Mercury Thermostats

Until the 1990s, Mercury thermostats were used extensively to control residential and light commercial HVAC systems. Together with a sealed glass bulb, mercury was used as part of the switch in the older style thermostats. There were numerous advantages to the mercury thermostat design. However, the toxic nature of mercury has resulted in these thermostats being phased out. Prolonged exposure to mercury may cause severe health effects. In addition, mercury is harmful to the environment.

Accurate Heating & Cooling is an established HVAC contractor serving the Omaha metropolitan area since 1998. We are happy to advise you on a replacement thermostat that will meet or exceed your needs. There are many thermostat designs available today to meet your needs. There are very simple thermostat designs where you just set the set point manually. Then you select heating or cooling mode. Maybe there is an auto or on fan setting. There are also programmable thermostats that have a 5 day or a 7 day schedule. Another popular design is the wireless thermostat that is accessible using a laptop or smartphone. Give us a call today, 402-238-2425. We will replace your current mercury thermostat with a new one that will improve the temperature control of your home.

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Honeywell Model T87 Mercury Thermostats

The photo below shows an older Honeywell model T87 thermostat that contains mercury. This thermostat came from one of our residential customers homes. They wanted to upgrade to a newer style thermostat that would provide improved temperature control.

Honeywell model T87 mercury thermostat

This T87 thermostat had settings for Cool, Off, or Heat. And it also had options for the blower fan to be On or Auto. When in auto, the blower fan in the furnace ran when either the furnace or a/c was running. The on position had the furnace blower fan run all the time. This blower on feature has some advantages. Keeping the blower on all the time helps to circulate air in the house. This helps to even out temperature differences in the house. And it also helps to circulate air across the air filter. This helps to remove contaminants from the air.

Honeywell Model T87 Manuals

A manual for the Honeywell Round Mercury Thermostat CT87A,B,J is available here.

The model of the Honeywell thermostat pictured above is the T87F model. A manual for this particular Honeywell Round Mercury Thermostat T87F is available here.

The photo below shows some of the internal mechanical components of the Honeywell T87 thermostat.

Honeywell model T87 Mercury Thermostat showing mercury bulb

The coil of metal wire in the photo above is the coiled bimetal strip. If the bimetal coil were removed and laid out flat, it would be a flat strip of metal. The metal strip consists of two different metals bonded together. The two metals that are commonly used are steel and copper. Consider the metal strip that was laid out flat. The top surface would be one metal, for example the copper. And the steel part would be on the bottom. The two different metals expand or contract based on temperature change.

Key here is that the rate of expansion or contraction is different for each metal. The bimetal strip is coiled so that it takes up less space. When the coil is heated, the coil uncoils slightly. When the temperature lowers, the bimetal coil will coil tighter. One end of the bimetal strip is connected to the glass bulb that contains the mercury puddle.

As the tightness of the bimetal coils changes, the orientation of the glass bulb also changes. The mercury puddle within the glass bulb can then occupy one or the other end of the glass bulb.

Glass Bulb Containing Quicksilver Puddle

The photo below shows three electrical wires that penetrate the glass bulb.

Honeywell model T87 Mercury Thermostat showing mercury bulb and terminals

The above thermostat is a cooling and heating thermostat. One of the three wires is the power wire. It connects 24 VAC power to the switch. One of the remaining wires is for heating and the third wire is for cooling.

A drawing of the arrangement of the three wires within the glass bulb is shown below.

Drawing of Glass Bulb Switch in Mercury Thermostats

Mechanical Thermostat Bimetal Coil

As the bimetal coil changes position with temperature, the glass bulb tilts. When room temperature is cold, the bimetal coil uncoils tilting the glass bulb. The mercury puddle moves to the end shown in the drawing above and closes the switch between the power wire and the heating wire. This gives a call for heat to turn on the furnace. When the room is cold, the bimetal coil contracts. This tilts the glass bulb the other direction. In the cold case, the puddle of mercury is on the other side of the glass bulb. In this case, the switch between power and cooling closes. This starts the air conditioner.

A video is available here which describes the operation of an older mercury thermostat. The thermostat in the video is a different model than the T87 described above. However, the principles of operation of the mercury switch and the bimetal coil are the same.

The design of the mercury thermostat was fairly robust as far as thermostats go. There are not a lot of things that can go wrong with them. However, even being fairly reliable, all of the mercury thermostats out there have gotten some age to them. We see these older mercury thermostats failing. They often will develop intermittent issues. The furnace or air conditioner may start and run for a while. But then the unit will just shut down before the thermostat set point is reached. If your HVAC system is experiencing this, it may be a failing thermostat.

One can pick up a new thermostat from the hardware store fairly inexpensively and replace it yourself. Or, give us a call and we will install a Trane, Honeywell, or Emerson thermostat for you.

Newer Style Round HVAC Control Options

If you are really attached to the older round thermostat design, there are even new round thermostats available.

Honeywell round thermostat in shape of older mercury thermostat

The photo above shows a Honeywell round thermostat that does not contain mercury. The customer with this thermostat replaced his failed older mercury thermostat himself. He wanted a simple thermostat that was not programmable. And he was accustom to the round thermostats. So he went with the model shown from Honeywell.

The customer reports that this particular thermostat does not sense room temperature all that well. He said that when he has us replace his older furnace down the road, he would like us to also install a newer thermostat that maintains the home at a closer temperature. He would even go with a programmable thermostat as long as it is of a simple design. We can definitely install a thermostat that he will be happy with when he is ready.

Why Replace Mercury Thermostats

The older mercury thermostat in your home may eventually fail. One doesn’t have to wait that long, however, to upgrade to a newer thermostat. The newer programmable thermostats offer energy savings and also create a more comfortable home temperature. Plus, replacing the mercury thermostat has some possible health benefits. There is always a chance that the glass bulb containing mercury could fail. The mercury would then be released into the home. Mercury is a highly toxic substance. It is known that Mercury causes poisoning from exposure to either liquid or vapor form.

The Nebraska Department of Health and Human Services offers the below guidance on their website here.

Liquid Mercury Toxicity and Spill Cleanup

Mercury is a chemical element that is also called quicksilver. It’s present in air, soil, and water. Elemental or metallic mercury is a heavy silvery-white metal in liquid form at ordinary temperatures. Organic mercury is released into the environment from fossil fuel combustion, mining operations, trash incineration, and industrial waste discharges. Exposure to mercury may affect and harm your brain, heart, immune system, kidneys, lungs, and central nervous system. Carefully manage mercury-containing products to prevent an accidental release and protect your health.


Did you know the most toxic effects from mercury exposure come from breathing in contaminated air? Short-term exposure to high concentrations of mercury may cause chest pain, chest tightness, chills, coughing, difficulty breathing, fever, and headaches. You may also experience abdominal pain, diarrhea, mouth soreness, or nausea. Symptoms may subside with time, or could get worse and damage your kidneys and lungs.

Short-term exposure to mercury may not lead to long-lasting health effects. Long-term exposure to mercury liquid or vapor may cause effects gradually developing over time like fine shaking of the eyelids, hands, jaw, lips, or tongue. Shaking could indicate damage to your central nervous system. If you have concerns about exposure, please see a doctor.

Other known effects of mercury poisoning are allergic skin rashes, indecision, insomnia, intellectual deterioration, irritability, and memory loss. Metallic mercury may also negatively affect the health of an unborn fetus.

Actions on Mercury Spills from Mercury Thermostats

As per the Nebraska DHHS website, in the event of a mercury spill, don’t vacuum! Heat generated by vacuums may turn mercury into a toxic gas that can be inhaled. Vacuums scatter mercury and may spread it to other areas of your home. Mercury may be easily spread to other areas of your home by clothing, pets, and even your shoes. Block access or shut the door to the room where the spill occurred and don’t let children or pets into the area.

Ventilate! Open windows, shut vents, and turn off your furnace fan. Mercury easily evaporates at warmer temperatures. If your air conditioning is on, turn it off. Open your windows to encourage off gassing.

If the spill is larger than a thermometer, follow these guidelines. In addition, call the Nebraska Mercury Call Line at (888) 242-1100, Extension 4. As a matter of fact, if the spill is more than two tablespoons, it’s mandatory to call the National Response Center at (800) 424-8802.

Clean carefully!

Use the “flashlight test” to find all mercury that spilled. Getting the area or room as dark as possible, shine a flashlight around the area to see where the spill occurred. Since mercury is metallic, it reflects light. Collect all visible recoverable spilled mercury using an eyedropper, or by gently scraping the mercury up with a card or paper. Put the mercury in a non-metal airtight container for proper disposal or recycling. Don’t put mercury in dumpsters, sinks, or toilets. Remove and dispose of all absorbent materials like carpeting or upholstery that were contaminated. Dispose of any items used for the clean-up such as brooms, cards, clothing, and gloves.

If you stepped through a mercury-contaminated area, it’s possible you might have trace amounts of mercury on the bottom of your shoes. Trace amounts of mercury evaporate to the surrounding environment quickly, especially in warmer temperatures. Moreover, remove your shoes as soon as possible to avoid spreading contamination further. Leave your shoes outdoors at least 2 to 3 days to off gas any remaining mercury vapors. You may also throw your shoes away in the regular trash if you wish.

Where to Take Mercury Thermostats

When we replace a customer’s older mercury thermostat with a new thermostat, we properly dispose of the old thermostat. We take mercury thermostats to local HVAC Wholesale Supply Houses. By the same token, they normally have a collection box for mercury thermostats. While they sell wholesale only, they usually welcome homeowners who want to bring in a mercury thermostat.

The photo below is of one such mercury thermostat reclaim location at a wholesale HVAC equipment supply house in Omaha, Nebraska.

mercury thermostat collection location at HVAC wholesale supply house in Omaha

Other Interesting Topics

Refer to our Home Page here to learn more about Accurate Heating & Cooling.

Check out this blog for information about Home Humidification. And have you ever wondered what a ton of air conditioning means? Check out this blog to learn about what a ton of refrigeration is.

For more information about Electrostatic Air Filters, refer to this blog.

Refer to this blog to learn what HVAC maintenance items they can do to improve the life of their Omaha HVAC equipment.

To learn more about which air filter is best for your home, refer to this blog.

Accurate Heating & Cooling is an established HVAC contractor with over 20 years of experience. We proudly serve the Omaha metro area and surrounding cities. Give us a call today. We can replace your older thermostat with one that will improve indoor air temperature control. There are thermostat options that will also improve the overall efficiency of your home HVAC system. Call us at 402-238-2425.

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