Furnace Limit Cycling - Potential fix?

[Witch Doctor]

Did you ever get your limit problem fixed yet,

[Allistah]

Nah. Going to have to wait until the weekend probably. I think I need to look closer at the routing of the ducts and maybe even replace one/some.

[Allistah]

Reporting back on this fix. Tore the furnace out yesterday and all of the vents. I thought I may replace some of the ducts completely but they cut 4" holes in multiple layers and have the ducting running through those so you can't just pull it through and string a new run in there. I did find that a 2 out of the four runs hit the register at a pretty steep angle though. This causes the flange on the register itself to block about 30% of the duct. So on one register I trimmed back that area so it wouldn't block it anymore. Here is a pic so you can see what I'm taking about. I'll do the same to the other one as well. Also rearranged what duct comes out of which port on the furnace to get the ports with less air pressure connected to the shortest runs. You can see this just by the design of the heat exchanger. Going to finish this on Monday. When I do I'll get back to this thread again.




Sent from my iPhone using Tapatalk

[Witch Doctor]

Originally Posted by Allistah

Reporting back on this fix. Tore the furnace out yesterday and all of the vents. I thought I may replace some of the ducts completely but they cut 4" holes in multiple layers and have the ducting running through those so you can't just pull it through and string a new run in there. I did find that a 2 out of the four runs hit the register at a pretty steep angle though. This causes the flange on the register itself to block about 30% of the duct. So on one register I trimmed back that area so it wouldn't block it anymore. Here is a pic so you can see what I'm taking about. I'll do the same to the other one as well. Also rearranged what duct comes out of which port on the furnace to get the ports with less air pressure connected to the shortest runs. You can see this just by the design of the heat exchanger. Going to finish this on Monday. When I do I'll get back to this thread again.




Sent from my iPhone using Tapatalk

You got to be kidding but I see the picture what is wrong with these people no QC at all. Do you have any cut outs on the pileum yet? I still think your a little short for CFM, but your finding so far might help. what a PITA on a new trailer, but you are better off doing it yourself....

[Allistah]

Sorry I don't know all the terms but what is the "pileum"?

Seriously though, it looks like these runs go through all sorts of angles, turns, through multiple 4" via holes.. It's crazy. But you can see if I had not cut out that section on the register, it would be protruding into the airspace of the duct, blocking it because of how they ran it to to register. I'll get some other pics tomorrow and you'll see what I mean. I really wish I could replace all the ducts but I think I'd have to really tear this thing apart to do so. I hope that with these mods it will be enough.

Another thing I did was to clip out every other cross slat on the register covers so that there are 50% less of them. This should reduce the air restriction at the register as well. It looks a lot more open and may not direct the air as well as it once did, but I think I would rather have the furnace working properly and not cycling off the limit switch. I guess this is part of the trade off given how they ran all the ducting. Will report back tomorrow with more pics. You'll laugh.


Sent from my iPhone using Tapatalk

[Witch Doctor]

Originally Posted by Allistah

Sorry I don't know all the terms but what is the "pileum"?

Seriously though, it looks like these runs go through all sorts of angles, turns, through multiple 4" via holes.. It's crazy. But you can see if I had not cut out that section on the register, it would be protruding into the airspace of the duct, blocking it because of how they ran it to to register. I'll get some other pics tomorrow and you'll see what I mean. I really wish I could replace all the ducts but I think I'd have to really tear this thing apart to do so. I hope that with these mods it will be enough.

Another thing I did was to clip out every other cross slat on the register covers so that there are 50% less of them. This should reduce the air restriction at the register as well. It looks a lot more open and may not direct the air as well as it once did, but I think I would rather have the furnace working properly and not cycling off the limit switch. I guess this is part of the trade off given how they ran all the ducting. Will report back tomorrow with more pics. You'll laugh.


Sent from my iPhone using Tapatalk

The pileum is the tin box that all the ducts hook to
(Sorry) with travel trailers they like to use the end to add ducts, that is bad the air has to mix, you have hot and cold spots coming off the heat exchanger
It is starting to sound like a crap duct insulation plus not dumping enough air, add another run, but before you do it remove a plate where a duct could be attached turn on the furnace and see if it cycles then. If it still cycles you more then likely have a bad limit, but I doubt it


Sent from my SM-N920V using Tapatalk

[Allistah]

Ah ok. Yeah there are two ports on the face that aren't knocked out. I fixed a bunch of stuff and am testing it now.


Here you can see under the shower how they ran the duct tight against the wall and made an abrupt turn at the end to the register (duct on the left) and also how that support is right in the way.



Here you can see where I made a small gasket out of that insulation stuff for furnaces. It's the same material that goes around the burner piece. This will minimize any direct transfer of heat from the heat exchanger going to the limit switch.



Here you can see the limit switch with the gasket installed.




Here you can see how I cut out some of the slats on the register covers to help with the airflow.


Testing in progress so we will see how it goes. I'll report back.


Sent from my iPhone using Tapatalk

[Allistah]

Damn. After about 18 minutes it hit the limit switch.

The only problem with adding another duct is that it faces the intake grate.






Sent from my iPhone using Tapatalk

[Allistah]

Took the front I take grate off to see if that helps at all and I'm testing again. What a pain in he a$s this is.


Sent from my iPhone using Tapatalk

[Allistah]

Hit the limit after 14 mins. Going to pull the register covers off and try again.


Sent from my iPhone using Tapatalk

[Allistah]

Punched out one of the front cutouts and stuck a piece of duct in it to see if this fixes the issue. We will see.





Sent from my iPhone using Tapatalk

[Allistah]

Hit the limit after 11 mins. Damn this sucks.


Sent from my iPhone using Tapatalk

[Allistah]

Cut an access port on the side and replaced one of the long runs with a short one to test. This also increases the intake area by a huge amount. Will see what happens now. Testing in progress.





Sent from my iPhone using Tapatalk

[Allistah]

This seemed to help. I think I need to replace part of the duct that goes to the bedroom. Replacing it with a short run improved a lot. Going to try again tomorrow after I repair the bad section at the end of the run. Unbelievable how difficult this is.


Sent from my iPhone using Tapatalk

[jgourlie]

I am so sorry that you are going through this, but all your documentation is going to help others and that is really valuable!

[TimVWulp]

Hi have you replaced the Hi limit? It may be weak and the more the bimetallic disk flexes the weaker it will become. just my 2 cents for what it is worth.

Hope this helps Tim

[Witch Doctor]

Originally Posted by Allistah

This seemed to help. I think I need to replace part of the duct that goes to the bedroom. Replacing it with a short run improved a lot. Going to try again tomorrow after I repair the bad section at the end of the run. Unbelievable how difficult this is.


Sent from my iPhone using Tapatalk

Trust me I know and FR should never have done this, like I said your short on air flow, like crushed ducts, 2nd, you mention you have ducts blocking the R/A that has to be open, are you drawing any air from the basement? Your rear of the unit R/A entry could also not letting enough CFM (air) in make sure you getting air. if the ducts are blocking from your R/A grill , that I have seen It's a major problem not enough free air to the R/A. That also will make you cycle and cause heat build up,
Since you made a knock out in the front, Try to move your furthest run to that cutout. Yes we might not mix the same but you should get atleast 1/3 more air on that run. Don't give up your getting close...

[Witch Doctor]

I looked at your picture again, is your furnace in that cabinet or is it in the basement as most are? If it's in that cabinet only your problem is not enough R/A the ducts are taking and blocking over 1/2 of your R/A sq. in. that you need to make it work. Do you have any R/A grills on the side in the back? We can add all the duct we want but if you do not have enough R/A that little piss ant blower motor will not make it up. So what happens is that you don't move enough air over the heat exchanger, You will then definitely cycle...

[Allistah]

It's in the cabinet directly below the oven/stove.

I was thinking about taking it apart and cleaning the fins on the blower real well. There's barely anything on them but I'm grasping at straws at this point. There is a tiny bit of dust buildup on the blades. How sensitive are those types of blowers to being dirty and losing CFM?


Sent from my iPhone using Tapatalk

[Allistah]

It looks like the furnace is in the airspace above the subfloor and in the area that goes under the cabinets if that makes any sense.


Sent from my iPhone using Tapatalk