Nest will not fire up furnace properly unless AC is disconnected

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Old 06-08-20, 09:41 AM
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Nest will not fire up furnace properly unless AC is disconnected

I have 2 separate heating and cooling systems and I am trying to wire them up to a Nest 3rd Generation thermostat. By separate I mean AC air handler is in the attic with its own set of flexible ducts going to ceiling registers on the second floor and Furnace is on the first floor with separate metal ducts going to lower wall registers on floors 1&2. (2 separate 5 conductor 18AWG cables are run to the thermostat location)
  • Heat: Trane XL80 Furnace
  • AC: Trane XR13 condenser with a TEC3F36 Air Handler

On my oldHoneywell (RTH7600) thermostat I had it all wired like this and everything worked fine:
  • Heat: R, W1, W2 andAC: R, Y, G
  • To: R, W, W2 and RC, Y, G on the Honeywell Thermostat

When installing the nest with the same wiring the AC works fine but the furnace will not fire up right and does a lot of stopping and starting and clicking but never fully fires up. If I remove the 3 AC wires however the furnace fires up just fine (both stages) and there are no issues. If I put the AC wires back on the nest the same problem happens.

I have also tried connecting the C wire from the furnace and had the same problem. Then I tried connecting the C wire from the AC unit instead and still the same problem. No matter what I try the heat doesn't work with the Nest while the AC is also connected.

It could be similar to this post though this poster seems to only have 1 W wire for heating as opposed to 2: https://www.doityourself.com/forum/t...connected.html

Would a relay help here as well, or am I stuck switching the wires from cooling to heating (and vice versa) when the season changes, or getting a 2nd Nest and controlling the heat and AC separately? Nest support failed to resolve. Please help! Thanks!

Happy to send photos of any equipment if needed.
 
  #2  
Old 06-08-20, 12:10 PM
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Verify the Furnace's R is going to the Nest Rh. Sometimes the connector gets stuck and you have to remove and re-insert.

The Nest's Rc should go to the AC's R and the Nest's C to the AC's C.
 
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Old 06-08-20, 07:21 PM
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As long as any A/C wires are connected...... the C must come from the A/C system.
 
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Old 06-08-20, 07:56 PM
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Thanks for the tips! I double checked that the Furnace R is going to Rh and the AC/s R is going to the Nest's Rc and also I tried again with the C from the AC System connected to the C on the Nest and even tried again with no C connected and the same result: Anytime heat is called for by the nest (either stage 1 or stage 2) the furnace spins up for a bit (like normal) then clicks (like normal) but then instead of igniting it clicks again and stops and spins up again and clicks and stops and so on... and never gets going.

It's like the Furnace definitely gets the call for heat and responds but its also getting something (signal or voltage,etc) it doesn't like or expect before it fully lights and then the extra clicking and fan stopping and starting happens.

Oddly enough the furnace works fine with the Nest when connected by itself but when connecting up the AC wires the problem occurs, even if I remove the blue (c wire) going to the Furnace from the Nest's C terminal and replace it with the the blue (c wire) going to the AC.

Is there anything else I can check? Thanks again!
 
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Old 06-08-20, 08:32 PM
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Verify that the R, W1, W2 wires at the thermostat go directly to the furnace and nothing strange going on.
 
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Old 06-09-20, 05:54 AM
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Thanks for the tip! I just re-traced the wires and they seem healthy (no nicks or cuts or anything) and they do in fact go directly from my wall in the hallway where the thermostat is to the AC in the attic (about 20FT) and to the furnace downstairs (about 40FT) Both runs are 5C 18AWG.
 
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Old 06-09-20, 07:51 AM
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Hi, does the Nest give any alerts? You do have an 18/5 going directly to each unit, not to familiar with Nest, but is there any programming required?
Geo
 
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Old 06-09-20, 08:56 AM
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Just curious - try without the W2 wire attached as most don't have a two stage furnace and aren't seeing your problem. Does the furnace work then?

Please check the voltages with and without the AC wires hooked up. Can't do it remotely but need to go to the Nest itself. It is under Settings - Technical Info - Power. Voc, Vin, Iin. Voltages explained here: https://support.google.com/googlenes.../9241211?hl=en

If voltages OK I'm thinking the base may be bad.
 
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Old 06-09-20, 09:34 AM
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Thanks again! No the nest does not give any alerts and thinks everything is fine. Yes both units have 5C 18AWG wire directly between them and the nest

Also I have tried without the W2 wire (with W1 and W2 jumpered at the furnace) This was the way it was wired originally since an older thermostat did not have W2 terminal. During this test the furnace experienced the same starting stopping and clicking issue.

I'll check the voltages later today and report back.

I'll check voltages with AC only hooked up which works fine
With furnace only hooked up which works fine
with both AC and Furnace hooked up - only AC works fine furnace does not.

Also if there are any other ways to check voltages (I have a volt meter) let me know between which wires to test and I'll check them as well.
 
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Old 06-09-20, 10:31 AM
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Hi, can we assume that you have programmed the Stat correctly, 1 heat stage and 1 cooling stage ?
Geo
 
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Old 06-09-20, 12:36 PM
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Thanks again! Yes its been programmed correctly. I tried with both 1 heat stage (W1 jumpered to W2 at the furnace connected to W1 on the nest) + 1 cooling stage as well as with 2 heat stages (removing the jumper at the furnace and connecting W1 and W2 from the furnace to the nest) + 1 cooling stage.

The nest is aware its 2 stage heat and both stages work fine (tested both) when the ac wires (Rc, Y & G) are disconnected. But unfortunately both stages fail the same when the AC wires are reconnected.
 
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Old 06-09-20, 02:57 PM
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Hi, if you were to remove the Stat from the sub base and jump out RH to W1 will the furnace fire up ? likewise if you jump out RC to G and Y will the cooling fire up, this should prove if the Stat itself is the problem.
Geo
 
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Old 06-09-20, 06:50 PM
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Yes the heating works with R and W1 connected without the tstat. Also the tstat controls the heating just fine with the AC wires disconnected it only has the problem if the AC wires are connected.

Furthermore the AC works fine from the tstat every time and never has any issues. It also works when Rc is connected to Y & G without the tstat.

Furthermore the old tstat controlled the furnace fine with this same configuration.

Based on that I can almost say with certainty that the nest behaves differently than the old tstat. Could the old tstat be electrically isolating the AC wires when the Heat mode is selected? If that's the case then I can see how that would work but the nest is not doing that which is causing issues.

So basically the nest can control the furnace fine by itself but always has issues when it tries to fire up the furnace with the AC wires connected. Its basically like It's like the furnace definitely gets the call for heat and responds but its also getting something (signal or voltage,etc) that makes it freak out and start the clicking and stopping and restarting etc...
 
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Old 06-09-20, 07:03 PM
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Based on that I can almost say with certainty that the nest behaves differently than the old tstat.
Yes and that happens a lot. You may want to contact Nest and have them send a replacement sub base. The sub base is where all the switching is done.

Although based on the problems you are having.... it may be prudent to look to another smart stat.
Possibly an Ecobee or something in the Honeywell line.
 
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Old 06-09-20, 07:20 PM
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All, just checked my voltages from the nest and here is the results under all 3 scenarios:

AC Only
Result - Works Fine
Wiring from AC to Nest: Rc, Y, G & C
VOC 36.25V
VIN 36.25V
lin 200 ma (c)

Heat Only
Result - Works Fine
Wiring from Furnace to Nest: Rh, W1, W2 & C
VOC 37.12V
VIN 37.12V
lin 200 ma (c)

Heat and AC
Result - AC works fine and Heat fails (clicking and restarting, etc)
Wiring from AC to Nest: Rc, Y, G & C
Wiring from Furnace to Nest: Rh, W1, W2
VOC 35.82V
VIN 35.82V
lin 200 ma (c)

Does this look ok? According to googles tech articles these are well within the ranges. I am thinking that based on what mentioned in this post: https://www.doityourself.com/forum/t...connected.html it may be worth it to try the relay. I am just not sure what type I would need and how to wire it. Any thoughts on that or any other ideas?

Thanks again for the feedback!!!

 
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Old 06-09-20, 08:37 PM
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Those voltages all look correct on the surface. I wouldn't think Voc/Vin would drop that much when furnace connections present. Pointing to a base issue. Assume none of the wires from AC and furnace touch one another.

If you do have a volt meter it would be interesting to know what the voltages were at the furnace. C to R, C to W1, W1 to R.
 
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Old 06-09-20, 08:48 PM
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In order to use relays...... you'd need two.
One for W1 first stage and the second for W2 for the second stage.
 
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Old 06-09-20, 09:05 PM
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I had google swap the nest and base so I don't think the issue is caused by the base. I can check the voltages tomorrow with a volt meter, but in the meantime I guess the options are at this point as follows:

A) try another brand t-stat: I am fairly invested in the nest ecosystem and would prefer not to manage another app
B) Switch the wires when the season changes: I am too lazy for that
C) Buy a second nest: I will consider this but prefer not to spend the extra $$$ and put another hole in the wall
D) Try a relay or 2 relays: I am willing to try this before relegating myself to option C but can anyone tell me what type of relay to try and how to wire it?

I just have a feeling the relay will work. fingers crossed. Thanks again!
 
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Old 06-09-20, 09:32 PM
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So you only have one thing to buy a dual relay like this should be easy to install: https://www.amazon.com/Electronics-S.../dp/B01B60RZPA
 
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Old 06-09-20, 09:45 PM
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Cool $12 is not bad at all and worth a shot. If I order this how do I wire it?

I assume on the furnace side leads for relay #1 go to W1 and R and leas for relay #2 go to W2 and R Is that right?

Now for the nest side what do the leads for relay #1 and relay #2 go to?
Leads for Relay #1 -> W1 and C on the nest?
Leads for Relay #2 -> W2 and C on the nest?

Let me know if that makes sense?

 
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Old 06-09-20, 09:54 PM
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Yes.... that makes sense and is correct.
You'll be using the 24v from the A/C system to run the relay coils.
 
  #22  
Old 06-09-20, 10:27 PM
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Relay Wiring

Please see the picture I just created. Is this right or do I have it backwards?
 
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Astuff voted this post useful.
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Old 06-10-20, 10:35 AM
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You had it correct in your text description. The diagram is also correct.
 
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Old 06-10-20, 10:50 AM
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Thanks! I'll get the relay and give it a shot.
 
 

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