Mercury bulb thermostats have been ‘mandatory equipment’ for decades. Chances are, however, you’ll never miss them now.

There’s lots of lamenting that the age of mercury thermostats is coming to an end. But I think it’s a positive step forward with not much lost at all. Mercury-free models look the same on the outside so you won’t get customer complaints. On the inside they work even better at temperature control, and the price is about the same.

When I was a kid, mercury was a cool thing to play with. Can you imagine! Now mind you, this was the 1960s, when folks thought spraying DDT promoted life and seatbelts were a Communist plot. So in context, it’s not crazy that among my toys was my own little jar of mercury.

Not all kids had mercury, but once they played with mine, they wished they did! I took my mercury to school for sharing-time only once. Everyone took the mercury out of the jar and rolled it around in their hands. My mercury was ruined by dirt from all those little hands. That’s OK, though, because my - ironically, safety-freak - electrician dad got me some more.

Today there’s not much debate that mercury-on-the-loose is a bad idea. Yet there is grumbling about the end of mercury bulb thermostats. I want to talk about why mercury bulb thermostats are being phased out, why they have been great, why their going away isn’t a big loss, and how you can responsibly discard mercury thermostats for free.

Before Electronics: First, let me explain how we got mercury bulb thermostats in the first place, and why we hate to see them go away. Back in the days before electronics, there was a problem making the switch in the thermostat sensitive enough to heat change that it could keep the temperature within just a degree or two of set point. That couple of degrees, by the way, is what’s known as “temperature swing.”

The heating industry standard is that for people to feel comfortable (and we are in the comfort business) the temperature can’t vary more than 2 degrees. That’s also known as “plus-or-minus-one-degree.” A nonmercury bulb, nonelectronic thermostat may operate at plus or minus 5 degrees - a temperature swing of about 10 degrees. That might be OK in the garage, but certainly not in the living room.

The temperature sensor in a non-electronic thermostat is a bimetal. A bimetal is two different metals sandwiched together under the principle that various metals expand at different rates. The two chosen metals create a predictable movement of the bimetal caused by change in temperature. The movement of the bimetal causes the switch in the thermostat to close and open. The switch closes, and the heat comes on. The switch opens, and the heat goes off. So simple! Such a big temperature swing, because it takes a lot of heat to get that chunk of bimetal to move!

What we needed was a metal that would move with a tiny bit of temperature change. Now think more about the properties we need inside the thermostat. The material needs to be metal so that it can conduct electricity and serve to close the switch. It would also be useful if that metal were liquid at room temperature. Is there such a thing? You betcha - it’s mercury.

So here’s what the thermostat manufacturers did. They kept the bimetal. But on the end of the bimetal they attached a glass bulb containing mercury. Inside the bulb, they also put the switch that turns the heat on and off.

The bimetal senses a degree or two of temperature drop. Because metal contracts when it cools, it moves a bit, but not enough to close a switch. However, its tiny movement is enough to tip the mercury bulb, and all of the mercury flows to one end of the tube. That’s the end where the switch is. The mercury flows to cover both terminals of the tiny switch, and the switch is closed. The heat comes on. When the mercury flows in the other direction, the heat goes off.

Electronics: Why do away with a great thing? Because for about the same money, we now can get even better temperature control from many electronic thermostats.

Notice that I didn’t say all electronic thermostats. Temperature control accuracy varies among manufacturers. But if you stay with the manufacturers who made mercury bulb thermostats, you’ll have great temperature control with their electronic thermostats, too.

Now, what are we going to do with those millions of installed mercury bulb thermostats as they’re replaced? Dispose of them responsibly and for free. If you’re a contractor, take the whole thermostat, any brand, to a thermostat distributor who’s participating in the national recycling program. There are lots of these distributors. You can find them on the Web at

Or if you’re a contractor with seven or more technicians, or if you’re located in a rural county, you can get set up to return on your own for a one-time fee of $15. The program picks up all other costs, including replacement shipping containers. There’s no paperwork and no tracking.

This recycling is done by the Thermostat Recycling Corp. TRC was established in 1998 by three major mercury bulb thermostat manufacturers - Honeywell, White Rodgers, and General Electric. NEMA stands for the National Electrical Manufacturers Association. It was founded in 1926 to provide a forum for the standardization of electrical equipment.

There is something else lost with the passing of the mercury bulb thermostat besides mercury. Tune in next month to find out why that’s going to be OK, too!