I have just moved in to a 30-year-old house and I am having trouble with the
thermostat. The original one is an old combined heat/AC rotatable type.
I am replacing it with a new programmable one.
There is a blue Honeywell type box on the face of the furnace with a
terminating stripe down the side, what is its function. I suspect it
supplies low DC voltage, but not sure. I notice that the line for the
heating goes directly to the furnace igniter, should it not go to the
Any help or schematics would help.
Normally, thermostats work on 24 volts AC. W is heat(White); G (Green)
is the indoor blower; y (Yellow) is cooling or compressor; r (Red) is
24 volts hot; 24 volts common can be any other color, usually blue,
brown or black.
Heat pumps get a lot more complicated. Common is not used on many
older, non digital thermostats.
Sounds like a T87F....best thermostat out there....period.
Why? Are you upgrading the heating and cooling system too?
Honeywell TYPE box?
Honeywell makes a ton of "boxes"...
How can anyone give you a schematic of a unit that you havent given anywhere
near enough information to even start to look?
You dont need all that information. All you need is to know the color and
letter designation of the wiring on the sub-base of the T87F.
Real simple here.:
Remove the outer trim ring off the old stat, remove the three set screws
that hold the actual thermostat to the sub-base.
BEFORE you do anything else, you get a pen, paper, and you write something
Etc...it might be a different color combination, but if it was wired
anywhere near right by anyone with a clue, that will be close to it, and the
Y is only if you have AC..
Now..if you have a power stealing stat, you will need a C wire, and if you
dont have one, its not going to work, and a new 18-5 or similar wire will
need to be pulled from the stat to the unit.
If you have a battery powered stat, you wont need the C wire, but it might
be required to activate some of the features on the stat...
Anyone, including myself here is stabbing in the dark, since we dont know
what stat you have, what brand or model unit airhandler, or furnace you
Your transformer BTW, does not supply DC control current.....its 24VAC.
Depending on the transformer, it will be anywhere from 20 to 35VAC if you
read it with a DMM.
And this is a case where if you are still unsure of what is there, call in
someone that does, as mounting a stat properly, and being able to see what
you have and do it right, is cheaper than if you short out the transformer,
or worse, other controls, and have to call someone in later to find out that
you have wired the stat wrong.
And yes...we see this about 10 times a month....Lowes or Home Depot runs a
special on some cheapy stat, and then the calls start coming, and they all
start this way:
I bought a new stat and wired it and now nothing works....
It ends this way normally, after the call:
After we replaced the transformer, and two heat control limits, rewired the
stat correctly, and pulled the new control wire, your bill comes to......
it would have been much cheaper had they stopped, considered that something
was not right about the way it looked, and followed the directions a bit
Not slammin on you, just advising that you have not given enough
information, and it might be in your best interest to call in someone that
can see exactly what you are talking about. I carry about 5 different
Honeywell "boxes" and each one does a different thing....so...consider that,
and read the instructions....if you still think you can handle it, post the
model of the stat, the model # and brand of airhandler or furnace, and if
you have AC or not, and someone, myself, Stretch, or another person MIGHT be
able to guide you a bit better.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.