Underlooping and max EWT questions.

Discussion in 'Maintenance and Troubleshooting' started by jbryant, Jun 3, 2011.

  1. jbryant

    jbryant New Member

    (note - updated incorrect model number from 072 to 049)

    EWT questions and system performance help - installed December 2010

    1) We live in Sterling VA (20165)
    2) Suggested cooling need 3T heating need 5T (based from manual-J calcs, which I don't have a copy of).
    3) Heat pump = Carrier GT-PX split system Model 049 (4T with AUX heat to meet the 5T heating requirement). Split system with air handler in attic. I believe model # is 50YDS-049
    4) Horizontal loop from Rehau - design parameters below

    We currently have total of (roughly) 1400 ft of Rehau PEXA geo pipe layed in a single trench with 4 loops; 2 runs at 7ft depth and 2 runs on top of these at 5ft depth - with horizontal separation of 4ft. The design was requested for a 4T system.

    The current problem : when outside air temp is 90-92 (earlier this week) the inside air temp only made it down to 78; with the system running at max.

    Observations: EWT during this time was 105F; and delta from LWT was approx 8F. Flow rate was 16gpm or over, which I assume indicates the low delta since it should be more like 12gpm. Since the EWT goes above the outside air temp, something was not right.

    Now; to explain the EWT being 105 I looked into more detail for the spec for the loops as designed and found the following:
    MAX EWT: 80F Cooling Peal load 25.8KBtu/Hr (2.15 tons)
    MIN EWT: 30F Heating Peak load 48KBtu/Hr (4 tons)

    Here seems to be an obvious problem, somewhere in the communication the Cooling peak load was set as 2.15 tons which is far from what our requirements are (3T as calculated), far from what was asked for (4T) and also far from what the Geo system can pull (4.55 tons).

    With some going back-and-forth with running these numbers; it seems that adding a Ton of cooling requirements to the system ups the max EWT by approx 10F with the same pipe length; which means that when the system runs on max setting (4.55T) the EWT is expected to get up to 100-105F according to the software.

    I currently suspect that since we hit these high EWT numbers; and because the efficiency of the system reduces drastically at these numbers now can't keep up with demand. Is this a valid suspicion?

    Rehau runs these numbers have provided me with a couple new calcs as below (the extra pipe is using the same layout and depths as earlier as a reference point)-

    3T cooling load, max EWT of 80 - need 500ft more pipe
    4T cooling load, max EWT of 80 - need 1100ft more pipe
    4T cooling load, max EWT of 90 - need 500ft more pipe

    My question right now is what should the max EWT of the system be configured for, and should it be based on the load required or the system capacity? We were sold a 4T system (4T heating with AUX and 4T cooling) - so that's what we had been expecting.

    If we assume the 500Ft option then if the system runs at max capacity of 4.55 Ton then the real max EWT could get up to 95 when bringing the house to the right temp; and then efficiency reduces... May this run into the same problem and get stuck on high?

    One other reference points; during winter the loop temps got down to below 30 (the calculated min EWT); but not by much and also the ground was not well compacted and may had cooled from the digging. This indicates that the winter-time cooling is pretty much on spec for the loops, maybe with a couple of degrees variance - but we will have to wait for next winter to confirm. There has been a fair amount of settling (up to 1ft) on the trench since winter-time so next winter the temps should be better.

    In summary; my questions are as follows:
    1) What is the normal setting for max EWT for the pipe calculations and should it normally match the equipment load or the calculated housing load?
    2) Even with today's setup there are problems bringing the house internal temp down; is this likely to be caused by the under-looping; or could the load calculations be off too?

    Then are there any other questions I should be asking our installer company or Rehau you guys would recommend?


    James Bryant.

    The pipe calcs were done with the following parameters:

    Flow rate: 3.0gpm/ton
    Fluid: 23.5% glycol (I think we actually have more like 35% according to installer)
    Fluid specific heat: 0.96BTU/(*f*lbm)
    Density (rho) 64.0 lb/ft^3
    Ground temp 57.0F
    Thermal conductivity: 0.75Btu/(h*ft*F)
    Thermal diffusivity: 0.62 ft^2/day
    _Trench Layout_
    Separation: 7ft
    Depth: 7ft
    Width: 48.0 in
    _Pipe configuration_
    Piping style[X x Y] : 2x2
    Vertical Separation: 24.0in
    Type: 1 in (25mm)
    Flow: Turbulent - RAUPEX
    Pipe resistance - 0.107 h*ft*F/Btu
    _Modelling time period_
    Prediction type : 1.0 years
    Long term soil temp: Cooling 57F, Heating 57F
    _Extra kW_
    (all left zero's)

    Then the cooling section has the following in; from the first (wrong) calcs which only assumed a 2T cooling load

    : Trench length (ft) 353.2
    : Trench number : 1
    : Single trench length: 353.2
    : Total pipe length: 1412.9
    : Unit inlet (F): 78.7
    : Unit outlet (F): 88.4
    : Total unit capacity 59.6
    : Peak load 25.8 (this is the wrong one as it's 2.15 Ton)
    : Peak demand : 1.4kW
    : Heat pump EER/COP: 18.0
    : System EER/COP: 18.0
    : Flow rate: 6.5

    The heating section has the following:

    : Trench length (ft) 353.6
    : Trench number : 1
    : Single trench length: 353.6
    : Total pipe length: 1414.4
    : Unit inlet (F): 30.0
    : Unit outlet (F): 24
    : Total unit capacity 48.0
    : Peak load 48.0
    : Peak demand : 3.7 KW
    : Heat pump EER/COP: 3.8
    : System EER/COP: 3.8
    : Flow rate: 12.0
    Last edited: Jun 3, 2011
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    Is it truly an 072 unit?
  3. jbryant

    jbryant New Member

    Hi, thanks for the reply!

    It should be; at least that's the unit as in the contract is specified as a Carrier "58YDS-049"; nominal tonage 4-ton 2-stage, with GT-PX split unit in basement area and air handler and duct mod's in attic.

    I can double check from the Serial number with Carrier tomorrow.
  4. jbryant

    jbryant New Member

    Ok maybe I mist-typed that number 072 - sorry! I'm not sure where that came from - I think I was just looking at the wrong spec sheet; I've been looking at too many data sheets recently :)

    The model number was 58YDS049 from the quote, and indoor GT-PX Split type and should be 4 tons. I'll confirm this with the actual serial number and let you know. Actually, I can't find info about this specific model number online so I certainly need to double check.
  5. jbryant

    jbryant New Member

    My current suspicion is a typo in that model number from the quote; I believe the system we have is the Carrier 50YDS049 (since I don't see 58x anywhere). This is the 4-ton system I hope? :) From the carrier spec http://www.carriergeo.com/downloads/Section_3_GT-PX.pdf

    I did mis-typed the 072, that was wrong.
  6. Howard Ek

    Howard Ek Member

    If it is hard to dig or you don't want to dig, I would suggest a CoilMaster FLS-104 fluid cooler and heater.
    Bruce Holt [Bruce.Holt@coilmastercorp.com]
    Director Sales and Marketing
    Coilmaster Corporation
    440 Industrial Drive * Moscow, TN 38057
    Ph 901-877-3333 Fax 901-877-3335
    Web: www. coilmastercorp.com
  7. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    To much emphasis can never be brought to bear on the initial design. I was at a job yesterday where the system was designed for cooling load of 2.5 tons at 95/75 temps. I feel that is short but anyways, a climatemaster 038 was installed, with 3-200' 3/4" vertical bores. We survived a full 10 days of temps near 100. The homeowner wanted 68 to 70 in the house. The unit ran non stop and never satisfied the t-stat. That drove up the loop temps, which decreased unit performance, and the homeowner is convinced that it is the loops fault due to his internet surfing.

    The point I am making is that with a initial design that is to tight and does not reflect homeowners wants, indoor 70 or less, the design has failed not the loops or the machine. If the design was 95/75, 10 days of 100/70 were not achievable. The loop temp is currently 80 ish which should deliver 2.5 tons of cooling per the cut sheet no problem. It does not satisfy the demand, i.e. the load is not really 2.5 tons of cooling.

    Hope this helps.
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Your data suggests that the loop is sufficient designed for your heating load, but quickly fails due to a higher cooling load, which is especially concerning since it is just the beginning of the cooling season.
    350ft/ton is usually a design for pond loops, but usually not enough to allow enough heat transfer to ground.
    Your data suggests that your heatload is around 2 tons. For cooling, at those temperatures the loop is running, the heatpump is running very inefficient, and does not have much cooling capacity anymore, which explains the symptoms your are describing.
    Loop capacity must be matched to house load, not to necessarily to equipment size.
    My calcs indicate that you need at least twice the lenght of loop to match your house load for cooling, not just 500 ft more.
    Also your glycol concentration is a concern, and can falsify your flow measurements, since a higher viscosity will give you a higher pressure drop, but in reality your flow is lower.

    In order to fix your problem and have your heatpump run within specs and more efficiently, you need to increase you loopfield by about 100%, and decrease your glycol concentration.

    Your installers load calculation were off for cooling load, and your loop is clearly undersized for the (real) load of your house in cooling. If you fix that your heatpump capacity will increase significantly, and potentially be enough to comfortably cool your house.
    Last edited: Jun 4, 2011

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