My furnace has a dedicated fan switch is this normal?

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Old 10-16-15, 06:20 PM
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My furnace has a dedicated fan switch is this normal?

Hello I recently purchased my first house and fall is not upon us. While the furnace was checked by the inspector to see if it worked it was not noted that something was odd.

I have gone to use it this past week and here is what I found.

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As you can see I have a power switch and a fan switch. This stuck me as odd because I either have to leave the fan running 24/7 or come in and turn it on and off every time I want the heat.

I looked at my thermostat

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and found it only had 2 wires

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The furnace is a Lennox 180DF.

Fuel Type: Gas

House style: single story 109 year old Craftsman i believe

And the thermostat is a Honeywell RTH7400

I have tested to see if the thermostat can control the fan at all and it can not. I tried leaving the fan off but the furnace on and see if it auto started. The furnace kicks on and I hear what sounds like it might be a fan but it is not the main one.

I do not know much about furnaces and have not been able to figure out is this is a normal situation or not.

I would like to know what I should do.
 
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Old 10-16-15, 08:19 PM
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I have tested to see if the thermostat can control the fan at all and it can not.
How did you do the test? Did you merely move the FAN switch from AUTO to ON? If this was your test it is no wonder it failed as you need a minimum of three wires between the thermostat and the furnace to be able to use the thermostat fan switch.

I tried leaving the fan off but the furnace on and see if it auto started. The furnace kicks on and I hear what sounds like it might be a fan but it is not the main one.
How long did you wait after turning up the thermostat? The normal ignition sequence would be for the furnace to first do a self check of the safety devices, start the induced draft blower (the fan noise you heard), prove the draft through the furnace, energize the hot surface ignitor and after a few seconds (when the ignitor is glowing white hot) energize the gas valve. Once all burners are lit, from the one closest to the ignitor to the one on the ether end of the row, the flame safety device will de-energize the ignitor circuit and after a short warm-up period, less than a minute, the room air blower will start. The whole process may take as long as two minutes from thermostat "calling for heat" until the room air blower starts, possibly even longer if the internal timing adjustments have been altered.

If it doesn't follow this sequence then you should have a professional come and give the entire furnace a once over.

It is obviously a newer furnace in an old house. It is not out of character to have only a two conductor thermostat cable although in modern installations with two-stage burners and central cooling may have as many as nine conductors. The "fan only" switch on the furnace itself was common several decades ago to allow the room air blower to circulate unheated air during the summer months. Some people without air conditioning still use this feature although normally by use of the AUTO-ON switch located on the thermostat. The normal position of YOUR fan switch is OFF.
 
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Old 10-16-15, 09:06 PM
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Thank you. When I had tested the fan on the thermostat it was by changing it from auto to on but that was before I pulled it off and found only 2 wires.

As for the other test no I only waited about 30 seconds because I did not know it did the entire safety check. I will do a test and wait up to 5 mins to see if it works.

As for the thermostat is that something that can easily be changed or will I need an electrician or HVAC expert to do that?

Also how can I confirm the proper safety check is happening?
 
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Old 10-16-15, 10:00 PM
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Changing your thermostat is an easy job but first tell me why you want to change it.

Also how can I confirm the proper safety check is happening?
Well, you could use the method that a former coworker used on the boilers (commercial sized) in a plant where I used to work. Tom's method was to simply turn the switch on and see if it lit. He assumed, incorrectly I'll add, that if the fire lit and after the flame stabilization period the burner went into modulation mode that everything was fine.

While that method was dead wrong for that equipment it IS a fairly good test for residential furnaces and boilers. A better method is to have the furnace professionally serviced every other year. I can go over the things a true professional will do on such a servicing if you would like.
 
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Old 10-16-15, 10:40 PM
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As for number one right now I mainly want to keep the thermostat but make it so I can actually control everything there instead of having to go in the closet. As for the future I am into home automation so I would want to be able to have all the proper controls set up.

I should probably just go with getting it serviced. Part of homeownership is paying for that kind of stuff.
 
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Old 10-17-15, 01:12 AM
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In order to control the fan/blower manually you need to have at least three conductors to the thermostat. Generally the minimum used is a four conductor cable to allow for cooling as well as heating. If you were ever to install a furnace with a two-stage burner you would need yet another conductor and some thermostats, including most Wi-Fi thermostats will require a sixth conductor. A forward thinking furnace installer would never use anything less than a seven conductor thermostat cable and I personally prefer having nine conductors.

That means that you will need to replace the existing thermostat cable before or at the same time as the thermostat upgrade. There are a few thermostats, called "communicating thermostats" that (I think) work with only two conductors but I doubt they will meet your desires.
 
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Old 10-17-15, 04:00 PM
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Thank You. When I got home last night I tested as you suggested and you were correct I was just not waiting long enough. All is good until I decide to upgrade.
 
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Old 10-18-15, 12:40 AM
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Thank you for letting us know that everything is working as it should.
 
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