Jan 22, 2013

K-Fail: The *Other* Hot Water Problem

This isn't really a K-fail, more of a K-mystery.

For months, I was dealing with a strange phenomenon. When I turned on my shower, I would have medium-hot water for approximately two minutes, at which point it would turn cold. You can imagine how unpleasant this was in an unheated bathroom in the dead of winter. My hygiene may have suffered slightly. I found myself showering at the gym a lot-- choosing the stares of old Korean women over the inhumane conditions of my own shower.

It wasn't until about 3 weeks ago that a friend of mine let me in on what is apparently an open secret in Korea: When showering, don't turn the water pressure on all the way. Only turn it about 80% on. I tried it, and was treated to a scalding (seriously: face-meltingly hot) shower that lasted (seemingly) forever. I was appalled. I still am. There is no logical reason for this to be happening.

First of all, my apartment is heated by hot water. It's pumped through the pipes under my floor that make up the ondol system I have gushed (pun intended) about frequently. My floor heats up approximately to the temperature of the heated seat in a car. This is no lukewarm rubber water bottle. That water is hot.

It stands to reason, therefore, that there would be enough of the stuff to give me more than 2 minutes of a  moderately hot shower at full pressure.

But say there isn't. Let's say that there is a limited quantity of hot water, and the pressure at which I run my shower determines how long I can take a hot shower. The math still doesn't add up. By reducing my water pressure by about 20%, I figure I should be able to extend my shower by about the same amount. Along those lines, the more I reduce my pressure, the longer my shower should be. But so far, I haven't found a significant difference between, say 50% water pressure and 80% water pressure. They both result in a very hot shower lasting around 40 minutes. Yet, if I turn the water up to full pressure, it turns cold in mere moments.

Let's keep my carbon footprint out of this for now. It's been really cold here, ok? Like... really, really cold. Also remember that I was taking 2-minute showers for months.

This is so baffling to me that I even tried to figure out a mathematical formula to illustrate the relationship between water pressure and length of hot shower. Seeing as I haven't taken math since junior year of high school, I obviously failed miserably.

The only theory that works is one posited by my more mathematically-inclined friend. She suggested that the lower pressure gives the heater time to heat up more water. While this makes sense, it doesn't explain the giant discrepancy between 80% and full power, or the fact that the water coming out is SO BOILING LAVA HOT that it should take much longer to heat up than the few moments a slight change in water pressure can give. Water has a high specific heat, meaning it takes a long time and/or a  lot of energy to heat up.  (How you like me now, Mr. Borland*?!) We've established that my water heater is powerful, but it's not a miracle worker. Moreover, if it's so powerful, how come it just can't heat up water fast enough for me to take a shower at full power?

Anyway, this is an ongoing conundrum for me, albeit one I thoroughly enjoy testing now that I can take gloriously hot showers.


*Mr. Borland was my high school freshman science teacher. We loathed each other despite the 95% I got in his class.


  1. also, its likely that you only get X amount of volume flow from hot water, (being that the source is a small tank, not the ocean), and to get you to the water pressure you want, your shower simply adds more cold water.

    like you demand a 16 ounce gin and tonic from your bartender and he says "well five dollars only gets you one shot of gin" but in korean so you dont understand, so he just fills it up with cold shower water. My bathroom (and bartender) in cali both do this

    1. First of all, thank you for the first comment. :)

      Second, good point about volume flow. I hadn't thought to take that into account. That probably explains it. Hot water can only flow from my tank to my shower at a rate limited by the size of the piping, so it would stand to reason that lowering the water pressure to equal or below that rate would result in a big jump in temperature and duration. Bravo! I think we solved the mystery!

  2. That sounds about right @gods_basement. From what I remember about pressure from gen chem both temperature AND volume are directly related and have to be taken into account. Also, nice post Meg! You did SCIENCE :)

  3. I've learned a lot of things regarding Korea and what things I should do once I live there that will absolutely help, especially about the water heater because winter in Korea can be very very cold. Of course, the smaller the tank, the limited the water supply is. So if you want more hot water, I guess replacing your tank would be better. Elia Lester