Scalding Water Coming From Cold Side of Faucet

My parents have a hansgrohe widespread roman-style lavatory faucet in their master bath. They have always had a problem with scalding hot water coming out of the cold side of the faucet. After about 20 seconds, the water would turn cold. Now, however, the problem has become much worse. The scalding water continues for 2 minutes or more, before it becomes cold. Once it is cold, it stays cold, until the hot water is used. Then the cold becomes hot again, after both the hot and cold are turned off for a while.

Anyone have any ideas what could be causing this?

If they have a hot water recirculation system, the check valve is probably shot. Depending on the system, if there's a valve under the vanity for the recirculation system, it may need replacement.

Or, there could be a defective cartridge somewhere that is creating a cross-over.

They don't have any hot water recirculation system. However, that is the same bathroom that has the hansgrohe thermostatic shower cartridge that has been acting up.

That shower also has scalding hot water coming out of the cold side for a minute, before it turns cold. Then, you can't get anything out of the shower except luke-warm water, regardless of the position of the temperature control knob on the handle.

What I don't understand is how can that shower cartridge cause scalding hot water to come out of the cold side of the lavatory faucet, when you can't even get hot water to come out of the shower, except for a few seconds when you first turn it on?

Some cartridges when they fail connect the hot and cold water together - i.e., a cross-over. If there's a small pressure difference, when you turn on a faucet, it can flow one way or the other.

thermostatic valves are mixing valves so they will alter the flow of hot and cold water automatically. The faulty valve must be allowing hot water to back flow into the cold water line instead of mixing the water towards the shower head.

Thanks Jad and Ckl!!

So, I guess all the hot water that I'm not getting out of the shower is ending up in the cold water line.

I've actually tried to remove that cartridge, but it is very frozen. You have to use a huge crescent wrench as a counter-wrench, while you unscrew the cartridge from the housing. I'll try squirting on some PB blaster, unless someone has a better idea.

The majority of thermostatic valves have integral check valves to prevent the water from going into the system and affecting other faucets, so that may NOT be your problem. BUT, only a plumber, on site, may be able to diagnose what the REAL problem is. WE can only guess using YOUR information, which may not be completely correct.

With no recirc system, it is NOT common for hot to cross over to cold. It is more common for cold to cross over to the hot side. Someone needs to see your system/

The tech-support manager at HG told me that it was impossible for their cartridge to cause the kind of cross-over situation I described. But before we hired a plumber, I decided to try to troubleshoot this myself. I followed the cold pipes starting at the meter, feeling them whenever they were visible, and the same thing for the hot pipes, starting at the HW tank.

The closer I got to that shower, the warmer the cold pipes were. And behind the shower access panel, the hot and cold were the same HOT temperature. As I ran the shower, the cold shower supply got colder and hot got hotter. But, when I shut off the shower, the cold instantly became just as hot as the hot. So, I knew the cartridge was at fault. I replaced the cartridge and there's no more cross-over. And the shower works perfectly again.

Thanks for all your suggestions.

Glad it worked out for you. To quote Sherlock Holmes, "When you have eliminated the impossible, whatever remains, however improbable, must be the truth".

If the hot and cold lines were reverse during installation, the water would ALWAYS be reversed. The cold lines would not be getting variably hot and cold. Without a recirc system, what else could it have possibly been?