Plumbing questions - no hot water in shower

Here is the issue.
All areas of the condo get hot water.
One shower unit gets hot water for a few minutes then it runs out.
The shower has standard hot and cold water taps. It does not have one of those single handle faucet controls.
I’m just wondering how this can be.
Any ideas?

Maybe it’s a bad anti-scalding valve? Some places will install a device in the shower to avoid drastic increases in temperature, like when someone flushes a toilet. If that was broken, it might let cold water come through.

What is the temperature of the hot water when it’s not hot anymore? Is it warm at all or is it as cold as the cold tap?

When you turn the hot (only the hot) water on and you say it runs out. Do you mean that it stops flowing or that it changes to cold water?

If it changes to cold, you likely have a bad mixing valve. It could be anywhere in the house. For example, a kitchen sink nowhere near the bathroom with a broken mixing valve, allowing the cold water to backfeed into the system would do it.
If that’s the case, go and close all the shut offs under every single handled faucet you can get at. If the shower now works correctly, you can start opening them until you find the one that causes the problem.
Sometimes you can even just let the hot water in the shower run for 10 or 15 minutes and see which sinks have a cold water pipe that’s warm to the touch.

On the other hand, if the water stops flowing, it’s likely something in the cartridge that’s moving around or expanding when it heats up. Shut off the water, pull the cartridge for the hot side and see if there’s something you can remove/clean/replace.

That happened at my parent’s house. The pressure balancer* had a torn diaphragm which was mixing the water. IIRC, it wasn’t too noticeable in the shower. The issue was that other parts of the house couldn’t get hot water. The reason being that sinks are the most likely place to have full hot water running. In the shower, you’re already mixing it…but I could be wrong about that part, it’s been a while.

*For what it’s worth, the anti-scalding valve is what makes sure you can’t turn the knob too far and/or make the water too hot. The pressure balancer is what keeps you from getting a faceful of scalding hot water when someone flushes the toilet. When it sees a drop in the pressure on one side (hot or cold) it reduces the pressure on the other side as well (or, volume, I guess).

I think it is cold as the cold water.

No, it just switches to cold water. It doesn’t shut off.

I agree with the above posts. Smells like a bad pressure-balancing mixing valve to me.

Then somewhere in your plumbing system the hot and cold are getting mixed. It’s fairly common.

To reiterate, you have two options to quickly find the culprit.
1)Run the hot water (even if it’s coming out cold) in your shower for 5 or 10 minutes. Then go to each single handled faucet and put your hand on the cold water supply. If you haven’t used it recently it should be, more or less, room temperature +/- a few degrees. If you find one that’s warm or hot or one that feels like water is moving through it, that’s your culprit. In fact, the hot water supply to the suspect faucet would be warm/hot as well.

2)Shut off the water, at the shutoff under the sink, to all the single handled faucets. Open both the hot and cold on one of them and see what happens with the shower. If it continues to go from hot to cold, close those valves and open another one, and keep going until you find the problem.
Another way you can do this, that may actually be faster, would be to run the hot water in the shower until it goes warm/cold. Then go to a single handled faucet and shut the water off to only that one. If the hot water at the shower is restored, that’s the problem, if not, move on to the next one.

In general, but not always, it’s going to be a sink on the same branch. That is, if your shower is on the second floor, start with the bathroom directly under it. If it’s a one story house, it’s likely to be the bathroom right next to it, so start close.

Having said all that, if none of the other single handled faucets in your house can be eliminated, it may be the shower valve itself. Though I don’t believe a two handled shower has a mixing valve or pressure balancer.
I know this can sound complicated, but if you can wrap your head around the theory, it’s easy to find the culprit.

It almost sounds like a mixing valve somewhere has a hose attached to it with a shutoff at the end of the hose and the water is being left on

That would certainly accomplish the same thing since you’re connecting the hot and cold water. But torn pressure balancing diaphragms and bad o-rings or cracked cartridges in single handled faucets are very common. If I had to guess, that’s probably more common than someone connecting a hose to a faucet and leaving both the hot and cold water ‘on’.

OP here.
Apparently it is a problem with the other shower in the unit as well.
I do not live in the unit, it is a rental and hard for me to get to.
I wish I could try some of the things you mentioned.
I don’t understand how hot and cold water could mix if there are no mixing valves.
There are two toilets, three sinks and two showers in the unit. All the sinks have separate hot/cold as do the showers.
I have called a plumber.

You mention it’s a condo. If water is included (that is, each unit doesn’t have their own water meter), this would be even more possibly if each unit doesn’t have their own water heater. But in any case, I could still imagine it’s possible.

FYI
The plumber says it is the water heater.
I suspect the water heater guys just did a cursory look probably because they did not want to fork out the cash to put a new one in.
The plumbers says the showers were affected more than the other faucets because they use more water.
Thanks for everyone’s help.

I’m apparently too late to post this but I will in case it gets searched.

A similar thing happened in my first house. The sinks would have plenty of hot water, but the shower would run out very quickly. The problem was twofold:

The “dip tube” in the water heater, that forces incoming cold water to the bottom of the tank, had broken off and so cold water was coming into the tank right at the top. After a few gallons of hot water, it would be mixed with the incoming cold water enough to become frigid.

You don’t notice it as much with a sink because you typically are not running gallons of hot water through it. But a typical shower (depending on your shower head) can easily use 15 - 30 gallons of water – or more if you like to luxuriate in there.

Once we solved the dip tube issue, the next issue was that the shower head was a really high-flow device, and our water heater was small. Wife liked to use a LOT of hot water, and so it ran out rather quickly. At 4 or 5 GPM, it will not take long to use 30 gallons of hot water.

Switched it out to a more efficient shower head, using no more than 2.5GPM, and showers don’t get cold until you’re the third person in line…