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  1. #1
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    Mar 2017
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    ORE.
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    Pressure switch errors carrier weathermaker 18 year old 90%

    Was troubleshooting a no heat. Inducer comes on then immediately flashes pressure switch error on board. Bypassed pressure switch to see if I could ignition, ignitor lights but then faults out on retrying. Didn't smell any gas.

    16 inches h20 on inlet to gas valve.
    Brought 24v power from board to gas valve to see if I could get it to open and show gas pressure on outlet side. couldn't see any gas pressure on outlet when bringing power directly from board. Could definitely hear the valve click.

    2.72 inches h20 on collector box tubing to pressure switch
    .74 inches h20 on "t'ed" tubing coming of combustion box to pressure switch.

    I replaced the board and it immediately flashes pressure errors again. Caulking on inducer motor falling off so I removed and resealed with red high temp. There were some bird feathers in the inducer when removed.

    Removed burner box cover and tried to get ignition but nothing lite off.

    I'm thinking exhaust is plugged or secondary plugged.

    The feathers in inducer were fluffy like they fell in through exhaust.

    When I removed the pressure tubing from the collector box the unit started trying to light.

    Don't know what to make of the immediate pressure switch error and no gas pressure on outlet side.

    Is 16 inches h20 to much pressure forms gas valve.

    Anybody experience similar problems.

    Sorry for the long rambling thread. 1 year junior service tech.

  2. #2
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    Mar 2018
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    Chico, Ca #StateofJefferson
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    Not sure if your burning NG or LP but that's to much inlet pressure, which could prevent the valve from opening. I'm betting your going to find a friend in that pipe with the feathers but what is the pressure switch rated for?

  3. #3
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    Thread Starter
    Natural gas. Didn't have time to inspect exhaust. Couldn't read pressure rating on switch. I will be going back to swap out new board for old one that apparently works fine. Hate swapping stuff only to have same issue, learning alot on this one.

    Inlet pressure seems high. Thought is was supposed to be around 8 inches h20 not 16. Regardless of gas valve why would it be throwing pressure switch fault immediately on call for heat? That's where I'm getting confused.

    Usually I can bypass and get ignition somehow. Not on this one.

    I guess Ill get my scope into the exhaust and see what happens.

    Furnace 14ft up in t bar ceiling horizontally at a church. Hardly ever gets used. Exhauts on flat rooftop.

    Is there a "normal" static pressure reading if I drill a small hole into exhaust PVC? Normal static pressure for fresh air intake? Shouldn't they be similar in values? Thought the secondary might be clogged because of .74(sealed burner box) vs. 2.74(collection box).

    Appreciate the help.


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  4. #4
    Join Date
    Jul 2002
    Location
    Chariton, Iowa
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    If the 16" reading is accurate I would have the gas company fix the over-pressure issue and then replace every appliance gas valve that has been exposed to it as a CYA measure.

    There is roughly .43 PSI per foot of depth of water.

    Appliance gas valves are generally only rated for .5 PSI maximum

    .5 PSI is therefore 13.84-13.85" of water

    We generally round that up to 14"

    Chances are that if your area uses a high pressure gas main system with pressure regulators at each meter that the regulator seat or disc is damaged or contaminated and so the regulator isn't "locking up" and is "creeping up".

    I had this happen at my own house, I only knew about it because such gas regulators generally have an over-pressure relief vent feature integrated into the sensing diaphragm that opens at roughly 2x the set pressure and I could occasionally smell the gas odorant outside.

    Sure enough when I measured the pressure using nothing more than distilled water in a length of clear tubing taped to a yard stick it crept up to 14" instead of staying at the proper 7". My only gas appliances were a Smart Valve equipped furnace and a natural gas Weber grill IE no standing pilots so I had 0 flow most of the time.

    Appliance gas valves internally use poppet type valves that open against the pressure/flow of the gas so that an over-pressure situation assists the spring and forces them closed harder rather than forcing them open, this is why makeitcold said that excess pressure could keep the valve from opening. It is a safety feature.

  5. #5
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    Jul 2007
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    The burner box pressure is usually higher (less negative) than the collector box pressure (more negative).

    The switch closes based on the differential between these two pressures.

    Post the model number of this unit, and later today I will try to post what the pressure switch rating should be.
    Instead of learning the tricks of the trade, learn the trade.

  6. #6
    Join Date
    Mar 2016
    Location
    Dayton Oh
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    That model furnace at that age, is probably in the secondary HX recall era.

    Pull the blower motor out and look at the back plate of secondary HX to see if it’s got goop dripping from it. If so, no need to waste a bunch of other time on it.

    And second check your gas pressure as others have said. Should have about 7 in.wc not 16.


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  7. #7
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    Another guy went out and vacuumed the exhaust, didn't change anything. Now I'm leaning heavily to regulator/gas valve/secondary.

    Appreciate all the responses.

    Ill update when solution is found


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  8. #8
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    Aug 2018
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    Piedmont Triad, NC
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    Does that model have “pressure switch stuck closed” flash code? Sometimes the pressure switch won’t open and the board won’t let the system get past that point. It will flash the code pretty much immediately and jumping the switch won’t make it go away since it’s already closed.

  9. #9
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    Mar 2018
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    Chico, Ca #StateofJefferson
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    I dont know how the exhaust pipe is ran where you are, but everytime I've had a critter in the exhaust pipe it is cooked until its gooey. Sometimes it's best to come back with a stick of fresh pipe and some couplers. Yum-yum!

  10. #10
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    Aug 2002
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    Southold, NY
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    Quote Originally Posted by 503 Hvac View Post
    Another guy went out and vacuumed the exhaust, didn't change anything. Now I'm leaning heavily to regulator/gas valve/secondary.

    Appreciate all the responses.

    Ill update when solution is found


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    Guessing or troubleshooting?

  11. #11
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    Mar 2017
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    Thread Starter

    Pressure switch errors carrier weathermaker 18 year old 90%

    Problem solved.

    Thanks to whomever said with 16inch h20 gas valve wouldn't open. Adjustment made to regulator. Able to get unit to fire. Will return to replace regulator and gas valve to ensure proper operation. Appreciate all the help and feedback.


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  12. #12
    Join Date
    Jul 2007
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    Quote Originally Posted by 503 Hvac View Post
    Problem solved.

    Thanks to whomever said with 16inch h20 gas valve wouldn't open. Adjustment made to regulator. Able to get unit to fire. Will return to replace regulator and gas valve to ensure proper operation. Appreciate all the help and feedback.
    Thanks for the update.
    Instead of learning the tricks of the trade, learn the trade.

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