Replacing a Lennox DSL-450LX with a Honeywell RTH6400

Lennox (old) Wiring Diagram

W1v >white
Y1 >yellow
G >green
R >red + jumper
24V > jumper to R
24V(c) >blue
O >dk green
B > unused

Trying to decide how to wire the new t-stat…

Honeywell (new) Diagram
(Ref: Page 21 in manual)

G> green
W O/B> QUESTION: Is it correct to connect both W1 and O wires together here?? Somehow I don't think this can be right.
Unused
Y > yellow
R> red plus jumper
Rc> jumper to R

Outside Unit is Lennox HP26-042-7P
Inside Unit is Lennox CB30-M51-1P

Thanks in advance for your response!

** Answer: The Honeywell RTH6400 thermostat isn't compatible with your system. **

Okay... well I searched long enough and finally found the search term that got me a definitive answer so I'll answer my own question here just in case someone else needs to know in the future and happens across this when searching.

As I suspected, the W1 and O wires do not go together in the "W O/B" terminal on the Honeywell thermostat. Having both the W1 and O wires means it is a two stage heat pump. The Honeywell RTH6400 thermostat isn't compatible. It is meant for single stage systems only. My situation requires a thermostat that is compatible with heat pump and auxiliary heat (2 stage).

And THIS, folks, would be why every single unit on the shelf -- yep, all 7 of 'em! -- was a resealed package that had obviously been returned. I knew that didn't bode well when I bought the thing. Next time I'll go with instinct. Ha ha!

Update: I successfully replaced my old T-stat with the Honeywell RTH7600D. The RTH7500D is also compatible with multistage heat pumps.

-- Helpful Tip Regarding Thermostat Replacement for Heat Pumps --

You'll spare yourself some annoyance and an exchange if you will take off your old thermostat cover and make a wiring diagram as STEP 1. (Obviously shutting the circuit breaker off is Step 0! ). Then go online and pull up the wiring diagram in the installation manual of the potential replacement you are considering to check that the terminal labels match up to your old t-stat and there's a place for all the wires to go.


I find it very frustrating that no where on the product packaging or in the manual did it address possible incompatibility with multi-stage heating systems. "Works with Heat Pumps" "Compatible with virtually all systems" Uh huh. I'm sure Honeywell customer support in New Delhi would have told me this -- after the manual had instructed me to drill holes and install the new wall plate -- had it been open during the times I was available to work on my t-stat replacement. "9 AM to 6 PM CST" a recorded message kindly informed me as I stood there looking at a bunch of neatly labeled wires hanging out of the wall, contemplating reinstalling my old t-stat or waiting for 9 AM CST to roll around with no power to my A/C and Heat system in the interim. Alas, I was left with Google as my tech support. Thank goodness for the internet. Hopefully this post will save someone other DIY homeowner the hassle and delay allowing you to realize an actual "15 minute installation". ;) Best luck!

Do you mind to post your Wiring Diagram?

I have the same lennox dsl-450lx, and am planning to have it replaced with a programmable thermostat.

Thanks!