Honeywell RTH8500 batteries died after a month?
#1
02-11-18, 07:56 AM
Honeywell RTH8500 batteries died after a month?
In my wife's store, there is a gas furnace. I replaced the old mercury thermostat with a fully programmable Honeywell RTH8500 about a month, maybe a month and a half ago. All of the wires from the old thermostat coincided perfectly with the terminals on this new thermostat so there were no questions at all. The heat works, the schedule works, everything has been fine until my wife called me this morning to say the heat was not coming on. I found the thermostat to be off/dead/etc. I replaced the three AAA batteries and it came back on.
Question: Shouldn't this thermostat use the batteries as a backup only? I haven't broken out my multimeter yet because the system was working fine, but I'm pretty sure the red wire coming in is the 24v supply the thermostat uses to in order to trigger the relay inside the unit itself - is it not? If I've got a 24v wire coming into this thermostat, shouldn't the thermostat be powered by this wire, and not the batteries?
Question: Shouldn't this thermostat use the batteries as a backup only? I haven't broken out my multimeter yet because the system was working fine, but I'm pretty sure the red wire coming in is the 24v supply the thermostat uses to in order to trigger the relay inside the unit itself - is it not? If I've got a 24v wire coming into this thermostat, shouldn't the thermostat be powered by this wire, and not the batteries?
#2
02-11-18, 08:01 AM
Group Moderator
#3
02-11-18, 08:59 AM
OK, that is probably the issue. My wires were R/G/Y/W/B. I've got the blue wire attached to one of the terminals on the left side of the unit for what reason I'm not sure. I wonder if the old thermostat was wired oddly so that when I put on the honeywell stickers, I did it incorrectly.
Can I check the blue wire to ground for 24vdc? Or perhaps I better just pop off the panel at the unit to see with my eyes where each wire is going to on the control board?
Can I check the blue wire to ground for 24vdc? Or perhaps I better just pop off the panel at the unit to see with my eyes where each wire is going to on the control board?
#4
02-11-18, 09:18 AM
Group Moderator
#5
02-11-18, 12:13 PM
Thank you very much for your help on this PJmax. I did open up the cover and access the board. I found the blue wire not in use. I've taken some pictures for your reference. It's not the end of the world if I have to replace the batteries monthly, but that's definitely not ideal. Would it be unwise to grab my 24vdc right off the transformer?
On a side note, where does my thermostat get its ground from?
I made a few notes while I was inside the unit. Here are where my 5 wires from the thermostat go once they enter the unit.
Blue - unused
Red - "4" terminal on board
White - "W" terminal on board
Yellow - splices to a wire which goes up and out of site along with the AC refrigerant lines
Green - splices and goes to a relay attached to the side of the unit
On a side note, where does my thermostat get its ground from?
I made a few notes while I was inside the unit. Here are where my 5 wires from the thermostat go once they enter the unit.
Blue - unused
Red - "4" terminal on board
White - "W" terminal on board
Yellow - splices to a wire which goes up and out of site along with the AC refrigerant lines
Green - splices and goes to a relay attached to the side of the unit
#6
02-11-18, 12:45 PM
Group Moderator
Yellow - splices to a wire which goes up and out of site along with the AC refrigerant lines
Just checked..... the X terminal is your C connection.
Connect the blue there.
Similar manual for your unit.... hvac partners/docs/1009/Public/03/39394D41.pdf
#7
02-11-18, 01:02 PM
My hero! Thank you! I'll make that wiring change tomorrow. Here's to not having to change out the batteries every month!
