help with gas furnace

Discussion in 'Shop Talk' started by tat, Nov 24, 2004.

  1. tat

    tat Active Member

    Aug 26, 2003

    I just bought a house with a gas central furnace. I just had the electricity turned on today and the gas last week. This morning I plugged in the furnace and the blower came on. I unplugged it and turned the thermostat off and plugged it in again and the blower came on again, I unplugged it again. I lit the pilot light and went to the thermostat again and turned it on and nothing happened. With the pilot light still lit I plugged in the furnace and the furnace turned on instantly and the blower started blowing. After about 10 seconds the furnace went out(pilot light still on) and blower still going. I unpluged it and turned the gas off. I had a gas furnace several years ago that I had to work on every now and then. I think that someone has hot wired this thing somehow. Does any one have any ideas on what I can look for? The furnace is 3 years old.

  2. mpillow

    mpillow Well-Known Member Supporter

    Jan 24, 2003
    The furnace is supposed to come up to temp. before the blower kicks on....and go off as it bottoms. I believe it is a limit setting but hubby is not here for definate answer...he is licensed burnerman...we had gas at home and now that furnace is at our camp....where he is...hunting...

  3. silosounds

    silosounds Well-Known Member

    Nov 12, 2004
    in the middle
    If this is an older furnace it will have a bi metal style fan switch, that when heated up, switches the blower motor on. then as it cools the switch switches off. sometimes the switch has a summer switch built into it, it usaully is a white plunger type switch marked "fan on", this switch needs to be in the pull out position to be automatic and not bypassed. this switch is usually located above burners and is either a black small box (this is robertshaw) or silver and is about 3"x4" (this is honeywell). the white plunger is usually sticking out of this. if there is no switch, then go to the t-stat and look for a "fan on" switch. this should be in auto mode, not fan on. just switch it to automatic. if there is no such switch, and if your handy, go to the furnace and pull the switch. this requires turning OFF the electricity to the furnace as this switch is 120 volt and will get you if its on. Pull off the cover to the swictch and take out the hex head screws holding it to the furnace. then slide the switch straight out as there will be a probe that is failrly fragile . this will be the by metal. if you cant replace it get w d 40 and spray it into the daisy wheel looking dial. dont turn this by hand as it is factory set and may be sprung by turning it too far. then slide it back in the furnace and try it again. if the problem still exists, try looking at the settings of the switch. these can be adjusted, but only as a last resort, as they are touchy and take a long time to get right because there is a high limit and a fan limit setting and these have to coincide with each other. for an example, if the fan doesnt come on fast enough the limit will sence a problem and shut off the burners causing a low air temp coming out of the registers. so then you have to adjust the high limt up and if this is too high its hard on the heat exchanger so do this as last resort.

    If this is a new furnace say 10 years or younger there most likely is a circut board that has a solid state timer built in. sometimes these have an adress changer in them and can be switched to make the blower come on fast or slow but I dont think this is the problem. these also have a summer switch or fan on switch and without knowing what brand you have i couldnt guide you to it, but its usaully on the schematic that are usaully on a door panel or if your lucky the previous owners left you the technical manual for this furnace, but they sometimes dont. so without knowing what you have as equipment this is all the help I can be. let me know, maybe i could help walk you through this problem. hope this helps and have a good holiday Terry in ks. :)