furnace fast cycling - thermostat adjustment

[emanon25]

i find myself at 6000 ft altitude in northern new mexico. some nights are already getting chilly, a low of 35 three days ago. so...i removed the furnace for cleaning and repair and fabricated a new sheetmetal front which was missing and performs as a plenum for the system. when i fired it up it would "short cycle" that is, quickly bring the air temperature up to the setting on the thermostat, then turn off, sit for a few minutes and do it again ad infinitum.

i did some research on honeywell thermostats and found that they could be adjusted for the heat anticipater index of the furnace. my 32 year old thermostat was quite unlike anything honeywell sells today but poking around i found just such an adjustment. the actual electrical contact is threaded so that it can be moved closer to or further away from a small magnet that holds the contact until the bi-metal coil produces enough force to overcome the magnet's attraction. or to put it more simply, by moving the electrical contacts closer to the magnet, one may increase the difference in temperatures that turn the heater on and off. i aimed for a difference of 5 degrees F. so if i set the thermostat at 70, if turns on the heater and runs it until the temperature reaches 75 and then turns off the furnace. by the way, the temperature continues to rise for a while even after the furnace and fan turns off.

nights have been toasty. i set the thermostat for 70 before i go to bed and reduce it to 60 at bedtime. the heater has been coming on close to dawn. it heats the cabin up to 65 for my coffee and crossword.

picture attached.

emanon

[paul k]

hi i had a simular problem last weekend myself are you saying that if i unscrew this screw out so that the head of the screw is farther away from the base that the heater will run longer mine never realy got up to the set temp and then started blowing cool air any help would be aprecieated 1976 safari 23 ft i believe that it has a nt 22 suburban furnace

[emanon25]

Originally Posted by Paulk

hi i had a simular problem last weekend myself are you saying that if i unscrew this screw out so that the head of the screw is farther away from the base that the heater will run longer mine never realy got up to the set temp and then started blowing cool air any help would be aprecieated 1976 safari 23 ft i believe that it has a nt 22 suburban furnace

yes, by turning, counnter clockwise, the small screw that is the stationary side of the electrical contact, the furnace will cycle longer. that doesn't mean it will solve your problem. do you know if your burner is lighting? if you do a search on this thread for "furnace" or "thermostat" you will find some very useful trouble shooting guides. i had a problem with low voltage because my 32 year old fuse panel is flakey. i keep the battery charger turned on at night to keep the voltage up. otherwise the blower motor runs a little slow and the sail switch shuts off the gas valve.

anyway, like just about everything else in an old a.s. the furnaced is well built and a little patient analysis should solve your problem.

emanon