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  1. #1

    Is this normal modulating behavior?

    I got a new Rheem modulating furnace and thermostat about 6 weeks ago. Love it.

    But in the process of recovery from night set back it does an odd thing (I think). It runs full speed for the first half hour or so to get toward the target temp at the target time. Then it throttles down steadily to finish up the recovery job. That's all fine. The odd thing is that after about 2 hours of run time the furnace ramps up again for a while. When I look at the thermostat I find the temperature has dropped a degree or two from what it was earlier in the recovery process.

    For a system that can fine tune the amount of heat to recover and maintain, it seems odd to me that it can lose track of what it's doing by as much as 2 degrees! This "losing track" behavior never recurs in the maintain process for the rest of the day.

    I asked my contractor about this a few weeks ago. He too thought it was odd and replaced the thermostat. No change. Still the same behavior.

    So, is this "normal?" Sure doesn't seem to me like it should be.

    Oh...one more thing. During the recovery, the +2 indicator may come on in the thermostat display, as described in the manual. But I don't notice a thing when it happens, and worse still, it STAYS ON even after the furnace is well into the modulating down phase. I am thinking that the +2 display is bogus, that it means nothing in a fully modulating set up. Right? Wrong?

    Thank you for any comments.

  2. #2
    Join Date
    Mar 2001
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    How deep is your set-back? It is possible you are setting back too much.

  3. #3
    Join Date
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    Your walls and furniture don't heat up as quick as the air does.

    So the thermostat senses the room air temp warming up close to set point, and mods down.

    Then the walls absorb more heat then the furnace is outputing, and cools the room air temp, and the stat responds by increasing firing rate.


    Thats my guess.
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  4. #4
    Moose -- our set back is 8 degrees, from 68 down to 60, starting at 11pm and resuming by 830am.

    Beenthere -- sounds reasonable. Just surprises me that the temp can drop so much before the furnace is told to respond.

    Thanks.

  5. #5
    Join Date
    Mar 2001
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    Recommended set-back is 3-4 degrees. Try that & see what happens. As Beenthere said, the mass in the walls etc. is taking a long time to recover, & affecting the t-stat. I set-back max. 4 degrees, usually 3. Don't have cycling issues.

  6. #6
    Join Date
    Feb 2007
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    Quote Originally Posted by beenthere View Post
    Your walls and furniture don't heat up as quick as the air does.

    So the thermostat senses the room air temp warming up close to set point, and mods down.

    Then the walls absorb more heat then the furnace is outputing, and cools the room air temp, and the stat responds by increasing firing rate.


    Thats my guess.
    I have occasionally noticed this behavior too, however, but it usually occurs an hour to an hour and a half after set point has been reached. Also have noticed it in the afternoon.

    When the sevice tech checked my stat (below set point) , we opened a nearby window (it was one of the recent cold mornings). Couldn't get that sucker to fire up.

    Amp

  7. #7
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    Quote Originally Posted by ampulman View Post
    I have occasionally noticed this behavior too, however, but it usually occurs an hour to an hour and a half after set point has been reached. Also have noticed it in the afternoon.

    When the sevice tech checked my stat (below set point) , we opened a nearby window (it was one of the recent cold mornings). Couldn't get that sucker to fire up.

    Amp
    By then the wall was warmed up.
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  8. #8
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    Quote Originally Posted by beenthere View Post
    By then the wall was warmed up.
    Maybe so. There was a breeze blowing and I was fanning the cold air toward the holes on the side of the stat. It got too cold for me; had to close the window.

    Still had to wait a few more minutes for it to fire.

  9. #9
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    Doc Holiday posted sometime ago that the mod stat has a delay built into it to prevent short cycling from breezes. And to prevent sudden up and down modulations.

    He's a Rheem rep, so I believe he would know.
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  10. #10
    ampulman -- Yes, that's how it is for me too. The thermostat has been reading 68 for nearly an hour before it drops to 67 and kicks up the modulation.

    Moose -- I can try that. I've not heard that the recommended set back was 3-4 degrees. If I had to guess at a recommendation, I would have guessed 6 degrees, i.e., the set back built into the thermostat. Do you know where the 3-4 degree figure comes from?

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