Tip & How-To about Heating & Cooling
Choosing your new thermostat:
Firstly you have to ask yourself why you are changing your thermostat. Is your old thermostat simply broken? Are you wanting a simple upgrade from analog to digital? Are you looking to save on energy costs using a programmable? Are you attempting to integrate your central system into a smart home network? Whatever the reason your choice of thermostat should not be simply the cheapest on the shelf. Different system types may require different thermostat types.
Before purchasing your new thermostat you need to figure out what kind of system you have. The general idea is *H/*C, denoting the number of heating/cooling stages and options, and fuel type (heat pump, gas furnace, electric furnace, etc.). An example would be a typical heat pump- it has a single cooling stage and a dual heating stage (heat strips, compressor) making it a 2H/1C heat pump thermostat. I usually install the Honeywell FocusPro 5000/6000 series for non-programmable/programmable applications for it's ease of use and general reliability.
Before beginning the swap always make sure power is off to both the indoor and outdoor sections!
Determining which wires go where:
First off, you may not ever rely on wire colors to guide you to the proper terminal. We have standard practices for wire color to function, but this varies from region to region and sometimes if a wire shorts out a substitute in the bundle may be used. When removing the old thermostat physically writing down or labeling the wires is not an exercise in tedium- it is almost necessary and something which I do every single time. You will usually have these terminals on the new device:
Rc- Cooling Power (usually jumpered to R/Rh)
R/Rh- Heating Power (usually jumpered to Rc)
Y- Primary Compressor
O/B- Reversing valve (activated in cooling/heating)(Heat pump)
W- Primary heat (Furnace or Electric heat)
G- Indoor Blower
E/Aux/W2/X2- Emergency/Secondary heat stage
Y2- Secondary Compressor (2 stage cooling)
L- Communicator for E-Heat (Heat pump)
If you do not have all of the terminals on the old as you have on the new it may not be the end of the world. Required terminals are an R, a Y (if AC is present), a W/W2/E/Aux/X2 (if heat is present), an O/B (if heat pump), and usually a G. Common is optional most of the time as it is used to power the thermostat only and batteries may be substituted. L is not commonly used. If you have any doubts as to which wires go where, stop now. Miswiring can destroy a system's low-voltage and potentially start a fire.
After labeling the old wires and removing the old thermostat, you must attach the new baseplate. Leveling the baseplate is not as important as it was with older mercury thermostats, but still applicable at least within +-5 degrees of rotation. If you are attaching the baseplate to drywall with no backing wood installing wall anchors (usually supplied) is key. Make sure you have a proper length of bare wire (not too long, not too short) before attaching to the terminals. Do not overtorque the screw lugs. Pull back on the wires after attaching them to ensure the connection is proper. It is a good idea to plug the hole through which the wires come with something (I use plumbers putty) to keep a draft from effecting the thermostats temperature. Many thermostats will require some pre-programming before hooking up (refer to installers guide)- I find this much easier to do via batteries before attaching the face to the baseplate. Once the face is on and the power returned to equipment, test the system. Keep in mind modern thermostats include a 3-5 minute delay for compressors as a protective layer.
Zone system wiring can be confusing and is not as standardized as it should be. If you have a zone system and plan to purchase new thermostats it is highly recommended to call in an experienced service technician.
High SEER and IQ drive systems use very different types of thermostats than conventional systems. For now the newer technology has yet to be perfected in the public market and these changeouts are best left to an experienced professional.
Some older thermostats use B as common (Trane Weathertron for example) This is not to be confused with B as a heating changeover valve and if it is can blow out a fuse,transformer, and/or your brand new thermostat upon startup.
Most face to baseplate interfaces are pin based- if one is not quite careful bending one of these pins during attachment can be irreparable.
Common is the least standard color across installers. Most use either brown or blue, but I've seen some use green and black, usually used for blower and secondary heat respectively. Never assume wire color as a standard.
Most of the service calls regarding thermostats I receive come from homeowners who have purchased and self installed incompatible thermostats. This is something I do everyday for a living, not so for the average homeowner. Take your time and do your research.
Save your manuals- all of them. Many cheap offbrands can be next to impossible to find manuals for online.
When in doubt, call in a pro. Most of us charge around $70-100 to install a thermostat (plus cost). This is almost always much cheaper than a service call to diagnose and repair a low voltage problem.
Posted by JD III on
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