Should I replace my mercury switch thermostat?

Question

We have an old mercury switch thermostat in our house, You know the type, gold ring, very common.

We are in new England on a gas/steam system. There is no central AC.

We have been told by more than one heating professional that there really aren't any decent alternatives. The problem is that the thermostat doesn't report the right temperature, so it basically functions as a switch. The other problem is that my wife hates it.

I think we are on a 2 rather than three wire system. The house was built in the late 1920's.

Does anyone have experience with alternative thermostats?

Answer 1

Almost every thermostat on the market today is a better alternative than a mercury switch-type.

Even the cheapest digital thermostats have schedules, so that they can have different heating points during the morning, daytime, evening and overnight, and range from having weekend vs weekday to 7-day indepedent schedules. This allows you to save money by turning down the temperature while you're not home or sleeping.

On the more advanced side of things, there are internet-connected thermostats you can program/control from your PC/phone/etc, and ones that can self-learn based on manual adjustments and motion-sensing. There is a corresponding increase in price, and these typically do require a C-wire so since you only have two wires you'd have to pull a new wire for this.

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To connect a basic digital thermostat to the 2-wires, the best thing is to follow the instructions that come with the thermostat. Typically you're going to connect the two wires between R (or Rh) and W.

Answer 2

I agree with John (?) - simple, reliable, doesn't require batteries or power. Indeed, if you have gas you can have normal heat during power outages. If you are frustrated with the lack of temperature sensitivity, you might try messing with the bi - metallic strip and change its performance characteristics.

Answer 3

Since you say this is an older system, is the thermostat loop low voltage (eg 24V) or high voltage?

A mercury thermostat will work with either.

Most modern thermostats are designed for low voltage, controlling a relay which switches the high voltage.

Since mercury is a persistent environmental toxin, and since most systems here in the states use low voltage control, many municipalities actively recommend replacing mercury thermostats with modern ones and delivering the mercury unit to a hazardous waste site for safe disposal.

Modern thermostats are more accurate, can be set more precisely, can often be programmed to adjust temperature based on time of day, and in the fanciest versions can be operated remotely and/or integrated into a smart home system to do things like lowering the temperature automatically when the space is unoccupied. Depending on the model, they do require batteries, or need to have power supplied to them in other ways; again, details and installation complexity vary from model to model.

If what you have is doing everything you needed it to do, and you don't have to worry about kids or clumsy adults managing to break the mercury capsule, I would say you can leave it alone for now if that's your preference. Personally I would replace it just to make sure that the mercury is handled properly, and I do like the additional features of programmable thermostats, so I would replace it.

Answer 4

For 30 years we had an old round mercury thermostat. It worked great. Then we bought a new Trane furnace with a new digital honeywell stat like the one above. The new stat freezes. It's programed and constantly gets stuck on the previous setting and doesn't adjust when it's supposed to unless I press a button, any button, then it wakes up and almost like it says "oh, sorry, I was sleeping" and corrects it's self. Im tempted to reinstall the mercury stat. And this isn't the 1st digital stat I've had problems with. It seems the digital stats I've encountered have a short lifespan, like smart phones. Perhaps they're designed not to last more than 2-3 years.

Answer 5

I have a mercury thermostat and I have been told by more than one heating and cooling guy that it's the best. It's simple and doesn't freeze up. Ours isn't the most accurate but it does the job.

Answer 6

Mercury thermostats are clearly the best for many reasons not the least of which is they will last forever and never need a battery. They also have fully adjustable heat anticipators which NONE of the digital or Nest thermostats have. ....... Digital and Nest thermostats have dozens of components to fail and die without battery power. So they simply turn what WAS a reliable furnace into a Rube Goldberg machine that breaks down every heating season.

Answer 7

Interesting post. I have mercury switch thermostats. Had the first problem with it ever. It was at the beginning of the heating season. I had set the themostat all the way down for the summer. But the mercury got "stuck" at the end of the glass tube and I had an apparent no heat situation. I took the cover off the thermostat, saw the problem which was easily fixed by turning the thermostat all the way up to the highest heat setting. I watched the mercury roll out of the tip of the tube where it had been "stuck" and turn on the heat. After that it worked fine as it has for years. Requires no internet or battery.

Answer 8

Mercury thermostat switches are the most reliable, but they are not programable, and not really adjustable. And people are funny about temperature. if you set the thermostat at 65 degrees, you'd get upset if it acts at 62 degrees. the other hard truth, is that they are an environmental disaster. They had to stop making them, because there was no good way to dispose of them.