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Thread: AO Smith FPSH-50, Power Vent & short cycling

  1. #1
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    Default AO Smith FPSH-50, Power Vent & short cycling

    Situation: Water heater works fine but something causes it to shut down during the intial proofing cycle. But, if I partially cover the intake vent for about 15 seconds after the gas has ignited (during the proofing cycle), the water heater will run perfectly until all the water is heated. The problem is that on the next call for heat, it will short cycle again and time itself out until I go down and partially cover the intake vent again.

    I checked the switch and it's fine, I bypassed the thermometer switch and that doesn't seem to be the problem either.

    I checked all the wires for continuity, VAC, and proper ground and all that is fine. I even tried a different air pressure switch and I know both of them are working. I can't figure out why partially blocking the intake vent somehow meets the proofing cycle criteria. I don't have a manometer to measure air pressue but I stuck the hose inside the intake vent to ensure the vaccuum was high enough and even that didn't work.

    It's not the vent piping either as I tried it unvented and it still short cycled.
    I want to get a job as someone who names kitchen appliances. Toaster, refrigerator, blender.... all you do is say what the shit does, and add "er". I wanna work for the Kitchen Appliance Naming Institute. Hey, what does that do? It keeps shit fresh. Well, that's a fresher....I'm going on break.

    Mitch Hedberg

  2. #2
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    Not sure what would be causing it, is your water heater inside or outside in your garage or shed?

    Jake
    Appliance Repair School 1987-1988
    Star Appliance Tech. 2 yrs. 1988-1990
    Wards Appliance Tech. 11 yrs. 1990-2001
    Sears Appliance Tech. 4 yrs. 2001-Oct. 2005
    Jake's Appliance Repair Nov. 2005-present

    Look-Up & Order Parts

  3. #3
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    The water heater is in the basement and vented to the outside. I've noticed that the outside air does flow down into the basement so the power vent needs to work against that but I also have tried running the power vent with no venting (directly into the basement) and that didn't work either. Plus, everything thing has worked fine for 7 years or so. Last year it started acting up but I went down and took things apart and put it back together and it worked fine again until about a month ago.

    The smart valve has an indicator light and when the system locks out, I get six flashes. From the owner's manual this indicates any of the following:

    1. Reversed polarity in 115 volt power supply.
    2. Heater/burner not properly earth grounded, check pin 3 on wire harness plug (valve end) to burner tube for 115 volts, check ground path to burner.
    3. Ground fault interruption (GFI) breaker causing intermittent operation, try non-GFI outlet.
    4. Flame sense rod dirty or incorrectly positioned.
    5. Vent system length exceeded, too many elbows, blocked exhaust/intake switch opens after light off.
    6. Flame sense/HSI wiring condition/connections.
    7. Low or no supply gas pressure.
    8. Fluorescent lighting on same breaker as heater.
    9. Damaged, defective, or incorrectly positioned HIS (Hot Surface Igniter).


    Since I can get hot water if I just partially block the intake during the start-up, I figure it has to be a venting issue. I checked all the electrical and that's fine and I tried it vented with no pipe at all.

    The proofing cycle goes like this:

    1. 115 - 125 VAC is applied to the water heater.
    2. Water temperature drops below set-point minus the differential.
    3. A call for heat is activated.
    4. The SmartValve control performs a self diagnostic routine. The air proving switch circuit must open within 30 seconds or the SmartValve will lockout and display a 2 flash error code, the blower will not start. (Blocked exhaust switch contacts should have re-opened after previous cycle completed. If air proving switch circuit opens at anytime during lockout the SmartValve control will continue the sequence from this point.) Air proving switch circuit will always have a blocked exhaust switch. FPD models will also have a blocked inlet switch. Some models will also be equipped
    with a vent temperature limit switch (see page 30 in this manual) in this circuit, other models will have the vent temperature switch wired into the 115 VAC neutral wire supplying the blower motor.
    5. SmartValve control sends 115 VAC to blower motor through wiring harness.
    6. Blower starts and comes up to speed.
    7. Negative pressure (slight vacuum) is induced in the blocked exhaust switch’s plastic sensing tube.
    8. Blocked exhaust switch (normally open) contacts close within 30 seconds. (The air proving switch circuit should now be closed) If the blocked exhaust switch contacts do not close within 30 seconds the SmartValve will de-energize the
    blower after an additional 30 second purge time. The SmartValve will then lockout for 5 minutes during which it displays a 3 flash error code. After 5 minutes the SmartValve will retry at step four above. The SmartValve control can
    cycle this way indefinitely if the air proving switch circuit remains open.
    9. SmartValve control sends 115 VAC to Hot Surface Igniter (HSI), igniter warms up 7 seconds on 1st trial for ignition, 12 seconds on 2nd - 4th trial.
    10. SmartValve control opens main gas valve.
    11. Main burner lights and proves flame within 4-5 seconds. If the flame is not sensed within 4-5 seconds the SmartValve will de-energize the gas valve, run the blower for a 30 second purge cycle, then try again at step nine for 3 additional tries. If after the 4th trial for ignition the flame is not proven, or if flame sense is lost a 4th time before the call for heat is satisfied, the SmartValve will run the blower for a 5 second post purge and then de-energize the blower. The
    SmartValve will lockout for 1 hour during which it displays a 6 flash error code. After 1 hour the SmartValve will try again at the beginning of the sequence for 4 more tries. The SmartValve can cycle this way indefinitely as long as a call for heat is present.
    12. Main burner remains on until water reaches set-point.
    13. SmartValve de-energizes gas valve, main burner goes out.
    14. Blower post purges 5 seconds and then stops.
    15. Blocked exhaust switch contacts open.
    16. Heater goes into standby mode.


    My system gets to step 11 on its own and then locks out and gives the 6 flash code. The only thing I haven't tried is the flame sense rod but seems odd since blocking the vent partially somehow completes that step and allows the system to go through to step 16. The only thing I can think is that by partially blocking the intake vent, I'm having some effect on the burner flame that may be allowing the flame sense rod to do its job. Does that make sense? Do you think it's worth a shot to replace the flame sense rod?
    I want to get a job as someone who names kitchen appliances. Toaster, refrigerator, blender.... all you do is say what the shit does, and add "er". I wanna work for the Kitchen Appliance Naming Institute. Hey, what does that do? It keeps shit fresh. Well, that's a fresher....I'm going on break.

    Mitch Hedberg

  4. #4
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    Sounds like to me you have a bad or partially clogged burner orifice, not allowing the full proper flame to keep it on and running.

    You could take the burner assembly apart to check for clogs, if non found, then its likely the flame sense rod.

    What series is your water heater?

    Jake
    Appliance Repair School 1987-1988
    Star Appliance Tech. 2 yrs. 1988-1990
    Wards Appliance Tech. 11 yrs. 1990-2001
    Sears Appliance Tech. 4 yrs. 2001-Oct. 2005
    Jake's Appliance Repair Nov. 2005-present

    Look-Up & Order Parts

  5. #5
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    It's an A.O. Smith FPSH-50 Series 252.

    The clogged burner orifice might be it. I think I'll take a look.
    I want to get a job as someone who names kitchen appliances. Toaster, refrigerator, blender.... all you do is say what the shit does, and add "er". I wanna work for the Kitchen Appliance Naming Institute. Hey, what does that do? It keeps shit fresh. Well, that's a fresher....I'm going on break.

    Mitch Hedberg

  6. #6
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    I went in and looked at the burner assembly, sprayed some compressed air through the gas tube and took a look at the flame sense rod. It was a little less than a quarter inch away from the spreader, the manual recommend between 7/64 and a 1/4 inch but suggests using a nickel to gage the gap to 7/64. I reset the flamse sense rod to that distance but it still short cycled. I then looked at the manual again and it suggested cleaning the flame sense rod with steel wool or emory cloth so I did that, re gapped the flame sense rod with the nickel and tried it again. This time, it cycled through correctly on its own. Hopefully it will continue to do so. The flame sense rod did not look that corroded but oh well, something worked.

    Anyone know where I can get a replacement flame sense rod just in case? I've found the complete burner assembly ($120) but it doesn't look like the flame rod comes separate. Probably at best it would come as a unit with the hot surface igniter as those are wired up together. Would it matter if you rigged it up yourself, just replacing the flame sense rod with a generic one and wiring in the HSI harness? What's my best bet in case something needs to be replaced but not buying the entire burner assembly.
    Last edited by jaboobie; June 16th, 2008 at 03:17 PM.
    I want to get a job as someone who names kitchen appliances. Toaster, refrigerator, blender.... all you do is say what the shit does, and add "er". I wanna work for the Kitchen Appliance Naming Institute. Hey, what does that do? It keeps shit fresh. Well, that's a fresher....I'm going on break.

    Mitch Hedberg

  7. #7
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    Glad it seems to be working good again.

    Anyone know where I can get a replacement flame sense rod just in case? I've found the complete burner assembly ($120) but it doesn't look like the flame rod comes separate. Probably at best it would come as a unit with the hot surface igniter as those are wired up together. Would it matter if you rigged it up yourself, just replacing the flame sense rod with a generic one and wiring in the HSI harness? What's my best bet in case something needs to be replaced but not buying the entire burner assembly.
    Nope, I sure don't know how to just order the flame rod separately, I found it like you in the complete burner assembly.

    I'd be careful trying to rig another flame sense rod to replace the original.

    I've always just replaced the burner assembly when I do them.

    Maybe someone else who has successfully replaced just the flame sense rod can assist you if they happen to read this thread.

    Jake
    Appliance Repair School 1987-1988
    Star Appliance Tech. 2 yrs. 1988-1990
    Wards Appliance Tech. 11 yrs. 1990-2001
    Sears Appliance Tech. 4 yrs. 2001-Oct. 2005
    Jake's Appliance Repair Nov. 2005-present

    Look-Up & Order Parts

  8. #8
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    As long as I don't have to get up every morning and start the water heater, I'm happy. $120 for a new burner assembly doesn't sound so bad in comparison.

    On the up side I now know a heckuva a lot more about AO Smith Water heaters than I ever wanted to. If Iget a digital manometer, I could go into business on my own since I went through every single freaking troubleshooting step trying to fix this darn thing.
    I want to get a job as someone who names kitchen appliances. Toaster, refrigerator, blender.... all you do is say what the shit does, and add "er". I wanna work for the Kitchen Appliance Naming Institute. Hey, what does that do? It keeps shit fresh. Well, that's a fresher....I'm going on break.

    Mitch Hedberg

  9. #9
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    I just came back here to find the date when I finally fixed the water heater. Just for informational purposes I thought I'd also post that just about two weeks ago the water heater went out again with the same symptoms. So I took out the burner assemble and cleaned the flame sense rod again with some emory cloth and it's been working fine ever since. Looks like something I can expect to do as yearly maintenance. If it starts happening on a more frequent basis, I'll get a new burner assembly.

    I wonder how many people pay a maintenance guy to come in and do the same thing? It's literally a 10 minute job at the most.
    I want to get a job as someone who names kitchen appliances. Toaster, refrigerator, blender.... all you do is say what the shit does, and add "er". I wanna work for the Kitchen Appliance Naming Institute. Hey, what does that do? It keeps shit fresh. Well, that's a fresher....I'm going on break.

    Mitch Hedberg

  10. #10
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    Good to see you again.

    Thanks for the update, yes it sounds like you may probably need to do that once a year now.

    Jake
    Appliance Repair School 1987-1988
    Star Appliance Tech. 2 yrs. 1988-1990
    Wards Appliance Tech. 11 yrs. 1990-2001
    Sears Appliance Tech. 4 yrs. 2001-Oct. 2005
    Jake's Appliance Repair Nov. 2005-present

    Look-Up & Order Parts

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