Water sometimes hot, sometimes not

Factual Questions
1

I’m looking to satisfy curiosity rather than fix a problem (because it’s not that big a problem.)

I live in a house built in 1937. There have been minimal upgrades to the plumbing. The hot water heater, located in the basement, is maybe 10 years old. The bathroom where we take showers is on the second floor.

Sometimes, the water coming out of the shower head is so hot it has to be blended with cold water to get it to the right temperature. Sometimes, I turn the hot water on full blast and it’s a comfortable shower temperature with no cold water added. It is more often the former than the latter, though.

External temperature doesn’t seem to be a factor here, as the variations seem to occur throughout the year.

Is there an explanation for this?

2

What kind of heat do you have? Is there a burner or boiler that also has a water heating coil? Your hot water heater may be set at a low temperature but you get much hotter water if it’s fed with preheated water.

3

I think it’s just your garden-variety gas water heater – no pre-heating.

4

So there could be something wrong with the temperature control. It may not heat until the water is colder than you’d like, or may not stop heating until it’s too high. I don’t recall ever seeing this on a hot water heater, but perhaps it has low and high temp settings. But I’d think it’s just not cycling properly, getting too cold before starting to heat.

Of course one of the obvious things is that someone else is using up the hot water before you shower. What’s the capacity of this heater? A shower with a low flow head doesn’t use that much water. The water may be really hot when the tank is fully heated, but after running the dishwasher it may be a while before it heats up again.

And another simple cause if this only happens in the shower is a single handle mixing faucet. Differences in water pressure can easily cause the hot water to dominate when cold water pressure is low.

5

I will check the capacity. I don’t think it’s a low-flow head, but I’ll check on that, too.

This morning, it happened when mine was the first shower of the day, so no other competing uses for at least 8 hours.

The shower/bath hardware is one of the non-updated parts. There are actually separate faucet handles for the tub and shower.

6

Your being rather vague to say more than it’s likely this or that.

The following applies only if nothing else uses water at the same time.
Do you get hot water at first and only warm later?
In that case it could be:

  1. not a big enough tank.
  2. a lot of sediment covering the electrical heating elements.
  3. one of two heating elements gone bad.
  4. a broken pipe in the tank that directs the incoming cold water flow.
  5. a bad thermostat

Do you get warm water at first and hot only later.
In that case it could be:

  1. long lengths of metal pipe that suck the heat out of the water and you don’t get hot water until all that pipe is hot.
  2. a bad thermostat.

You could have a bad thermostat if the hotness changes randomly during the shower.

There are also tankless heaters you can install near your shower that will heat the water instantly.

7

You know how you turn the hot water faucet on and let the water warm up? That’s what I do. It either starts out cold and gets hotter and hotter until I have to turn on the cold water faucet to make it comfortable, or it reaches a degree of warmth that is comfortable but doesn’t need cold water added, and it just stays there. When it does that, it does sometimes fit your second description (warm then hot). But sometimes it’s just hot.

It’s never hot then warm, unless there are competing uses. That’s pretty rare.

8

Could there be a timer to cut power and save energy on this system?

9

This is what it sounds like to me. Especially since you said this happened first thing in the morning. The workaround is to run the hot water just long enough to get the heater to kick on 20 mins before you shower. If it were me I’d also crank up the thermostat just a wee bit depending on where it’s at now.

10

OK, in the interest of science, I am going to start keeping notes to see if I can identify a pattern as to under what circumstances the water is really hot and not so hot.

11

Water heater dip tube troubleshooting.

I had a similar problem, and figured out it was a broken dip tube. The dip tube replaces the hot water you use with cold water. Ideally, the dip tube delivers water to the bottom of the tank, mixing with the remaining hot. If the dip tube crumbles away, you get colder water more quickly, as cold is introduced higher in the tank.

This was a huge problem with specific tanks produced during a certain time. The dip tubs were faulty, and aged quickly. Worth a thought.

Another discussion of dip tube problems.

12

The dip tube is what I was referring to in point 4 when I couldn’t think of the name.

13

If it was the dip tube this would cause the water heater to behave as if it had a much smaller tank: Hot at first, then very quickly seem to run out of hot water. This differs from the OP’s description in that s/he has plenty of hot water some days.

Certainly this snippet rules out dip tube in my mind:

14

Not on a gas water heater.

15

But it’s an easy thing to rule in/out. I thought my hot water issues didn’t fit for a dip tube problem, but it was.

16

The water heater my just be getting old. Ten years is getting on in age for your typical hardware store water heater.

17

A bad dip tube lets the cold water in next to the hot water out so you have instant hot water dilution at the hot water exit. The cold water doesn’t enter near the thermostat so you get slow response to starting the heating cycle. Once the heating starts it’s more likely to shut off faster as the water near the thermostat heats and and doesn’t have the cold water forced in at the bottom near the heating elements.

18

As far as we can tell, this is what happens to us.
The first shower of the mornings is warm. But if you wait a bit, letting the tank rehear, water after that is hot. It is especially noticable if we have been away for the weekend.

19

It sounds like it is probably not the dip tube, but dip tubes are cheap and easy to replace so it may be worth checking out.

I assume you are trying to avoid calling a plumber and/or replacing the water heater but it sounds to me that that is probably the solution. First I would recommend calling the manufacturer (or Rheem or some other big manufacturer of water heaters). If it is an issue with the heater itself they will probably recognize the problem and solution.

One possibility is that a fitting somewhere else has been incorrectly installed and is mixing the hot with cold before it gets to the shower. The variability could be due to what the other valve is set to at the time. I had this problem with a new home where the plumber accidentally installed the cartridge in the basement shower valve backwards, giving us luke-warm water on the second floor.

20

That’s certainly an interesting possibility.

freckafree, to quickly check for such a crossover condition you can shut off the cold water suppy to the water heater and then go turn on a hot water faucet somewhere. After an initial spurt the water should stop coming out.