Getting heater fan to start

Discussion in 'HVAC Heating & Cooling' started by tabnab, Nov 24, 2008.

  1. tabnab

    tabnab New Member

    Messages:
    2
    Location:
    Texas
    I have a house that sits empty a lot but when I go to turn on the heat I have to turn the fan off of auto and put it on the fan position. When I do this I can then go to auto and the heat will cycle most of the night. It than quits cycling and I sometimes can get it to cycle again by turning the fan switch to fan then back to auto. Sometimes I have to play around with the thermostat a lot to get it back on. The thermostat is an older unit and controls heat and a/c. Do you think it is the thermostat?
  2. Verdeboy

    Verdeboy In the Trades

    Messages:
    2,051
    I would definitely check the wiring in the thermostat. Make sure no wire is touching another and that they are all screwed in tight.

    If that doesn't solve the problem, you may want to replace the thermostat. After that, it's time to call an HVAC tech. Depending on what model furnace you have, there could be a plethora of other things to check, such as a bad fan switch, fan limit control, faulty wiring, etc...
  3. burleymike

    burleymike New Member

    Messages:
    118
    Location:
    Idaho
    Probably not the thermostat, check the limit switch. If you don't know what the limit switch is you are better safe than sorry, call an HVAC tech.
  4. tabnab

    tabnab New Member

    Messages:
    2
    Location:
    Texas
    thanks for your replies.I am planning to call a HVAC tech but the company that my home warranty company uses is about 35 miles away and if I do a little troubleshooting and can explain what may be happening to the tech he can bring some parts with him. I was going to put on a new thermostat if you thought that might be a problem as it is cheap and simple to start with. A little further info I have is that I think the heat is working but not the fan. My thinking on this is that when I turn on the fan(not on auto) the heat works fine and after switching to auto it cycles correctly. I think the fan just doesn't come on correctly on auto. I may be full of it but just trying to narrow my problem down. Thanks for your help.
    Last edited: Nov 25, 2008
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    OFten, there's a thermal switch in the furnace that tells it to turn the fan on once the temp reaches a certain point, then, (sometimes) a different switch to turn it off as it cools after the burner is turned off. Simpler systems just turn the fan on when the burner is turned on and off when it is off. One that turns on with the heat is more efficient, since first you aren't blowing cold air as it starts up, and second, you get the heat you paid for after it shuts off rather than letting it just sit there.

    The auto switch position on the thermostat allows the furnace to do its thing. The ON position, bypasses that and turns it on continuously. If you have to jiggle the switch to make either of those things happen, it sounds like it is the thermostat. If it never works right, it could be either the thermostat or the furnace control.

    If the thermostat is old, I'd consider just replacing it with a more modern (and likely more accurate) electronic thermostat with a setback capability. This would eliminate the thermostat as the problem, and provide some energy and therefore cost savings on your heating bills if you choose to use the setback function (you don't have to, you can use it just like a conventional, non-setback thermostat).

    Your utility company may offer a rebate for a setback thermostat...I'd check with them, and the cost could end up free or very minimal; regardless, it should pay for itself in the first heating season easily.
  6. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    The fan switch on the thermostat bypasses the normal fan\limit switch on the furnace. If the fan is not cycling normally with the thermostat fan switch in the auto position then fan switch on the furnace is faulty or the blower motor itself may be have a bad spot on the starter winding.
  7. gwarthur

    gwarthur New Member

    Messages:
    1
    I have a 20+ year-old Carrier gas fired furnace that is logic controlled (that is, it has an electronic control unit in the top of the fan compartment). For some time, instead of starting the blower fan as it should after the burner has come up to a certain temp, the burner just runs until it (apparently) shuts down from over-temp. THEN the fan immediately comes on and runs for about a 90 seconds. This is really inefficient! I know the unit is old, but I'd like to squeeze another season or two out of it. Since the problem obviously lies with the control unit or with a sensor, I'm thinking of replacing all of the sensors and the control unit. This would be expensive, but this seems to me to be what a service tech would probably do, and then I'd be out the bucks for the service call AND the new hardware.

    Any thoughts about this irritating type of failure? -GA
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    Most of the sensors can be tested...replacing them without testing will potentially be expensive, and if it is the control module, won't help the situation at all. Same is probably true of the control module. A good tech may save you some money, but it depends...some would do the same thing...just replace parts until it works. A better one would know how to troubleshoot and fix what is broken. Now, depending on the cost of the parts, sometimes it IS cheaper to just replace, but (I don't think anyways), not all that often. On a unit that old, you may have trouble finding parts.

    With electrical parts, once you buy it, it is yours, so you can't take it back.

    Some time with a multimeter and checking some things would probably help to isolate the problem. If you aren't comfortable with that, then it's time for a pro.
  9. Grayfeathers

    Grayfeathers New Member

    Messages:
    27
    Location:
    Pleasant Grove, Alabama
    I've seen this several times. The fan usually has two speeds, fast and slow. During the AC mode the fan runs on the fas speed and during the heat mode the fan runs on the slow speed. I've replaced moters where the slow speed has failed. I'll be willing to bet that this is your problem. Need to have an experieced hvac tech check this out and to replace the moter if this is indeed your problem.
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