Results 1 to 8 of 8
  1. #1
    2 ton Carrier 38YXA heat pump.
    Balance point set at 40*F

    Defrost thermostat closes contacts 30*F plus or minus 3*F
    Defrost thermostat opens contacts at 80*F plus or minus 5*F

    Questions:
    1.- Given the high balance point is there any need for defrost operation at all?
    2.- If there will be defrost, it will be minimal and 80*F appears to be too high for defrost thermostat reset. Are there thermostats with lower contact-opening temperatures in order to open the contacts of the thermostat before the timer calls for a defrost cycle start?

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,179
    1. You can have frosting at 40 out if very humid

    2. Carrier must think 80 on the open is appropriate


  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,097
    Yes, you need the defrost, 80 with duel fuel maybe not, but I don't think Carrier just picked 80 randomly.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Mar 2004
    Posts
    1,042
    I have a 38YXA 2 ton myself in a dual fuel configuration. My balance point is more like 30, though. It's set for a 90 minute defrost interval.

    So far under the worst-case sceneraio, the 90 minute setting has done fine and I'm starting to wonder about switching to 120. Worst case has been temps hovering around freezing with freezing rain. Unit ran continuously for a full 90 minutes. It had a full winter coat of frost going by the end, but discharge temperatures were still the usual 95-100 degrees when the defrost cycle began. When there is that much frost, though, the defrost cycle is by no means too long. It barely gets everything melted.

    Bottom line we are both losing some efficiency by having to defrost every 90 minutes, when there's really minimal or no frost most of the time. A demand defrost control would work better, but we're stuck with time-temp control. Depending on your application and current defrost time setting, you may be able to get away with bumping the defrost timer to a longer setting. Short of replacing the defrost control completely with a demand-driven system, I wouldn't mess with the temp settings of the defrost thermostat.

  5. #5
    Thanks for the reply.

    The contractor set the defrost time at 30 minutes and I jacked it up to the factory 90 minutes setting. Now I'm going to raise it to 120 minutes. Too bad there isn't a higher setting than that.

    I looked at other brands of heat pumps and one resets the defrost thermostat at 65*F and another site mentions a range of resets, all below the 80*F used by Carrier and Bryant.

  6. #6
    Join Date
    Mar 2004
    Posts
    1,042
    Keep an eye on it when you're close to your balance point in humid or rainy conditions to make sure that it's not getting so frosty that it restricts the airflow. You might also monitor your discharge air temperature to make sure it stays in the normal range when the frost does start to build up. I have a cheap digital "indoor-outdoor" thermometer with a wired probe; I put the probe in one of the registers to measure the temperature and have just left it there since. Then whenever I want to know the discharge temp I can just look at the display.

    It seems like this particular unit can handle a lot of frost, though, because it has a huge single-row coil instead of something more moderately sized in two rows.

    This is a dual fuel system we're talking about, isn't it?

  7. #7
    Originally posted by wyounger
    Keep an eye on it when you're close to your balance point in humid or rainy conditions to make sure that it's not getting so frosty that it restricts the airflow. You might also monitor your discharge air temperature to make sure it stays in the normal range when the frost does start to build up. I have a cheap digital "indoor-outdoor" thermometer with a wired probe; I put the probe in one of the registers to measure the temperature and have just left it there since. Then whenever I want to know the discharge temp I can just look at the display.

    It seems like this particular unit can handle a lot of frost, though, because it has a huge single-row coil instead of something more moderately sized in two rows.

    This is a dual fuel system we're talking about, isn't it?
    Right, dual fuel system. Backup is a natural gas furnace.
    I watched the heat pump operation last winter and all I ever saw was merely the early beginning of frost formation.

    Thanks a lot for the additional info.

  8. #8
    Join Date
    Jun 2003
    Location
    Florida
    Posts
    9,548
    [QUOTE]Originally posted by deme
    [B]Thanks for the reply.

    The contractor set the defrost time at 30 minutes and I jacked it up to the factory 90 minutes setting. Now I'm going to raise it to 120 minutes. Too bad there isn't a higher setting than that.

    A couple of questions....
    1) Why are you changing these settings without discussing this with your contractor?

    2) Is this unit under warranty?


    The contractor set this up for a reason....maybe he/she knows something you don't. Why don't you ask instead of prying fingers?
    If everything was always done "by the book"....the book would never change.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event