What are the differences between on/off and modulating thermostats?

An on/off thermostat and a modulating thermostat each have distinct advantages depending on your heating system and budget. Here’s an overview to help you choose the right one.

On/Off Thermostat

  • Operation:

    • The thermostat switches the boiler on when the temperature drops below the set point and off when the desired temperature is reached.
    • The boiler always operates at full capacity when on, but with a well-set hysteresis (e.g., 0.5°C), the temperature remains stable and comfortable.

  • Energy Usage:

    • While an on/off thermostat might be slightly less efficient than a modulating one, the difference in energy consumption is often minimal, especially in well-insulated homes. Modern boilers are designed to handle frequent switching efficiently.
    • The simplicity of an on/off thermostat ensures reliable operation without complex settings.

  • Advantages:

    • Wide availability: On/off thermostats are broadly compatible with most boilers.
    • Affordable: They are less expensive to purchase and install compared to modulating thermostats.
    • Ease of use: Ideal for older boilers or situations where modulation is unnecessary.

Modulating Thermostat

  • Operation:

    • A modulating thermostat adjusts the boiler’s power output to match the actual heating demand, avoiding full-power operation. This can lower energy bills slightly and provide more consistent indoor temperatures.
    • Only works with modulating boilers, usually via protocols like OpenTherm.

  • Limited Choices:

    • The market for modulating thermostats is more restricted. Manufacturers like Vaillant or Bosch often use proprietary protocols (e.g., R-Bus or eBus), limiting you to their thermostats.
    • Universal modulating thermostats (e.g., OpenTherm-compatible models) are also available but are usually significantly more expensive.

  • Cost and Compatibility:

    • Modulating thermostats can cost two to three times more than simple on/off models.
    • They require a compatible boiler, which may involve additional investment.

  • Advantages:

    • Better temperature control and slightly lower energy consumption, particularly in larger homes or poorly insulated spaces.
    • Suitable for homeowners seeking maximum efficiency.

Why Choose an On/Off Thermostat?

  1. Simplicity and reliability: Works without concerns about compatibility or complex settings.
  2. Affordable solution: Significantly cheaper than most modulating thermostats in terms of purchase and installation.
  3. Wide compatibility: Fits almost any boiler, regardless of brand or age.
  4. Practical energy use: In well-insulated homes, the energy consumption difference is often negligible with a low hysteresis (e.g., 0.5°C).

Conclusion

An on/off thermostat is an excellent choice for those prioritizing simplicity, affordability, and wide compatibility. While a modulating thermostat may offer slightly better efficiency, its higher purchase price and limited options—often tied to specific brands or protocols—make it less appealing for many households. For most, an on/off thermostat with a 0.5°C hysteresis provides a perfect balance of comfort, reliability, and cost-effectiveness.