Water heater not staying hot more than 10mins

I live in a one bedroom apt. Our water heater is not the normal 40gallon but a smaller 20gallon. When we moved in it was hot for at least 30-45 mins. Now it stays hot for less then 10. My maintance man came and "fixed" it which it only got hotter faster and then got cold. Obviously he only turned up the heat. What can I do besides continually bother them or just replace the damn thing myself? He told me yesterday that I'm using too much water, yet I think I've been using less.

• 10 minutes of what? – isherwood Jan 20 '16 at 14:27

TL; DR: When the temperature is colder outside, your cold water is colder so you have to use more hot water for the same shower temperature.

Facts:

• A shower head puts out 2.5 gallons per minute (GPM) (or close to it).
• To get the temperature you want, you have to mix hot and cold water.
• In the summer, the "cold" water temp is much warmer than it is in the winter.

Given the above facts, consider this (numbers and calculations made up)... In the summer, to get your shower where you want it, you need to mix 70 degree "cold" water with 120 degree "hot" water to get a final temperature of 100 degrees. This means that you have to use about half hot and half cold, so 1.25 GPM cold and 1.25 GPM hot. That means that 20 gallons of hot water will last (20 / 1.25 = ) about 15 minutes. This doesn't match your 30-45 minutes, but just stay with me... It's just an example.

In the winter, the incoming "cold" temperature is much cooler. It could get down to 40 degrees. When you mix the 40 degree water with 120 degree water to get a 100 degree shower, you don't need nearly as much cold water. You still have to add up to 2.5 GPM for the shower head, so the ratio ends up being more like 0.5 GPM cold and 2 GPM of hot. That will reduce your shower time to 10 minutes - a 50% drop, and nothing in the house has obviously changed. You probably like the shower hotter than 100 degrees, so the ratio will end up using even more hot water.

So, regardless of what the real numbers are, in the summer you end up "having more hot water" even though your water heater hasn't changed at all.

Also

As this shows, you have been using more hot water even tho your total water consumption may have gone down because of the shorter usage time.

And if you're renting an apartment, you can't replace the water heater. Plumbing and electrical repairs must be done by their personnel so they can remain liable for any problems.

• I would change your TL; DR to "When the temperature is colder outside, your cold water is colder so you have to use more hot water for the same shower temperature". My main point of contention isn't just that it seems that way. It is that way. – Dean MacGregor Jan 20 '16 at 16:44
• And this is why I set my hot water heater to my preferred shower temperature (that and it seems like a waste to "cool down" water that I'm paying to heat). +1 – Comintern Jan 21 '16 at 0:01

Another possible culprit beyond what JPhi1618 mentioned is that electric water heaters often have two heating elements. One at the top, one at the bottom. If the bottom one fails, the top one only heats a portion of the water, giving a short run time. Wikipedia has a nice description: https://en.wikipedia.org/wiki/Storage_water_heater

A few searches in google didn't show me any as small as 20 gallons that used two heating elements though.

Of course, you're right to complain. I don't know your locale, but radiator heating would be a main culprit depending on the setup. Other than that, this can also be due to a water leak, which you might hear as a constant hissing sound.

I've had pipe leaks under the concrete & sometimes you can hear those also hissing in the plumbing & also feel a warm or even hot spot on the floor with your bare foot.

One thing that will test for a water leak & get you a hot shower is to turn the hot water off at the water heater. It stops the water from leaving & lets the heater do a complete heating.

Turn the hot water back on only right when you need it & if there's a noticeable shower difference from before then a hot water line is definitely leaking. Label the water heater & keep the hot water off, especially if you're paying the utilities. You can use it this way until the leak is fixed for a semi-normal life.