I'm trying to update the thermostat in the house this weekend, and I am having some issues, and I wonder if someone could help set me straight with what I'm doing wrong... Just to be upfront - I don't know much about HVAC, we didn't really have them where I came from. I did replace the thermostat in my old house a couple of years ago with a Chronothem IV, and I wanted to do the same in this new place.

So, the equipment...

The new thermostat is a Honeywell Chronotherm T8601D2019. The old thermostat is a Honeywell T87F2873. The furnace is a Lennox G8

The wiring...

The T87 was wired up with 4 wires, Y, W, R and G... The blue wire was not connected to anything. I powered off the heating and air at the main fusebox, removed the wires from the T87 base unit, and fitted them to the corresponding terminals on the T8601.

When I powered everything back on, there was no display on the T8601. I checked for 24V across the R & W terminals, and that was fine. I went down to the furnace to check there, and everything was fine there too. Double checked that the blue wire was not connected to anything at the furnace end either.

The T8601 has terminals for C, B and O - none of which are connected at the moment, as I don't have the wires for it (there's also a couple of terminals for the outdoor temp sensor, which I'm ignoring).

I suspect that the T8601 isn't getting power.. In what I've managed to find, I think I need to jumper R to Rc on the base... But there is no Rc to jumper to...

I'm kind of guessing that I need to do something with the B terminal, as I don't think the circuit is being made to power the T8601. Connecting it to the B wire isn't going to help, as it's not connected to anything at the furnace end, and I know that I don't know enough about messing at that end without asking for help.

Does anyone have any suggestions please?


Graham

On the old stat:

R = 24 volt power into the stat
W = power out to heating relay or control
G = power out to fan relay
Y = power out to outdoor cooling compressor

I cannot see how you got 24 volts across the R & W, as the red is the power in , and the W is the power out.

Honeywell website is useless, so I cannot see a wiring diagram for the new one.

So hopefully I helped with explaining how the old one worked.

I cannot see how you got 24 volts across the R & W, as the red is the power in , and the W is the power out.

I am kind of confused now too... But I did double check... I'll have another look tomorrow...

On the new stat, there are 3 versions, one which is powered from batteries. One is powered via a common line, and one via some magical method which doesn't need a common... It looks like I've got the wrong version, and need the power stealing version... sigh... :)


Graham

I think you need the battery version, since at the stat you only have a power feed, the red, and no return, to create a 24 volt source at the stat.

Or you can use the spare blue wire as a return. This needs to be connected to the control transformer, on the opposite terminal at the transformer, not the same as the red. If you can get this done, use a mter at the stat, and you should measure 24 volts across the red and blue.

I think you need the battery version, since at the stat you only have a power feed, the red, and no return, to create a 24 volt source at the stat..

I was hoping not to get that one... I am terrible at remembering to change batteries and the like.


Or you can use the spare blue wire as a return. This needs to be connected to the the control transformer, on the opposite terminal at the transformer, not the same as the red.

I'll need to head downstairs and check later, but I seem to remember that all 4 wires were connected to what looked like the transformer... I know they couldn't be, it doesn't make sense. I'll head down when there is a little daylight and double check.


Graham