Yesterday’s Thoughts

September 1, 2006

Hot Water/Cold Water

Steve Yegge wants to hear about my gross conceptual misunderstandings. I’ve had a couple in the hopper to blog about for a while, so I’ll take the opportunity of this new blog to mention two I’ve become clear on over the past few years. Both relate to plumbing and water temperature.

Once when I was about five, I had to go to the bathroom while my dad was in the shower. In that house on Zorn Avenue, we only had one bathroom, so I knocked on the door and asked if I could go. He said, “Yes, but don’t flush the toilet.” “Why?” I asked. “Because flushing the toilet will make the shower water hot,” he said.

To this thought went into my five year old brain in exactly those words. And stayed.

A few years later, I’ve grown up a little and taking showers myself. I notice that it often takes a while after turning on the tap for the water to become hot enough to use and I’m left standing naked in the cold bathroom. Accessing my memory I find this nugget, “flushing the toilet will make the shower water hot.”

OK then, flush away.

For 30 years whenever I was in a particular hurry, I’d turn the shower on and flush. Living in Northern California, the water isn’t ever that cold, and we now have insulated pipes, but in Pennsylvania winters in a 200-year-old farmhouse, getting the shower water hot often seemed pretty urgent.

Eventually I realized that it made no sense.

You would think that I would have realized that it didn’t work, except of course, it did work eventually. Just like the medical patients that got well no matter what the doctor did, or didn’t do, the shower water did eventually get hot. Every time. I had no control in my experimental protocol.

Why this I was wrong When my dad was in the shower, he had the water adjusted to the proper temperature. Essentially he was mixing hot water at a constant pressure with cold water at a constant pressure to create the proper temperature for his bath. If the toilet were flushed, the pressure on the cold water entering the shower would go down by about half, assuming the shower and the toilet pipes were the same size. Suddenly there was less cold water and the water coming out of the shower head was hotter.

This isn’t what happens when you first turn the shower on. When you turn the shower on the water is cold because there is a certain volume of water in between you and the hot water source and that water has normally cooled off. Until this water is flushed from the pipes, there won’t be any hot water into the shower and the water will be cold.

How to get hot water faster A year or so after I realized my error, Queenie pointed out the real way to make the shower water warm faster. If you have a tap that flows into the tub and a diverter that redirects the water to the shower head, don’t turn on the diverter until the hot water is flowing into the tub.

Why this is works All the clues are in the previous answer. When you turn on the diverter, the hot water pressure is lower. It is lower because 1) the shower head is higher than the tap and 2) because the diverter and the shower head constrict the flow of water. The result is that it takes longer to flush the cold water from the hot water pipes when the water is coming from the shower head that when it is coming from the tap. This actually does make the shower water hot faster.

Sorry, comments for this entry are closed at this time.