Mercury-bulb thermostat myths

April 3, 2012
The world of thermostats for hydronics is full of myths and misunderstandings. Some of these are misinformation about thermostats in general. Some are specific to mercury-bulb thermostats. And some thermostat myths apply just to hydronics.

The world of thermostats for hydronics is full of myths and misunderstandings. Some of these are misinformation about thermostats in general. Some are specific to mercury-bulb thermostats. And some thermostat myths apply just to hydronics.

Typical of electro-mechanical, non-electronic, mercury-bulb thermostats are the Honeywell T87 and the White Rodgers 1E56. These are sometimes thought of as old-fashioned thermostats since they are not electronic. But the purpose of thermostats is temperature control, and these are still great at doing just that.

So here are some mercury-bulb thermostat myths I want to address, that you may have heard of:

Myth:Mercury-bulb thermostats should be replaced because they are inaccurate. 

Fact:Mercury bulb thermostats are among the most accurate for temperature control within the HVAC industry. Cheap electronic thermostats provide inferior temperature control.

Myth:Mercury bulb thermostats are hazardous and should be removed. 

Fact:The small amount of mercury in a thermostat is safely encased in a glass bulb. Unless the bulb is broken, which isn’t going to happen unless someone smashes it, the mercury is perfectly safe. On the wall with the cover properly mounted, there’s not much chance of the bulb being broken.

Myth:Mercury should be removed from a thermostat when the thermostat is replaced.

Fact:Replace the cover on the thermostat and return the intact thermostat to your wholesaler for mercury recycling. If your favorite wholesaler does not participate, it should be easy to find one that does.

Myth:For hydronics, slide the anticipator all the way to the end of the dial. (This myth is so bad that it does not even say which end.)

Fact:But first, what is an anticipator? Have you ever wondered what the purpose is of the dial inside a non-electronic thermostat — the dial that has decimal numbers on it? This is the thermostat anticipator. It is there to make sure the room temperature is comfortable. The HVAC industry definition of comfort is that the temperature in the living space does not vary by more than a few degrees.

The anticipator is a winding of tiny wire. As the heating equipment runs, a small amount of electric current goes through that winding. This causes a bit of heat inside the thermostat. The thermostat is tricked into thinking the room is a little warmer than it is. So, it turns off the heat a little early. The rest of the heat is supplied by the over-run of the heating system after the call for heat ends.  Without the anticipator, the room would be too hot at the end of every heating cycle.

When we set the anticipator, we are setting cycle rate. It has been found that for most hydronic and forced air systems, the ideal is six cycles per hour (cph). This means that on-time plus off-time is 10 minutes. (Sixty minutes divided by six cycles per hour equals 10 minutes per cycle.) As the temperature gets colder outside, on-time takes more of the cycle and off-time takes less. Cycle rate for cast iron radiators is usually three cycles per hour. That’s because it takes longer for cast iron to give off all its heat. Electric heat is usually nine cycles per hour.

For six cycles per hour, you simply set the anticipator to match the amp draw of the load. For example, a zone valve load might have an amp draw of 0.3 amps. (It’s written on the valve and on the outside of the box. It’s also in the technical literature.) Or you can use an ammeter to measure the amp draw of thermostat circuit. New equipment comes with instructions from the manufacturer as to where to set the anticipator.

For hydronic fin tube radiation, set the anticipator to match the amp draw of the zone valve or zone pump. That will give you six cph, which means that the heat will come on (if it’s needed) six times an hour or every 10 minutes. For cast iron radiators and infloor radiant, multiply the amp draw of the load by 1.2.  That will give you three cph, or the heat on every 20 minutes. 

The bigger the numbers on the anticipator, even though they are merely decimals, the longer the cycle. That means the heat comes on less often, but is on longer. It also means greater temperature swings and less comfort. So setting the anticipator at one extreme end or the other will get you either rapid cycling or big temperature swings — neither desirable.

Myth:Mercury bulb thermostats need to be calibrated.

Fact:This myth started long ago. The T87 round thermostat came with a calibration wrench for adaptation to a unique steam heating situation. It was never because thermostats work themselves out of calibration. When a thermostat seems to be out of calibration, it may be out of level. 

Another reason for a thermostat to seem to be out of calibration is that the anticipator is not set correctly. The living space gets too hot during the heat-on portion of the heating cycle, and too cold during the too long heat-off cycle. Or the heat goes on and off too rapidly. See the explanation above about the anticipator and cycle rate.

Myth:Leveling a thermostat is for appearance only.

Fact:Leveling mercury-bulb thermostats is critical for comfort control. For every degree out of level, the thermostat’s accuracy is off by one degree of temperature. To level a round thermostat, remove the ring and notice the two clips at the top that hold the ring on.  Lay a small level across these two clips.

Myth:A cheap thermostat works just as well as a fancy one.

Fact:A thermostat is one of those things where you get what you pay for. Really cheap ($10 or so) electro-mechanical thermostats have their purpose — where tight temperature control isn’t important, or where the thermostat is likely to get damaged. 

The reason they have big temperature swings is that they have no mercury bulb.  Temperature sensing is done only by a relatively heavy piece of metal called a bi-metal.  It takes a lot of temperature change before the bi-metal senses it. Temperature swings will be big — maybe 10 degrees or so — but in a warehouse it probably doesn’t matter.

Myth:If a six-inch ring comes in the box with a round thermostat, you need to mount it on the wall.

Fact:The ring has no function other than covering wall blemishes, for example if you’re replacing a rectangular thermostat. The ring is called a “beauty ring.”

All in all, old fashioned mercury bulb thermostats are still great to use on your hydronic systems. The mercury poses no hazard as long as it remains in the glass bulb. The accuracy of a mercury-bulb thermostat is close to the best electronic thermostats, and far superior to the cheap ones.

In my next column I’ll discuss electronic thermostats!

Carol Fey is a technical trainer who has been in the HVAC industry for over 25 years. You can find her books and DVD at  To see her adventures while a heating mechanic in Antarctica, go to

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