I'm having some odd fan cycling problem on my Carrier furnace. It has
behaved oddly ever since it was installed. At the furnace controller
card no yellow wire is attached. The voltage on the Green wire is
varying and when it gets down to 18VAC the blower fan kicks off. The
red wire is a solid 27VAC.
I'm still using an old round Honeywell thermostat and I'm guessing the
mercury switch in the thermostat has gone bad. Is that a good guess?
When I get a new thermostat should I add a yellow wire?
Also, is the wiring diagram below essentially accurate as a typical
To the best of my knowledge, Honeywell has sold over 27 billion yo-yo
thermostats since their inception, and none have ever gone bad. Should you
add a yellow wire? Why, what for and what would you connect it to?
Thermostat cables come with a variety of colors, so don't necessarily expect
colors to have definitive meanings. Your best bet is to contact a HVAC
professional and let him check it out
You right about having the system checked out by his service company.
In the business you try to stay with the same code which I learned over 20
RED = Power= Hot (24volt)
Those old round thermostats are only for heat, with 2 contacts (R & W), no?
You need a FAN ON switch on the thermostat to activate the green.
The furnace fan will turn itself on when heating, but not when cooling.
I think my problem may be that the switch contacts in the base plate
have gone bad. I cleaned them with alcohol but they may have lost
their plating. Also I did find appropriate schematic info on the
Honeywell website (CT87B thermostat looks like it). Thanks for the
If you think the switch contacts may be bad, bypass them. No need to
buy a test replacement thermostat. Either bypass the switch with a
separate wire**, or disconnect the wires and touch them together***.
**Everyone should have 10 or even 20 jumper wires with alligator clips
on the end. You can make special ones, like 6 feet long, and you can
buy them in bags of 10 of 5 different colors from radio shack. You
may be able to clip the alligator clips to the screws on the back of
the thermostat, or to the wires.
***It's only 24 volts and you won't even feel it. Or you can probably
use even a single alligator clip, paper clip, wire nut, or twist them
together a little bit. It's only for testing.
Or disconnect one wire and mount it under the screw that goes to the
P&M, reply by post.
You're right. Except, Is it possible for a mercury switch to be
Could his 24 volt transformer be failing? Maybe not since he has 27
volts most of the time. But I like this story, so I'll tell it again
anyhow. I moved into this house in late May, and had 4 ffriends
from NYC for July 4 weekend. At noon on Saturday, the AC failed. At
6PM the water failed. And at 6PM Sunday, the electricity failed.
And the house was almost new!
But we had a good time, and after they left, I checked the oil furnace
and the 24 volt transformer that powered the AC control system also
had failed to zero. They wanted 150 for a whole control unit, but
sold me a transformer for 20. Too big for the space so I mounted it a
foot away. STill working fine 24 years later.
How wet switch makes connection? Mercury bubble bridges two electrodes.
Those metal parts can get you know what. Easy thing to prove that is
measure across the switch with meter tilting the bulb back and forth.
I suspected the mercury switch at first but the problem turned out to
be the mechanical ON/AUTO switch in the thermostat base.
I guess connecting the yellow wire to the furnace would increase the
blower speed but I think I prefer running the blower continuously at a
lower speed. I don't want the compressor and blower fan to turn off
simultaneously and I don't know if connecting the yellow wire would
activate a delay.
His diagram actually is sound. The only problem I see is that with the
[newer] computer boards on some furnaces require that the "Y" terminal be
connected to the furance computer board as well. Some newer furnace boards
run the "continuously fan" at a slower speed than with the air conditioning
["G" terminal]. The computer recgonizes the demand for air conditioning
["Y" terminal] and runs the blower at highest speed.
Stormy, go back and check the wiring again. You will see it is correct.
2-wire condenser - 4 wire thermostat.
Other than the Y signal to the furnace, the rest of it looked OK.
The lower fan speed won't do as good a job of pushing cold air up
hill. Would work fine if the air handler was in the attic. Where
I am, furnaces are typically in cellars.
Christopher A. Young
You can\'t shout down a troll.
The reason for the "higher" air speed is that cool air is dense [heaver]
than heated air. Take a look at a pscyometric chart and plot... you'll see
the cooled air carries more weight per foot than heated air, thus needs more
Actually, you can run air conditioning down to 200 cfm per ton Stormy, but
the results are dismal. You will remove a considerable amount of moisture
from the air but will pay for it in performance. 25 years ago, 325 cfm per
ton was the norm and 400 cfm per ton on heat pumps. Now the norm is 400 cfm
per ton on air conditioning and 450 cfm per ton on heat pumps. Why the
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.