Increasing Existing Vertical Loop Length by Adding a Pond Loop?

Discussion in 'Geothermal Loops' started by wjgreen, May 31, 2010.

  1. wjgreen

    wjgreen New Member

    I am going to replace my 15 year old closed-loop geothermal heat-pumps (1.5 ton upstairs & 3.5 ton downstairs) with new larger units (2 ton & 5 ton). I am told by my contractor that the existing separate vertical ground loops might need to be lengthened to accommodate the larger units.

    In hindsight, the originally installed loops probably should have been placed into the 4 acre pond which is located 60 feet behind my house. The water is 16 feet deep and I presume the original contractor had never heard of pond-loops.

    QUERY: If the existing loops (or either of them) prove insufficient to handle the larger unit(s), is there any reason I cannot or should not add the additional needed length from a pond loop instead of drilling additional vertical loops? It seems like a no-brainer to me. But then again, that is how I always start getting into trouble.


    Bill Green
    Pensacola, FL
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Are you quite certain you need the extra tonnage?

    Have Manuals J and D design calculations been done? What do they call for?

    How do present systems stack up against design calculations, if any? Are the units making expected tonnage and is each room getting close to required airflow? I'd want those answers as well as present ESP across air handlers before I invested one dime toward increasing system size.

    Many, if not most systems (heat pumps in general, not geo) are oversized but connected to insufficient ducts, a miserable combination causing some or all of discomfort, excess noise and severely reduced efficiency.

    All that said, let's suppose for my next remarks that you do now in fact need all that extra tonnage. Could you instead make envelope improvements to bring the load down? Add insulation, reduce infiltration, reduce solar heat gain. You might find such improvements are cheaper than upsizing the units and enlarging ductwork to flow the air the larger units will need. One of my favorites is to sprayfoam underneath the attic sheathing and seal up the attic. That drastically cuts infiltration, civilizes attic conditions, and brings ductwork there into indirectly conditioned space, greatly reducing duct thermal losses as well as reducing the significance of duct leaks.

    Finally to the pond question. Ponds can work well, but I'd be concerned that in the deep south a pond could become so warm in late summer that it drastically cuts system efficiency and capacity. If that pond is much above 85 in late summer it may not serve well as a heat exchanger. That it is 4 acres and 16' deep suggests that it might be usable
  3. wjgreen

    wjgreen New Member

    Is it possible to add a pond loop to an existing ground loop?

    Good Evening Kurt:

    Thanks for your insightful reply. Let me respond in order to your expressed concerns:

    ● The local power company (Gulf Power) engineers did a written load evaluation which was reviewed by the three geo dealers who have provided competitive bids. The cooling area has been significantly increased since I built the home 15 years ago. All agree the 1.5 ton unit upstairs could possibly be sufficient but I have not ever been completely satisfied with the cooling. The downstairs unit must be increased due to the extra square footage subsequently added to the living area.

    ● Each HVAC contractor agrees the return air for each unit was originally undersized. Each bid includes the cost of adding additional return capacity for the units. (Sorry, I don't know what ESP means.)

    ● With regard to additional insulation; interestingly the contractor who built my house won national awards 15 years ago for superior insulation design and construction. It was his "schtick" and the reason I chose him to build. For example, all exterior walls are 2" x 6" (not 2' x 4") insulated with bats within the walls. There is additional insulation outside the OSB and the entire exterior was then wrapped to further seal the envelope (all under brick veneer). Attic insulation batting is (to me) similarly impressive. However, the attic ceiling was not foamed as you have suggested nor is the attic sealed. Even interior room penetrations (electric wall plugs and the like) are all foam filled to restrict the passage of room air inside the walls into the attic. Windows and doors are similarly highly insulated.

    ● My effort to check the water temperature in the pond was cut short recently when I broke the thermometer while I was preparing it for the task. I will get another thermometer and try again. Although the hottest part of the summer is not yet upon me, I can tell you from swimming in the pond in past summers, it is pretty cold at 16 feet.

    My present HVAC contractor suggested that the new units be installed with the larger returns but without initially modifying the loops. He simply cautioned about the possibility that it might have to be done. He will apparently run tests and compute efficiencies after installation before deciding whether or not additional loops would be necessary or beneficial.

    My Real Question is: IF additional loops become necessary, COULD a pond loop be used in conjunction with my existing ground loops. It is my understanding that I would not have to completely abandon my existing vertical loops to add additional VERTICAL loops but that I could simply tie (add) onto them. If I am understanding that correctly, why can I not similarly tie onto the existing vertical system and "throw" the loop into the pond instead of burying it vertically in the ground?

    Again, thanks for your obviously knowledgeable prior reply. I look forward with a sense of confidence to your continued input.


    Bill Green...
  4. wjgreen

    wjgreen New Member

    Adding Pond Loops to Existing Ground Loops

    Good Evening focusonz:

    Thank you for the information. I too am a numbers guy and your numbers seem to say I'm barking up an empty tree.

    My response to the previous post by engineer suggests that additional insulation is probably not the answer either. I am now more interested than ever in completing the effort to determine the actual temperature of the pond water at depth. But again, those folks who make charts and maps for a living seem to be telling me that the water is not going to be as cold as I had imagined.

    As I previously posted, it looks like the new units will go in with the old loops in place, an increase in return air capacity and fingers crossed that the loops will handle the new load. I have a feeling though that at some point there is going to be some digging going on in my back yard.

    Thanks again for the really great information.


    Bill Green...
  5. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Simply - yes

    Yes you can add the pond loop to the existing vertical. The best fit might just be to take the previous vertical loops and pair them with your new 5 ton and have your separate 2-ton on its own pond loop. You could pair all the loops together with a bit of effort in balancing flows.

    As a side note - ponds don't warm up so much under geo cooling as they increase their evapotranspiration rate. Somewhat a self-regulating mechanism. Unless they dry up - then that is a different story.
    Littleriverslim likes this.
  6. wjgreen

    wjgreen New Member

    Simply, Thanks!

    Good Afternoon Uthbuoy:

    Thanks for your "simply" answer. It confirms my guess regarding "can I?". I can now concentrate on "should I?". I particularly like your suggestion that I join my existing loops (if necessary) to support the 5-ton unit and consider a pond-loop for the smaller one.

    I am concerned about Focusonz's calculations which suggest the pond may not be cold enough during the hottest part of the summer to act as an effective heat-sink. I am also going to check into "foaming" my attic ceiling as suggested by Engineer.

    All-in-all I am much further along in making a rational decision than I could have ever been without the kind assistance to my inquiry. Thanks again to one and all.

    Bill Green. . .
  7. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Fix the return air situation and then...

    ...and only then contemplate increasing tonnage.

    Insufficient return air (or supply ductwork) is quite common. If airflow through the unit is compromised by crappy ductwork then it'll never make nameplate tonnage or operate anywhere near advertised efficiency.

    ESP stands for External Static Pressure and refers to air pressure drop across the unit. Typical blowers won't make rated CFM at ESPs greater than 0.50 inches water column but often operate at 0.75 or higher in a typical installation. I've measured as high as 1.4" in new construction where a 6 ton unit was installed in place of a 4 ton unit in response to complaints of inadequate cooling. Result was continued inadequate cooling, horrific noise, and air velocities in the master bedroom such that the unit just about blew the grills off the walls...all in a $million beachfront house.

    It is almost impossible to successfully condition a substantial addition to a home without substantial rework of the ductwork. The temptation to graft a few registers onto the ends of the present system might have been overpowering given the relative difficulty of doing it right. Things might seem fine until extreme weather occurred, at which time the addition contractor's payment check has long since cleared and he has sailed over the horizon.

    Given the known issue of inadequate return air I would demand, at minimum, an ESP measurement before I considered upsizing the units. If the ductwork is undersized now it'll be a complete disaster if the units are upsized without modification. In addition this is all the more reason to demand present room-by-room airflow measurements.

    The instruments needed to measure this to a reasonable level of approximation cost well below $1000 and are operable by any tech able to consistently see lightning, hear thunder, and fog a mirror. The software needed to run house, system, zone, and room load and airflow calculations is similarly cheap.

    If your present contractor is at all hesitant or mystified by these concepts, find someone else. No one should contemplate spending 5-digits of your money without a minor investment in the gear and skills to back it up

    2x6 walls with batts vs 2x4 is a good upgrade, but it takes a whole house blower door test to be sure the construction details really work as designed.

    BTW, I use RHVAC....
    Last edited: Jun 1, 2010
  8. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    closed loop capacity test

    It is called a thermal conductivity test. You hook up a known load to a known loop size or configuration and run continuously for a minimum of 40 hours using a bigggg generator. During that 40 hours the the data is logged and stored and sent away to be annylised by some serious math heads. They send you back capacity data, rated in btu per hour. Average cost $6,000.00 and up per test.
  9. Bergy

    Bergy Member Industry Professional Forum Leader

    6K...Is that all? Every residential job should be required to perform the test just to prove the loop field delivers as promised!! ;)

  10. Bergy

    Bergy Member Industry Professional Forum Leader

    Before you increase the tonnage check the heating, and cooling, outputs of the new units. Todays heat pumps put out more Btu's than the 15 year old machines. We are in the process of replacing a 10 year old 6 ton synergy with a 5 ton GeoComfort Combo unit AND we are gaining 1,800 Btu's/hr.

  11. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Among my favorite mantras......

    It is never too late to spend more money.

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