Flaky Furnace Thermostat

BFL13 wrote:
"
Leaving the red wire connected to the existing tstat, I plan on tapping off the red wire to provide 12-volts to the new tstat. Then I would disconnect the white wire from the existing tstat and connect it to the new tstat"

Does a voltmeter confirm you got 12v to the new Tstat? ISTR on my furnace- only Tstat, the two wires are "positive" from the furnace and are switched by the Tstat. Not sure where "negative" is.

Note the fan switch position. It is confusing that you have a heat pump, but this is for the furnace.

EDIT--the furnace operates its own fan, so not sure what that is about.

Not clear on whether taking the red and white from old Tstat gets you the wires to put on W and R with the new. Also the new has its own battery, does not use 12v from the RV, not sure where that applies in this case.

Here is how the original one is supposed to work --if the wires from that are in cut -out with the heat pump even with that white one removed, not clear what is going on.

http://old.rvcomfort.com/rvp/pdf_documents/t_stat_electric_heat_function.pdf

Thanks for the help. Your mentioning that the CT87K had an internal battery caused me to re-read the owner's manual (more closely this time). It is for 24 VAC systems - not 12 VDC systems. So I think my plan is sound, but I need to get a compatible thermostat.

Here is how I understand the OEM tstat works:

It connects to the furnace via three wires. It gets 12 VDC on one wire, ground on another wire, and uses the 3rd wire to control the furnace. (The 12-volts and ground are used by the tstat for the digital display as well as other funtions so I have to leave them connected to the tstat. That's why am tapping off of the 12 volt line to go to the new tstat.)

The OEM tstat "energizes" the white wire from the tstat which connects to the furnace's control wire. Basically, (I think) it receives 12 volts from the furnace and when heat is called for, it returns the 12 volts to the furnace via the control wire. It essentially shorts the red wire from the furnace to the control wire from the furnace to turn it on.

As for the fan mode and fan speed switches on the OEM tstat, they control the air conditioner's fan both in cooling mode and in heat pump mode.

I'll let you know how it goes after I obtain the right tstat.