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BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22115
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a 1993 Lennox Pulse furnace with LP when I turn up

Customer Question

I have a 1993 Lennox Pulse furnace with LP when I turn up themanual thermostat it fires up and heats the house but when it shuts down it goes off about 20 sec. then restarts and runs another 8-10 sec. then repeats on off and on off. Any Ideas whats wrong?
Submitted: 1 year ago.
Category: HVAC
Expert:  BillyHvac replied 1 year ago.

hello,

it is a fan control out of calibration. On the front wall do you see a rectangle box..and in the box is a dial with numbers...put the first pointer at 100 and see if it runs better,,,,basically the fan is sensing heat after shut down and kicking back on....sign of an old fan switch

Customer: replied 1 year ago.
I tried setting the fan switch at 100 but furnace continues to go on and off any other ideas? thanks
Expert:  BillyHvac replied 1 year ago.

try 110-120...if that works then your switch is way to weak to be reliable and will need to be replaced

Customer: replied 1 year ago.
Billy, I raised the control to 120 but the furnace still does the same thin goes on and off repeatedly. anything else to try?
Expert:  BillyHvac replied 1 year ago.

so after the unit shuts down and the fan cycles a few times...eventually it stops and remains dormant until new heat cycle?

Customer: replied 1 year ago.
illy yes after it shuts down and cyles a few it does go dormant.
Expert:  BillyHvac replied 1 year ago.

Thank you..you have a bad fan switch. Yours is beyond adjusting and needs to be replaced if you want the cycling to stop.

Billy

Customer: replied 1 year ago.
purchased a new fan limit control switch. Istalled the new one . Now it heats up to the set temp then off and on for a shorter cycle. usually just a miniute or so between cycles. This is after I manually set the thermostat up. Any other ideas? Thanks
Expert:  BillyHvac replied 1 year ago.

set the "on" temp a little lower by 10 degrees...we can balance this with timing and settings

Customer: replied 1 year ago.
still cycling on and off. what is the sequence on the fan limit dial? is lowest setting fan off, next setting fan on then the high temp limit off about 200 degrees. I have tried several settings. now it is at 60-90-190 degrees
Expert:  BillyHvac replied 1 year ago.

Set as follows..

110

140

200

Customer: replied 1 year ago.
Billy, I set the switch at 110-140-200 but it still short cycles . warms the house to the set temp then goes off for about 10-15 sec. then on for 10 + sec. then off and on again for several cycles. anymore ideas?
Expert:  BillyHvac replied 1 year ago.

Yes we have the next route but I save until the switch is adjusted the best we can...

The blower motor in bottom...are you comfortable changing speed on the blower?

Customer: replied 1 year ago.
I do feel comfortable adjusting th blower motor.
Expert:  BillyHvac replied 1 year ago.

Great...the wires from the blower...there are different speeds. I do not know if you still have oem motor..but we usually have red wired for heat and black for cooling...then there could be spare speeds in between of yellow, blue or orange...what is your set up?