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  1. #1

    Question Brand New Rheem RGPS Gas Furnace Short Cycling........ WHY ??

    Long story short.....

    We just had a new Rheem Gas Furnace installed in our home the other day. The Contractor is NOT finished with the install. They still need to hook up the return pipes as well as reconnect and recharge the A/C coil.

    In the meantime, they got enough of the furnace installed to operate and supposed to heat until the can return to finish the job.

    Before they left the furnace when through a heating cycle successfully. After they left, it started short cycling. Coming On for about a minute and turning right back OFF. We called them back. He checked some things out and said our thermostat was old and a POS and it will need replaced as well. He said he would replace it when they come back next week.

    We've been on electric space heaters since.

    I have my doubts about the thermostat causing the short cycling as our old furnace did not do this. A bearing went out on the squirrel cage and well as a bad blower motor.

    I do know incorrectly adjusted fan limit switches can cause short cycling, but I can not find and documentation on our RGPS that shows a fan limit switch and/or it's adjustments. I'm assuming is it's in the electronic controls.

    So why is short cycling and how can it be fixed ??

  2. #2
    Join Date
    May 2012
    Location
    Upper Michigan
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    3,589
    I have had that happen from old stats a few times. Try turning it up all of the way and maybe it will act different.

  3. #3
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    2,070
    A possibility is the heat anticipater on thermostat has not been set to match the new furnace. That's something the contractor should take care of. Or he could replace the thermostat.
    It is not likely to be a fan/limit switch as new furnaces us a PCB for fan timing. The limit will cause short cycling only if air flow is restricted.

  4. #4
    Thanks for the quick replies.

    This the type/style thermostat that is currently hooked up.

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  5. #5
    Thanks for the quick replies.

    This the type/style thermostat that is currently hooked up.

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  6. #6
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
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    7,748
    Quote Originally Posted by Labrat0116 View Post
    Thanks for the quick replies.

    This the type/style thermostat that is currently hooked up.

    Name:  old_honeywell_thermostat.305221753_std.jpg
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    That's probably the problem, the heat anticipator on that stat is not set properly. A new digital stat should fix it.

  7. #7
    Quote Originally Posted by jtrammel View Post
    That's probably the problem, the heat anticipator on that stat is not set properly. A new digital stat should fix it.
    The anticipator is set at .1 right now. I just turned it up to .4 and see how it responds.

    Well, the furnace blower came on and blew cold air for about (5) minutes and then it turned OFF. NO HEAT though. ??? It came back ON about a minutes later and ran until it came up to the thermostat setting and finally went OFF again.

    The air coming out to vents is Warm....NOT Hot like it should be.

    Suggestions ?
    Last edited by Labrat0116; 01-12-2013 at 10:36 PM.

  8. #8
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    Quote Originally Posted by Labrat0116 View Post
    The anticipator is set at .1 right now. I just turned it up to .4 and see how it responds.

    Well, the furnace blower came on and blew cold air for about (5) minutes and then it turned OFF. NO HEAT though. ??? It came back ON about a minutes later and ran until it came up to the thermostat setting and finally went OFF again.

    "The Contractor is NOT finished with the install. They still need to hook up the return pipes as well as reconnect and recharge the A/C coil."

    The air coming out to vents is Warm....NOT Hot like it should be.

    Suggestions?
    Depends on how things are situated, but with less negative Return static the blower could be delivering more airflow than it should. However, these newer furnaces operate at a lower temp-rise therefore the supply air may not be as warm as the old furnace's airflow.

    Is the furnace the same Btuh Output?

    The Tech should have checked on what the heat anticipator setting needed to be, & then set it properly for you.

  9. #9
    Join Date
    Jul 2005
    Posts
    5,576
    Quote Originally Posted by Labrat0116 View Post
    The anticipator is set at .1 right now. I just turned it up to .4 and see how it responds.

    Well, the furnace blower came on and blew cold air for about (5) minutes and then it turned OFF. NO HEAT though. ??? It came back ON about a minutes later and ran until it came up to the thermostat setting and finally went OFF again.

    The air coming out to vents is Warm....NOT Hot like it should be.

    Suggestions ?
    Somewhere on a data tag on the furnace there's a temp rise range listed. It would look something like

    Temp Rise --- 35 - 65 F

    The actual temp rise should fall somewhere near the middle of that range, or about 50 in my example. If the return air is 65 and the temp rise is 50, then the supply air temp would be about 115. If the ducts run through an unconditioned space such as a crawlspace or attic, then you can easily lose a few more degrees before the air makes it to the supply grilles. So the air is probably WARM, just like it should be. As udarrell noted, older furnaces tended to run much higher temp rises, in part due to their design, and in part due to airflow issues such as dirty evaporator coils and undersized ductwork.

    FYI, there is no adjustment of any of the limit switches. They're all fixed and are there to save your life. Don't mess with them.

    The short-cycling could be due to any number of issues, but at .1 your anticipator was definitely set too low. That number is actually supposed to be the measure of amperage flowing through the t-stat. On some newer furnaces that current can be as high as 1 amp. I had to set an anticipator the other day to 1 to accommodate the new furnace we installed. It was short-cycling, running about 1 minute max at a time before I made that adjustment. The installer had set it to .1 thinking that he had it on 1. Those numbers are pretty hard to read sometimes and the decimal points can be invisible in low light.

  10. #10
    Quote Originally Posted by udarrell View Post
    Is the furnace the same Btuh Output?

    The Tech should have checked on what the heat anticipator setting needed to be, & then set it properly for you.
    Close I believe. The old one was 80,000 btu.......the new one is 75,000 btu 80%. The Tech said the old one was about 35 years old and was about 50% efficient.

    I know he "should have" set the anticipator correctly before he left. Oh well...... They'll be back Mon AM to finish the job and install a new thermostat.

  11. #11
    Quote Originally Posted by hvacrmedic View Post
    Somewhere on a data tag on the furnace there's a temp rise range listed. It would look something like

    Temp Rise --- 35 - 65 F

    The actual temp rise should fall somewhere near the middle of that range, or about 50 in my example. If the return air is 65 and the temp rise is 50, then the supply air temp would be about 115.
    Very good info!

    The return air ducts are not yet connected to the "inside - 1st floor" return vent work. BTW, they are a pair of (12") ducts that run up there. The furnace is in a crawl space under the house. The return air is being drawn from that area. Not ideal by any means.

    Anyways, the ambient temperature under the house now is probably in the 40s F. So that is probably why the supply air right now is luke warm. ??

  12. #12
    Join Date
    Jul 2005
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    5,576
    Quote Originally Posted by Labrat0116 View Post
    Very good info!

    The return air ducts are not yet connected to the "inside - 1st floor" return vent work. BTW, they are a pair of (12") ducts that run up there. The furnace is in a crawl space under the house. The return air is being drawn from that area. Not ideal by any means.

    Anyways, the ambient temperature under the house now is probably in the 40s F. So that is probably why the supply air right now is luke warm. ??
    That would do it.

  13. #13
    Quote Originally Posted by hvacrmedic View Post
    That would do it.
    I found a document On-line that shows the Temp Rise --- 40 - 70 F for this unit.

    Going down to 23 degrees (where is the degree sign on the keyboard ??) by tomorrow AM so the Contractor best be here at 0830 like he said he would be!

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