The high-tech revolution has swallowed up that quaint old rotary thermostat which used to adorn the living room wall and replaced it with a sophisticated computer which seems to require a specialist technician to operate it, an advanced degree to understand what it can do and yet is rarely used to do more than a couple of basic operations. The Waterfurnace chart of thermostats gives an idea of what is available today and demonstrates the problem facing us when we try to select the best for our purpose. The following comments and brief descriptions of each thermostat may help you to determine exactly what functions you need and narrow down your options.

1. Many programmable thermostats are sold today but few are actually programmed to do anything but keep a constant temperature. You may want to consider what you actually want from your thermostat before investing in an all-singing-and-dancing model. Of course a sophisticated model can behave like a simple model so there is no harm in getting one that you might not use fully if you don’t mind the expense. However, many service calls are caused by wrongly programmed thermostats so simplicity may be the key.
   
   
2. Note the number of heat and cool ‘calls’ available with each model of thermostat. Many furnaces and heat pumps have 2 or more levels of operation these days. A furnace may have extra burners that are only called for in colder conditions. The compressor of a heat pump or an air conditioner may have 2 or more levels of ‘power’, the higher levels being called for in particularly cold or hot conditions. Heat pumps often have ‘back-up’ electric strip heaters which can be called into operation if the heat pump cannot produce enough heat. Having various levels of operation results in significant energy savings and improved comfort. You are not blasted with very hot or cold air for short periods in relatively mild conditions. Heating and cooling is supplied by longer periods of comfortable air. This also gives more continuous filtering of the air in the home and helps to mix hot and cold pockets of air. This is especially true when it is combined with an ECM fan. To control these more sophisticated heat pumps and furnaces you need a more sophisticated thermostat which will decide when to call for the next level of heating or cooling. The logic used to make these decisions can be fairly complicated but they all aim to provide the selected temperature at the lowest cost.
   
   
3. Be warned of the potential problem of large nighttime temperature setbacks. When the setback occurs in the evening there will be no circulation of air for a longer period, not a problem in itself but it also usually means no filtering or mixing for that period. More importantly, when the setback reverses in the early morning there will be a large call for heat which will often trigger the thermostat to call for the 2nd or 3rd level. This will cause increased, unnecessary power usage, particularly if the 3rd level is electric strip heaters. You may be better off not to ask for a large setback or to program in a gradual increase in temperature in the morning.
   
   
4. The most modern ‘thermostats’ are designed to go beyond simply sending calls for heating or cooling to the units in the basement or outside. They can receive operational information from the units and display it either in words on a screen or by means of flashing lights. This can give you early warning of a problem and allow you to give vital information to a service technician.
   
   
5. You may have noticed that one thermostat is made for 4-wire installation. This is designed for retrofits where running a new 6 or 8 core lead is impractical. It uses a sophisticated communication system to send all the information down fewer wires.