The Awkward Engineers Uses Bits of Wax to Make His House Better

As winter approaches, I am 100% still geeking out about some old school mechanical technology I installed in my home that let me move from a single thermostat to 5 zone temperature control.

I live in an old house (built in 1900!), and we have two floors, heated by single pipe steam radiators. The problem was trying to keep the temperature throughout the house even, particularly at night.

Although a steam expert will tell you it's possible to "balance" a steam system by sizing vents to control how fast steam displaces air in the radiators, I never managed to get it in tune. Typically, on a very cold night, the downstairs would barely stay warm, while the upstairs would get super toasty.

Enter the thermostatic radiator valve, which adds a simple, mechanically driven control loop to the heating system, all built around a bit of wax.

First, Some Background on Steam Heat

Before we dive into that, a couple quick points on single pipe and dual pipe heat, and how it works.

Two pipe systems are rather intuitive - you pump hot fluid in one end, it dumps heat into the room via the radiator, and cooled off fluid flows out the other end, where it returns to the boiler.

In single pipe steam heat, one pipe is used both for outward flow and the return, which, not coincidentally, saves on both material and labor to install.

  • When the thermostat calls for heat, the boiler kicks in and generates steam, which then pushes air out of the pipes via the radiator vents.
  • The vents are temperature sensitive (usually driven by expansion of an alcohol vapor), so when the hot steam hits them, they close, which avoids venting scalding hot steam into the room.
  • The steam cools off, condenses back into water, and the radiator vents open again, letting air back into the pipes. Gravity then pulls the water back to the boiler.

We've skipped over some of the details, but the important part is the steam pushing the air out of the radiator vents. The whole system is rather clever, in my opinion.

The Thermostatic Radiator Valve

The thermostatic radiator valve improves on the system by sensing the ambient temperature in the room and shutting off airflow out of the radiator, so that steam can't enter in the first place.

The fully mechanical sensor uses a wax element (rather than a thermistor, or some other electronic device), which has a high coefficient of thermal expansion. In my installation, the sensor is placed several feet away from the radiator, where the expanding wax can't drive the valve directly, but get's a better reading of the room as a whole. Instead of direct actuation, it's connected through a small capillary tube, which then connects to a small piston, which actually drives the valve.


By mechanically adjusting the dial, you can adjust the starting position of the piston, and therefore, the temperature at which the valve shuts off.

And BOOM. You've just used a small piece of wax to control the radiator. Good for several thousand cycles and 15 years of operation. And no electronics, whatsoever.

Now, to be clear, these slick little mechanical controls can't call for heat when it's too cold, but they do prevent rooms from over heating, which was the problem I was trying to solve in the first place.

So now that I have these sensor on all my upstairs radiators, I effectively have 5 zone temperature control retrofit onto a 100 year old heating system.



Best regards!

P.S. Fun fact, I've seen wax driven technology used on satellite telescopes as a "pin puller" on a latch mechanism. When it was time for a mechanism to deploy, a heater melted wax, causing it to expand, driving a piston which released a latch pin. Simple, reliable, and completely free of explosive bolts which could dirty up optics. Just keep it out of direct sunlight, on orbit :-).

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