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HotPi is a Raspberry-Pi based HVAC control system. It differs from other products both in that it is very cost-effective and that it was designed as a generic controller rather than a task-specific device. This means it may be used for a wide variety of purposes, such as controlling more sophisticated systems like solar collectors or geothermal loops, or even environments other than HVAC.

Features

HotPi provides the following options in its standard configuration:

• A Raspberry Pi-based Head Unit with Python-based support software. This is the "brain" of the system.
• A Google AppEngine-based mobile-friendly Web interface that provides reporting and control functions.
• A variety of 1-wire network interface devices, including:
  • A zone controller that can sense a thermostat demand and control a zone valve or circulator.
  • A generic, high-power relay controller that can drive draft fans, circulators, or other devices.
  • Direct connections for raw temperature sensors for things like storage tanks and outdoor temps.
  • A thermocouple interface for sensing high temperatures, such as inside a flue.
  • A capacitive level sender that may be used in unpressurized storage tanks.

Design Principles

HotPi was designed with two primary design principles in mind. First, whenever possible the system should "fail safe." When properly wired, HotPi will not interfere with a backup system like an oil boiler, and removing power from the system releases all control and returns the system to a default state (which can be whatever you want it to be - usually a fail-over operating mode.)

HotPi was also designed to mimic commercial products as much as possible. HotPi is not UL-listed, and carries not warranty or guarantee of any kind. It is an experimental device, and you assume all risk in constructing and operating it. However, its design still follows the principle "Honeywell might sell this":

• All electrical wiring is done in UL-listed cabinets and junction boxes using appropriate wiring and fittings matched to the loads involved.
• Important components such as relays and terminal blocks are high-quality versions from Omron, Amphenol, etc.
• Sensor networks use isolated 1-Wire busses.
• High voltage components are isolated from the rest of the system.
• PCBs use surface-mount technology wherever possible, and if assembled properly comply with RoHS standards.

Cost

HotPi is generally inexpensive, but cost varies widely depending on the components you need. Allowing for the cost of the PCBs, components, wiring, thermostats, and other elements, expect a typical system to run around $600. You might get this as low as $200 for a minimal system using mechanical thermostats and providing very few features, but you would be giving up a lot of functionality to get there.