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Are you wondering what to do with your old mercury thermostat? Are they actually dangerous for human health and the environment? Keep on reading to get an understanding of how mercury thermostats work, how to properly discard them, and how to upgrade to new digital versions.

History of mercury thermostats

Mercury thermostats and thermometers have been widely used in households, medical facilities, and other organizations over the past century – from measuring the room temperature to medical use. The use of mercury-filled devices has significantly decreased since 2005 due to mercury’s hazardous nature. The thermostats have been replaced with new, innovative digital versions.

At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.


The accuracy of a mercury thermostat depends on whether the thermostat was properly calibrated, mounted in the right location, and leveled on the wall. With many influencing factors, especially when it comes to imprecise calibration, traditional thermostats can fail in providing a high accuracy temperature reading.

Working Principle

Traditional thermometers use mercury to read and control the temperature in a space. The mercury is contained in a glass tube, and based on the fluctuations of temperature, it can reduce or increase its volume. Moving up and down on the reading scale, it will display the current temperature in a room. To get an accurate reading, the thermostat must be properly calibrated by using a thermal equilibrium technique. 

A traditional analog thermostat consists of a mercury-filled thermometer that is attached to a mechanical switch, mainly used to operate and control heating, cooling, and air conditioning systems. Built with bimetal coils that trigger the switch, the components of your HVAC system are being turned on or off – so that your space is warmed up or cooled down depending on the room temperature. Depending on their use, commercial or industrial, mercury thermostats can have multiple switches thus activating different heating and cooling systems. 

Are Mercury Thermostats dangerous?

There are many myths and stories around the toxicity of mercury, the only metal found in a liquid state. But is it really dangerous considering it has been used in thermostats and a variety of other household appliances over the years? The fact is, as long as the mercury does not leave the thermostat, it is not dangerous. However, direct contact with mercury can be extremely hazardous. 

Thermostats contain only a small quantity of mercury and even in the unlikely event of the housing breaking, the mercury can be safely cleared away by specialists using strict precautions. Bigger quantities of spilled mercury can be problematic though. Once it comes in contact with air, it evaporates and forms toxic mercury vapors which can cause serious health issues if inhaled. Similarly, when it comes in contact with skin when you’re not wearing special protective gloves, mercury can be extremely harmful. 


At the same time, this silver-colored liquid metal can be quite fascinating if it is spilled on a surface. Also called ‘quicksilver’, mercury’s unique properties make it roll fast on most of the surfaces and will divide into smaller parts if pressure is applied. Then it has the ability to quickly spread and contaminate different areas. 

If you encounter this situation, make sure young kids are not around – as it might be quite tempting and entertaining for them to fiddle with the small droplets of mercury.

Having said that, unless you are directly exposed to a large quantity of mercury, having a mercury thermostat is safe. 

Disposing of Mercury Thermostats

As we already know, mercury can be extremely dangerous not only for our health but also for the environment, being a source of pollution for air and water. If a mercury thermostat is just thrown out in the garbage, the inevitable mercury leakages can quickly contaminate the surroundings.  

Considered to be hazardous waste, mercury thermostats have to be recycled properly. That’s why it is illegal to dispose of mercury thermostats along with regular garbage. Some states have completely banned the use of these thermostats in households and recommend switching to newer digital ones.


But how can you discard your old thermostat in a responsible and environmentally-friendly manner? 

Falling in the category of hazardous waste, recycling mercury-filled thermostats is not cheap and this can lead to inappropriate disposal. 

This concern has been raised by the authorities and, thankfully, along with the HVAC industry specialists, a new and simple solution has been developed to mitigate the risks of the potential mercury waste and contamination – the Thermostat Recycling Corporation. 


With little costs involved, this government-approved program is meant to facilitate the proper disposal of mercury thermostats and it is available for both contractors and consumers. This program clearly encourages a safe and conscious approach towards keeping our natural environment free of dangers. 

So if you want to get rid of your old thermostat, make sure you contact the official organization that handles the recycling of items that contain mercury in your area. A quick search can indicate the nearest mercury recycling center based on your location.

Upgrading from Mercury Thermostats

Modern digital thermostats are definitely safer than traditional mercury thermostats and they also have many more benefits. From energy-saving to remote access via a smartphone app – the new digital thermostats not only help you save money but also eliminate the potential dangers caused by hazardous substances like mercury. So even if your old thermostat is still working, give it a second thought and consider upgrading to a newer and smarter device, built to offer a hassle-free experience.

Mercury-free thermostats

With more and more laws and regulations in place, mercury-filled thermostats are being phased out. The new generation of digital programmable thermostats has been rapidly introduced and adopted by many house owners. Offering increased flexibility and accuracy, mercury-free thermostats are also environmentally friendly and can greatly reduce overall energy consumption. 

How to replace an old mercury thermostat?

Replacing your existing mercury thermostat can be done as a DIY project as long as you have an understanding of how to deal with the wires. The first step is to remove the faceplate of the thermostat so you can get to the wires and determine the correct wiring system. However, if you are not comfortable with this, you can reach out to an HVAC technician that can easily get this done for you.

Wiring systems

While most thermostats have an older two-wire system, some do have the modern four/five-wire system. Simply remove the thermostat and see how many wires are coming out of the wall. Wires come with color codes, so it might be useful to take a picture of the original wiring or sketch it on a piece of paper. 

Also check if the old thermostat was using line voltage, 24V or mV. Then, you can place the new thermostat on the wall and start the rewiring process. 

For understanding how to wire the new thermostat, check out our wiring guide here – Guide to Thermostat Wiring

The Verdict

The usage of mercury thermostats in households is slowly coming to an end – not being available for purchase in many states already. They are quickly replaced by smart and digital versions, which are now extremely popular thanks to their flexibility and efficiency. 

If you are considering replacing your old thermostat, check out our guides on smart thermostats and choose the one that matches your needs.


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