I have the following currently installed:
- heat pump 4TWX6036C1000AA
- furnace TUH2B060A9V3VAB
- thermostat TCONT803AS32DAA

I am planning to replace the TCONT803 with a Nest. I'm not 100% confident that the current thermostat wiring is correct for my configuration. The installers who put it in 3 1/2 years ago seemed to struggle with getting it set up correctly. It heats & cools, but I'm not sure I've getting correct 2-step operation of the heat pump in both heat & cool modes.

I'm in Atlanta, and have an economic balance point in the low teens, so the furnace should rarely be used as long as the hp can keep up with the heating load. The switch over in the current t-stat is set much higher than this (don't have exact value at hand, but think it's above freezing). From what I've read, the Nest should be smart enough to only switch over to the furnace when really needed to keep up with heating demand based on its actual measurement of house temp rise (or lack of it).

My main question is this: exactly what should the wiring be between the Nest, furnace, and heat pump to ensure that I'm getting 2 cool, 3 heat operation?

One thing I've been looking at to understand this is the TCONT803 installation manual: http://www.butcherdistributors.com/I...isntallers.pdf
I think Figure 15 is the right one to be referencing, but I don't completely understand the difference between Figure 14 & Figure 15.

 

Figure 15 is for the 16seer and figure 14 is for the 20seer.

I wouldn't use a Nest stat for that setup.
The Honeywell Prestige IAQ would be a much better option since it supports up to 4 stages of heat and 2 stages of cooling and supports an outdoor temperature sensor as well as duct sensors.

 

So by going with the Nest, I'm giving up 1 stage of heat on the furnace, which should rarely be used, at least given my current electric & gas prices & climate. It appears that I'm at least no worse off than I am with the current TCONT803?

A couple of questions regarding the connections shown in Figure 15:
- BK vs. YLO at furnace - what is the difference between these two?
- W1 jumpered to W2 at furnace, and connected to X2/BK at hp - I assume jumpering these two is what locks me in to only one stage of furnace heat (really my 3rd stage). What is the purpose of the X2/BK connection? I'm assuming that my two stages of heat from the hp are controlled by Y1/Y2/O at the hp, with O reversed from the cooling position.

Thanks!

 

BK and Y1 are fan speeds. Figure 14 and 15 are different ways to wire the controls to allow for the fact that the scroll compressor unloads to 70 percent capacity and 20 seer is 50 percent capacity in low speed.

The TCON803 and Prestige thermostats will keep the heat pump and gas furnace from running simultaneously when properly configured. Running a gas furnace and heat pump simultaneously can cause high head pressure lock outs.

When used with an outdoor temperature sensor, both of those Honeywell stats give you the option to lock out the heat pump at an adjustable ambient temperature setpoint.

You can also lockout the gas furnace if the outdoor temperature is above an adjustable setpoint.

 

I'm double-checking that the units are wired correctly & noticed the following differences compared to Figure 15:
- O from t-stat wired to O at furnace & O at hp, diagram shows no connection at furnace.
- no jumper from R to O at furnace
- Outdoor temp sensor wired to t-stat using two leads in the thermostat cable, vs a seperate cable as specified.
- no B/C wire between t-stat & furnace

What problems would be expected from these wiring differences?

What should furnace controller DIP switch settings be for use with the XL16i hp?

Thanks!

 

The fan would deliver less cfm of air than it should.

This can cause an inaccurate reading of your outdoor temperature. This can be checked with a digital thermometer. The stat has the option of calibrating the sensor. I have seen this done without problems on short runs. A separate wire would be better.


You would need to use batteries in the stat. It seems like this also defeats the compressor protection time delay function of the stat.




The BK jumper should be labeled W14.