Fan in furnace keeps turning on.

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  #1  
Old 04-30-04, 11:52 AM
TheDog
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Question Power vent Fan in furnace keeps turning on. is this normal ?

Hello,

I recently replaced the thermostat with a Rite Temp touch screen programmable thermostat. Everything seems fine except that when I have the thermostat switched off I can hear something turning on in the furnace.

I opened the furnace covers and I can see the main blower is off while there is a smaller fan about two feet above the main blower which turns on for less than a minute and then turns off .

What does this fan do ? Is it blowing out the exhaust fumes from the pilot ?
 

Last edited by TheDog; 05-04-04 at 09:33 AM.
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  #2  
Old 04-30-04, 04:57 PM
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that is the inducer fan that only comes on with a call for heat, or if a safety is open. first remove all wires from new tstat, if it stops, tstat is incompatible. if not, look at circuit board for diagnostic light, what make/model is it?
 
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Old 05-03-04, 06:54 AM
TheDog
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Thanks for your reply, I will check and get back to you. By the way is there a site where I can learn more about forced air systems and how they work, I would like to know what the different components are in a furnace and their functions so that I can trouble shoot better.

Thanks.

Dog
 
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Old 05-03-04, 10:48 AM
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Wink

Might try and go To http://homehowstuffworks.com

Also try the .com for the furnace that you have there



ED
 
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Old 05-03-04, 06:48 PM
TheDog
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Ok I some more info -

Manufacturer - Goodman Manufacturing Company
Model GUPS100-3

I found a circuit diagram for the furnace and it has two fans, one is marked PV Motor ( Power Vent) and the other IDB (Indoor Blower), I have no idea which is which.

Also the fan in question, the one that turns on at times with the thermostat off, has a vacuum switch connection to it ( a tube running from some kind of pressure switch), the fan also has a duct running from it to the hat atop the water heater (gas) and then this duct goes out (I guess some sort of exhaust system).

I hope I'm making sense, as you can see I know very little about furnaces.

One more thing I'm not sure if this situation existed before I replaced the thermostat. I've haven't had a chance yet to remove the thermostat and see if the fan turns on or not as the fan turning on takes a while, it isn't very frequent.

TIA
 
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Old 05-04-04, 01:25 PM
TheDog
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Ok, so I now know that the fan is the power vent blower and it's function is to blow out waste gases, basically an exhaust system.

Now my question, is it normal for this blower to turn on periodically even with the thermostat in the off position ?

Two ,what is the pressure switch connection for ?

Thanks.

TheDog
 
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Old 05-05-04, 04:28 AM
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no, it is not normal for that fan to cycle with no call for heat. here is how it works

tstat calls for heat by sending 24 volts back to furnace on white wire

this initiates the inducer motor through a relay on the circuit board

the negative pressure through that tube into the pressure switch closes a set of points, telling the board that the motor is running, allowing the sequence to continue.

after checking all safeties during a "prepurge" period of 15 to 30 seconds, the board then powers the ignition system, in your case sending 120 volts from the board to the ignitor.

after 10 to 17 seconds depending on brand, gas valve opens, gas passing across the ignitor, lights the burner.

the flame travels across to the rest of the burners through small gas openings between the burners, to the farthest burner from the ignitor

in front of that burner is a flame sensor that senses the flame and signals the board to leave the valve open, as ignition has been achieved.

this way, the valve will not stay open, spewing gas if the furnace is not lit.

when call for heat is satisfied, 24 volts is dropped from the white wire from the tstat which immediately closes the gas valve, but leaves the inducer motor on for a set "postpurge" period to clear the combustion chamber of any left over gas.

after 90 or 120 seconds depending on the setting on the board, the indoor blower will then go off.

my suggestion is to disconnect the tstat wires from the furnace and see if this cures it. if so, replace the tstat, if not i would think that the circuit board is the culprit, possibly bleeding voltage over to the inducer relay.

sometimes these fans will spin if there is a draft down the chimney.

hope this helps
 
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Old 05-05-04, 04:34 AM
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BTW just noticed you changed the tstat.

be sure if there is a switch for elec/gas it is in gas

i have seen Hunter brand tstats do this, they are "power stealing" tstats, powering the display or memory off of the 24 volts powering the stst, using other unused wires as common by barely bleeding through their circuits
 
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Old 05-05-04, 06:44 AM
TheDog
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Thanks for your response, I will make some more observations and also remove the thermostat this weekend and post back.

Thanks.

TheDog
 
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Old 05-06-04, 06:49 AM
TheDog
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Well, it turns out there was no problem, I was looking around inside the furnace and came across a note stuck on the main blower (indoor blower or whatever it's called) , according to this note the "Venter blower" will turn on every hour for two minutes as long as power is supplied to the unit.

So I guess all is well.

Thanks HVAC4u for educating me and taking the time to do it. I will be back with more questions.

Would you advise having my furnace serviced ? I bought this house recently and it's 12 years old and I guess the furnace is also 12 years old.

thanks.
 
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Old 05-06-04, 10:09 AM
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i have never heard of that...

very good info, thanks, we all learn here.

yes i would have it serviced, a good cleaning and check up at 12 yrs is a good idea. goodman is lower end, and you are approaching the end of it's life span
 
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