02-23-2025 09:55 PM
Is my Honeywell/Lennox T87F Thermostat 2 Wire Red/White, compatible with Nest Thermostat 3rd Gen?
I currently have a older Honeywell/Lennox T87F Thermostat for forced air furnace. It has a Red Connection and a White connection. Note both wires are black, but contact to a R connector, and a W connector on the Honeywlll thermostat base plate. Many of the older houses in San Francisco have this type of forced air heating system. The heating comes up through the floor and wall vents.
I attached the R wire to the RH connector on the Nest Thermostat and the W wire to the W1 connector. However the Nest Thermostat doesn’t power up. I don’t want to hire a HVAC technician to come out and tell me, he can’t do the work and charge me.
Thank you for any advice that the community can provide me.
The Honeywell T87F thermostat controls a old huge 4 foot x 6 foot long Mars forced air Furnace Model #L15000
with a BTU Input Per Hr: 150000 and uses Natural Gas. Is my Honeywell thermostat compatible with the Google Nest Thermostat 3rd Generation?
02-24-2025 02:49 PM
The Nest Learning Thermostat will work, but you also need the Nest Power Connector and an external 24V transformer. If the furnace has an available C terminal, you might not need the transformer.
Wire it like this if you need the transformer,
Or like this if there is a C terminal,
02-25-2025 01:16 PM
I have printed out your diagrams and I will research, I've spend to much time trying to install it. I can also share with the HVAC contractor to ensure that he can do the work. One contractor wanted to charge me $77 for coming to give me an estimate, without any promise that he could install it. Thank you very much.