Most older gas furnaces that have standing pilot systems use a thermostat switch located in the hot air plenum that detects when the furnace is hot and turns on the blower.

These thermostats (more commonly called the Fan/Limit control) are usually made by Honeywell or white-Rogers. The typical settings for the stops are 70 to 90 "Off", 130 to 150 "On", and the "Limit" to the bottom of the slot (lowest setting) as long as you don't get nuisance shutting off of the fire. As you might guess they also control the gas valve if the furnace gets too hot.

If you have problems with the blower not shutting off, its probably sticking. There made up of bi-metal strips that eventually wear out. They come in different sizes in length. Various from 5", 8", and 11" long. Need to pull your old one out to see what size it is. Here is a picture of your control






Here is a link for your parts, but shop around. I have seen them alot cheaper than this:

5" Fan & Limit Time Delay Switch (6 wire hook up)(Honeywell): American HVAC Parts[URL="http://americanhvacparts.com/Merchant2/merchant.mvc?"][/URL]

ok, that helps. Your blower is actually controlled from the circuit board with a glass relay. Alot of the older ones, u can actually unplug the glass relay from the circuit board.

Turn off the 120v breaker to the furnace and remove the metal cover over the control board and you should see the relay. If it's glass (you can see through it), should be able to unplug it from the board and just replace it.

If it's small and black however, you will have to replace the entire control board, since this type will not unplug from the board.

Maybe remove cover and take a pic of that. Hopefully you have a glass relay since this is somewhat cheaper route than replacing the entire board.

I thought that looked like a Bryant control board before you took the cover off..lol.. good pics. That relay will not unplug, so you will have to replace the board.

I found a exact replacement board, well sorta of. Its the replacement board as close as you are going to get.

Only biggest difference i can see right off, is the extra terminals for the A/C, which on the current one you have now, there is none.

So you can actually hook this one up the same way the one you have now is.

RED= 24V...WHITE= Heat...Green= Fan for A/C or Manual Setting @ thermostat.(which is HIGH speed),Bigger Black wire on control board going to blower motor.

Bigger Red wire you have now on control board, is LOW speed, which is for heat.

2 wires going to transformer which is LINE Voltage(120V) to power transformer. Other 2 wires leaving transformer is the LOAD side, which is 24V going back to your control board.

Not too hard really. Think of it like this: The control board is a Motel that holds all the switching and working of the system.

The control board first has to have 120V supplied to it. Then from there, the control board supplies 120V to Transformer, which then the transformer makes 24V, and plugs back into control board to work all your 24V controls.

Your thermostat is fed with 24V from the red terminal on the control board. The 24V @ your stat, is just sitting there, waiting for YOU to tell it what to do. For example,

You click stat to Heat, and then the 24V is moved from the red wire, to the White wire on your stat, which is sent back to the control board. Once the (W) terminal (which happens to have a white wire) is energized on the control board with 24V, it starts the heating cycle. As long as (W) Terminal is energized, the heating cycle will stay on, or call for heat.

It then sends 24V through all your limits, and if there "closed" as they should be, it completes the circuit and the relay for the spark ignition will Begin sparking, and pilot light will light, or hot surface ignition will glow, depending on your application.

Once it lights, then the thermocouple(or flame sensor if hot surface) will let the gas valve open and ignite the burners. in 120 seconds(rule of thumb), the control board will energize the blower relay(close), and give 120V to the RED wire and the fan will begin running.

Once (W) terminal is de-energized due to you turning off the stat, or the stat satisfying due to temp setting, the gas valve will close, and the relay will continue to be closed and let the fan run about 180 seconds(rule of thumb), then open the 120V it was supping to the red wire, thus stopping the blower..... In a nutshell.

Knowing this, you should be ok to change out the control board. Here is a link from carrier itself i found.

Furnace Control Circuit Board Replaces two Clear Relays.(BDP, Bryant, Carrier, Payne): American HVAC Parts

Let me know what you think, or what else i can do to help ya.


Also, do me a favor. Bryant, Carrier were notorious for having cracked chambers. So before you spend money, remove the cover over the 3 burners, and get a mirror and a flashlight and look up into each chamber,(which you have 3), and make sure you see no cracks, or gashes in them. here is a couple of examples:

John;

I did a quick search for the circuit board you found and, unless I'm mistaken, I found the identical board on a certain auction site for considerably less money. As I tried numerous times to post the link to the page or mention the site by name, without success, here is the item number. When you enter it in the search box of this auction site, it should take you right to the correct page. the item number is 180459964151 . It is described as Carrier Bryant Payne HH84AA020 New Upgraded Circuit Control Board.


Now to hope that I find no cracks and I might get out of this relatively unscathed and have the old "girl" make it through the remainder of winter.

Gerry

Rather than worry about what I might find tomorrow when I do the inspection, I'll settle for being content that the bargain I found will work.

Thank you once again for all your help.

Gerry