Heating, Air Conditioning, Fridge, HVAC/Multizone nest question


multizone wires
multizone wires  

I have a furnace with 3 zones. I have installed nest thermostats on each zone and 2 of them work perfectly. The 3rd one tells me the Rh has no power when i connect the C (blue). if i remove the C, it works correctly with the exception that the nest is not powered and works on battery. Somehow, it doesn't receive power. When I looked at the multizone board, the 3 zones' wires are correctly wired, but  when i look at the  the wires going to the furnace itself, i see 2 red wires, one connected to Rh, the other red wire connected with the blue wire to C. Somehow, the installer did not install the other red wire to Rc.

Could that be the issue or is that correct to connect one red and blue together?


ANSWER: David,

Where does the red wire that is on the C terminal with the blue wire . You would need to see if it's hooked at the thermostat . Usually if you use one transformer it's not uncommon to have a jumper from RC to RH . If the red wire at RC at the thermostat it should go to RC at the board. If the other red wire at the thermostat is on RH that wires should go to RH at the board You don't mention what wires are what at the thermostat .what does the instruction of the nest say in regards how to wire it .  . If you can't find where the red wire at the C terminal hookup at the board is on the thermostat , you may not need it. In that case you may just have to add a jumper from RH  RC at the board . Also your board 24 volt power which says 24 VAC transformer  the two hookups wire are suppose to have a dedicated transformer . Your not suppose to use your heat/ cool transformer. It looks like the nest does not use the jumper from RH to RC like standard thermostats .

If you have a meter you can check to make sure you have 24 volt AC  on terminal C and RC or C and RH where it says SYSTEM  on the board.

---------- FOLLOW-UP ----------

QUESTION: Thank you for getting back to me so quickly. Actually, the picture I attached goes to the furnace itself. and probably to the transformer. On the other side of the multizone board, the wires for each of the zones are connected. I have for each thermostat a Green, White, Red, Blue and Yellow. The nest actually connects Rc and Rh automatically, so only one of them needs to be connected. The wires seem to be connected correctly at the board and what i don't understand is that i get the problem only for one of them. If I disconnect the Blue and leave only the Green, White, Yellow and Red, the thermostat works but doesn't seem to recharge the battery of the nest. So that's why I was wondering about those 2 wires connected together and whether the Red shouldn't have been connected to the Rc instead.


Those wires going to the " system " you would need RC terminal . the RC is one side of the transformer for your cooling , I would remove the red  wire from C and hook to RC at the zone board , BUT double check that the other end does NOT go to the common side of the transformer . That RC red wire once you hook it up at the board the other end it should hook to the R side of the transformer . The blue wire on the C board should go to the common side of the transformer . Then check with a meter , you should have 24volt AC at C and RC at the board . Then check RH to C if you do not have 24volt AC and only if your using one transformer for heat and cool you can jumper RC to RH at board . If you use one transformer for cool and one transformer for heat ,do NOT jumper. As stated in other post  ,the board is suppose to have its own dedicated transformer . ( I'm sure the wiring diagram of your zone board shows that. )  You can also look at the wiring diagram of the zone board . It usually located on the inside cover.

The Y terminal on the board is a dummy hookup ,as that is your AC compressor contactor , one of those wires comes from the thermostat Y terminal the other should come from your outside unit one side of the compressor contactor labeled Y

As far as why one of your stat does not charge the battery it may or may not be due to not having the RC terminal hooked up at the board

Heating, Air Conditioning, Fridge, HVAC

All Answers

Answers by Expert:

Ask Experts




40+ years diversified experience.An HVAC/Refrigeration Self Employed Contractor since 1986. NATE Certifield. Answer questions pertaining to Residential and Commercial Air Conditioning , Warm Air Heating, Heat Pump systems.Mechanical and Electrical troubleshooting of these systems.Be as detailed as possible when describing problem.Packaged unit or split systems.No appliance questions.


Installation and Troubleshooting, Mechanical and Electrical, in following areas, Refrigeration ( Walk In and Reach In Coolers and Freezers ) Commercial Roof Top and Packaged Heating/Cooling ( Natural Gas,Propane, Electric, and Heat Pumps ) Computer Room Air Conditoning Systems,Commercial Residential Packaged and Split Systems Air Conditoning and Heat Pumps.Warm Air Oil , Natural and Propane Gas Heating systems.

Graduated from Private Technical School in 1975. An HVAC/Refrigeration Contractor . Have Unlimited Heating/Piping and Cooling Contractor License.Limited Sheetmetal Contractors License.NATE Certifield. York Certifield Master Heat Pump Technician 1986. Served 33 years combination Active Duty Air Force , Air Force Reserves and Air National Guard in the HVAC/Refrigeration Shop/Mechanical Shop. Troubleshooting/Installing HVAC/Refrigeration Equiptment Worldwide. Retiring in 2003 as a Senior NCO ,and a Mechanical Superintendent.

©2017 About.com. All rights reserved.