Hi All,
Really strange one here. I have a G43 Lennox in my home and this season been having some issues. My furnace has been tripping on limit/watchguard, the furnace would come on for a bit then ventor motor would shut off and the blower would ramp up to cool down then kick back on. I changed the limit, problem continued. The filters have been the same basic pleated merv 6 I've been using since day one. I've checked the AC coil and it's fine. I checked the heat exchanger and its fine. Put my clamp on on the ventor and blower and both amperage draws are fine. Pulled the blower, cleaned the squirrel cage and cleaned the secondary which wasn't that bad anyway and still same problem. All returns are free/clear and pulling in air. All vents are open as well.
Finally, when I put the filter in all the way is when it trips, if I leave it out a quarter then no issue. I get that it's starving for air but why all of the sudden is this an issue with these filters?
Should I try cutting another return in the basement trunk line? Any help is greatly appreciated!
Thanks
That is strange. Is the fan blower wheel dirty? Washing that may improve airflow. Yes, adding a basement return air would be an easy thing to do. Hopefully that will solve the problem.
It may be worth having someone double check the gas pressure. It's weird that this hasn't been an issue since day one.
Whats the gas pressure of the burner firing at???
Thanks guys, manifold pressure is at 3.4"-3.45, rating plate is 3.5 so no worries here
Quote from: Admin on January 17, 2021, 09:51:13 AM
That is strange. Is the fan blower wheel dirty? Washing that may improve airflow. Yes, adding a basement return air would be an easy thing to do. Hopefully that will solve the problem.
It may be worth having someone double check the gas pressure. It's weird that this hasn't been an issue since day one.
I cleaned the fan blades one by one, "so much fun that is eh!" But they weren't really dirty to begin with.
Just for poops and giggles I removed the patch that was on the return branch where they cut to clean the ducks a few years back just to see if this helped. It did not and same thing happened.
I wonder if I adjust the fan setting, highest to lowest black, brown, yellow, red. right now factory setting is black is on AC (yup), yellow heat, brown and red on park. What if I swapped yellow and brown for heat and gave the fan more on heat?
Its very strange that this happened all of a sudden, and was not an underlying issue. Are you able to do an ESP check across the furnace? it should generally be below 0.5 inches WC. If you follow a fan curve, you will see that as ESP increases CFM reduces, so not necessarily does High speed equate to more air flow. You may have to reduce the fan speed to drop the ESP to control the limit tripping.
If this is the issue, essentially the fan cannot overcome the residual pressure in the duct that it has already created.
Quote from: scarey8 on January 17, 2021, 01:23:23 PM
Its very strange that this happened all of a sudden, and was not an underlying issue. Are you able to do an ESP check across the furnace? it should generally be below 0.5 inches WC. If you follow a fan curve, you will see that as ESP increases CFM reduces, so not necessarily does High speed equate to more air flow. You may have to reduce the fan speed to drop the ESP to control the limit tripping.
If this is the issue, essentially the fan cannot overcome the residual pressure in the duct that it has already created.
Unfortunately I do not have the testers for my mannometer to measure external and internal static pressure.
At this point I may just regress to a less restrictive filter. I mean it's builder grade single stage Lennox that owes me nothing, maybe it's time for an upgrade 😁