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eviljuan 03-19-2013 08:06 AM

New thermostat - Heat Pump - Mystery "K" wire
Hello all,

I searched through other topics and did not see anything that was quite the same issues as I'm having with my heat pump - hopefully someone here can help me! I think the answer will be simpler than the question itself:

I live in a condo with a heat pump. Unfortunately I don't know the brand or the age, but we've lived in the unit for almost 7 years and never had a problem with it. Now that we're selling, I thought it would be a good idea to replace the thermostat, since the current one is most likely the original (and is hideous!)

I bought the RTH2510 from Home Depot. The wires that I found inside the current t-stat match up to the wiring guide of the Honeywell, except for a mystery "K" wire. Incidentally, it also has a W2 wire, which is "Aux" ?

The heat pump itself is inside the unit, connected to a grate to the outside. In 6 cold Toronto winters, we've never had seen the "EM Heat" light turn on, and it's only in the past couple years that we even had the EM Heat switched turned on.

There seem to be few of the Honeywell products that reference a "K" wire in their literature, and the only one I found that did, said "Do not use K wire". I have, however, deduced that the "K" wire does in fact refer to the unit's emergency heat functionality.

Would it be OK to simply cover the K wire with electrical tape and not connect it to the new t-stat? Fingers crossed, we will only be living there another few months, and really, I just want the t-stat to look nice and modern.

Being that the pump itself is not outside, can I just leave the "K" wire disconnected (provided the regular heat still works when tested) ?

Thanks in advance for any help!

Done That 03-19-2013 08:24 AM

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.

Technow 03-19-2013 09:51 AM

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.

eviljuan 03-19-2013 09:52 AM

Ok I'll look into that, thanks.

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

eviljuan 03-19-2013 01:13 PM

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?

beenthere 03-19-2013 06:02 PM

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.

eviljuan 03-20-2013 10:27 AM

1 Attachment(s)
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)

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?

beenthere 03-20-2013 04:19 PM

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

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