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cal91752 · Nov 25, 2008, 11:05 AM

I am installing a Venstar T1800 thermostat in my home. I have a heating & AC system--no heat pump or any other devices. Just the furnace and AC. (California State) I checked my system with the old existing (12 years old... with mercury bulbs) Homeywell thermostat before removal & the system worked fine. The thermostat runs off 24VAC. I notice that the Honeywell thermostat had a jumper wire between the RH and the RC connection on its connection board.

The instalation went as per instructions. No issues yet.

Here are the questions:

1) Do I put a jumper on the new Venstar T1800 between the R and C terminals as was on my Honeywell? If not, does the T1800 need only one 24VAC to work both the AC and the furnace?

2) Since I have just a furnace & AC system... what jumpers are to be placed on the Venstar circuit board? I am confused with the MISC1 MISC2 MISC3 jumpers as well as the other ones on the board... do I remove them completely or set them a certain way?

I have no display on the T1800 nor do I have any functions at all. (Looks like no power)

Could it be the "C" terminal that is the issue. If so, where do I connect the wire from the "C" terminal to? (Would probably use the blue wire as it is not being used)

To check if I was getting 24VAC from the system (the 24VAC transformer, I touched temporarily the "R" terminal to the green (fan) & it worked.

The wires from the unit to the thermostat are white/green/yellow/blue & red. The blue is not being used. The red is the 24VAC.

Thanks for the help.

MarkwithaK · Nov 25, 2008, 06:42 PM

1) The jumper from RH to RC on your old stat is there because you have a single transformer system. From what I know about your new stat you do not need this jumper, furthermore you do not have the option of this jumper. If you jump out R to C on your new stat you will blow your transformer.

2) From what I could gather about your new stat you will have no jumpers. The MISC1, MISC2 and MISC3 are for humidifier, dehumidifier and 3rd stage heat.

It appears that you will need a common on this stat, the C terminal. If you have a wire not being used then you connect it to the C terminal on your stat and also on the C terminal on your furnace's circuit board.