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Lennox

OEM Lennox Supco AT021 Adjustable Thermostat 74T12 Style — Y8840

SKU: LNX-Y8840 Brand: Lennox
$39.95 $54.95

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Texas CMBL #19328542014
SAM.gov Registered
Govt POs Accepted

The Lennox Y8840 is an OEM Supco AT021 adjustable thermostat in the 74T12 style, replacing part number 310708, designed to provide reliable and precise temperature sensing and control within Lennox HVAC systems. If your system is short-cycling, failing to maintain set temperatures, or not responding correctly to temperature changes, a faulty thermostat like this may be the cause. This genuine Lennox OEM replacement part ensures correct fit and performance for compatible Lennox heating and cooling equipment.

Shipping Information

  • Orders placed before 3PM CST ship same business day
  • Standard delivery: 2-5 business days
  • Express shipping available at checkout
  • Free shipping on orders over $75
  • Tracking number emailed upon shipment

Return Policy

  • 30-day returns on unused parts in original packaging
  • Electrical parts (capacitors, contactors, control boards) are final sale once installed
  • Email info@nationalhvacparts.com to initiate a return
  • Refunds processed within 5-7 business days

Frequently Asked Questions

Does this Thermostat fit my Lennox OEM Lennox Supco AT021 Adjustable Thermostat 74T12 Style — Y8840?
Yes. This is the factory-specified OEM thermostat for the Lennox OEM Lennox Supco AT021 Adjustable Thermostat 74T12 Style — Y8840. It is a direct drop-in replacement that fits without modification. Always verify the model number on your equipment nameplate before ordering.
How do I replace the thermostat on my Lennox OEM Lennox Supco AT021 Adjustable Thermostat 74T12 Style — Y8840?
Refer to your equipment service manual for model-specific steps. General approach: turn off power, photograph connections, remove old part, install new part, restore power and test. For gas components, hire a licensed technician. Email info@nationalhvacparts.com for guidance.
What symptoms mean I need a new thermostat?
Common symptoms: unusual noises, intermittent operation, complete failure, or visible damage. Visit our diagnostic hub for symptom-based troubleshooting.
Is this thermostat compatible with Bryant or Payne too?
Many Lennox parts are cross-compatible with Bryant and Payne since all three brands are manufactured by Carrier Global Corporation. The same physical part is often used across brand labels with different model numbers. To verify: check your Bryant or Payne equipment service manual for cross-compatible part numbers, or email info@nationalhvacparts.com with your model number.
Should I use OEM or aftermarket for my Lennox thermostat?
OEM recommended for reliability and warranty compatibility. OEM parts match exact factory specifications for proper fit, performance, and safety. While aftermarket alternatives cost less upfront, the risk of improper fit and reduced lifespan usually makes OEM the better value long-term.
How fast does National HVAC Parts ship?
Orders placed before 3:00 PM CST ship the same business day. Standard delivery is 2–5 business days anywhere in the continental United States. Express shipping available at checkout. We ship from Houston, TX to all 50 states. 30-day return policy on all parts.
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Mike Rivera
HVAC Parts Specialist — Online