Question about Honeywell Programmable Thermostat Heater

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Replacing aging erratic honeywell chronotherm III

I have an older (~20 yrs) honeywell chronotherm III programmable thermostat controlling an oil furnace and a seperate AC. It will skip over periods, get stuck in periods, not raise the temp, not stop running funace when temp is reached, etc. I've tdecided it's time to replace it, but I don't know what with. Is there a cross-walk somehwere that says "if you had x, you should now get y"? How do you know what's compatible? How hard a job is it for a person (my son, not me!)with computer hardware and automotive skills but no experience in heating to do? Should I call a pro, or is this a DIY project?



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Hi, your replacement part can be purchased on amazon, take a look here: Amazon com honeywell chronotherm III thermostat. I hope this was helpful. Cheers

Posted on Dec 06, 2014

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Honeywell has a great newer thermostat.

This is what I sell, it is easy to program and use and has a large touch screen. I hooked up 4 in a office and it saved them 15 % of heating bills this is the link for Honeywell thermostat for the page.

Posted on Nov 30, 2008

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You will be fine with another Honeywell T-stat.

You need one designated for single stage heat, single stage cooling.
You can also use one for universal applications as long as the single stage heat and cool are one of the options.

These can sometimes be a little more confusing to hook up for beginners, as you have many different options.

Honeywell's instruction booklets are as good if not better than most all the others.

When he removes the old one. Have him draw a diagram indicating what color wire goes to which terminal.

This will help hook up the new one.


White wire = W2
Red wire = R
Green wire = G
Yellow wire = Y1
Blue wire = C (common on most HVAC systems)
Some T-stats have batteries and do not need a common wire.
If you have a brown or black wire hooked up and you have no humidifier/dehumidifier, air cleaner etc then they are most likely unused unless they are going to the W1/W2, Y1/Y2, R, C, or G terminals.

Posted on Dec 06, 2008

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Janet, the problem most likely is not in the thermostat. It is probably the fan relay on the circuit board in the furnace. Some furnaces have an individual fan control board, some furnaces have all the controls integrated within one control board. If your furnace is 20 years old or older, it could be the fan/limit control because at that time all furnaces did not come with a control board. If it is much older than 20 years, it is much more likely a stuck fan/limit.

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-AC appears in the display of chronotherm III

I have had the same problem- twice. It was a similar but different problem each time. The -AC means the ac control voltage isnt making it to the thermostat. In my case both times it was a switch in the control circuit that wasn't closing properly. The easiest was the front cover switch. If the front cover is removed from the furnace or has vibrated loose, the switch will open, preventing the furnace from running. The other instance I ran into was a flow switch on the exhaust blower which ensures the there is flow OUT the chimney. The switch wasn't functioning, so the furnace wouldn't start. The blower would come on for a short time, but the burners wouldn't light.

Hope this helps,

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