One of the most common first-world gripes is getting the water temperature right when taking a shower. It’s a common experience that the water is too cold, you move the shower handle a tiny little bit to make it warmer, and suddenly instead of slightly too cold, you now get way too hot water.

Ben Holmen recently conducted a little experiment, and proposed a better way to design a shower valve, it makes for a fun read.

This prompted someone who is more familiar with the subject to comment on the related Hacker News discussion:

As a friendly heads up to anyone exploring this space — temperature control is a complex dynamic between many components, chiefly the mixing valve, any local thermostatic dampers in the valve (rarer, but definitely a feature of high end mixers), and the source of hot water itself.

Mixer taps are not simply mixing hot water at temperature H and cold water at temperature C. The temperature H which your hot water source is capable of reaching is highly variable and certainly not constant. Higher flow for example reduces temperature as more cold water cools the heater. Heaters also cut out and oscillate as they overload.

Throughout the year the C temperature may also fluctuate by several degrees in a way that can trigger non linear effects in the hot water generator. (This is all assuming you have a just-in-time hot water source of course — more common in Europe than, say, NYC apartment blocks.)

Study the whole system and be prepared for unexpected non linearity everywhere. Heat loss per foot of pipe is a real thing, as is thermal damping of uninsulated tubing, to give two more examples.

If you can model the system then next up, you might try your hand at predicting the weather.


This is obviously very random, but something I’ve thought of often, and I love it when smart people take the “thinking about something” to the next level.






Leave a Reply

Your email address will not be published. Required fields are marked *