How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Phil Your Own Question
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5648
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
Type Your HVAC Question Here...
Phil is online now
A new question is answered every 9 seconds

I have a honeywell T87F 2873 I just bought a digital replacement

Customer Question

I have a honeywell T87F 2873 I just bought a digital replacement for and the help desk can't tell me how to terminate the old O and B wires. I have 2 zones, forced hot air heating and cooling
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.
Hello, The T87F series including the suffix 2873 is not designed for use with heat pumps. You need to buy another thermostat. These can range in price from $50 or so to, $700... I personally prefer the simpler easier to program ones. such as this one below. Look those over to get an idea of what the options are, and I will help you select one.
Customer: replied 1 year ago.
Hi, I don't have a heat pump. We have oil fired furnace with an outdoor AC unit. I know the wires go to a box on the side of the furnace to a temp - trol zone controller . I thought
There might be wires landed on the T stat and not connected in this box but they are all landed. I tried running AC with the B and O wires not connected and it didn't turn on the AC compressor
Expert:  Phil replied 1 year ago.
Thanks, O and B are commonly seen on heat pumps only. But not in this case apparently. Tell me how many wires and what colors were connected to the old thermostat and to which terminals, and its brand and model number please, I can tell what you have from that. .
Customer: replied 1 year ago.
Here's what we had:Old t-stat- wire color
Y- yellow
W- white
Old t-stat: Honeywell T87F 2873 3
Old t-stat back frame: Q39A1436, mcr5, 9717 installed 1997
Expert:  Phil replied 1 year ago.
Hello again,
I am unable to find a wiring diagram for the T87F of any suffix with O and B terminal. This leaves me at a loss.
If you can tell me where the O and B wires terminate and take a photo of that module, I might be able to figure it out.
If there is a wiring diagram glued to that unit and you can take a photo of it and post it here using the 'attach files' link at the bottom left that will help... photo needs to be well focused in good light and taken straight on close up.
We can go from there if you like.

Related HVAC Questions