Bryant Furnace Limit Circuit Fault - 33

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GCC

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I’ll check to see what wires are plugged where but do you men the exhaust motor or the main blower motor?

The A/C company thinks all of the ducts and distribution boxes need to be bigger. They want to replace all of them - which is going to be too expensive to just fix the heat that gets used a few nights a year down here.

I’ve done some more experimenting. There is one of those black light things I stopped using years ago in the supply plenum (I am not sure on terminology but it is the top of the air handler where the air is being supplied to the house). I took that out which left a 3-4 inch hole for the air to blow out. It didn’t help. Even with air coming out of there it shut down. I also cut a 5-6 inch square out of the return plenum in that little closet. (See pic - I taped it back up) I figured that would get more air into it but it had no affect - it still shut off after a few minutes.

So - if I add a larger return duct - is that effective enough or do all the other ones need to be replaced. I guess what I mean is, can you compensate for inadequate sized ducts by adding more, or is the real fix replacing. I’d like ro add a vent to a room that always os difficult to cool in the summer anyway, so I could add a supply vent and and return vent, but I’d hate to do it if it isn’t going to matter.

Thanks!
 

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John Gayewski

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I’ll check to see what wires are plugged where but do you men the exhaust motor or the main blower motor?

The A/C company thinks all of the ducts and distribution boxes need to be bigger. They want to replace all of them - which is going to be too expensive to just fix the heat that gets used a few nights a year down here.

I’ve done some more experimenting. There is one of those black light things I stopped using years ago in the supply plenum (I am not sure on terminology but it is the top of the air handler where the air is being supplied to the house). I took that out which left a 3-4 inch hole for the air to blow out. It didn’t help. Even with air coming out of there it shut down. I also cut a 5-6 inch square out of the return plenum in that little closet. (See pic - I taped it back up) I figured that would get more air into it but it had no affect - it still shut off after a few minutes.

So - if I add a larger return duct - is that effective enough or do all the other ones need to be replaced. I guess what I mean is, can you compensate for inadequate sized ducts by adding more, or is the real fix replacing. I’d like ro add a vent to a room that always os difficult to cool in the summer anyway, so I could add a supply vent and and return vent, but I’d hate to do it if it isn’t going to matter.

Thanks!
If you cut holes in the return you should also do it to the supply. That would give you more air flow and simulate more/ larger ducts. Take your filter out. Give it as much air as you can, this will help your experiment. I don't think ducting is the problem. But it's hard to have am intuitive guess from iowa.
 

GCC

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If you cut holes in the return you should also do it to the supply. That would give you more air flow and simulate more/ larger ducts. Take your filter out. Give it as much air as you can, this will help your experiment. I don't think ducting is the problem. But it's hard to have am intuitive guess from iowa.
I actually cut that hole in the return and had that black light thing out in the supply at the same time. I thought for sure it would help but it didnt. I also took the filters out and that didn’t help - although I just did those one at a time.

I’m going to try experimenting but I am running out of ideas. I dunno - is there a way to test the inducer blower wheel for the exhaust? It looks fine but maybe it isn’t getting the hot air out of the flue fast enough?
 

GCC

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A bad roll out switch would do exactly what it is doing.
When I test it with a multimeter it is closed. The limit switch is closed as well before it starts. When it turns on they all read closed at 0 ohms but then when it shuts off I quickly test them all and find the limit switch open. The one on the right. The other switches, including the roll out, are still closed and show zero ohms. I switched the limit switches around - swapped the left with the right. The same limit switch location closed and shut down the unit. So if 2 different switches fail in the same location and work in the other location, it’s got to be the location getting overheated, right? To me, I would think either the exhaust motor/blower wheel isn’t moving the air out of the unit fast enough or something is causing the unit to get too hot.

I don’t know how many cold days/nights we’ll end up having - but at this point it is more about the mystery/challenge than anything else.
 

John Gayewski

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When I test it with a multimeter it is closed. The limit switch is closed as well before it starts. When it turns on they all read closed at 0 ohms but then when it shuts off I quickly test them all and find the limit switch open. The one on the right. The other switches, including the roll out, are still closed and show zero ohms. I switched the limit switches around - swapped the left with the right. The same limit switch location closed and shut down the unit. So if 2 different switches fail in the same location and work in the other location, it’s got to be the location getting overheated, right? To me, I would think either the exhaust motor/blower wheel isn’t moving the air out of the unit fast enough or something is causing the unit to get too hot.

I don’t know how many cold days/nights we’ll end up having - but at this point it is more about the mystery/challenge than anything else.
When a switch is closed it should conduct current which means there should be continuity (ohms). I think your just getting the terminology wrong.

Is there any way you can see what temp the switches are opening (not conducting current) at? That would tell you.
 

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When a switch is closed it should conduct current which means there should be continuity (ohms). I think your just getting the terminology wrong.

Is there any way you can see what temp the switches are opening (not conducting current) at? That would tell you.

I don’t have that type of tool, just a basic multimeter. I tried another experiment - took both filters out at the same time, opened the hole in the return plenum, and took out the air knight thing that I never use in the supply plenum. So there was an extra outlet (albeit 3 inch hole to act like an extra supply vent), no filters in either of the 2 return vents, and a 5x5 hole in the return plenum, and it still shut down! So, even with that little bit more flow it didn’t work.

But if I just leave the bottom door open where the blower wheel is it works. I know I can’t leave that in all the time . I am happy to add an extra return if it will help, but I want to make sure it will.
 

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