Honeywell T822A Mercury Thermostat Question

MikeC_3 · February 24

Hello -

Spent the weekend at a friend's house out in the Poconos. Two story farmhouse, single zone hot water system with the thermostat located on the first floor. This is the first winter that he has owned the place.

When we arrived, raised the thermostat to 70 degrees and the system fired. It continued to do so approximately every 15 minutes until it reached the set temp. However, once the temperature was achieved, it continued at this rate unless the set temperature was decreased to the mid-60's. Thermostat seems to be accurate.

I removed the cover to see what the anticipator was set to only to find that there was not one. I pulled the manual for the T822 line and it confirmed that the A model does not have an anticipator - or at least one that is adjustable.

So, my first question is are/were thermostats without an anticipator common? Do all systems benefit from having one? And finally, what would the recommended modern replacement be?

Thank you!

EBEBRATT-Ed · February 25

No, I think you found the issue. I would replace that stat. The newer electronic stats have dip switches which allow you to set the CPH…cycles/hour.

mattmia2 · February 25

is it a 24vac or millivolt system?

the thermostat needs to be level to be accurate.

what makes you think it was shutting off on the stat and not on a high limit? Did you see the mercury tipping back and forth?

I believe there is a little hex in the middle of the t-stat element that allows you to adjust the calibration with a small allen wrench as well as one for the thermometer. I might compare it to a reasonably accurate thermometer and observe the mercury bulb before making any assumptions about if it is functioning correctly. without a heat anticipator it would overshoot but not until after it turns off. The heat anticipator is to anticipate the heat that is in the system and will be released after the burner is turned off.

it also looks like the screws that hold it to the subbase and make the electrical connections are loose or cross threaded or something.

Jamie Hall · February 25

https://forum.heatinghelp.com/discussion/comment/1889416#Comment_1889416

Yup. That's one of the very very early mercury switch thermostats (you can tell by the size of the mercury vial). It will shut off — eventually. There is no anticipator, and the "swing" (difference between when it switches on and switches off) is controlled by the mercury at one end of the tube — or the other.

Absolutely reliable — but pretty crude (you have to start somewhere!).

You can replace it with a mercury T87 — if you can find one — which DOES have an anticipator and is as reliable, or you can replace it with a battery digital, which is great if you remember to change the batteries. I only see two wires there, though, so you would have to run new wires if you wanted a fancier digital which needed external power.

HVACNUT · February 25

That must be an early one. I remember the T822D had an anticipator. They were great for when converting to low voltage controls.

MikeC_3 · February 25

Yes, definitely only two wires. Not sure if it is 24V or not. I'll have to check.

I did compare it to a digital thermometer, and as far as I could tell without a single hashmark between 60-90, it was very close. As soon as the thermometer dropped a degree however, the thermostat re-fired the boiler.

So, without an anticipator, is that how these old stats worked? It is located in a hallway without a radiator so I'm sure that plays a role in the short/frequent cycling.

mattmia2 · February 25

the anticipator would make the cycles shorter, it would make the t-stat shut off sooner. if the building was cold and you are heating it to the desired temp, it may cool off faster for a while until the structure warms up. there may be a way to set the hysteresis on there but 1 f degree seems about right. if the structure was cold i would give it a day or so and see what happens. I would also make sure the wall is sealed behind the t-stat so cold air isn't blowing in from the wall.

EdTheHeaterMan · February 25

@mattmia2 started to touch on something with this question: is it a 24vac or millivolt system?

There is a T822C or version of this thermostat that is made exclusively for millivolt systems. Perfect system for a vacation home. If there is no electricity from the grid. the millivolt system will still heat. Gravity flow will keep the pipes from freezing. That particular thermostat did not have a heat anticipator

The more common T822D was for 24 VAC systems and did have a heat anticipator. I might suggest that you look at the heating system that the thermostat is connected to. You may find that there is a millivolt gas valve on that system. As far as the set-point and the overheating is concerned, I can't really see a setting for exactly 70° on that dial. Do You?

I believe it works this way: If you are too hot turn it down, if your are two cold turn it up. That's my kid of thermostat!

109A_5 · February 25

Since the Red wire could have be routed to the Mercury switch the same basic way as the White wire. However they did not do it that way. I believe the Red wire is connected to the Mercury switch via the bi-metal spring. When used with a 24 VAC system I believe there is some (nonadjustable) anticipation from the current through the bi-metal spring, producing slight amount of heat.

Is there no base plate ? Why do there screws look loose (Red arrows), typically they may be for the electrical connections to the base plate.

MikeC_3 · February 25

I didn't notice but it doesn't look like there is a baseplate. Lookin at the voids in the plastic, there doesn't seem to be anything behind them.

HVACNUT · February 25

https://forum.heatinghelp.com/discussion/comment/1889470#Comment_1889470

Those thermostats screw directly to a Gem box. There is no sub base.

Big Ed_4 · February 25

It is a millivolt or steam only thermostat .

Honeywell T822A Mercury Thermostat Question

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