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Old 11-11-2019, 12:13 AM   #15
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Quote:
Originally Posted by Old-Biscuit View Post
410*F...???
TYPO
Did you mean 140*F?


High Temp Limit Switches range from 130*F to a High of 190*F
They are sized based on orifice/burner and air flow specs





Installed an orifice 2 sizes smaller......???
Your LP Supply Pressure was TOO HIGH causing the Overfiring.










What Brand/Model is YOURS??
Sfv-42 and yes the high temp switch does open at 410f inside temp of the fire box. Not in the ducts. We had my heater out on a test bench at the NASA lab. Suburban didn't know about the cycling circuit in the board. They didn't know what the snap temp was to be on the high limit switch. We bought 2 of them and they both snapped at 410 degree inside temp of their box. They didn't know what the RPM of the motor is either. We contacted the mfg. of the motor. Speed is 3500rpm. We tested every component individually and watched it run through its cycles on the bench. Cant beat the team at NASA lab.of coarse had a manometer on the propane pressure. 10.5 inch of water. High limit switches may be 130 to 190 as it is fastened to the outside of the fire box.
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Old 11-11-2019, 10:02 AM   #16
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Quote:
Originally Posted by oldbeek View Post
Sfv-42 and yes the high temp switch does open at 410f inside temp of the fire box. Not in the ducts. We had my heater out on a test bench at the NASA lab. Suburban didn't know about the cycling circuit in the board. They didn't know what the snap temp was to be on the high limit switch. We bought 2 of them and they both snapped at 410 degree inside temp of their box. They didn't know what the RPM of the motor is either. We contacted the mfg. of the motor. Speed is 3500rpm. We tested every component individually and watched it run through its cycles on the bench. Cant beat the team at NASA lab.of coarse had a manometer on the propane pressure. 10.5 inch of water. High limit switches may be 130 to 190 as it is fastened to the outside of the fire box.

SF-42........
CORRECT parts are serial nmber dependent.
2 1/2" motor.....400 CFM
3" mtor............430 CFM


Number 49 orifice
Number 50 orifice


195*F High Temp Limit Switch (heat exchanger metal temp)


3 different gas valve assemblies



If wrong parts were installed during repairs..furnace is NOT going to fire correctly
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Old 09-13-2021, 08:26 PM   #17
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Originally Posted by Old-Biscuit View Post
Need to pull that intake/exhaust ports back off.....remove burner assembly and CLEAN it


Also blow compressed air thru heat exchanger while you have burner assembly removed and make sure no critters have set up house (nests etc)


Intermittant firing can be from heat exchanger getting too hot (High temp limit switch opening)....obstructed exchanger
OR can be due to heat NOT being removed....blocked registers/collapsed ductwork

OR can be due to lack of return air (stuff being stored around furnace cabinet)
I guess I never closed out my post but I did pull the entire unit out - cleaned the heat exchanger and cleaned the nozzle - there was a piece of rust or something stuck in it. The unit and controls are fine. I cleaned the fan too and the compartment all with compressed air. Working fine now. Thanks.
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