Hi,

I hate it when my first post in a forum is a question, instead of a contribution, but maybe I can field something else in another forum for Karma :)

Anyway, I'm installing a new Honeywell RHT7400D Heating/AC thermostat. Its replacing an analog Weathertron thermostat, hooked up to a heat pump.

I'll try to be as detailed as possible, but I was an idiot, and left my detailed notes 30 miles away, so I'm doing this from memory with the Honeywell manual in front of me.

I labeled the wires into the old thermostat, and ended up with the following wires:

R, O, Y, G, B, X2, W1, T

If those aren't exact, I will check and correct tomorrow. It shouldn't matter (much) to my question, which is:
I've hooked all the wires up to the corresponding connects in the Honeywell, except the black T wire. I still have the L connector empty; it's the only empty connector, but I don't want to do something stupid without asking someone smarter than I am. What is the T wire for, what is the L plug for, and can the wire labeled T go into the spot labeled L? :confused:

All the other connectors corresponded perfectly, the R and the RC were jumpered, the B wire is in the C slot, and the O wire is in the O/B slot.

Thanks for the help!

- Dan

Your questions are outside the basic R, G, W, Y, and B or C I am familiar with. Does the Honeywell manual explain the function of the T and L? Could they relate to switching the heat pump from heating to cooling modes from the thermostat? Have you tried operating the system without the T and L connected? Unconnected wires are less likely to damage something than ones connected wrong. Some thermostats and heating systems provide options such as different firing rates and outside temperature sensors. There could also be controls for supplemental resistant heat for use in too cold of weather for the heat pump.

Thanks labman.

OK, from what it sounds like from other reading, the T Terminal is for an outdoor anticipator reset. I have no clue what that is, but I have a feeling nothing will be broken if I just cap it off and leave it be.

I'm going to have to test this, of course, but aren't there guidelines for heat pumps about never turning the heat on if the temperature is above a certain point, and vice versa for A/C?

Anyway, found this:
http://yourhome.honeywell.com/yourhome/Applications/Faq/FaqDetail.aspx?faqid=10135
So I definitely no, the T does not go into the L. I'm just wondering what "feature" won't work if I don't put the T lead into something :)

Other internet posts elsewhere have indicated that changing from a GE Weathertron to a Rite Temp, the T lead isn't needed either.

Also, https://www.askmehelpdesk.com/showthread.php?t=11446