We have a brand new air handler at work, but the thermostat is mounted on a wall next to a server/computer room which is always warmer than the rest of the office. I want to install a thermostat that allows for the use of a remote sensor.

The C wire is hooked up at the air handler, but is snipped short at the thermostat. There is plenty of slack for me to add more wire.

The voltage between R and C is only 15-16 volts. The air conditioner and gas furnace work perfectly fine. Is this lower amount of voltage alright for use with a modern thermostat that requires the 24 volts flow on the C wire back to the air handler board?


Model: N8MSN0701716A Supply Voltage: 18 to 30 Vac or 750 mV

I don't think the thermostat will work with the current wiring setup.

I have 28 volts between the R and Y terminals, but 15 between R and C; at the thermostat. I don't see a splice of any sort with the C wire, but who knows in the walls or in the drop ceiling between floors. The Y terminal is spliced at the air handler before it enters the Y terminal on the board. I would have expected the voltage drop to be between R and Y, not R and C.

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  • 2
    You'd have to check with the thermostat manufacturer, to determine what voltages are acceptable. They likely step down, rectify, and smooth the input (24 VAC to 3, 5, or 12 VDC). Acceptable input voltage will depend on the components on the circuit board. Do you get 24 volts at the transformer?
    – Tester101
    Jun 8, 2016 at 13:14
  • that's strange. So you're losing 10 volts between the air handler and the thermostat!? Is the cable really long?
    – Tester101
    Jun 8, 2016 at 17:08
  • 1
    No it's not actually. I would guess only 15 feet; tops. I have re-tested over and over again, and the voltage is never above 16 volts.
    – Evil Elf
    Jun 8, 2016 at 17:24
  • COULD be a long run as the office building is old with brick walls, but I doubt it.
    – Evil Elf
    Jun 8, 2016 at 17:34
  • Wow, found this discussion: reddit.com/r/HVAC/comments/197kxo/…
    – Evil Elf
    Jun 8, 2016 at 17:39

2 Answers 2


Think twice before you do that, and talk to your sysadmin. Do not do anything which would compromise the cooling in the server room.

If you don't like the balance, consider "portable air conditioner" units which dump the cool into the server room and the waste heat into the office space. We had a big industrial unit, and it worked rather nicely. Now they exist as consumer grade units for a few hundred dollars, so you could get 3 or 4, and only lose a little cooling if one fails.

  • 2
    The server room has its own standalone cooling unit. The office space just happens to have a thermostat on a common wall, thus freezing us out.
    – Evil Elf
    Jun 8, 2016 at 22:05
  • 1
    @EvilElf - is a simple relocation an option?
    – Mazura
    Jun 9, 2016 at 1:05
  • I may try although with the brick walls it can be a pain.
    – Evil Elf
    Jun 9, 2016 at 1:29

The Honeywell VisionPro 8000 thermostat will work. You can purchase up to an additional 6 remote sensors and they will average the temperature or just get 1 and it will get the temp from that spot. It will work with a common wire or off batteries. As far as your lower than normal voltage. Take the reading at the board and at the thermostat. If it drops more than a volt or two you have a wiring problem that will likely show up sooner or later.

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