I switched to a new Hi-Tech stat from HAI called the Omnistat2. It connects to my PC and has a color changing background display. Very nice unit. However it can draw up to 60mA at 24VAC.

My old thermostat worked great, it was an old Janitrol. It was a "power robbing" type, but my DMM says it only used 10mA max. Again, no problems.

Now here is the strange part. As soon as I disconnect the fan wire and gnd from the old stat (with everything else connected to the new stat) the 24VAC power kind of shuts down; the new stat powers up for about a second then powers down then back up and repeats. The 24VAC supply becomes "weak" and can't power the stat. For some reason the new stat will only work with the old stat connected to the fan wire. If I disconnect the fan wire completely or connect it to the new stat the 24VAC is not strong enough to power the new stat.

Is this something that commonly happens with these furnaces and power robbing stats? In all the reading I've done here I've not seen anyone mention the fan wire connection as having any relation to their problem.

Thanks.

Do a search since something like this has already been posted. You might search the hvac link. You might have to use a larger VA transformer. Or use the resistor trick,

https://www.askmehelpdesk.com/appliances/comfortmaker-rpj-ii-2-gas-furnace-short-cycling-during-idle-298133.html

Thanks hvac1000, I did read that post. I even commented on it.

However my situation appears different than that one and others I've read. My problem is the fan/gnd wire must be connected to my OLD stat, not my new stat.

Let me explain it this way.

Step 1
Start with my old stat connected to the furnace, as normal.
Result: My old stat draws 10ma and everything works fine.

Step 2
Add the following Omnistat2 wires in parallel to the existing old stat: Red (24V), Green (Gnd), White (Heat).
Disconnect the White (Heat) wire to the old stat.
Now only the Omnistat2 is connected to White and only the old stat is connected to Yellow (Fan). Both get power and ground.
Result: My Omnistat2 runs heat (and the fan with it) just fine. There is plenty of 24VAC current for the Omnistat2. It turns on the heat as I set the thermostat. However I don’t have Fan-only mode since that wire is not connected to the Omnistat2.

Step 3
Disconnect the Yellow (Fan) wire from the old stat. (remember the Omnistat2 is not connected to Yellow)
Result: Now the Omnistat2 can’t get enough current from the Red wire and it endlessly power cycles.

Step 4
Connect the Yellow (Fan) wire to the Omnistat2.
Result: It still power cycles endlessly.


So for some reason it only likes the Yellow (fan) wire to be connected to the old thermostat. Once that wire is disconnected from the old stat there is not enough current for the new stat, the Omnistat2. Is this the same problem? It does not sound like it to me.

OK first off the color code is not correct/standard
Normally they are

Red= transformer +
Yellow= contactor A/C
Green= fan
white=heat
blue= reversing valve energized in heating.
Orange=reversing valve energized in cooling

Could this be your problem?
Yellow usually goes to the outside units contactor/starter and Green is the fan

Now how about a brand name and Exact model of the furnace. I will see if I can look it up.

Sorry, I left that out. It's a Comfortmaker or AprilAir brand, strangely it's not clear which. What I do know is that the model is a RPJ II conventional gas furnace.

Your color code made me realize I got the fan wire identification wrong. It's really the disconnecting of the AC wire (yellow) from the old stat that makes the new stat have insufficient power. Still don't get that part of it...

So my root problem is I'm missing the common wire. I think I need this for my high current consumption stat. Can I get the common wire from the furnace wiring? I have extra wires in the cable available.

That makes sense. You need a common wire.

The common is the other side of the secondary transformer. One side of the secondary is connected to R, the other side to common.

The gas valve, the cable to the AC will share a connection. This shared connection is common.

You will have 24 VAC at all tmes from R to Common.

An ending to my story...

Added the common wire to my stat. Just had to make the connection to the terminal strip at the furnace end. That, and the correct fan wire, makes everything work fine.

Thanks for the help.

Enjoy the new thermostat