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Old 02-28-2012, 08:22 AM   #1
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Leave "E" wire disconnected from thermostat?


Hello all,

I want to install a new thermostat. I have checked the wires connected to my current thermostat, and there is an "E" terminal not present on the new thermostat (all others are present). I understand this is for an "emergency heat" feature, though I don't even know how to trigger the emergency heat with my current thermostat.

I am wondering if I can safely connect the new thermostat and simply leave the "E" wire disconnected (and protected)? Is it crucial to operation of any heating unit?

Unfortunately, I do not have the model information of the unit handy. It is a combined AC/heating unit (two-stage).

A photo of my current thermostat's wiring scheme is attached.

(And, though it sounds strange, "just get a thermostat with an E terminal" is not an option in this case.)

I sincerely appreciate any help you can provide.
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Leave "E" wire disconnected from thermostat?-thermostat.jpg  

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Old 02-28-2012, 08:53 AM   #2
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Leave "E" wire disconnected from thermostat?


Well it sounds like you have a package heat pump, based on the presence of the "O" terminal for the reversing valve. The "E" terminal is there as you say to turn on emergency heat, which in your case is probably electric strip.

You should be able to manually switch your thermostat to emergency heat....for example if the compressor was not running so you could still get heat.

You would lose this capability without having your new stat have an E terminal. Yes, you could simply not connect the E and protect it, or also disconnect it at the outdoor unit.

I must be missing something because you should be replacing with a heat pump thermostat that should also have an E terminal??

Looking again at your picture the plastic tab at the bottom when moved to "SUPL HT" should energize that E terminal.

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Last edited by Done That; 02-28-2012 at 08:55 AM. Reason: update
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Old 02-28-2012, 04:37 PM   #3
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Leave "E" wire disconnected from thermostat?


Great, thank you for your help.
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Old 02-28-2012, 05:35 PM   #4
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Leave "E" wire disconnected from thermostat?


Post brand and model number of your new thermostat.
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Old 02-28-2012, 05:48 PM   #5
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Leave "E" wire disconnected from thermostat?


It's the new Nest learning thermostat. http://www.nest.com/

I am friendly with someone on the development team, which is why I am considering switching out my thermostat (besides it looking like a quality product). So I either get it and forfeit my emergency heat option (which I don't think I'd mind) or just stick with the one I have now.
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Old 02-28-2012, 06:37 PM   #6
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Leave "E" wire disconnected from thermostat?


It my become an antique soon. Your friend will know why I said that.
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Old 02-28-2012, 07:46 PM   #7
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Leave "E" wire disconnected from thermostat?


How do you mean?
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Old 02-28-2012, 08:31 PM   #8
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Leave "E" wire disconnected from thermostat?


This chart shows what systems the Nest thermostat is compatible with. The chart was created by trying all of the different combinations on the Nest compatibility tool Your system should be compatible with the nest thermostat. Do you know where the E and the W2 wires from your thermostat are landed at in the air handing unit?
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Old 02-28-2012, 08:34 PM   #9
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Leave "E" wire disconnected from thermostat?


Talk to your friend in R&D.
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Old 02-28-2012, 08:38 PM   #10
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Leave "E" wire disconnected from thermostat?


No, I don't know where the E and W2 wires are landed.

I used the compatibility tool, and it says I'm compatible when I leave the E unchecked, but then says I am incompatible once I check the E.
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Old 02-29-2012, 12:32 AM   #11
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Leave "E" wire disconnected from thermostat?


Besides your friend, why would you want that thermostat? There are much better thermostats that can do what the NEST can and much more. They also do what the nest cant do which is operate your system properly by providing you the option of emergency heat.

I think Beenthere may know about a new version coming out? (just guessing as he has me intrigued now)
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Old 02-29-2012, 05:14 AM   #12
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Leave "E" wire disconnected from thermostat?


Nope. They're being sued for patent/technology infringements. I believe 7 infringements.
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Old 02-29-2012, 06:50 AM   #13
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Leave "E" wire disconnected from thermostat?


I've read about the patent infringements. One of them is something like "displaying grammatically complete sentences while programming a thermostat". I'm not even sure how you can get a patent on that. In any case, the patent suit doesn't sway me in one way or another.

hvactech126, can you point me in the direction of a similar thermostat? I suppose I wouldn't be closed to the idea of another model, but I have personally not found one that can do what Nest can.
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Old 02-29-2012, 07:06 AM   #14
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Leave "E" wire disconnected from thermostat?


what feature of the nest do you want most? internet capability? sleek design? If you are looking for a thermostat that can program itself, then that is probably not going to happen. Programming a thermostat is not hard to do. You do it once and then it is done.
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Old 02-29-2012, 08:14 AM   #15
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Leave "E" wire disconnected from thermostat?


So then there aren't thermostats that "can do what the Nest can do and much more". You just don't think it's worth buying the Nest.

I appreciate the help with my original question about the "E" wire, but I didn't ask to be talked out of my purchase because of the personal feelings of those responding. I shared the model I am looking at assuming that it was relevant to my question.

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