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  1. #1
    Join Date
    Oct 2015
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    Help! Makeshift C wire from Oil furnace?

    Hi,

    I'm hoping I include all information for this question.

    Up at the cottage we have an oil burning furnace. We decided we would like a wifi thermostat so we can get the heat going before we get there. Attachment 1 shows how the old thermostat was wired. Attachment 2 shows the unit we have. Attachment 3 show the wiring diagram for the unit. It looked to me that the 24v coming from the transformer is going to that lower "F" terminal, so i used that as my C wire. I turned on the power and turned the heat on and everything is running okay.

    My issue is, i'm a little nervous about this, because of the cad cell sensor. How much power does it need to function? Would the thermostat be taking too much of the 24V?

    This is the new thermostat
    Honeywell
    Wifi 7 Day Programmable Thermostat BasicName:  1.jpg
Views: 2072
Size:  57.0 KBName:  2.jpg
Views: 2630
Size:  185.9 KBName:  3.jpg
Views: 6457
Size:  71.8 KB

    Thanks for any help

  2. #2
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
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    I don't see a happy ending. That ignition control has NO true low voltage common. Me thinks the factory installed smoke is going to vacate.
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  3. Likes lkapigian liked this post
  4. #3
    Join Date
    Jan 2012
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    Broomall, PA
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    ...My issue is, i'm a little nervous about this, because of the cad cell sensor...

    It will work great until you burn your house down. The F F terminals are for flame sensing and are a safety device to shut down your burner due to flame failure. If that fails to happen, because you are tricking the safety device, the burner will keep running, fill the chamber with oil and if you lucky not ignite. If you're unlucky...
    I recommend pre-paid funeral and a will.
    Don't discount my sarcasm...this is at best dangerously stupid.

  5. Likes lkapigian liked this post
  6. #4
    Join Date
    Oct 2015
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    Thread Starter
    Ok, so the sensor will not work if i am running the thermostat off the F terminal. How often does the flame fail? Is this something I would need to drive up to the cottage for to take care of now or could it wait until I am there on the 31st. the furnace is only set to turn off if the temperature inside drops below 45F.

    Would a 500mA AC to 24VAC adapter be enough to power a wifi thermostat?

  7. #5
    Join Date
    Jun 2014
    Location
    Dover, DE
    Posts
    9,859
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    There should never be anything connected to the cad cell terminals other then the cad cell. What you did is highly dangerous and a large fire hazard.
    I would shut the furnace down until it can be properly wired.
    I havent failed. Ive just found 10,000 ways that wont work. - Thomas Edison

    Its not whether you get knocked down, its whether you get up. - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  8. #6
    Join Date
    Oct 2015
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    Thread Starter
    Ok.

    My thermostat was not connecting to the wifi since 8am this morning. so I started driving up to the cottage... I checked the thermostat again 20 mins into the drive and now it is connecting.

    So if I just set the system to off, am I safe until I can get up there and put the wires back to the way they were? Or do I need to actually be up there to cut power to the furnace?

  9. #7
    Join Date
    Jun 2014
    Location
    Dover, DE
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    I'd turn power off to the burner
    I havent failed. Ive just found 10,000 ways that wont work. - Thomas Edison

    Its not whether you get knocked down, its whether you get up. - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  10. #8
    Join Date
    Oct 2015
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    Thread Starter
    Even though on the thermostat I have set the system to off? I didn't think anything would actually turn on if the thermostat isn't telling it to.

    Or is turning of the power just as a precaution... Would be another 6 hours of driving so if it isn't necessary I would avoid it.

  11. #9
    Join Date
    Sep 2015
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    Ontari-ari-ari-o
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    Flame don't often fail it's everything controlling them that can fail. Bypassing a safety is beyond crazy and kills people that try every year. An oil furnace with too much fuel in it is the nightmare of every oil burner tech and no word of a lie can cause your furnace to explode. Even just a bit of extra oil can blow the doors off the unit. If you want the WiFi nonsense you'd have better luck seeing if you can replace the module with a compatible one.

  12. #10
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
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    You CAN NOT power a thermostat off that primary control. An additional transformer and relay could be added to "upgrade" the controls. this would also allow a fan function if you wished. An oil man who has worked on older furnaces could do that in an hour or so.
    Col 3:23


    questions asked, answers received, ignorance abated

  13. #11
    Join Date
    Jan 2004
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    PA
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    Moved to AOP Residential forum.


    Sorry, this is not a DIY site.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  14. #12
    Join Date
    Oct 2015
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    6
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    Thread Starter
    I really didn't realize i was bypassing anything... i just thought i could use that power to run the thermostat, i didn't realize i was making the cad cell not work at all. I had been looking online and everywhere i looked said you can you the 24v from the transformer as a C wire, but maybe this is not for oil systems. I had also read a post in the Nest forums saying you can use the lower F terminal no problem as a C common, but i guess the guy didn't know what he was talking about.

    Im glad I decided to take another look into it as it sounds like it was an incredibly stupid thing to do.

    I have turned off the system at the thermostat, i assume no oil will be flowing if the system is off. I will go up tomorrow to remove my mistake from the F terminal and put the old thermostat back in.

    I have already contacted Honeywell and they said I can use a 24VAC wall adapter and hook it up to the C and Rc terminals. So i will go ahead and buy one of these, although they were not specific with the amperage, i would get 500mA would be okay?

    I guess now i know better than to go messing around with my furnace.

  15. #13
    Join Date
    Sep 2002
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    23,146
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    I have never seen it done, nor heard of it being done - but from studying the diagram you posted, I can't see where harm can result. Well; what I really mean is danger. <g>

    So far as I can see the only risk is that the thermostat amp-draw load is greater than the control's internal transformer can handle. The control's transformer may burn out - but that's inherently safe as nothing will operate. <g>

    No wait: I have a question - is the control's low voltage actually 24 volts? Does anyone know? What is the voltage reading if the meter is connected to the top T terminal and the bottom F terminal?

    There is no way to electrically 'trick' the cad cell. It has to start out open / high resistance - or the burner will not start. And once the flame is established it has to go closed / low resistance - or the burner will lock out.

    Does the wifi function actually work? Can you control the stat from where ever you are now? Because if so; the control's low voltage much be at, or near, 24 volts.

    It's an interesting idea - I've just never considered it before.

    PHM
    ----------



    Quote Originally Posted by jwhiteney View Post
    Hi,

    I'm hoping I include all information for this question.

    Up at the cottage we have an oil burning furnace. We decided we would like a wifi thermostat so we can get the heat going before we get there. Attachment 1 shows how the old thermostat was wired. Attachment 2 shows the unit we have. Attachment 3 show the wiring diagram for the unit. It looked to me that the 24v coming from the transformer is going to that lower "F" terminal, so i used that as my C wire. I turned on the power and turned the heat on and everything is running okay.

    My issue is, i'm a little nervous about this, because of the cad cell sensor. How much power does it need to function? Would the thermostat be taking too much of the 24V?

    This is the new thermostat
    Honeywell
    Wifi 7 Day Programmable Thermostat BasicName:  1.jpg
Views: 2072
Size:  57.0 KBName:  2.jpg
Views: 2630
Size:  185.9 KBName:  3.jpg
Views: 6457
Size:  71.8 KB

    Thanks for any help
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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