[Dr_Zoidberg57]

Please bear with me as I know little if anything about heaters but am patient when it comes to sorting through a home repair problem, handy with tools and can follow directions.

I have a 25 year old central forced air gas furnace. According to the label inside, it is a Carrier model number 396GAW000075. I first noticed that occasionally after the fan would shut down, following its cool down cycle, about 30 seconds later it would come back on for about a minute and then turn off. Then I discovered, over the last two nights, right after the set-back time on the thermostat, if the heater happened to be on, the fan would continue to run long after the burners shut down. I checked the thermostat and confirmed that the fan was set to "automatic". I feared that I would have to unplug the furnace as the only means to shut off the fan. As a last resort, I over-rode the temperature set on the thermostat and got the burners to light again. This time the fan shut off following the cool down cycle. As yet, I have not noticed the continuous running happen at any other time.

In searching this and other sites for a diagnosis of my situation, it seems that I may have a failing fan limit control switch. If that is the case, none of the answers I looked at described where it would be located on the furnace or what it looks like.

Any assistance would be greatly appreciated.

Gerry

[John @ Fischers]

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

[Dr_Zoidberg57]

Thank you for your assistance. I believe my furnace has electronic ignition. I did manage to remove the schematic from the upper cover of the furnace and scan it. Hopefully it will be legible enough and will enable those with vastly more knowledge than I have to figure out what I need to do.

[John @ Fischers]

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.

[Dr_Zoidberg57]

John;

First, thank you for your time and help. Attached are the photos you requested and if I am interpreting what I see correctly, there is indeed the relay which you described. If that is indeed the case, could you steer me in the direction of where to get a replacement?

[John @ Fischers]

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:

[Dr_Zoidberg57]

John;

You've been a big help and again thank you so much, especially for the very detailed explanation. I learned a very long time ago that the only stupid question is the one you don't ask for fear of sounding stupid. That said, I do have a couple of question regarding the information you've provided. First, do I need to verify the part number on my circuit board to make sure that the new one is indeed the correct replacement. If so, where would the number be located? Second, I think your idea of checking the chambers for cracks is spot on. I am a bit uncertain as to where the chamber cover is located and depending on where it is, how to remove it for inspection. Is the cover what I see right behind the control valve in the photo of the lower section? If so, then removal looks pretty straight forward. I should also probably invest in an appropriate sized inspection mirror so as to be thorough because I get the feeling that, sould I find any cracks, I'm pretty much "hosed".

Again, I appreciate your patience in dealing with my lack of expertise and once I know what to do here I will get back to you ASAP with the results.

Gerry

[Dr_Zoidberg57]

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

[John @ Fischers]

Yes, that circuit board will work. When they first made them, they only had single relay control boards, which you have now.

Wasn't too much longer after that, A/C became more popular, they added another for the A/C.

Unfortunately you probably wont find the exact board as you have now. The Link i listed will defiantly work for your application.

I am a bit uncertain as to where the chamber cover is located and depending on where it is, how to remove it for inspection. Is the cover what I see right behind the control valve in the photo of the lower section

Yes, either 2 Phillip head screws, or 1/4, or 5/16 nut driver. Remove 2 screws and remove burner cover and look up in chambers where burners are laying in bottom of. Look around and you will see the welds, and keep looking around and make sure you don't have any cracks. Imagine the chambers as 4 inch wide walls, going from where the burners are to the top where they got to your flu box.(where the vent pipe hooks up).

As i think about it, since that furnace is as old as it is, it probably has cast iron chambers, and they take forever to crack. But just check in case.

But the board will work, and once you get it and need help, take a pic and ill give you a wiring diagram for your application.

[Dr_Zoidberg57]

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