Huge electric bills after 4th gen upgrade

[H20_goals]

After getting hit with two huge utility bills — $600 for August 2025 and $760 for September 2025 — I knew something was wrong.

For comparison, August and September 2024 bills were roughly half that (I can post exact kWh data if helpful).

Looking at my 15-minute Con Ed interval data, my kWh pattern totally changed starting right when I upgraded to the Nest Learning 4th Gen.

I’ve done some serious diagnosing with ChatGPT (yes, really), but can’t seem to fix it.

📍 Core System Details

• Location: Manhattan loft apartment
• System type: Gas-fired forced-air furnace, single zone (one thermostat for entire apartment)
• Thermostat history: Replaced older Nest 2nd Gen with new Nest Learning 4th Gen on July 29, 2025
• Wiring:
  
• Old Nest 2nd Gen: White → Y1, Green → G, Red → RC, Yellow → W1 (reversed from normal but worked fine)
• New Nest 4th Gen: Tried correcting to Yellow → Y1, White → W1, but cooling blew hot air — reverted back for now
• Mode: Heating = gas Cooling = standard A/C (not heat pump)
• Fan: Auto
• Stages: Single; no humidifier; no common wire
• Power: Individual apartment meter (verified isolated from other units)

⚡ Problem Summary

• Since the upgrade, daily kWh usage is up ~40–50 % vs prior year (verified in Con Ed data).
• Two huge 15-minute spikes when A/C turns on:
  
• 1.86 kWh @ 6:15 PM 10/6
• 1.68 kWh @ 9:30 AM 10/7 ≈ 7 kW average draw
• Controlled test: Turned HVAC off 11:45 AM → 1:55 PM 10/7 → usage dropped to near zero ⇒ meter and other loads normal.
• Cooling now feels slightly weaker than before; when temperature is reached, I hear a short (~5 s) pressure-release sound.
• Con Ed meter otherwise stable — no shared-load signature when HVAC off.

🔍 What I’ve Confirmed

• Baseline draw near zero when HVAC off → not shared power or faulty meter.
• Spikes only when compressor kicks on → inrush or dual-call issue likely.
• Nest reports “Gas and forced air.”
• Wiring at furnace may still have Y1/W1 swapped from the old configuration.

❓ What I’m Asking

DOES ANYONE HAVE ANY IDEA WHAT COULD BE GOING ON?

Has anyone seen the Nest Learning 4th Gen cause a continuous blower or cross-signal between heating and cooling on gas systems?

Could the Nest’s auto-detect logic misidentify stages and energize both Y1 + W1?

Or is this more likely a stuck relay / contactor issue after the thermostat swap?

saw similar post with

@CoolingWizard @brookevc 

[CoolingWizard]

@H20_goals ,

First of no thermostat can draw large amounts of power. The thermostat actually calls for heating or cooling and the HVAC equipment draws all the power.  If the Thermostat does not cancel its call for heating or cooling, then the power usage would be higher and at same time your home would be colder or hotter than desired.

Here are the facts. 
The W, W1 and W2 thermostat terminals are used to call for Heat.  In your case the gas furnace.  
The Y, Y1, and Y2  used to call for Cooling.  In your case it activated the Outdoor Compressor. 

If you have a conventional HVAC system, you cannot activate the compressor with the W1 terminal. No thermostat made can see the color of the wire attached to any terminal.  
If you put the Yellow wire on Y1 and when you call for cooling and the gas furnace turns on, then the yellow wire is attached to W in the gas furnace. Likewise, then the White wire when attached to Y1 is  activated the AC cooling would come on.

The AC Cooling Wizard

[H20_goals]

Thank you for that response. I’m less concerned about the wire swap (which I’ve corrected for ) and most concerned about the energy. I also understand the thermostat isn’t the cause of the power spike, but the actions it causes (longer compressor times etc). To reframe my question: ever since I upgraded to the nest 4, my power bill has skyrocketed, any ideas of what might be going on here ?

[CoolingWizard]

@H20_goals ,

As I stated, if the compressor is running longer in order to satisfy the demand, then it is not the thermostat.  If on the other hand you are state the Nest Gen-4 set at same temperature ad my Gen-2 makes my apartment feel colder.  Then I would say we need analyze the temperature sensors.  

Your complaint is energy consumption not comfort issues. Now if your HVAC system is running longer to cool your apartment, it has to be an HVAC system performance issue.  

The AC Cooling Wizard

[H20_goals]

Thanks.  I would state my apartment actually feels warmer and performance on the same temperature has felt slightly less cold. Seems like the HVAC system is 'working harder.'  If it's an HVAC performance issue, then the next step would be to get my unit serviced? I'm just confused on why this happened the same exact day I upgraded the nest. 

[CoolingWizard]

@H20_goals ,

I would carefully monitor the temperature set point and the system run time.  There can be a slight variance but the Nest Learning Thermostats use three separate thermistors and averages the three; the temperature reading is very accurate.

Make sure your air filter is new.  

AC Cooling Wizard

[H20_goals]

These are the settings.  I adjusted yesterday. Do these make sense ?

[CoolingWizard]

@H20_goals ,

that all looks fine I would just let you know that most HVAC systems need to run about 15 minutes to reach their peak efficiency before a technician will measure its performance

the AC Cooling Wizard