Core answer:
Yes. You can have a smart thermostat work without a C wire, using battery power, a power adapter, or a thermostat that supports power stealing or C-wire adapters. Below are practical options and quick guidance.

Quick options

• Battery-powered smart thermostats
  • Pros: Easy install, no C-wire needed.
  • Cons: Need periodic battery changes; may have fewer advanced features.
  • Examples: Several models in the market advertise no C-wire operation.
• Thermostats with power-stealing technology
  • How it works: They draw small amounts of power from existing wires (often R, W, Y, G) to charge internal electronics.
  • Pros: Keeps most smart features without a dedicated C wire.
  • Cons: In some systems, you may see occasional connectivity quirks or slight HVAC cycling changes.
  • Common models: Some Nest, Ecobee, and other brands offer this approach or compatible adapters.
• C-wire adapter kits or external power adapters
  • What they do: Provide a dedicated 24V common reference via an included adapter, often installed at the furnace/air handler.
  • Pros: Allows a wide range of thermostats to function as if a C wire existed.
  • Cons: Requires access to the HVAC wiring compartment; may involve a bit more setup.
• Thermostats that explicitly advertise no C-wire need
  • Pros: Designed for homes without a C wire; simplified installation.
  • Cons: Some models may use batteries or apps for power management.
  • Examples you’ll often see: various budget and mid-range models from popular brands.

How to choose safely

• Check your HVAC system compatibility
  • Some systems (especially older or oil/dual-fuel setups) may behave differently with power stealing or adapters.
  • Look for a compatibility checklist on the thermostat manufacturer’s site and confirm your furnace/air conditioner control board supports the method you choose.
• Consider features vs. power method
  • If you want advanced features (voice control, room sensors, occupancy routines), a power-stealing or C-adapter solution is usually preferable over a battery-only unit.
  • If you’re okay with basic scheduling and remote control, a battery-powered model may be simplest.
• Professional guidance
  • If in doubt, a quick consultation with a HVAC technician or an installer can confirm the best option and avoid any risk to control board wiring.

Common caveats

• Some power-stealing setups can cause the thermostat to briefly reboot or cause faint thermostat flicker in rare cases, especially with certain HVAC configurations.
• If your system uses a millivolt or non-standard control circuit, a C-wire adapter or professional install is more likely required.

If you share:

• Your thermostat model or preferred brand
• Your heating system type (gas/oil/electric furnace, heat pump, or boiler)
• Whether you’re comfortable with a simple adapter install or prefer battery-only

I can tailor a concrete recommendation and a step-by-step setup plan.