Question about Honeywell Consumer Products Mechanical Heat/Cool Thermostat Heater

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Honeywell Old T874D1512 dual heat/cool thermostat

Dual heat/cooling mercury-2 controls with an automatic setting on each side--when I set the heat to come on the unit will not react--I can set the heat control to ON and the unit will run warming slightly--the unit will not turn on when it is set to automatic it shuts off--
I do not know what the 2 half moon dials under the thermo cover are for and how to set them properly - this is a commercial large unit. It is in the factory area. Thru the years the thermostat cover has been removed and I feel these small dials have been tampered with--
Is it possible to explain to me the purpose of the dials? I know this is an old thermostat but I really need to work with what I have--times are ruff and I really don't have the money to replace the thermostat if it is something I can adjust. Of course, I donot have the paperwork that came with the unit or the thermostat. I also donot have the knowledge to replace the thermostat. It cycles on and off OK through out the day once I get it on. I've done so many different things I'm not for sure what has worked. This thermostat had 1 control for the heat and 1 control for the cool. On the left side on the bottton of the unit is Auto---On The right side is Off Heat Auto Cool
I set it to Auto and Heat and put the top gage to 80 degrees and the unit will not turn on--after setting it to On and let it run for a while, sometime I can slide it to Auto and it will stay on but usually it turns off. After leaving it on On for 20+ mins it may stay on when I slide to Auto
I hope I'm making sense. I really think it may have to do with the moon shaped dials. I do not know what their purpose is. I have looked for information on these dials on the site but everything has gone to programmed thermostats. I figure my employees could really screw one of them up. Should I try to dust the coiled wire as in all factory
there is some dust inside the cover.
If I haven't given you enough information just let me know. You need to keep you information to me in "woman's terms" if possible. I pretty handy with things or I think I am.
But some of the termonology may be greek. Help me work with this unit and not replace it yet.

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I think I understand what you are saying....First, that switch marked On and Auto, is to control the blower fan only. So when you switch that to On, it turns on the blower, but not the actual heat. It sounds more like a problem at the furnace then at the thermostat.

Posted on Jul 20, 2009


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