Tennessee New Geothermal Not keeping up/High Cost

Discussion in 'Maintenance and Troubleshooting' started by gorilla33, Jul 30, 2020.

  1. gorilla33

    gorilla33 Member

    Hi, new to the board. I've been doing lots of reading on the internet trying to understand all I can, but I wanted to get directly in front of the experts for some feedback. I'll try to be straight to the point and complete all I can from the suggested information to be provided.

    New Home-3200sqft with an unfinished basement. Layout is 2500sqft on main level(kitchen, laundry, living,master suite and 2nd bed and bath, 700sqft above garage(2bedrooms,bath,playroom).

    Unit-Bosch SM060(5Ton Geothermal with 2 stage compressor). 3 zones. 1 for master suite, 1 for main area, 1 for upstairs.

    Original Symptom-Early in June I noted that my upstairs would not maintain setpoint. Reaching 77/78 with setpoint at 72ish. During this time it wasn't that hot outside. Mid 80's most days, maybe 90. I called the installer and they came out and found that my construction filters at the unit had never been removed. Therefore my DAT was in the low to mid 60s. They removed the filters, cleaned the system and went on their way. DAT dropped to low 50s. Later that afternoon, upstairs returned to the hot temps, so I called back. They came back out the next morning and found that my unit was not kicking on the 2nd stage. They reprogrammed the zone board and boom, it came on. Temps maintained setpoint for a week or so during fairly mild temps outside. Highs in the 80s.

    It was also noted during this service that my air upstairs is loud when on. Not a whistle, just the loud roar of air. The tech mentioned that if problems persisted they would install a bypass damper, and if that wasn't enough, they would increase the main trunk line in the basement that supplies the upstairs. All of this has now been completed and the unit is coming closer to maintaining setpoint upstairs. With outside temps in the mid 80s, the unit runs from noon to well after midnight to maintain 73. If the temps get a little hotter, it falls behind and will still make it up to 75. Also, my RH is measuring really high>80% upstairs.

    My electrical usage is really high. This is whole home, not just HVAC, but June was 4100kWh, and July was 4500kWh.

    1) Where you live-Knoxville, TN
    2) Heat loss/gain calculations for your home-Cooling Load is 54880BTU/hr
    3) Brand, size (model) and type of heat pump-Bosch SM060
    4) Type of loop field (open/closed/vertical/horizontal) size and design parameters-Horizontal, 4 pipe, 3/4” Polyethelyne SDR11, 795’
    5) Average cost/Kwh of electricity and consumption-4000kwh each for June and July
    6) Entering and leaving air temperatures (EAT, LAT) measured immediately upstream and downstream of the heat pumpunsure, DAT reads 51-58 on avg
    7) Entering and leaving water temperatures (EWT, LWT) measured at the heat pump-Measured 105 LWT and 96LWT in afternoon
    8) Percent of load to be covered by geo and balance point-Not sure what this means
    9) Installers assessment of your systems operation.
    10) Projected operating costs, actual operating cost and previous heating and cooling costs
  2. SShaw

    SShaw Active Member Forum Leader

    I saw your thread on HVAC-talk. Good move coming over here. There's not as much activity on here, but there are a couple experts on geo.

    Your best bet is to get someone there to make some measurements, but you could do most of the measurements yourself, and folks on here might be able to figure out what's wrong.

    5T should be plenty to cool new construction. It's unlikely your heat pump is undersized.

    Your high EWT is high and seems to indicate some problem with the loop. There's insufficient heat exchange between the water and the ground. It could be something simple, like one or more circuits not active and the loop needing to be purged. Or the loop could be undersized.

    Can you elaborate on the pipe configuration for the ground loop? Is that 795' total pipe, a 795' trench with four loops in it, 4 loops each 795', etc? In other words, how many total feet of pipe is there in the ground?
  3. gorilla33

    gorilla33 Member

    I cant confirm what is in the loop off the top of my head. I know they told me that it would accommodate a 6 ton unit if I ever needed additional capacity.
  4. gorilla33

    gorilla33 Member

    Based on photos I have of the trenches being dug, where my grass currently grows differently, measuring tools and aerial photography I think I have 3 trenches, each with a down and back. Each loop measures approximately 750 ft (375 ft long trench).
  5. SShaw

    SShaw Active Member Forum Leader

    Your original note says 4-pipe, which would imply two loops per trench. If you have three trenches, each with two loops, that's 3 x 2 x 750=4,500' of pipe in the ground.

    I plugged some numbers into LoopLink for your location, cooling load, and GSHP. If I entered things right, 4,500' should be oversized for 5T. According to LoopLink, your EWT should never go above about 85. Even if you only had one loop in each trench, you should still be OK, with a max EWT of about 92. The target is usually 90.
  6. gorilla33

    gorilla33 Member

    That makes sense because the quote I was given said 4-pipe. I didnt know what that meant. Your measurements support what i was told about the loop being big enough to accommodate a larger unit.

    That all said, what is the impact to my units performance? Is that why i'm struggling to maintain setpoint, thus using excessive kWh's?
  7. SShaw

    SShaw Active Member Forum Leader

    Your EWT is not consistent with an oversized loop, so that's fishy.
    Your electric usage sounds very high (but I don't think you have figures for what the unit alone is using.) For reference, my 4T unit used 252 kWh in June, and 420 kWh in July, which has seen a record heat wave.
    Your humidity is very high. It should be between 45% and 50%.
    Your system's capacity to cool your house might be slightly undersized for the latent load if your cooling load is really 55K, but it shouldn't be struggling at 80-deg outside.

    See what @docjenser says. He's one of the experts.
  8. gorilla33

    gorilla33 Member

    someone on the other site mentioned aux heat spuriously kicking on...could that explain humidity and electric use?
  9. SShaw

    SShaw Active Member Forum Leader

    If something was mis-wired, causing the AUX to kick on, that would cause high electric, and would work against the AC, raising the cooling load.

    There could be a problem with the zoning. When mine was first installed, the dampers were wired wrong, and were not opening and closing as they should. Something like that could prevent the house from cooling and raise the cost.

    A systematic check of basic operation would be useful--verify damper operation, measure water delta-temp and flow rate, measure air delta-temp, measure wattage used by fan and compressor, calculate heat of rejection, cooling capacity, and efficiency.
  10. gorilla33

    gorilla33 Member

  11. SShaw

    SShaw Active Member Forum Leader

    It varies.

    I'm sorry no pro has weighed in to help you yet.

    You might have multiple issues going on. The high EWT is a concern, but doesn't explain the high power usage or inability to cool. Your unit should operate with EWT up to 110-deg. Higher EWT has a small impact on capacity and power consumption, but it's not huge, so the unit should still be able to cool your house at 96-deg EWT.

    Your issues could be mostly airflow/duct related. Looks like the Bosch units operate at 80%/100% capacity in low/high stage. That's not a great fit for three zones. If only one zone is calling, the unit will push 4 tons of cooling and airflow into that one zone. That would make the unit oversized and it will come on for short cycles. The fan will use more power if your ducts cannot hand the airflow. This will decrease efficiency and lead to higher humidity. Adding a bypass damper isn't a great solution, as it just dumps the cold air back into the return ductwork. You might be able to set the unit to a lower CFM, if it's not already on low. Or you could disable the zoning and keep all ducts open.

    Your loop pumps could be consuming significant power. How many pumps do you have and what model are they?
  12. gorilla33

    gorilla33 Member

    I have 2 geo-flo UPS26-99F. Also. The unit definitely doesn’t short cycle. The upstairs starts running around noon and doesn’t turn off until after midnight. The main living area is basically the same, although it typically maintains setpoint. The third zone, which is smaller and has less heat load,cycles on and off all day what I would consider normal cycles.

    I wondering if I need a larger unit. Does the extended run time gradually cause my EWT to increase?
  13. SShaw

    SShaw Active Member Forum Leader

    The 26-99 is a decent pump. Each pump could be drawing close to 200W when on though.

    Loops are normally sized to have a maximum EWT of 90 degrees at the end of the summer. That’s driven by the house cooling load and the local climate. Extended runtimes are desirable and won’t hurt a properly sized and functioning loop.

    Your unit has a 65,000 BTUH cooling capacity. Unless your load calc is all wrong and your house has zero insulation a larger unit is the last thing to be thinking about.

    As mentioned above, measure the fundamentals of air and water flow and delta-temps, see if things are operating to spec, then go from there. It’s all simple stuff for a tech to measure. Sounds like your system has sensors to report some of the data already.
  14. gorilla33

    gorilla33 Member

    Thanks for the input. I feel like my HVAC contractor wants to get everything correct, they just seem perplexed. The unit has been checked out and told that everything was working within spec. He did note the EWT being a little higher than desired, but that it shouldn’t be a deal breaker.
    I have to figure out the electrical usage. For the most part, the unit seems to be keeping up now that the upstairs duct was increased.
    I also ordered a remote thermostat to install in an upstairs bedroom instead of right at the top of the stairs. Hoping the temp is just hotter right at the top of the stairs and that may have been much of the problem.
    I also ordered the Sense Whole home Energy monitor that should be delivered tomorrow. I’m hoping that will help we figure some things out.
  15. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    "105 LWT and 96LWT in afternoon"
    I assume that you mean 105 LWT and 96 EWT. Still pretty high.

    It also seems that your ductwork might not be well balanced and not large enough to direct enough cool air upstairs.

    No bypass damper on a geo heat pump! Who suggests that does not understand how heat pumps work. Plus what are they going to accomplish? They do not get more cooling BTUs upstairs that way. And they will increase the refrigerant pressure, since you are only cooling down air which is already cooled down.

    Apart from any control/damper issues or refrigerant issues, it seems like a combination of things:

    1) High pumping power, reducing efficiency (450 watts constant)
    2) You likely use a lot of fan power, reducing your efficiency, due to ductwork not large enough.
    3) Loop field too hot, reducing efficiency.

    Now one thing leads to the other.

    Higher pumping power heats up that loop field more, since all the pumping power ends up in form of heat in the loop. High fan power will end up as heat in the conditioned space, now you need to cool it more, meaning you have longer run time fro compressor, pumps and fans, meaning you heat up the loop more. Now the cooling capacity decreases with increased loop pump, meaning now the compressor, pumps and fan runs longer. It is almost a vicious cycle....
  16. gorilla33

    gorilla33 Member

    Thanks Doc. Now that the 16” supply duct is feeding upstairs, it maintains 73 all day. Maybe now I need to focus on these efficiency issues. Any suggestions?
  17. gorilla33

    gorilla33 Member


    I have attached a file showing date,daily usage, high temps, CDD. Please note that the usage is total, not just HVAC. Does this seem out of line?

    Attached Files:

  18. SShaw

    SShaw Active Member Forum Leader

    It's very easy to fit a regression line to your usage data on degree days dot net.

    Your data indicate 69 deg is the best base temperature. Using that, your base electric usage is around 100 kWh per day with an added cooling component of about 5.2 x CDD69.

    The best fit to your data is:

    KWH = 100.96 + 5.2 x CDD69

    So, 3000 kWh is your base usage per month. This implies most of your electric is not coming from the cooling load. Maybe one third.

    Attached Files:

  19. gorilla33

    gorilla33 Member

    I'm gonna need some help digesting what you just said. I have no idea what you mean by 69 being the best base temp. I think you are saying that 3000kWh is non cooling usage, which is shocking. Can you elaborate at all on this?
  20. SShaw

    SShaw Active Member Forum Leader

    The base temp is the temperature above which the building requires cooling. You posted an excel file that included cooling degree days with a 65-degree base temp. Your house data fits slightly better to 69 degree base temp. It depends on the house and the thermostat set point.

    You can go to degreedays . net and read about degree days and regression there. If you are not a scientist or engineer, it might be difficult to understand regression and the underlying math though.

    Let's just say that the daily kWh usage data you posted for your house are best predicted using the equation "KWH/day = 100.96 + 5.2 x CDD69"

    To estimate your daily usage, get the cooling degrees for that day using a 69-deg base temp, and plug it into the above equation. For days when no cooling is required, the cooling degrees are zero, so your usage works out to be 100.96. That's your usage with no cooling load. That implies you would use 3000 kWh every month with no cooling.

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