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Old 10-29-2011, 09:55 AM   #1
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20 Year Old Airco Furnace


I need a new thermostat for this older Airco model MM1402C purchased in 1992. The thermostat that was used is an old single lever with a mercury bubble in it. I purchased a new programmable one and it just keeps the furnace on. I checked the wires coming to the stat and the voltage is only 12 volts and the thermostat is for a 24 volt system. My problem is I can't find a programmable one that is for 12 volts so I will go back to the slide lever or turn knob style but no one lists a thermostat for 12 volts. I checked right at the furnace and it is 12 volts coming off the transformer. Anyone run into this before or have any ideas what to do.

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Old 10-29-2011, 10:21 AM   #2
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20 Year Old Airco Furnace


on the secondary of the furnace transformer you are actually reading 12Vs off the wires? yes the stat is cycling for heat but the rest of the stat need a constant 24Vs at the subbase to power the show....if you went from the TR hot R to ground it will be 12Vs...get back on that reading...maybe a Canadian unit.if so you might have to add a 24V TR and isolating relay with N.O. contacts(totally for a stat controlling an older system) to close the furnace pair of wires that the old stat closed to heat..cheap easy install.lets hear back

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Old 10-29-2011, 12:45 PM   #3
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20 Year Old Airco Furnace


Quote:
Originally Posted by biggles View Post
on the secondary of the furnace transformer you are actually reading 12Vs off the wires? yes the stat is cycling for heat but the rest of the stat need a constant 24Vs at the subbase to power the show....if you went from the TR hot R to ground it will be 12Vs...get back on that reading...maybe a Canadian unit.if so you might have to add a 24V TR and isolating relay with N.O. contacts(totally for a stat controlling an older system) to close the furnace pair of wires that the old stat closed to heat..cheap easy install.lets hear back
Thanks for the reply and suggestion's.
I have been waiting for either Honeywell or White Rogers to get back to me and WR finally did
Yes it is a Canadian model and have double checked and it is 12 volts. I found a schematic behind one of the panels and it states it is 12 volt so my worries that the transformer was shot is for not. I received a e-mail back from White Rogers and they have given me a number of one of their thermostats to buy, they said it is rated for 24 volts but will work with 12 volts as it is a direct replacement for the old mercury switch units. It is 1E30N-311 and I have just finished hooking it up and it works fine.
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Old 10-29-2011, 01:10 PM   #4
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20 Year Old Airco Furnace


that was simple enough where suppose to get flurries today in NYC and people down here are flippin' out...stay warm any questions on setbacking the temps don't hesitate...
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