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  1. #131
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    Jun 2001
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    Quote Originally Posted by RoBoTeq View Post
    So why would it not also keep the t-stat powered through the R terminal, indicated by the wiring to the R terminal from the CPU?

    Do you know for a fact that this model furnace kills the power to the thermostat when the limit switch is open? I'd like to see the circuitry on the back of that board for myself.
    The line from the Transformer before the fuse powers the CPU. The trace from the R and W lines are inputs to the CPU to tell it mode of operation and set codes. Power goes to the CPU through the thin RED line, the CPU detects inputs on the blue lines. If the CPU detects a loss over voltage on the R terminal it sets a "33" code. If you were to look at the physical board you would just see circuit board traces going to the CPU. You would probably find some sort of AC-DC convertor to power the CPU that's not shown on the wiring diagram. I've never seen a CPU powered by AC directly, most run on 12v, 5v or 3.3v DC.

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  2. #132
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    Seen 2 of them that did it. Work on a fair number of them, but it wasn't a problem of an open limit, roll out, etc.

    The wire from/between the CPU and the R terminal doesn't feed power to the R terminal. Its only so the CPU knows if the limits have tripped. Thats why the open limit, roll out, etc is all the same error ode, cause the CPU doesn't know which one opened, just that there is no power from the R terminal.
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  3. #133
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    Houston,Tx.
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    This is such an easy "SIMPLE" fix without changing the way any limits, or anything else acts/responds on the board, it's so easy even a caveman could do it, I am having to bite my tongue.
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  4. #134
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    I'm still trying to figure out why someone who is supposed to be helping consumers with energy costs would think that putting a thermostat that goes dead because furnace limits open wouldn't let the consumers know that they had furnace/ducting problems rather than just charge them to have an easily killed thermostat installed....I'm just saying...

    And I would never keep a furnace operating in a condition where if a limit opens, it kills power to all external controls.

    I'm just sitting here thinking of all of the possible scenarios that could occur by an open limit de-energizing the R terminal. Someone goes away for a day or two, the furnace limit opens because of 1) the filter finally hit it's limit...2) the blower motor fails...3) There is a power failure while the furnace is running full tilt...etc. So, the HO comes home to find a dead thermostat and assumes that there is a problem with the thermostat. This just doesn't make good sense.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  5. #135
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    When the limit resets, the thermostat is re-energized. So when the home owner comes home, the thermostat will have its display again.

    Heck, lots of customers think its a thermostat problem when the thermostat does have a display.
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  6. #136
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    Quote Originally Posted by RoBoTeq View Post
    I'm still trying to figure out why someone who is supposed to be helping consumers with energy costs would think that putting a thermostat that goes dead because furnace limits open wouldn't let the consumers know that they had furnace/ducting problems rather than just charge them to have an easily killed thermostat installed....I'm just saying...

    And I would never keep a furnace operating in a condition where if a limit opens, it kills power to all external controls.

    I'm just sitting here thinking of all of the possible scenarios that could occur by an open limit de-energizing the R terminal. Someone goes away for a day or two, the furnace limit opens because of 1) the filter finally hit it's limit...2) the blower motor fails...3) There is a power failure while the furnace is running full tilt...etc. So, the HO comes home to find a dead thermostat and assumes that there is a problem with the thermostat. This just doesn't make good sense.
    Current protocol is to alert a customer when the furnace shuts off on limit, we don't bypass anything in order to keep the thermostat alive when it kicks off on limit. If we discover a furnace kicking off on limit (or anything else not working/unsafe) during the pre-installation test we don't do the install. A lot of the "limit problem" went undiscovered since most installs are done in the summer as part of the Smarthours Program. Like yourself, we weren't even aware that furnaces could cut power to the thermostat due to high limit until the service calls started coming in. If they have a system problem we refer them their HVAC contractor to repair the problems. The thermostat, installation, and our service calls are all free of charge to the customer.

  7. #137
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    I still don't like the idea of killing the thermostat because of a furnace/ducting issue.

    So, if this thermostat is wiped out because of a high temperature issue, the HO then has to call an HVAC company that will most likely condemn the thermostat and sell one that they prefer?
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  8. #138
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    Dec 2002
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    Houston,Tx.
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    Quote Originally Posted by RoBoTeq View Post
    I still don't like the idea of killing the thermostat because of a furnace/ducting issue.

    So, if this thermostat is wiped out because of a high temperature issue, the HO then has to call an HVAC company that will most likely condemn the thermostat and sell one that they prefer?
    New stat and new furnace, because if the duct work is related to tripping the limit, you know the HX must be crying out, HELP ME!
    __________________________________________________ _______________________
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  9. #139
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    Quote Originally Posted by RoBoTeq View Post
    I still don't like the idea of killing the thermostat because of a furnace/ducting issue.

    So, if this thermostat is wiped out because of a high temperature issue, the HO then has to call an HVAC company that will most likely condemn the thermostat and sell one that they prefer?
    If the constant cycling of the limit switch blows the thermostat we install a new one free of charge to the customer. For testing purposes of "intermittent blank thermostat" we run the heat for 10 minutes. If it's going to shut off on high limit it will typically do so before the 10 minutes is up. If the furnace shut off the power to the thermostat on high limit, then they will need to contact their HVAC company to fix the issue. If the contractor sells them a new thermostat that "hides" the limit switch issue, so be it. I've do my best to inform the customer of the furnace shutting off on limit issues. Some get it, some I wonder about. I suspect many do as Mr Bill suggested and sell them a new furnace due to heat exchanger issues. Hopefully they fixing the cause of the issue and not just replacing the furnace. I've seen old Acoils on top of new upflow furnaces, and new Acoils under old downflow furnaces. I guess a customer didn't want to pay for a coil/furnace, but they have already paid for the labor since it's being removed/replaced anyway...

  10. #140
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    Quote Originally Posted by RoBoTeq View Post
    I still don't like the idea of killing the thermostat because of a furnace/ducting issue.
    You need to take that up with Carrier.
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  11. #141
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    Quote Originally Posted by beenthere View Post
    You need to take that up with Carrier.
    Some Lennox and Goodman models do this also. Haven't seen any Rheem/Ruud units do this. It seems the Carriers we're discussing in this tread seem to cause more service calls than most. Perhaps the blower capacity is marginal compared to BTU output of the furnace, any extra static pressure causes the furnace to kick off on high limit. 70k BTU burners with a 800cfm drive (070-08/024070) is pushing it IMHO. I don't know why these are even installed in Oklahoma houses, a house that can be cooled with 2 tons of AC (based on 800CFM, the maximum capacity of the 070-08's blower) doesn't need 70,000BTU of heat.

  12. #142
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    54, I sent you an email to the one that's in your profile, guess it went into your spam box.
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    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  13. #143
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    Quote Originally Posted by Mr Bill View Post
    54, I sent you an email to the one that's in your profile, guess it went into your spam box.
    I looked through my junk folder, didn't see an email from you.

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