Multistage thermostats are available for any heat pump system in any configuration. The speeds may be controlled by sensing outdoor temperature or space temperature or a combination of both. Some deluxe model heat pumps now come with two speed compressors. Most heat pumps have two stages for heating and one stage for cooling. The second stage will turn on the compressor. The first stage will energize the reversing valve.
#DISTECH CONTROLS THERMOSTAT LOWER TEMP MANUAL#
The thermostat manual heat/cool switch can either energize or de-energize the reversing valve, depending on how the system is configured.Īutomatic changeover usually requires a thermostat with an additional switch or stage. For instance, a system that’s changed over manually to cool, the reversing valve would be energized by a sub-base switch. Changeover can be for heating or cooling. But there are many heat pumps in use with outdoor reset systems and many that do not use an electronic thermostat, so knowing how outdoor reset is applied is still necessary.Ĭhangeover can be either manual or automatic. Cycle rate is programmed into an electronic thermostat. The advent of electronic thermostats has eliminated droop and heated anticipators. In cold weather, the thermistor has high resistance, resulting in low current flow and little or no heat being added to the thermostat by the anticipator. The thermistor has low resistance in warm weather, allowing high current flow to the heat anticipator.
#DISTECH CONTROLS THERMOSTAT LOWER TEMP SERIES#
The reset heater in the thermostat is connected in series with a thermistor sensor that measures outdoor temperature. The thermostat is actually being de-calibrated to compensate for differential and droop so it appears to still switch near setpoint.Īt warm outdoor temperatures, heat is added to the thermostat by the heat anticipator and at cold outdoor temperatures, this extra heat is automatically removed. Outdoor reset has the effect of automatically raising the setpoint under high load, cold outdoor temperatures. There could be a large offset between the setpoint and actual room temperature. Outdoor reset was often applied to heat pump systems to minimize droop under high load conditions. In this way, the cycle rate is designed into the thermostat. A constant amount of heat is produced during the “on” cycle. Using voltage anticipators, the heat added to the thermostat is fixed, no matter what the load is. Some heat pump thermostats have voltage heat anticipation instead of current anticipation. Being a refrigeration system, a heat pump is better off being cycled 2.5 to 3 times an hour. At 50% load, fossil fuel thermostats cycle a furnace about 5 or 6 times an hour. The cycle rate of a heat pump thermostat is different than a thermostat used with fossil fuels. There is a degree or two of offset between stages one and two so that the second stage, auxiliary heat, is made only when the heat pump can’t handle the load. The two stages make contact sequentially. Auxiliary heat and compressor will not start at the same time, and under high load, the heat pump will run constantly instead of being cycled. Using a multistage heat pump thermostat will govern the use of auxiliary heat by actual demand. The use of auxiliary heat is not only determined by outdoor temperature, but also by actual heating requirements of the building. The two-stage heat pump thermostat is designed to do this. No auxiliary heat should be utilized until there is a demand for more heat. When the compressor output exactly equals the heat needed, the compressor will run 100% of the time. The idea of heat pump control is to use the most economical heat source first, and the auxiliary heat source, usually electric resistance heat, only when the heat pump can’t handle the load. The space temperature sensor needs to be at least a two-stage thermostat. are the same on a heat pump as on a cooling unit only.) (Contactors, time delays, high and low-pressure controls, etc. We will look at only those devices associated with a residential heat pump. Breaking out the individual requirements for each control will show the logic involved and make it easier to grasp once you understand each control’s function.Ĭontrol systems utilize low voltage circuits since line voltage systems would be difficult and expensive to wire. When you first look at a heat pump control system, it can be overwhelming. Of all the heating and cooling systems used in residential applications, the heat pump is the most complex.
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