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  1. #1

    No Aux Heat During HP Defrost

    Hello HVAC Pros,

    I have been in a 3000 ft2 cape cod in western NY built in 1988 for the last 4 years. 2x6 exterior framing and thermopane casement windows. No natural gas available here, we are on a municipal electric system that buys hydro power from Niagara Falls. Elect rates are $.035 for first 1000 KWH and $ .0525 there after. The house is all electric, with 4 people here, and a large 400 gal hot tub. My usage was 5142 KWH in Jan with 1068 heating degree days. Last year was 6168 KWH in Jan with 1333 degree days. I try to keep the place at a constant 68 degrees.

    The (I believe) original Trane XE 900 HP system seems to work OK until the defrost cycle when the unit blows cold air. The aux heating coils (in a Trane air handler) are not energizing during the defrost cycles. I can hear the relays click on and generate the resistance heat when I raise the thermostat's set point 3 degrees (as the jumpers are set to do) above the current indoor temp.
    The outdoor coils never seem to ice up so far this heating season.

    The defrost control was replaced 10 years ago, based on a service sticker on the air handler. Last week I shut down the system including the service disconnect, and looked at the defrost control board. It is an American Standard unit CNT 1924 with only 1 jumper for diagnostics. The board is smaller than the original since I see two mounting holes in the sheet metal that are not used for this replacement board.

    Is the board the most likely reason for no aux heat during defrost? Should other hardware be looked at also?

    When a service person comes in next week, if this board is replaced should I make sure a different defrost control is used that has more configurability in defrost settings?

    When finances allow in the fall/winter 2012-13 should I look into replacing this unit? Will I gain enough in energy efficiency to have a decent payback period? We will probably be here another 5-6 years minimum.

    Sorry for being so wordy, I wanted to minimize requests for more info.

    Thanks in advance,

    Greg.

  2. #2
    Join Date
    Jan 2010
    Location
    North Huntingdon,Pa
    Posts
    356
    If it is indeed energized via good contacts,I'd suspect bad elements,time to call a pro.
    PA#9377
    Your Satisfaction...Our Goal.

  3. #3
    Join Date
    May 2004
    Location
    Frederick, MD
    Posts
    964

    Did I

    read this correctly? The system is 24 years old? If so, you will definitely do yourself a favor by replacing the system.
    GO NAVY, BEAT ARMY!

    A DECADE OF DOMINANCE! +2

  4. #4
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,535
    This is not a DiY board so most will simply recommend you get a pro to check out. Many times you are only getting part of the heat elements working and when having to "outrun" the defrost function, you may feel you are not getting the back ups to work when in fact you have not got enough working to create warm discharge air. I'm sure a competent person will quickly trouble shoot this and repair it. I'd not spend much on a system this old however!

  5. #5
    Thanks Wahoo, Greg P, and Openclasspro

    I did read the FAQs and rules about no DIY instructions prior to posting. I just like to be armed with enough info to make an informed decision when a service person presents the repair/replacement options.

    If the repair is beyond what I think makes sense for 2 more months of winter, what brands do you guys like for new heat pumps? I'm impressed with the longevity of the existing Trane, should I stick with them?

    Greg.

  6. #6
    Join Date
    Sep 2003
    Location
    Western PA
    Posts
    1,357
    What's the problem? I have been thinking about disconnecting my aux. heat during defrost. My electric rates are not as good as yours.
    Local 449?

  7. #7
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,670
    Nods,

    this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

    You can find the rules for posting and qualifications here.

    Further infractions may result in loss of posting privileges.

  8. #8
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,048
    Trane units today are not built with the same quality as your units were.

    Brand name is not a major factor in getting a efficient, reliable system.

    Good installation and proper duct systems are the major factors to spend your budget on.

  9. #9
    Targetman,

    During defrost the vents are blowing cold air around 55 degrees which drops the temp in the house 2-3 degrees. When it's in the 20's outside it takes forever for the HP to get back to the set point after the defrost.

    Greg.

  10. #10
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    Greg

    I have not read the entire thread.

    But if your last statement is accurate, then it suggests your heat strips are not energizing in defrost mode as they should be.

    what size heat strip do you have? If you don't know, you should.

    Is your heat strip locked out down to a certain temperature? It should not take hours for thermostat setting to be reached following a defrost call. Again this suggests your heat strip is not energizing.

    Now the question is why?

    Is the situation you describe new or has your system always performed correctly.

    One suggestion for you to try is to shut regular heat off and go to emergency heat. Post back and tell me your supply temperature as well as outside temperature.

    IMO

  11. #11
    Tigerdunes,

    The cold air during defrost is new this heating season.

    When it was 32 outside this AM I went to emergency heat, and my registers were blowing air at 98 to 93 degrees depending how far the rooms were from the air handler. The temp in the house at the time was 70.

    A while after the system was changed over to normal operation when it was 36 outside, the registers would blow 89 degree air.

    The size and lock out temp of the resistance heat bars is not known - unless you mean that the aux heat is only allowed to kick in at a 3 degree minimum difference between the set point and the actual temp.

    Thanks in advance,

    Greg.

  12. #12
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    Greg

    Well you do know your unit is going into defrost mode but without AC air being tempered.

    And you do know your heat is operating just not in defrost mode but apparently in normal heating mode and emergency.

    Could be a loose wire.

    I suggest a call to your hvac pro.

    IMO

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