Making pneumatic thermostats energy efficient

Resources Cypress Envirosystems, www.cypressenvirosystems.com Cypress Semiconductor, www.cypress.com Wikipedia page on thermostats, http://en.wikipedia.org/wiki/Pneumatic_thermostat

In the last decade or so, owners of commercial buildings have relied on building automation systems (BAS) to minimize energy use, with the biggest savings generated from better control over HVAC systems. Now the big buzz is over wireless BAS which, because there is no new wiring involved, should save up to 50% of its initial cost by minimizing installation expenses.

Though BAS is a great idea, unfortunately millions of commercial buildings can’t take advantage of it, neither wired nor wireless. That includes most commercial buildings erected before 2000. That’s because these buildings use pneumatic thermostats to control heating and air conditioning. Pneumatic thermostats use compressed air actuation and control. They contain no electronics or remote controls and provide no diagnostic data. Pneumatic thermostats employ a simple bimetallic strip to sense temperature and each must be set manually.

The only way to retrofit these buildings with BAS is to cut open walls and ceilings to replace the pneumatics with electric motors and digital controls. This is expensive and disruptive, particularly when such work exposes asbestos and other harmful materials to the building occupants. In other words, it is largely impractical for most buildings.

With a clear need like this, one might think a solution would have been found years ago. One tactic has been to replace the pneumatic thermostat with an wireless electronic version. The typical approach has employed a pulse-width-modulated solenoid (PWM) and microprocessor to wirelessly control it. The problem has been that such systems require battery power, and the constant sampling and pulsing quickly drains the battery. Also, a PWM valve isn’t reliable. It can easily clog with water, dirt, or oil, all of which are found in most older pneumatic systems.

Cypress Envirosystems took a different approach with a device called the Wireless Pneumatic Thermostat or WPT. A version of the Wireless Pneumatic Thermostat, or WPT , directly replaces a pneumatic thermostat and turns it into a BAS node.

The WPT enables remote temperature setpoint adjustment by moving a slider lever. In conventional pneumatic thermostats, the slider lever lets occupant control temperature setpoint by adjusting the position of the bimetallic element which in turn is connected to a small air valve. When the room is too warm, the bimetal bends one direction and opens the valve. When it’s too cold, the bimetal bends the other direction and closes the valve.

The WPT interacts with the slider lever using a stepper motor that connects to the lever through a cam. The motor itself is off-the-shelf and quite small, measuring about 1×1×2 cm. When the WPT receives a signal to set room temperature, it begins rotating the stepper motor in the correct direction. Optical feedback ensures the stepper motor moves to the correct position.

The WPT also employs a pressure sensor to monitor the air pressure line controlled by the thermostat. The air pressure line serves as a control signal for the building’s heating and cooling system. The WPT taps into it via a small tube connected to a MEMS pressure sensor. The WGR monitors air pressure and beams air pressure status back to the BAS.

Electronics in the wireless thermostat stays mostly in “sleep” mode, waking up every 15 minutes to check status and do set-point changes. The heavy lifting is still done by the pneumatic thermostat. This stretches the life of the battery (a standard lithium battery called a CR123a) to as much as five years.

Data passes over a wireless mesh network which uses several repeaters. This network is self-healing (when one path is blocked, another path may be established using different repeaters). In a mesh network, data may go through multiple redundant paths to reach a central hub. The hub stores historical data and hosts a user Web interface through which one can access specific thermostats. Typical tasks include looking for trends, doing set-point control, and downloading temperature data to Excel.

The hub, called the Green Box, has a built-in server that runs BACNet, the open protocol used in the HVAC industry. The server makes every gauge in the system look like a BACNet object. This lets the hub talk with an existing BAS.

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