusted, AND one or more of the tanks has developed a large air bubble. It's more likely to happen during periods when the municipal water supply pressure falls (because other nearby buildings are also using a lot of water). The hot-water tanks have been pressurized by high municipal pressure, including the one with the bubble, which makes it into a large surge tank. When municipal pressure falls, pressurized hot water backflows through the failed or misadjusted tempering valve into the cold-water system, where it's delivered to faucets, toilets, etc.

A really good plumber should be able to track down the source, but it'll take some time. They'd have to carefully measure the temperature of the pipes in the pipe chases, following the hottest pipes towards its hottest end until they come to the hottest spot in the cold-water system - that'd be the tempering valve in question. It'd be BEST if they could work while the occupants of the affected units were not in the building, so the water to their units would remain pretty static... making measurement easier.

The only other cause I can think of would be backflow (intentional or accidental) through a bathtub or laundry outlet, where hot and cold could be mixed without actually dispensing water - in the case of a bathtub (which has much larger water connections than a faucet or toilet), it'd require that the spout be blocked while both hot and cold were turned on. This would allow hot to backflow into the cold-water system.

I'm leaning very hard towards the tempering-valve problem, though, because otherwise we're talking about collaboration between numerous occupants in different units or a really odd set of coincidences.