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  1. #14
    Join Date
    Oct 2007
    Posts
    12
    I also have a question about how a 2 stage TStat works. In particular, I'm interested in information on how a Honeywell RTH7400 works with respect to it's stage 1/2 heat/cool algorithms. Is there a PDF file on-line somewhere that explains it?

    My equipment is an Amana AMV80905CXB/ASX160361A. The fan is set to profile D, i.e 50% for 30 seconds, 82% for 7.5 min, 100% until done, 50% at the end. The fan is set for 1210 CFM (3 ton) high speed. The low speed spec at 787 CFM. (This is the B+ setting.)

    I would like to understand when the TStat should switch between low and high demand. And I'd like to understand what fan blower speeds I should expect during that process and the interaction between high/low stage speeds and the profiling.

    My guess, for cooling, based on 787 low speed and 1210 high speed:

    787*50% for 30 seconds, 787*82% for 7.5 minutes, 787 until set point reached or after some time (how long?) if set point not reached then 1210 until set point reached, then 50% of either 787 or 1210 depending on if stage 2 was used for the final 30 seconds.

    I have two newly installed units. The current behavior on both units is that it's always running at 1210 (modified by the ramp up/down of profile D). I've never noticed the outdoor unit fan speed at anything other than a single fan speed.

    At least for one of the units, which from the bedroom I can easily monitor at night, it comes on for about 4 minutes and shuts off. It seems this is way too short a cycle.

    Also, one of the units (the other one) gets into a state where the fan stays on a low speed for a very long period of time but the compressor doesn't kick in.
    Last edited by ptbrunet; 10-09-2007 at 12:56 AM.

  2. #15
    Join Date
    Oct 2007
    Posts
    12
    >787*50% for 30 seconds

    Actually, The AMV80905CXB's lowest speed is 563.
    Last edited by ptbrunet; 10-09-2007 at 11:51 PM.

  3. #16
    Join Date
    Oct 2007
    Posts
    12
    Response from Honeywell:

    The thermostat has an internal algorithm that determines the use of second stage of heating and cooling. The algorithm evaluates how far the room temperature is away from the set point and how long it has been away from the set point. Based on this information, the thermostat will determine if and when it is necessary to bring on another stage of heating or cooling. Other factors that affect the thermostat’s determination include outdoor weather conditions, insulation levels in the home, manual changes and the recovery ramp. During the recovery ramp, the thermostat will come on early to bring the temperature up to set point by the next program time, and may be less likely to use second stage heating or cooling. During a manual temperature change, the thermostat will increase the number of stages used more quickly than when it is maintaining a set point or following a schedule.

  4. #17
    Join Date
    Oct 2007
    Posts
    12
    >Also, one of the units (the other one) gets into a state where the fan stays on a low speed for a very long period of time but the compressor doesn't kick in.

    I determined today that what's happening here is that the TStat is calling for stage 1 and the furnace fan is coming on but the compressor is not. So it stays in stage 1 for a very long time until the temp gets far enough away from the set point so that stage 2 is called. Then the compressor kicks in and when the stage 2 call completes it's either back to stage 1 or completion of the cycle.

  5. #18
    Join Date
    Feb 2007
    Location
    Southern California, LA Metro
    Posts
    389
    Carrier/Bryant 2stage heat variable speed blower furnaces are capable of controlling heat staging with a single stage thermostat. Algorithm built into the furnace control board is set up by default from the factory to control the heat staging.
    Larger Duct, Larger Duct, More Air, More Air
    READ THE MANUAL
    What if I train them and they leave?
    What if you Don't train them and they Stay!
    DSG member

  6. #19
    Join Date
    Feb 2007
    Location
    Southern California, LA Metro
    Posts
    389
    Quote Originally Posted by ptbrunet View Post
    >Also, one of the units (the other one) gets into a state where the fan stays on a low speed for a very long period of time but the compressor doesn't kick in.

    I determined today that what's happening here is that the TStat is calling for stage 1 and the furnace fan is coming on but the compressor is not. So it stays in stage 1 for a very long time until the temp gets far enough away from the set point so that stage 2 is called. Then the compressor kicks in and when the stage 2 call completes it's either back to stage 1 or completion of the cycle.
    It sounds like someone mistakenly connected the tstat second stage to the condenser instead of first stage. The blower would come on because that is controlled by a different terminal.
    Larger Duct, Larger Duct, More Air, More Air
    READ THE MANUAL
    What if I train them and they leave?
    What if you Don't train them and they Stay!
    DSG member

  7. #20
    Join Date
    Oct 2007
    Posts
    12
    Looks like it's working now after a rewire. Amana has purple for the low stage and yellow for the high stage and that threw the installer off. Now getting 4 amps on low and 6 amps on high. I am not 100% sure it's quite right yet though because the condenser fan (Amana ASX16) always runs at the same speed and one of the manuals I have (page 45 of http://www.acdirect.com/xcart/images.../RS6200006.PDF) indicates the fan would run at different speeds.

  8. #21
    Join Date
    Oct 2007
    Posts
    12
    The condenser fan does change speeds on an ASX16 but it's a very subtle change.

  9. #22
    Join Date
    Oct 2007
    Posts
    12
    How do I tell if my TStat (Honeywell RTH7400) is making a stage two heat request? I was able to tell if it was making a stage two AC request by bumping down the set point one degree each time the panel backlight went off and eventually I would hear a relay click and my system would respond accordingly (higher furnace fan speed, more current drawn by the compressor, higher condenser fan speed). However, so far, doing the same thing in heat mode and bumping up the set point each time the backlight goes off I have not heard a relay click or noticed any difference in my heating system response. (Maybe it needs to be colder in the house?)

  10. #23
    Join Date
    Oct 2007
    Posts
    12
    Tech support from Honeywell told me that the RTH7400 will make a stage 2 call if the temp is bumped up 5 or 6 degrees. So I bumped it up 10 degrees (not 1 degree at a time as I did to test the stage two cooling request, but by a full 10 degrees) and that caused a stage 2 request. That helped me discover that one of my units is responding correctly and the other is not.

  11. #24
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Quote Originally Posted by skippedover View Post
    Some do all and all do some. (Elton John?)
    Now you've done it. You pissed off the GTSA (Gay Thermostat Society of America). They will be picketing your business next week. Elton John is their spokesperson.

  12. #25
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    2nd stage should kick in any time you bump up the temp a couple of degrees.

    Is the set up menu set right?

    #170 to 8

    They wire up W2 at both t-stat and furnace?

  13. #26
    Join Date
    Sep 2007
    Posts
    21
    Quote Originally Posted by oldertech View Post
    Carrier/Bryant 2stage heat variable speed blower furnaces are capable of controlling heat staging with a single stage thermostat. Algorithm built into the furnace control board is set up by default from the factory to control the heat staging.
    How does the algorithm work? Dealer told me it simply switches to stage 2 after 10 minutes. Is it more complicated than that? One of the installers also told me that if the temperature it needs to catch up is more than 5 degrees it goes automaticlly to 2nd stage but I don't see how it'll know that with a single stage thermostat.

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