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Using fan relay to ad a/c to mobile home

What do you mean the n/c switch hooks up to the heater box

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Remember a fan switch has 2- contacts pus coil power (24volts) a normaly open --and a normaly close thus n. o. contacts and a n. c. .. contacts and and you hook the 24volt coil contact to cool and the hi-speed fan switch to the n.o. contacts... SO when it don't calls for cool the n.c. contact wil send power to the low speed wire for heat (red to the fan motor which connects to the temperature control which cycle on the fan when calling for heat! HEY, you didn't give enough information, like WHAT___WHERE___WHEN__WHO ??? ____ model #-- name of unit-- age-- gas fire or electric ---upflw or downflow?

Posted on Sep 09, 2008


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Heater still runs.

Most furnaces are equiped with a two speed blower. Hi speed is controlled by a relay sytem initiated by the thermostat. short of a stuck relay, hi speed should not remain on unless your thermostat "fan swith" has been moved from "auto" to "on", if this is the case turn the switch to "auto"

Low speed is controlled solely by the furnace itself. Depending on the age and brand of the furnace it is either brought on by time or temperature.

From your description of "tapping the relay of the limit switch" I assume you have an older furnace with a combination fan/limit control. The most common of these were a grey box with a removeable rectangular lid. In some cases there was a (commonly white) switch poking through this cover which you could manually turn low speed on if you pushed it in, make sure it is pulled out.
Otherwise the switch is refered to as a bi-metal limit. When heated the two dis-similar metals (coiled together) expand and contract at different rates. When the furnace cycles on, the metals heat and, due to design, twist a dial, which you should be able to see under the rectangular lid. The dial should have three pin settings for the purpose of switching. the lowest setting, usually around 105, is the fan OFF setting. The second, usually aroun 135, is the fan ON setting. The highest should be fixed around 250 it is the high limit and should not be part of your problem.
....After your thermostat has been satisfied and the flame shuts can physically watch that dial move clockwise and hopefully reach the setting of around 105, unlesss that setting is set below the temperature of the room the furnace is in (try setting this setting higher to see if it helps).
(make and age of furnace helps a lot with trying to trouble shoot, also is it a mobile home unit?)

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1 Answer

I have a Coleman Presidential 7670 furnace in a mobile home. customer said it came on a couple of times by itself. I ran new wire, fan won,t shut off. Replaced fan switch, fan still won't shut off....

Brian, it sounds as if possibly the fan relay is stuck however, if the fan will not shut off I would suggest the following, Follow the wires from the fan to the relay on the relay disconnect the fan wire there and see if the fan shuts down if so there is the problem. If not you possibly have a short in the thermostat wire What can be done is take a small section of thermostat wire and disconnecting the existing thermostat (red 24 v power feed, green, is fan motor control, yellow is compressor control, white is heater control,) once you have disconnected the existing thermostat wires take the short peice of wire and connect it to the terminal block, once this done the other end (do not hook the wiring to a thermostat have the wires bare at the ends about 1/4 inch) Manually connect the red and green wires togather and see if the fan activates. If it does disconnect the r,g wires if the fan still runs then the fan relay is stuck, if not then it sounds as if there is a short in the wiring between the thermostat and the terminal block. Also check to make sure that you have set the thermostat setting itself to either gas or electric since the newer thermostats have the option of being used as gas or electric. Good Luck on your repairs.

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