Enertech Geo-therm w/ HBX CPU-0500 not entering stage2

Discussion in 'Maintenance and Troubleshooting' started by TimF, Jan 23, 2013.

  1. TimF

    TimF Member

    There is an outdoor thermostat connected to Pin8 and Pin9 of the CPU-0500, however, the Design Outdoor Temperature = OFF. The comments associated with this setting say "Outdoor reset is not applicable if the control is set to OFF", hence, it looks like I'm not taking advantage of the Outdoor Reset functinality. I did see the other day where the HBX folks mentioned this setting in the second Georthermal Applications example (very bottom) found here: http://www.hbxcontrols.com/Resources/marketing/cheat%20sheet.pdf
    In the referenced example they show the Design Outdoor Temperature = 10.0: with commments "Set this temperature at which you would like to hit your maximum temperature from the control." If I interpret this correctly, this means that I would expect the HP to hit my Design Room Temperature = 70F when the outdoor temperature reaches = 10F.

    Warning! Here come the questions:

    1) Do I understand this correctly? If so and assuming my avg outdoor winter temp is 25F, would I want my DOT = 25F or would the HBX example of DOT = 10F still apply?
    2) The other variable here is that most of the time, we keep our indoor temp at 65F (we raise it to 70-75F if we are expecting company), therefore, should my DRT be set tp 65F instead of the default 70F?
    3) Does the CPU-0500 need to have a thermostat connect to the CPU-0500 HD input pins for the outdoor reset functionality to work (currently the HD pin1 and CD pin3 are both jumpered to pin2)?

    Yes, when we entered our first "shoulder period" last spring, we were away from town. We came home to a frigid house and realized the HP was dumping the hot water during the day, filling up with chilled and doing the reverse at night. To put a stop to that, I went into the CPU-0500 and set the Cold Weather Shutdown = 150F. At that time, the HP thermostat had been installed under the outside basement bilco door and the first floor thermostat was in direct afternoon sunlight . Both temperature sensors were subsequently relocated to an area under the porch which alleviated most of the problem. As you noted, it is still possible for us to hit outdoor temperature conditions which fall within the normal CPU-0500 settings, I particularly like the switch idea. Actually a TPST (HEAT - OFF - COOL) switch sounds ideal...then I can also effectively control the HP at the HP versus at the breaker in the garage.

    I look forward to your suggestions regarding the outdoor reset and room temperature settings so that I can optimize those.

    Thanks again,
    Last edited: Jan 29, 2013
  2. TimF

    TimF Member

    I did a little research and found an example on the web of outdoor reset using a conventional boiler. From what I can tell, the same principle applies but the difference seems to be that unlike the boiler example, with the CPU-0500, I wouldn't specify an lower limit to the buffer tank temp. Since the Design Outdoor Temp = OFF, my system currently has a constant System Temperature1 = 110F. Since it seems the outdoor reset functionality will dynamically change the System Temperature1 based on the outdoor temperature, in order to get the equivalent 110F at my assumed average winter temp of 25F, I need to make the following setting adjustments:
    1) System Temperature1: From 110F to 135F

    2) Design Room Temperature: Stays at 70F for now so that I'm only changing two of the three variables.

    3) Design Outdoor Temperature: From OFF to -10F.

    When I do this with an outdoor temp of 44F, the CPU-0500 reports that my new buffer tank target is 90F...about 5 degrees below where my chart projects the temp should be...close enough for now I think.

    My only concern is that compared to 110F, the System Temperature1 value may be too high.

    Your thoughts?


    Attached Files:

    Last edited: Jan 30, 2013
  3. zacmobile

    zacmobile Guest

    Tim: It sounds like you're on the right track, allow me to ramble a bit to help clarify & hopefully not confuse.

    I just want to define/clarify some terms first. Indoor Design Temperature: your desired indoor temperature eg. 70F. Outdoor Design Temperature: The coldest anticipated winter temperature. Boiler Design Temperature: The hottest your boiler (or heat pump) needs to run at the coldest day of the year to achieve the Indoor Design Temperature.

    Your IDT is a static setting, as that is your desired indoor temperature you never want to change. The control will vary the other two: ODT & B(HP)DT to always try & satisfy the IDT provided your settings are accurate.

    Try to find some meteorological data on your area to determine what the 4 year coldest record has been and set your ODT a degree or so above that. The B(HP)DT you will probably have to experiment with until you get it dialed in just right. I recommend starting at the high end (Say 125F) and over the course of a few days turn it down until it's too cold in the house then turn it up to the last setting that was warm enough.

    The Warm & Cold weather shutdown points are also important to have set right. Set the WWSD at a couple degrees below what your IDT is set at (or a temperature that you do not want the heating to come on, this could be quite low depending on how stingy you are) Set the CWSD at a couple degrees above the IDT (or the inverse of your stingy WWSD setting)

    The CPU-0500 needs no thermostat input to operate, the only function of the thermostats is to take/reject heat via the air handlers from the buffer tank as it is provided.

    135F sounds a little high (I really doubt your structure would really need that temperature on a design outdoor day) but it may be OK as long as the return temperatures going back to the heat pump are below that.
  4. TimF

    TimF Member

    Thanks much for the reply and confirmation I'm close to understanding the outdoor reset strategy. I'm assuming the lower the BDT is the less the HP has to work and the less heat loss while there is no demand...saves energy and money while maintaining a relatively stable IDT. Since the original installer had the BDT fixed at 110F, I'm assuming he felt that was a good average temp for my region. Since we're in the middle of a relative heat wave now (55F), I think I'll go with the settings I mentioned in my last two posts to see what the impacts are of having the BDT reduced to the 85-93F range due to the outdoor reset being active. Rarely does the temp drop and stay lower than 15F, so I'm not quite so concerned about the higher BDT at this point as I am to the effects of the BDT being below 110F. I will keep your suggestions in the back of my mind though throughout the remaining heating season as saving money is always a good thing (but only if my wife isn't complaining she's cold!).

    I wish all of this staging and outdoor reset stuff had been explained up front or that it was more easy to ferret out. I'm sure others will find this thread as useful as I!

    Thanks again to you, Palace and Mark for your patience and assistance!

  5. zacmobile

    zacmobile Guest

    Another way to look at heat pump work is the amount of energy it takes to "lift" the heat from the original ground state to it's final form that is being distributed through your home. The closer you can keep these numbers together the less energy your heat pump will use (COP). Here is a spec sheet from a similar model to yours showing the COP at different setpoint temperatures.
    geocomfort cop2.GIF
  6. TimF

    TimF Member

    Wow...a picture is worth a thousand words! OK..I'm a believer...just after your last post here I changed my System Temperature 1 from 135F to 125F. I've been tracking the changes to the BDT as a result of the outdoor reset readings for the past day as outdoor temps went from 59F to 17F overnight. The BDT has been tracking just 2-3 degrees below what the chart says so I think I'm good there. We haven't noticed any change to comfort so I'm really happy at this point. I'm anxious to see what energy savings may result albeit I'm not sure I will get an apples for apples comparison in the short term unless we get another real cold spell.

    Again, many thanks...I feel like I'm in more control of my geo environment now! :D


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