Gas and Oil Home Heating Furnaces - furnace goes off then right back on
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dg6542
02-16-09, 11:42 AM
I changed a filter in my gas furnace, but now the furnace will come on and stay on until the set temp. is met, then the furnace will go off. After only about a minute or so, the furnace will come back on and only stay on for about 30 seconds. Then the furnace will stay off until the thermostat calls for it to come back on and it will stay on until the desired temp. is met and the whole process starts over, please help.
Grady
02-16-09, 04:44 PM
Is it actually firing or is it just the blower coming on? Old furnace?
dg6542
02-17-09, 06:54 AM
I believe the blower is just coming on. When it comes on, I quickly look at the thermostat and it is not at setpoint and should not be coming on. Its an older furnace, but I just started noticing this when I changed the filter
ecman51`
02-17-09, 08:44 AM
A blower that comes back on again is the result of the left-over heat in the furnace heating back up the blower fan switch again.
The blower, when it runs the first time in the cycle, keeps pushing the hot air through the heat exchanger, so it remains at a steady temp. But after the furnace shuts off, then that air is no longer moving, and the heat in the metal comes back out into the air inside the furnace, and causes the fan switch to come back on.
This only happens if the "ON' set temp of the fan switch is set too close to what the OFF fan setting is. And/or if the OFF setting also is too close to room temperature. If you have a REAL old furnace that has thermodisc fan switches, then you may have to replace one.
But if you have the Honeywell type that is a rectangular silver box mounted above the burner, you can remove the cover to it and repostion the temp settings on the dial. If you have one of these, from left to right the fan settings are OFF, ON and LIMIT. You want to seperate the 2 left ones some, so that the OFF is maybe about 90 degrees and the ON is about 115. If you have one of these, I bet the two adjustable tabs are touching each other. You'd probably leave the first left OFF one alone and move the 2nd ON one to the right a little.
[If you make tab setting adjustments, you must hold the dial while you just move the tab(s)!]
The blower, when it runs the first time in the cycle, keeps pushing the hot air through the heat exchanger, so it remains at a steady temp. But after the furnace shuts off, then that air is no longer moving, and the heat in the metal comes back out into the air inside the furnace, and causes the fan switch to come back on.
This only happens if the "ON' set temp of the fan switch is set too close to what the OFF fan setting is. And/or if the OFF setting also is too close to room temperature. If you have a REAL old furnace that has thermodisc fan switches, then you may have to replace one.
But if you have the Honeywell type that is a rectangular silver box mounted above the burner, you can remove the cover to it and repostion the temp settings on the dial. If you have one of these, from left to right the fan settings are OFF, ON and LIMIT. You want to seperate the 2 left ones some, so that the OFF is maybe about 90 degrees and the ON is about 115. If you have one of these, I bet the two adjustable tabs are touching each other. You'd probably leave the first left OFF one alone and move the 2nd ON one to the right a little.
[If you make tab setting adjustments, you must hold the dial while you just move the tab(s)!]
dg6542
02-18-09, 07:11 AM
ecman51`, thank you, I believe that is the problem, I'll try and reset the switch