Choose the best Thermostat for the job.

by G&S Mechanical:

With emphasis on repair not replacement. 

    We get many inquiries as to what thermostat one should choose for your home or business and how to troubleshoot problems. Thermostats get blamed for many problems because they are one of the most prominent part of your equipment that you see all the time and get replaced like spark plugs and in many cases mis-wiring makes the problem worse. Please do not replace your thermostat as a means of diagnosing a problem also see the bs page and wiring page, you will only make a problem worse. And please do not attempt to replace a working thermostat as an upgrade when a wiring mistake that disables your system would put you in danger; anytime in the middle of Winter or Summer and especially around the holidays. Just don't do it!!!

     One caveat a programmable will NOT save you any money if you are home all the time and you leave the temperature the same all the time. I recommend using a conventional mercury bulb thermostat for maximum reliability (that is what I have) or a good electronic with a cycling algorithm. Once again please don't replace your thermostat as a diagnostic unless you are absolutely sure that it is bad, it just makes it hard to solve the real problem by introducing more and gets very expensive too.

    Programmable thermostats work by allowing the space temperature to rise or fall closer to the outside temperature. The heat that is lost through walls and windows is measured in Degree Days, this is a linear unit less measure of how much heat transferees based on temperature difference. The closer you can let the temperature inside be to the outside the less heat is lost or gained, the problem with that in a well insulated building, is it will take a longer time for the temperature to change and it will not make nearly the difference that it will in a poorly insulated building. Insulation is good.

     Some recent additions to the market are the Invensys series, these have some fancy features such as outdoor air temperature monitoring and the 98XX series will perform humidification and dehumidification functions. The downside to these units are that they are more expensive than most and they require a new run of wire to accommodate all the terminals, which can be a problem in a finished basement... My take on wireless thermostats: run like Hell, it is hard enough to make wired thermostats work correctly!!!

    If you are home all the time an electronic thermostat will not help much for saving energy but if you want the best then read on:
Honeywell T7300 one of the best comercial thermostats on the market. This one has two remote sensors.
    For the ultimate in control and reliability I recommend the Honeywell 7300 (old version shown). This marvel will do automatic changeover from heating to cooling and doesn't run the equipment one more minute than it absolutely has to, using a timing algorithm derived from temperature change. It features a remote sensor capability so the thermostat can be located in the boiler room, machine room or closet where it won't be tampered with or damaged (thermostat guards are for lazy engineers). It is rather expensive (about $350-600 installed). But it is worth the price. They make a newer version that I don't like as much as the older ones because the battery backup never seems to work and they are a real pain to set up and maintain. You will need to chose an appropriate sub base for the system. A heat pump will require a heat pump sub base to operate the electric heat and reversing functions.

Note: Shown is the old style 7300. The new version looks a bit different and is a little more complicated. There is a fancier version called the 7350 which has communications capability and other features. The 7300 is no longer made and is replaced with the 7350, you can still find some old 7300s if you look.

I have noticed that the Honeywell 8600 series ("Chronotherm") definitely has a timing algorithm that will give you so many cycles per hour. This feature will spread the heating and cooling activity out so it will make for a very even temperature. The 8600 is discontinued and the Vision 8000 and 6000 takes it's place. The Honeywell thermostats that you buy in the Home Store are no where near the same quality as the ones that you would buy elsewhere. I know this because I have replaced so many of them and I have had customers call me for service only to find out it is the Honeywell thermostat they bought in the home store (Lowes, Home Depot ect. ) that is the problem.


     For home or commercial use at a lower cost I recommend the White-Rogers 80, 90, 91 and 97 series on Conventional systems.
    For fancier systems the White-Rogers 97/91 series, Invensys, Honeywell 8000 and 6000. have multiple outputs and other features like auto change-over, programmable fan and intelligent recovery.  But none will beat the honeywell 7300 for fine control and fuel savings (except when used on a heat pump due to electric heat usage).

    Feb. 2007:

    Home Depot is now stocking the Rite Temp thermostat line. Be careful because some of them which claim to replace 99% of all thermostats. This is B. S. They will not support the electric heat, auxiliary or emergency functions on a heat pump. If you can live with the electric heat only working on the defrost function then it will do. But there is no second stage electric heat output. You will have to chose something else. Rite temp has however modified their product line so most will handle the auxiliary heat function.

Jan 2010: RiteTemp has improved their line quite a bit and they are reducing the number of models and adding more features. One version does not have an Emergency Heat switch but instead you need to move a junper. You would only do this if you had a problem.

    With all that said one of my favorites is the 8085C and 8082C which have the distinction of humidity control in heating and cooling, and a nice offset feature for electric heat. According to the manual these thermostats will control humidification with an H terminal and will operate the cooling function to control maximum humidity in the cooling mode.

Problems: The 8085C has 1 major flaw (which Rite Temp can fix after reading this). in the cooling mode it will turn on your cooling system based on humidity which could in the end make the problem worse and stay on for a long time. The correct way to deal with humidity in the cooling mode is to slow the indoor fan down, this can be done with the same H terminal that is used in heating mode. Minor flaw: No E terminal, can be dealt with via a relay on a heat pump electric furnace if you need to activate more electric heat, otherwise the W output will take care of this problem. No big deal.

   I get many inquiries and hear talk about what is the difference between a thermostat for conventional systems and heat pumps. By conventional systems that means gas, oil, electric, forced air or hot water with or with out cooling (air conditioning). Heat pump means that the outdoor unit compressor is used to provide heating and cooling with or without the above mentioned fuels for back up heat. Do not confuse this with a "packaged" system that can use gas, oil or electric as backup heat with a heatpump or straight air conditioner and is ducted into the house.

    On a conventional system for cooling the compressor (Y terminal) is activated on a call for cooling and obviously not on a call for heat ("W" terminal). The fan/blower is activated by the G terminal on a call for cooling and not on a call for heat unless you have the fan switch set in the on position. Or your system requires it like some electric furnaces need the "G" signal to make the blower come on but should be backed up by contacts on the sequencer. If you have sequencers for your electric heat (heat pump or not) your fan should be controled by the "last off" sequencer, on a system that uses relays or contactors you may have to provide a "G" signal by setting your thermostat in the "electric" mode for fan control.

    On a heat pump system the compressor ("Y" terminal) is activated for both heating and cooling the only difference is that to switch to cooling the O terminal is activated to reverse the outdoor unit unless it is a Rheem/Ruud Then it will use a "B"  (see this page). The W terminal that is normally activated in heat mode is reserved for the back up heat when the heat pump can't keep up and the fan/blower (G terminal) is activated anytime there is activity. See above.

    When the thermostat is placed in the "emergency heat mode" it disables the outdoor unit ("Y" terminal) and makes the "W" terminal live on a call for heat as well as "G". This allows you to have heat if the outdoor unit should fail or it is so cold that it just cant keep up. Keep in mind while your house may get warm in a hurry you are burning electric heaters unless you have a fossil fuel kit. Activating the "emergency heat" function should be used as a last resort.

    If the thermostat you are buying says "For single stage heat pumps" then there is a very good chance that it will not work. Or if you have any of the following: need to jumper the "W" and "Y" terminals but have no W2 or AUX terminal, you have the designations "Rc" and "Rh" (in some cases where the thermostat doesn't have a heat pump configuration) or there is no Emergency Heat function. Then there is a very good chance it will not work. If you know you have a heat pump and there is no "O"/"B" terminal, then it definitely will not work.

    If you have a fossil fuel kit with oil you should wire the system so that when the thermostat is placed in the EM heat position (E terminal is activated) you disable the oil burner and allow the heat pump to run. The reason for this is that it is more likely that you will run out of oil than have the heat pump fail. The way most fossil fuel systems are wired that I have seen is if you run out of oil or the burner controls fail your house and pipes will freeze. You should also wire the system so there is more current to the heat anticipator (mercury thermostats only) in oil or gas mode than in the heat pump mode so the temperature will not over-shoot. Some thermostats activate the "E" terminal on a setting of Emergency heat and some on a call for emergency heat with "W" or "W1" or "W2" disabled. More on this to come...

 I hope this helps. Scott.

Just for the record I don't keep new spare thermostats on my truck and I only change 1 or 2 a year. Not every visit like some mechanics.

Common thermostats and my feelings:     If you need help connecting a new thermostat or diagnosing what you have I would gladly help. You can call me at 301-591-1646 but I would ask for a donation of at least $20 to cover time. This is nothing compared to the expense of wiring it wrong or worse doing damage.

    I will spend as much time as you need and be happy to review your system if need be, but I would expect to be compensated.

    I can also assure you that I will not let you do anything dangerous or that will damage you equipment as long as you follow my instructions.

Related pages:
    Wiring terminal designations. With trouble shooting help.
    Fossil fuel kit.
    Electric furnace troubleshooting.
    Heat pump problems.
    Blower and fan control problems.
    Gas valve system list.
    Heat anticipator setting.
    Combustion blower used on newer furnaces.
    Heat pump defrost cycle if unit freezes or ices.
    Trane heat pump thermostat wiring terminals.
    Amana Furnace (with combustion blower) trouble shooting. A new window will open.
    Millivolt gas valve thermostat fix.

Thank you for visiting our site Scott Meenen N3SJH

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Written By:  Scott Meenen N3SJH of:
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, Air-conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation.
Anything having to do with Heat and Energy.
Serving Maryland, DC, and Northern VA.

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