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  1. #1
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    Irregular draft with inducer running - causes?

    I've been dealing with an erratic problem on a Goodman furnace in a dual fuel system.
    Model GDT070-3 Serial 0203610075
    There are some obvious installation problems but I can't fault them for the furnace's behavior. Among the installation issues with this downflow furnace is the coil mounted on top of the furnace and oversizing of the furnace. I'm sure a 40,000 Btu furnace and 1.5 ton heat pump would be enough.


    The customer called because the furnace does not reach the setting of 72* on the t'stat. The house was 70*. I confirmed the call for heat at the furnace. There was a fault code for pressure switch open. The switch is rated -0.04" WC. I measured draft at less than -0.01 but the switch closed anyway and the burners operate. When the burners are on the draft increases to a little over -0.04". I checked draft with my Bacarach Insight as well as a Dwyer 460.
    The 4" B-vent has a 2' rise, a 90 (less than 90*)18" horizontal with an incline, another 90 (also less than 90*), 3' vertical, another 90 (same as the others), 6' horizontal with incline, final 90 (same) and 5' vertically which penetrates the roof. The pipe extends well above the roof and the cap is clear and unobstructed. I looked into the vent pipe from the top, middle and bottom and found it clear.
    Is it possible for a single speed draft inducer to have both poor draft and proper draft even though the motor does not drag or change speed during operation? The wheel is intact and the housing is clear. The vacuum port and hose are clear as well.
    BTW, the combustion analysis is as follows: CO 42 PPM (steady); O2 7.4%; draft (when burners are on) -0.04" WC.
    I'm considering replacing the draft inducer and pressure switch. Ideally I'd replace the equipment with a properly sized furnace with coil under it as it should be, but the system is only 12 years old and the homeowner naturally doesn't want to replace it yet.

  2. #2
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    From my experience with commercial heaters that use B vent, the cause could be intermittent winds at the outlet.

    One way to reduce this effect is to put a star kap on the pipe.

    http://www.fieldcontrols.com/starkap.php
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  3. #3
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    Quote Originally Posted by kdean1 View Post
    I've been dealing with an erratic problem on a Goodman furnace in a dual fuel system.
    Model GDT070-3

    There was a fault code for pressure switch open. The switch is rated -0.04" WC. I measured draft at less than -0.01 but the switch closed anyway and the burners operate. When the burners are on the draft increases to a little over -0.04". I checked draft with my Bacarach Insight as well as a Dwyer 460.
    The pressure switch from the parts manual is rated at -.47" wc.

    Are you tee'd into the Pressure switch hose when you are seeing what negative draft the inducer is pulling?

    Your -.04"wc flue draft is different, and not where you should be measuring to see how well the draft inducer is working.
    Instead of learning the tricks of the trade, learn the trade.

  4. #4
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    Check the inducer blower wheel, could be packed with white rust.
    Last edited by Chuck; 10-17-2014 at 07:32 PM.

  5. #5
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    Thread Starter
    Thanks for the replies everyone.

    The inducer wheel is clean and clear of rust. A little coating on it but no clogging.

    RunDawg, I was using my Draft Simulator with Fieldpiece manometer head. However I was rushed as the customer was preparing to leave. I connected hoses directly to the switch and the manometer. The switch closed at around .25" WC which makes me suspicious. I think I'd better put the manometer on the hose from the draft inducer to check what it pulls.

    timebuilder, the weather conditions were calm when I was checking it. I'll keep the starkap in mind. Thats' the one Jim Davis recommended at the NCI combustion training. The thing that baffles me is that the vent has very little draft during the pre-ignition phase. I understand that the draft will increase during burner operation as the heated gases will improve it. The frustrating thing is the furnace sequence was okay when I was there with one exception. The cycle started but evidently did not close the pressure switch as it did not energize the ignitor. The cycle succeeded on all subsequent tries.

  6. #6
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    With fan assisted furnaces, on inducer startup, you will have a positive draft. After the burners ignite you should produce negative draft within 1 min. You should be checking your draft 18" away from the inducer or 90's. You should also install a barometric with a automatic reset spill switch wired in with pressure switch.

    I would check the port where the hose connects. Sometimes they clog up. Make sure the gasket is sealed on the back side of the inducer.

    From the port at the heat exchanger or inducer you should hook your manometer to and make sure you have -.47 or greater. If you do, the switch is bad.

    Make sure the hose isn't cracked.


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  7. #7
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    Thread Starter
    Thanks Gravity. I have never tested draft before the burner ignition so I didn't know that draft would be positive then.
    The gasket is intact and the hose port is clear. I'm leaning towards the draft switch more and more as the comments come in.

  8. #8
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    Like Rundawg stated, I think you need to T connect into the draft press switch port to get a proper reading of the safety control operation.

    Keep in mind, the readings in the stack will be different than in the heat exchanger draft port.
    As far as start up pressure "in the HX draft inducer port", I've always noticed a pressure increase after the burners fire. Using your switch numbers rated at -0.47", I would typically notice somewhere around -0.80 pre-ignition purge, then -0.60 post ignition. It may be possible that you're HX pressure may be increasing above the -0.47", in turn tripping the switch.
    I would also watch the manometer after the blower energizes.
    What was the stack temp?
    Not to mention, that's an awful lot of lateral and 90s for such a short verticle.
    "The only true wisdom is in knowing you know nothing" Socrates

  9. #9
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    Quote Originally Posted by mgenius33 View Post
    Like Rundawg stated, I think you need to T connect into the draft press switch port to get a proper reading of the safety control operation.

    Keep in mind, the readings in the stack will be different than in the heat exchanger draft port.
    As far as start up pressure "in the HX draft inducer port", I've always noticed a pressure increase after the burners fire. Using your switch numbers rated at -0.47", I would typically notice somewhere around -0.80 pre-ignition purge, then -0.60 post ignition. It may be possible that you're HX pressure may be increasing above the -0.47", in turn tripping the switch.
    I would also watch the manometer after the blower energizes.
    What was the stack temp?
    Not to mention, that's an awful lot of lateral and 90s for such a short verticle.
    I need to check the pressure switch when I'm not rushed.
    At this point I don't suspect the HX. The problem has been failure to complete ignition. From advice I've been getting and the furnace behavior I suspect the pressure switch is the culprit.

  10. #10
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    I love to join the discussion and sorry for my English as my 2nd language.
    You said the house had not reached the temp. setting 72? and the error code indicated "open PS"?
    Well I will not tell you to go through all the essential checks You've done so far again I just like to ask you to do some tricks to find out what causes the failure.
    To eliminate the possible cause of a stuff up flue pipe,you can disconnect the flue pipe right on top of the furnace and try to restart the furnace. If the furnace runs fine afterwards then the culprit is the flue pipe and good luck with that.
    If the furnace still cut off (it may take a while ,I guess) with the code PS open, then you should try to restart the furnace again and use a jumper to short out the PS and let the furnace runs if the the furnace runs and satisfies the temp. setting then replace the Draft Inducer.I've rarely seen a bad PS,Its cheap anyway but my guess is a bad DI.Good luck
    Quote Originally Posted by kdean1 View Post
    I've been dealing with an erratic problem on a Goodman furnace in a dual fuel system.
    Model GDT070-3 Serial 0203610075
    There are some obvious installation problems but I can't fault them for the furnace's behavior. Among the installation issues with this downflow furnace is the coil mounted on top of the furnace and oversizing of the furnace. I'm sure a 40,000 Btu furnace and 1.5 ton heat pump would be enough.


    The customer called because the furnace does not reach the setting of 72* on the t'stat. The house was 70*. I confirmed the call for heat at the furnace. There was a fault code for pressure switch open. The switch is rated -0.04" WC. I measured draft at less than -0.01 but the switch closed anyway and the burners operate. When the burners are on the draft increases to a little over -0.04". I checked draft with my Bacarach Insight as well as a Dwyer 460.
    The 4" B-vent has a 2' rise, a 90 (less than 90*)18" horizontal with an incline, another 90 (also less than 90*), 3' vertical, another 90 (same as the others), 6' horizontal with incline, final 90 (same) and 5' vertically which penetrates the roof. The pipe extends well above the roof and the cap is clear and unobstructed. I looked into the vent pipe from the top, middle and bottom and found it clear.
    Is it possible for a single speed draft inducer to have both poor draft and proper draft even though the motor does not drag or change speed during operation? The wheel is intact and the housing is clear. The vacuum port and hose are clear as well.
    BTW, the combustion analysis is as follows: CO 42 PPM (steady); O2 7.4%; draft (when burners are on) -0.04" WC.
    I'm considering replacing the draft inducer and pressure switch. Ideally I'd replace the equipment with a properly sized furnace with coil under it as it should be, but the system is only 12 years old and the homeowner naturally doesn't want to replace it yet.

  11. #11
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    Quote Originally Posted by kdang View Post
    If the furnace still cut off (it may take a while ,I guess) with the code PS open, then you should try to restart the furnace again and use a jumper to short out the PS and let the furnace runs if the the furnace runs and satisfies the temp. setting then replace the Draft Inducer.I've rarely seen a bad PS,Its cheap anyway but my guess is a bad DI.Good luck
    That's a bit of a hasty diagnostic don't you think?
    No doubt, the inducer could very well be faulty, but there are a few more steps before charging someone 3-4 bills for a part.
    "The only true wisdom is in knowing you know nothing" Socrates

  12. #12
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    Hasty? sorry I do not get that

    when you jump out the pressure SW and the furnace runs fine afterwards without anymore code"open PS" THEN what else could be causing the fail code"open PS" beside the Draft Inducer does not produce a consistent draft?
    I'm all ears and still learning.
    Quote Originally Posted by mgenius33 View Post
    That's a bit of a hasty diagnostic don't you think?
    No doubt, the inducer could very well be faulty, but there are a few more steps before charging someone 3-4 bills for a part.

  13. #13
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    Is your vent sized right? Should be in the manual the sizing for the diff btu. Sounds crazy but have had some that run fine for years and then all a sudden pressure switch lockouts due to wrong vent sizing, sometimes vent termination can cause it too

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