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What Is a Differential Thermostat?

By Mal Baxter
Updated May 16, 2024
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A differential thermostat is a controller installed with water heating or space heating and cooling systems. It regulates multiple temperature responses between elements such as solar panels and hot water heaters. Similar to a household thermostat, it typically consists of an electronics component with temperature displays. Devices may also possess automatic settings to adjust heating or cooling. What makes this technology differ from standard thermostats is that it compares temperatures between multiple environs or heating and cooling elements to activate and deactivate the equipment.

To illustrate, solar water heating systems rely on differential thermostats to regulate temperature as water moves through different points in the system. A rooftop collector heats water pumped through it or delivered through natural convection; these are referred to as active and passive systems. Cooler water collects into a cylinder and is fed to evacuated tube or flat-panel solar collectors. The sun heats the water and the system returns it to a storage tank located elsewhere. Such a system is sometimes backed up by a boiler or immersion heater to ensure hot water in overcast weather.

When water temperature in a solar panel rises above the water temperature of a storage tank, the thermostat will activate the pump. This will deliver hot water from the solar collector to the tank. As the tank water temperature exceeds the water temperature in the solar panel, the thermostat will deactivate the pump. Doing so can prevent cooler water from entering the storage tank.

Units come available in numerous products, though they are not difficult to construct for electronics enthusiasts. They occupy systems in a number of configurations. These might include installation with solar panels, pumps, tanks, or simple drain-back systems.

More complex configurations can also be accommodated, such as split drain-back systems. Pressurized systems use multiple storage and service tanks with their own controllers. Additionally, multiple differential thermostat units may interface with each other to control recirculation or closed-loop systems.

Differential thermostat products vary in complexity and may feature simple plug-in technology or require professional installation. Units are designed to accommodate product types, including electrical, gas, or solid-fuel hot water heaters. They may be compatible with most common brands of solar collectors, making these controllers a versatile and powerful addition for energy-efficient home automation.

By using thermal sensors to compare temperatures between a solar panel and a water tank, the differential thermostat may activate within a specified temperature range to run a pump. Hot water from a solar panel may be sent to a tank; when the water in the tank gets warmer than the water in the solar panel, the pump will cut off. This prevents cooler water from entering the tank and reducing efficiency in storage. These settings can be modified to user-preferred temperature ranges.

Common constructions of differential thermostat products may include temperature sensors, which detect temperatures in various equipment. Units can include comparator chips that compare the temperature variations, or differentials, between equipment. A transistor might be employed to activate a pump or piece of equipment when a specified comparator output triggers it.

These units come with simple knob interfaces or with more complex digital readouts with information displays and various control switches. Adjustable differential ranges, freeze protections, and system tests may be offered. Additionally, units may display output power levels to and from the controller. Products are sometimes sold with additional temperature sensors or remote monitors for snap-in system diagnostics.

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