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Field Test Best Practices: A Resource for Practical Residential Building Science

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Thermostat Operation

It is common today to have programmable thermostats in new homes and in many existing homes as well. The sensor technology generally consists of thermistors with a heat or cool anticipation function. These thermostats generally have three or four wires connected to their terminals for sending and receiving heating, cooling, and fan signals to and from indoor air handling unit. In older homes, bimetallic switching thermostats are still used.

In a field test setting, a thermistor is commonly mounted close to the thermostat to monitor the thermostat sensing temperature. Coupled with the indoor air handler power and/or outdoor condensing unit power reading, and/or the furnace gas flow or supply and return air temperature readings, thermostat setpoints can be easily concluded. In simple heating and cooling air handling units with cycling on/off supply fans, the indoor fan status alone with a thermistor mounted next to the air handler can indicate the thermostat setpoints.

Understanding the thermostat setpoint is critically important in field tests since the setpoint is the main driver for house heating and air conditioning system. Many energy saving measures are set around thermostat night setback, unoccupied setback, or pre-cooling to take advantage of house thermal mass, so controlling and monitoring the thermostat setpoint is a central activity for any field test looking at HVAC systems.