Replace Lennox AC Thermostat with NEST v2


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Old 07-09-15, 05:47 PM
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Question Replace Lennox AC Thermostat with NEST v2

Hi all, I've been able to replace my downstairs Gas heating's old Honeywell mercury thermostat with a Nest v2 no probs but im stumped by our upstairs Lennox reverse-cycle AC system.

The Nest compatibility checker refers me to Nest who wont help because it's using stranded wires (im pretty sure it's still 24v, but will check with my multi-meter next chance I get). Changing to single core wire is a cinch and I have a connetor block and suitable wire ready to go, however I have no idea what goes where and there are no Nest Pro's in Melbourne Australia.

here is what I have:

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White and Yellow terminals are bridged:
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and the nest v2 connectors (wiring is for my downstairs gas heating)

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any help is greatly appreciated.
 
  #2  
Old 07-09-15, 06:16 PM
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Welcome to the forums from down under.

Lennox reverse cycle A/C is called a heat pump here.
Does your unit have electric reheat (emergency) heat ?

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Lennox stat - color - Nest
G ------------- green - G
R ------------- red ---- R
B ------------- blue -- W2/aux ???
O ------------ brown - O/B (program stat for O operation)
W ------------ white - W1
Y ------------ yellow - Y1
Combined to energize compressor for cooling "Y" and Heating "W"

I know the Lennox units require voltage on the O terminal for cooling.
Usually the O and B are not used together. I think the B may be used to energize a relay or contactor for heat.

Nest/support/article/What-s-Emergency-Heat-and-does-Nest-support-it

The Nest may not work for you. My partner Houston is more well versed in specialized thermostat wiring. He'll stop by and add to this.
 

Last edited by PJmax; 07-14-15 at 07:30 PM.
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Old 07-09-15, 06:37 PM
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Cheers for the Welcome.

Im honestly not sure if it has "electric reheat (emergency) heat" or not, I'll have to check and see if I can find the manual or the ODU model number and look it up - we only just bought the house and arent even living in it yet (re-painting the interior atm before moving in).

PS I doubt it tho, there is nothing on the old controller that indicates this that I can see.

PSS. Im not sure what the switch on the wall is for and havent checked it yet, I suspect it's a kill switch tho as the downstairs heating had one too which I bypassed so my kids dont disable the Nest controller/heating system (they seem to like light switches). I was able to turn the AC fan on when it was in the Off position so I've no idea what it's for (may not even be for AC).
 
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Old 07-09-15, 06:40 PM
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How cold does it get there in the winter ?
What size breaker or breakers feed the inside air handler section ?
 
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Old 07-09-15, 06:54 PM
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Normally over night winter temps are 3c-7c with very infrequent 0c temps and winter day time temps range from 10c-14c. During the Summer overnight temps range from 15c-28c and during the day range from 20c-45c.

i reckon it'd be on a 15amp single phase 240VAC circuit but I'd need to check the meter box to confirm.
 
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Old 07-09-15, 08:49 PM
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I was googling the "Lennox q539" thermostat and sicovered it's made by honeywell.

https://customer.honeywell.com/resou...0s/69-0537.pdf

https://customer.honeywell.com/resou...0s/60-2246.pdf

the only difference in the spec sheet I can find it the lack of an "X" terminal (not on the "A" model I have anyway).
 
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Old 07-10-15, 03:26 AM
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The Honeywell T87 round thermostats only have 1 mercury bulb and would not support a heat pump with heat strips.

I agree that it sounds like you have a heat pump. Lennox heatpumps would use the O terminal for the reversing valve. A model number for the outdoor unit would help clarify this.

The B terminal would get 24 volts the entire time that you are in heat mode, regardless of heat setpoint. It is unlikely that B is connected to heat strips.

I have seen an ambient control that required both O and B but it was made by Ruud. While it unlikely that B is connected to anything at the air handler end of the wire, I would open the panel and verify this as well.
 
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Old 07-10-15, 03:41 PM
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I'll be back at the house later today or tomorrow so I'll check the ODU model and wiring.

Last time I quickly checked it I noticed it did have 3 pairs of control wires going into it which matches the thermostat.
 
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Old 07-14-15, 12:30 AM
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So I couldnt take the cover off the unit as it was bucketing with rain so I quickly grabbed a pic of the OEM plate:

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Old 07-14-15, 03:24 PM
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So Nest finally got back to me with the following wiring based on the PDF's and Pic's I put up here:

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Old 07-14-15, 07:06 PM
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Is it working ?
I'm wondering how they arrived at blue on C as your old Honeywell stat did not use/need a C connection.
 
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Old 07-14-15, 07:14 PM
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I haven't had a chance to try it, we haven't moved in to the house yet as it's still being repainted.

I'll try their diagram 1st but if it doesnt work I'll try your suggestions.
 
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Old 07-14-15, 07:28 PM
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Before you wire it as they suggested..... just check where the blue wire is connected at the air handler end.
 
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Old 07-14-15, 09:04 PM
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Im just having a look thru the Service Manual now:

https://dl.dropboxusercontent.com/u/...29030to060.pdf

and the thermostat wiring shows as C R W1 O Y1 and no "G" wire at all .... Hmmm.

based on those terminals Nest Compatibilty checker gives me:

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So I have no idea where the "G" wire is coming from in my system. I did count the wires going into the ODU and there where 6, I just couldnt take the cover off at the time.

 
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Old 07-14-15, 09:14 PM
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The manual your are looking at is for the outside unit.
The G wire is only used for the inside unit... the air handler.
 
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Old 07-14-15, 09:34 PM
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Hmm, I'll try their suggestion first and see what happens - after all I did give them the thermostat schematics and pic of the old unit including pointing out the bridged Y/W terminals.

PS. I really appreciate your help.
 
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Old 07-14-15, 09:48 PM
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https://customer.honeywell.com/resou...0s/60-2246.pdf

I have not found anything to indicate that B is common with that thermostat.
I would snap a picture of that blue wire connection to the air handler ( furnace).

I would also connect green to G to avoid damage to your compressor.
 
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Old 07-14-15, 10:02 PM
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Nest Support's recommendation based on that very document is G = green too.

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If I get a chance I'll get up in the roof and check the wiring on the Air handler, i'm assuming the thermostat is wired back to it based on all your comments.
 
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Old 07-14-15, 10:10 PM
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The answer to the B and G questions cannot be resolved by looking at the heat pump.

http://www.lennox.com/pdfs/brochures..._Heat_Pump.pdf

The air handler is in a closet, an attic, a basement or a crawl space. It is not the heat pump.

Lennox uses O for the reversing valve.
G controls the indoor fan.
The B wire will also run to the indoor unit. It is possible that it then runs outside but it may be a different color. It must be viewed at the indoor unit.

B is not common in that document.
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Old 07-15-15, 04:43 AM
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So Houston I got up in the roof-space tonight and realised im going to have to go back on the weekend wearing old jeans etc, it was filthy up there and I was wearing suit pants and a shirt!!!

I also didnt take my socket set with me so I couldn't take the panel cover off even if I had managed to circumnavigate the air handler (it's a big bugger, I'd luv to know how they got it up there!!!).
 
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Old 07-16-15, 06:47 PM
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I'm hoping to visit the house tomorrow and check the wiring at the Air-Handler end ... in the mean time i've been googling 24VAC control systems and comparing what I've found with the thermostat schematics unearthed earlier.

I found this wiki:

https://wiki.xtronics.com/index.php/...als_and_wiring

and comparing that info to the thermostat wiring, info provided by PJmax etc, logically i'm thinking the following:

Lennox terminal - color - Nest - Function
G ------------- green - G ------ Fan
R ------------- red ---- R ------ 24vac
B ------------- blue -- W2/aux - set to Heating
O ------------ brown - O/B ----- set to Cooling
W ------------ white - W1 ------ Compressor
Y ------------ yellow - Y1 ------- Compressor
Combined to energize compressor for cooling "Y" and Heating "W" (I wonder if the bridge is still needed for the nest)

I guess the results from the Air handler should be interesting.

Thoughts (seems logical to me)?

PS, just to amuse myself, I put "R G O W1 W2 Y1" into the Nest compatibilty checker and got the "your system is compatible" but "We recommend professional installation".
 

Last edited by dJOS_75; 07-16-15 at 07:42 PM.
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Old 07-16-15, 08:16 PM
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Just to add to the fun, I reckon I finally worked out which model air-handler I have - im 99% sure its the Lennox CB29M:

http://www.lennox.com/pdfs/installat..._CB30M_IOM.pdf

Page 17+ contains the the schematics etc (heat pump is on p20).

Definitely need to see how mine is wired now!
 
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Old 07-16-15, 08:18 PM
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Lennox terminal - color - Nest - Function
G ------------- green - G ------ Fan
R ------------- red ---- R ------ 24vac
B ------------- blue -- not used unless you have a Rheem heat pump
O ------------ brown - O/B ----- set to Coolingcontrols the reversing valve
W ------------ white - Not Used currently but connect both air handler side and stat side if you have aux heat strips
Y ------------ yellow - Y1 ------- Compressor
Combined to energize compressor for cooling "Y" and Heating "W" (I wonder if the bridge is still needed for the nest)It is not needed
 
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Old 07-16-15, 08:51 PM
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Thanks Houston, I might give that a go and see if it works before climbing into the roof:

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Old 07-16-15, 09:04 PM
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Nest thermostats operate funny without that C wire. I would connect blue to C at the air handler then connect it to C at the new stat.

Remember to turn off the breaker to the indoor unit before replacing the stat to avoid damage to anything.

Air handlers are not water tight and not meant to be installed outdoors. Do you have a mechanical closet on your roof that the air handler is in?

I rarely see mechanical closets on roofs.

A 1/4 inch nut driver, a 5/16 inch nut driver and a screw driver as well as a pair of wire strippers are some tools that I would take to move that blue wire to C at the air handler. (A 6 in 1 screwdriver and a pair of strippers would also work)

Just to clarify what I think I see. Is yellow connected to Y and W on the old stat or is yellow on Y and white on W?
 
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Old 07-16-15, 09:18 PM
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Just to clarify what I think I see. Is yellow connected to Y and W on the old stat or is yellow on Y and white on W?
yes currently yellow = Y and White = W however they are bridged in the Thermostat.

Nest thermostats operate funny without that C wire. I would connect blue to C at the air handler then connect it to C at the new stat.
Hmm, i wonder if that is why nest suggested I use this wiring config:

[ATTACH=CONFIG]53457[/ATTACH]

Remember to turn off the breaker to the indoor unit before replacing the stat to avoid damage to anything.
I'll just turn off the dedicated breaker for the entire system in the main electrical meter box.

A 1/4 inch nut driver, a 5/16 inch nut driver and a screw driver as well as a pair of wire strippers are some tools that I would take to move that blue wire to C at the air handler. (A 6 in 1 screwdriver and a pair of strippers would also work)
Tools arent a problem, it's more that im not living in the house yet (doing some renovations 1st) so most of my tools weren't with me last time i stopped at the house.

Air handlers are not water tight and not meant to be installed outdoors. Do you have a mechanical closet on your roof that the air handler is in?
By Air-handler, I meant blower unit (or Heat Exchanger), it's mounted inside the roof cavity like most of them are for Ducted-AC Systems here in Aus. Most houses in Australia are built like this:

[ATTACH=CONFIG]53460[/ATTACH]
 
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Last edited by dJOS_75; 07-16-15 at 09:44 PM.
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Old 07-17-15, 05:14 AM
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Is the air handler in the conditioned space or is a ceiling between the living space and the air handler?

Do not connect white.
Nest will energize the compressor with heat and cool when configured for heat pump.

I would snap a few pictures of the stat wire connections while up in the attic. We need to verify that yellow is compressor.
 
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Old 07-17-15, 07:17 PM
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Is the air handler in the conditioned space or is a ceiling between the living space and the air handler?
It's mounted above the ceiling, horizontally, inside the roof cavity (sitting on #6 in that A frame diagram I posted previously).

Do not connect white.
Nest will energize the compressor with heat and cool when configured for heat pump.
ok cheers.

I would snap a few pictures of the stat wire connections while up in the attic. We need to verify that yellow is compressor.
Im heading out there today and will report back what I find.
 
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Old 07-17-15, 07:37 PM
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In your diagram that is called a truss system. Usually you don't see the air handler sitting on the lower part of the truss. You may have load bearing framing under that truss to support the extra weight.
 
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Old 07-17-15, 10:56 PM
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Well I came prepared today and took lots of photos too. What I found surprised me tho, I have the lennox B24Q3.5 Blower + CH22-41 Chiller??? I think (assuming ch= chiller?).

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and the for surprising bit, the wiring is a bit of a dogs breakfast! Only the Brown, Yellow, White, green & Red wires from the thermostat are in use, blue and black arent connected to anything at all.

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Unfortunately while up there I didnt take a photo of the blower area so have no idea if I have electric heat or not:

https://www.google.com.au/url?sa=t&r...98197061,d.dGY
 

Last edited by dJOS_75; 07-18-15 at 12:34 AM.
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Old 07-18-15, 12:40 AM
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Looks like an attic.


The heat strips would not be in the blower section.
Looks like you don't have heat strips but white is connected to something.

I see a contactor and a relay.

Blue does not appear to be connected to anything in the picture.
 
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Old 07-18-15, 01:21 AM
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It's not an attic, it's a roof cavity and a royal pain to get into and out of, you need a ladder to get to the man hole in the ceiling and then you crawl under beams to get to the air handler and only have enuf room to sit our crouch next to the unit. Directly above you are the roof tiles and the space is full of insulation and spider webs.

Correct, blue is not connected at all. I'm going to try a standard wiring setup next time I'm at the house as I reckon it should work fine now that we know the blue wire was a red herring.

I punched the remaining used terminals into the nest compatibility checker and got a stock wiring diagram straight away of y1 w1 o/b rh & g.
 

Last edited by dJOS_75; 07-18-15 at 01:57 AM.
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Old 07-18-15, 01:56 AM
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It still sounds like an attic? Just a bad one, like mine.


We still don't know what white is doing. It is not controlling a heat strip or we should see it in your pictures.



I like the attics in the River Oaks area that have the potential to be finished and have high ceilings and decking with more than 1 light.
 
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Old 07-18-15, 01:59 AM
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The White and yellow were both wired directly back to the ODU using individual wires (as does Brown) but without a 2 piece continuity tester and a 2nd person it's difficult to know what terminals they connect to.

With only a 20 (or 25) degree roof pitch, most Australian homes have a mostly useless roof cavity like mine.

Edit: looking at the ODU service manual again.

https://dl.dropboxusercontent.com/u/...29030to060.pdf

There are clearly y1 & w1 terminals plus R O & C Which all map directly back to the thermostat. The only one my stat doesn't connect back to is the Common.
 

Last edited by dJOS_75; 07-18-15 at 04:57 AM.
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Old 07-18-15, 04:51 AM
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PJMax and Houston, thankyou so very much for all your help over the past few days - I got it all working thanks to your help. The blue wire was indeed the key and was a total red herring.

I wired it all up like this:

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and after the usual setup process I got this:

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and this:

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Interestingly, I suspect the old thermostat was wired incorrectly as I'd never felt it produce heat before tonight (seemed to blow cool air before) - now it pumps out heat almost instantly!

I also finally worked out what the switch below the stat is for, it's a Zone control to disable the AC vents in the ensuite and bathroom.

if either of you find yourself in Melbourne, PM me and I'll buy you some real beer!
 
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Old 07-19-15, 03:24 PM
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Ok, so the only issue im noticing now is nest says "aux. heat" instead of heat, Im using W1 not W2 and pretty sure I dont have heat strips or anything.

I've selected "O" and not "B" in the nest at setup and this matches with the ODU terminal labeling etc from the ODU service manual (O energised = Cooling, de-energised = heating).

Is this just how a HP system works as it does seem to be heating, just slowly?

PS. It was 1c outside at 6am when I fired it up.

FIRST STAGE HEAT:
10 − Heating demand initiates at Y1.
11 − 24VAC energizes compressor contactor K1.
12 − K1-1 N.O. closes energizing compressor and
outdoor fan motor.
13 − See step 4 or 5.

END OF FIRST STAGE HEAT:
14 − Heating demand is satisfied. Terminal Y1 is
de-energized.
15 − Compressor contactor K1 is de-energized.
16 − K1-1 opens and compressor (B1) and outdoor
fan motor (B4) are de-energized and stop
immediately.

DEFROST MODE:
17 − During heating operation when outdoor coil
temperature drops below 35F (2C) or 42(5.5C)
(see defrost system description for specific unit dash
number) unit defrost switch (thermostat) S6 closes.
18 − Defrost control CMC1 begins timing. If defrost
thermostat (S6) remains closed at the end of the
30,60 or 90 minute period, defrost relay energizes
and defrost begins.
19 − During defrost CMC1 energizes the reversing valve
and W1 on the terminal strip (operating indoor unit on
the first stage heat mode), while de-energizing outdoor
fan motor B4.
20 − Defrost continues 14 + 1 minutes or until thermostat
switch (S6) opens. When defrost thermostat opens,
defrost control timer loses power and resets.
21 − When CMC1 resets, the reversing valve and W1 on the
terminal strip are de-energized, while the outdoor fan
motor B4 is energized.
 

Last edited by dJOS_75; 07-19-15 at 03:55 PM.
  #37  
Old 07-19-15, 04:12 PM
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Do you have warm air when you set the stat for emergency heat?
 
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Old 07-19-15, 04:22 PM
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Yes I have warm air but I don't have an emergency heat option, just "heat".

Does that excerpt from the ODU show it is a 2 stage Heating system and the nest is recognising when w1 is active(defrost mode)?


Ps on the actual nest unit I have seen it simply say "Heating" but I'm not at home atm and just playing with the nest app and trying to understand how hp's work (curious type I am).
 
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Old 07-19-15, 06:35 PM
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Ok so I turned it back on (had it off for a couple hours) and now it says "Heating".

It's 10c outside so i'm speculating the nest was showing "aux. heat" indicating the ODU was in defrost mode due to the extreme cold outside?

It also seems noticeably faster at heating the upstairs area when it just says "heating" - is this due to HP's not liking really low outdoor temps?
 
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Old 07-19-15, 07:34 PM
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When it says heating...... normally the heatpump should be running with no aux heat.
If the amount of difference between the setpoint and the actual room temperature is more than a few degrees..... the aux heat will be activated. As that gap closes.... the aux heat should shut off and just the heatpump should run.
 
 

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