# Failure mode of mercury-switched thermostat



## davey (Aug 14, 2010)

A customer's hydronic, separately controlled heating system is not holding the way he's used to. It's controlled by a Honeywell 87A thermostat, whose contacts connect through mercury. Unless wires have gotten loose, the cause of the problem seems likely to be either the furnace or the thermostat. I had assumed that the contacts would not wear appreciably even over many years, so the thermostat wouldn't fail off or on until the beast actually broke; and its failure modes would not include reduced temperature-maintenance accuracy. 

Does anybody know these beasts well enough to offer information, as opposed to my speculation? 

I used to encounter mercury toggle switches now and then, but haven't for quite some time. Not surprising, given that they haven't been made for something like a quarter century. Brian Rock, of Hubbell and a number of electrical safety groups, gave me to understand that only GE made the mercury modules, and they stopped making them that far back.


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## LGLS (Nov 10, 2007)

Even the simplest of those round thermostats had a heat anticipater. No idea how the sucker is supposed to work. Know that there's 2 ways to wire boilers. The old tried and true was to allow the boiler's thermostat keep the boiler hot and ready to pump hot water through a zone on a moment's notice. But the newer way is that the boiler completely shuts down when there is no call for heat from any zone.


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## davey (Aug 14, 2010)

Good thoughts, LGLS. This does indeed have a heat anticipator, which I understand heats the thermostat faster than ambient air heats, so it doesn't keep the furnace going till the temperature overshoots the desired set point. Undamped oscillation would switch the furnace off and on too frequently. Perhaps the resistor has burned out; I don't know if the circuiting allows it to continue working if that happens.


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## Signal1 (Feb 10, 2016)

davey said:


> Good thoughts, LGLS. This does indeed have a heat anticipator, which I understand heats the thermostat faster than ambient air heats, so it doesn't keep the furnace going till the temperature overshoots the desired set point. Undamped oscillation would switch the furnace off and on too frequently. Perhaps the resistor has burned out; I don't know if the circuiting allows it to continue working if that happens.


Basically yes, the anticipator is a variable resistor that acts as a small heat source to avoid "swing" or overshooting the room temp.

By providing a small source of heat in the thermostat, it shortens the heating cycle a little early by kind of fooling the thermostat that the room is a little warmer than it really is. It "anticipates" residual heat from hot radiators, baseboards, or the blower that will continue to heat the room after the heat cycle has ended.

It should be set to the ampere rating on the primary control, or the reading across the terminals with an ammeter. 

It is in series with the control circuit, so if it has failed the furnace wont run.

Simple check with an ohmmeter.


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## emtnut (Mar 1, 2015)

davey said:


> A customer's hydronic, separately controlled heating system is not holding the way he's used to. It's controlled by a Honeywell 87A thermostat, whose contacts connect through mercury. Unless wires have gotten loose, the cause of the problem seems likely to be either the furnace or the thermostat. I had assumed that the contacts would not wear appreciably even over many years, so the thermostat wouldn't fail off or on until the beast actually broke; and its failure modes would not include reduced temperature-maintenance accuracy.
> 
> Does anybody know these beasts well enough to offer information, as opposed to my speculation?
> 
> I used to encounter mercury toggle switches now and then, but haven't for quite some time. Not surprising, given that they haven't been made for something like a quarter century. Brian Rock, of Hubbell and a number of electrical safety groups, gave me to understand that only GE made the mercury modules, and they stopped making them that far back.


"not holding the way it used to" is kinda vague .

I'm not familiar with hydronic heating much ... but depending on the customers complaint, it could be something with the furnace.

If you think the thermostat (bad anticipator) is to blame, you can replace it with this one. 
http://www.honeywellstore.com/store/products/the-round-non-progammable-thermostat-ct87k1004.htm

Not mercury, but the same. You don't need mercury to close a contact


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## davey (Aug 14, 2010)

Okay, that's a useful confirmation. The anticipator hasn't opened, or the customer wouldn't have reported the furnace keeps them fairly comfy. 

It looks like resetting the anticipator is not something that could happen by chance, and the customer reports that the thermostat remains firmly secured. Is there a plausible mechanism, say an approaching-end-of-life phenomenon. through which it could have started to heat a bit more without somebody moving the setting, so the furnace regularly cuts off a little too soon? How do these thermostats behave as they're dying?


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## davey (Aug 14, 2010)

emtnut said:


> "not holding the way it used to" is kinda vague ."
> 
> Valid point. He reported that the house doesn't stay as close to the set temperature as it used to, and tends to be a little cooler.
> 
> ...


Good to know. Though I hate to replace something out of damfool ignorance--especially when replacing it doesn't help.


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