# New thermostat - Heat Pump - Mystery "K" wire



## Done That (Apr 28, 2011)

You may be able to ignore K.

Google THP9045 Wiresaver Module if you want to see details, it's an add-on for systems without a common wire.


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## Technow (Nov 12, 2010)

Just about every real estate sale the home inspector covers his butt by putting "heatpump should be checked by a licensed contractor" and you are going to end up paying for a checkup anyway. You could pre-empt this by just going ahead and getting the system checked out and thermostat installed and when the future buyers request the inspection you produce what was recently done.

When I sold my last house, even though I am a licensed contractor...I hired an independant company to check out the systems to limit any liability.


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## eviljuan (Mar 19, 2013)

Ok I'll look into that, thanks.

Do you know what the K wire is actually responsible for?


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## eviljuan (Mar 19, 2013)

Apparently the K wire is used with a transmitter which controls the power for the A/C, allowing the A/C to be disabled/non operational in certain months - and yes, now that I think of it, if you set the t-stat to "Cool" during the months or say November to April, nothing happens.

Does that change your opinion of whether or not I can simply not connect the K wire to the new Honeywell?

If it means that it can no longer be disabled remotely, I don't really care, but if it means the A/C won't function _at all_, then that it obviously a concern.

On another forum, the user was trying to connect a Nest t-stat and couldn't due to the mystery "K" wire. Although Nest support originally said it wasn't compatible with their units, he ultimately connected K to the connector which controls the power of the A/C, which in his case was K --> Y1

Any thoughts?


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## beenthere (Oct 11, 2008)

Perhaps if you posted the model number of your old thermostat. We might be able to find out if you need to use K on your R of the new stat or not.


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## eviljuan (Mar 19, 2013)

The current thermostat says Chromolox on it - looks very old and is likely the original.

I've taken and uploaded to the thread a picture of the current wiring. The new thermostat is a Honeywell RTH7600.

The current thermostat has these wires:

W1 (yellow wire in the picture)
X
G
R
K
C

According to the user manual for the 7600, it should be wired as such:

W1 --> AUX
X --> E
G --> G
R --> RC
*K --> K* wire not mentioned at all in 7600 literature
C --> C

I originally had a lesser model and the phone rep said it would not work, and that I should get something in the 7000 series - and said the K wire would connect to the "E" terminal. As you can see, the E terminal is already spoken for by the X wire.

Does any of this info help?


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## beenthere (Oct 11, 2008)

Is there a model number on the back of the stat itself.


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