Honeywell VisionPRO 8000 Wiring: TH8321WF1001, U1 and K Wire

1. Which thermostat this covers

This article follows Resideo’s VisionPRO 8000 Smart installation guide (33-00065EFS-01, Rev. 06-20), written for model TH8321WF1001, the Wi-Fi VisionPRO 8000 sold through contractors under the Honeywell Home name. The RedLINK VisionPRO models (TH8321R, TH8320R, TH8110R) are not covered by this guide or this article. The short guide also says where the longer document lives: the product data sheet, with fuller diagrams, is on customer.resideo.com.

The wiring diagrams in the guide are all 24 V systems. If your old thermostat is wired to L1 and L2 with thick wires, read our line voltage thermostat guide first.

2. The terminals, column by column

The wallplate is printed with a Conventional column and a Heat Pump column. Several screws carry a different letter in each, which is the main source of confusion on this thermostat. Resideo’s “Terminal Designations” table reads:

If your old thermostat has a letter that is not in this table, our terminal letter index says what each one usually means before you decide where it goes.

3. Power: C wire and the R-Rc jumper

The guide states the power rule twice: “24VAC power is required. Connect common side of transformer to C terminal”, and again at the wiring step, “24VAC (C wire) is required.” There is no battery option in the guide, so a missing common wire has to be solved before the thermostat goes on the wall. Our C wire page explains how to find an unused conductor at the furnace.

R and Rc come joined by a factory installed jumper. The guide’s rules for it:

• One transformer: leave the jumper in. Every single-transformer diagram shows the power wire on Rc with “R+Rc joined by jumper”.
• Two transformers: “Remove factory installed jumper for two transformer systems.” The heating transformer’s wire goes on R and the cooling transformer’s wire on Rc.
• Common with two transformers: the “common connection must come from cooling transformer”.

Not sure whether you have one transformer or two? Our Rh and Rc page shows how to tell from the old thermostat.

4. Resideo’s diagrams as one table

The guide draws each system on its own. This table reads across four of the most common ones. Each cell is what the terminal connects to at the equipment.

The guide also draws heat only (Rc, W, C), heat only with fan, cool only (Rc, Y, G, C), heat pumps without auxiliary heat (1H/1C and 2H/2C), and a Series 20 valve system, where the thermostat’s R, W and Y go to valve terminals “R”, “B” and “W”.

5. Wiring steps

1. Turn off power to the heating and cooling system. The guide’s warning is to disconnect power before installation.
2. Photograph the old wiring and label each wire by its terminal letter, not its colour. Our wire colour guide explains why colours cannot be trusted.
3. Separate the wallplate. Older units have a button on top of the wallplate: press it and pull. Updated models have no button; pull evenly along the sides and bottom.
4. Mount the wallplate with the supplied screws and anchors. Resideo gives the drill sizes: 3/16 inch for drywall, 7/32 inch for plaster.
5. Connect power: the common side of the transformer to C, and the power wire to Rc (or R and Rc with the jumper removed for two transformers).
6. Wire the rest from the column that matches your system, using the tables above.
7. Mount the thermostat. Hinged models hook in at the bottom and snap shut; models without hinges push on evenly.
8. Restore 24 VAC and follow the installer setup prompts on screen.

If the screen stays blank, check for 24 V between R and C before anything else. Our multimeter guide shows the test.

6. No C wire: the K terminal

K is not a general-purpose terminal. The guide’s footnote says the THP9045A1098 C-wire adaptor “is used on heat/cool systems when you only have four wires at the thermostat and you need a fifth wire for a common wire”. You use K in place of Y and G, on conventional or heat pump systems, so the fan and compressor are controlled through a single wire, and “the unused wire then becomes your common wire”. The adaptor at the equipment splits K back into Y and G; Resideo refers you to the THP9045 instructions for that side.

Because the trick combines Y and G, it needs a system that has both. Our K terminal and wire saver page shows the furnace-side connections, and the no C wire guide lists what to do when the adaptor does not fit your system.

7. Heat pumps: O/B, AUX-E and L

Read the Heat Pump column for these. The guide draws four heat pump systems, from 1H/1C up to 3H/2C, and three notes carry the rules:

• O/B, changeover valve: “O/B set to control as either O or B in installer setup.” One terminal serves both valve types, so the wire from your old O or B terminal lands here and the setting decides when it is energized. Our O/B page explains the difference.
• AUX-E: a single terminal labelled “Auxiliary/Emergency heat relay (Heat 2)”. The short guide shows one backup heat wire and does not draw separate Aux and E wires. If your old thermostat has both, check the product data sheet or ask your installer before joining them. See aux heat vs emergency heat for what each does.
• L: “Heat pump reset (powered continuously when thermostat is set to Em. Heat; system monitor when set to Heat, Cool, or Off).” Connect it only if your old thermostat had an L wire; our L, S and T page covers it.

The compressor stays on Y, a second compressor stage on Y2, and the fan on G. For the bigger picture of heat pump terminals across brands, see our heat pump thermostat wiring guide.

8. The U1 terminals

The two U1 screws are the two sides of one relay. Resideo shows four ways to use it:

• Powered humidifier, dehumidifier or ventilator: the accessory has its own transformer, so the two U1 terminals simply switch that circuit. No jumper.
• Non-powered humidifier, dehumidifier or ventilator: the accessory runs from the system transformer, so the guide shows a “field install jumper between R and U1”.
• Dehumidification with low speed fan: the relay is wired to the equipment’s low speed fan input, and it can be set to normally open or normally closed in installer setup.
• An extra stage: heat stage 3, cool stage 3, backup heat stage 2 for heat pumps, or geothermal radiant heat. Here the guide says the U1 terminals “are normally open dry contacts”, and you must install the field jumper if the stage is powered by the system transformer, but not if the stage has its own transformer.

The guide rates U1 at 30 VAC maximum and 0.50 A. If nothing was connected to a U terminal or an accessory on your old thermostat, leave both U1 screws empty.

9. Installer setup and test

On first power-up the thermostat asks a series of questions: residential or commercial application, a thermostat name, programmable or not, temperature scale, whether to use an outdoor temperature (wired sensor or internet), and the type of heating system. The answers must match the wiring: a wire on O/B means choosing a heat pump, and that is also where O or B is set.

To go back in later, touch Menu, then Installer Options, enter the password and choose Installer Setup or Installer Test. The password is the date code, printed on the back of the thermostat and shown under Menu, Dealer Information. The Installer Test lets you perform an equipment test, which is the quickest way to confirm that every wire is on the right screw. Comparing models? The contractor-grade T10 Pro and T6 Pro use a different base with slider tabs instead of a jumper.

10. Frequently asked questions