Heat Exchange in Vertical Loop - EWTs

Discussion in 'Vertical and Horizontal Loops' started by DavidCraig, Jun 6, 2014.

  1. DavidCraig

    DavidCraig Member

    When heating, how much temperature drop do you usually see in a few hours?

    For example, say ground water is 50°F and it takes 15 minutes for water to cycle through the loop once.
    For the first 15 minutes, source EWT would be 50°.
    The next 15 minutes, it would be less that 50°.
    The next 15 minutes, it would be less that that. Etc...
    The longer the heatpump runs, the colder it will get until the field reaches an equilibrium.

    The bigger/better the loop design, the more efficient the heat transfer -- 'efficient' meaning as high an EWT as reasonable.

    With my system, the first hour it drops 4°F. The next hour another 2°. Then progressively less as time goes by. When running continuously for a month, its down ~12°F. The original computer design shows it dropping as much as 20° from original temp near the end of winter.

    I've heard technicians say, it should never be as low as 40°. If that is the case, then the field needs to be expanded. Yet less than 40 seems to be common from what many report on forum.

    Any thoughts or comments?
  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    There are many BTUHs in 40*F loop fluid.

  3. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The entire purpose of the loop is to bring the EWT within the operating range of the heatpump. Yes, higher EWT is better for heating, but there is a point of little return. Meaning that you can increase your peak EWT by significantly increasing your loop field, but it is might cost you thousands, and gives you maybe $50 lesser operating costs annually. The rule of thumb is that you try to have a good compromise by designing your loops down to a minimum of 30F and to a maximum of 90F. But is it not detrimental to see you loop drop to 25F and go up to 100F during some peaks.

    You can see some of our loops monitored over the season at

    Saying that a loop should never be below 40F does not understand efficient ground loop design. In heat dominated climate, a loop field staying above 40F indicates that the designer made it twice as large as it would need to be, with only minimal operational cost savings.
    BMGEO likes this.
  4. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    "I've heard technicians say, it should never be as low as 40°."

    Yes and I heard a man discussing a very high profile installation saying we designed to never stray more than a degree or 2 of the earth temp at our depth. I thought to myself "what a collosal and ignorant waste or money". A fairly reknowned engineering firm was involved as well.

    I often relate the story of a man having trouble with his well that wished to go to closed loop on a 20 year old CM that could not have EWT below (I think it was) 42F. That meant the loop field itself would have cost more than a new 5 ton with a 30-90F field (as Doc suggests) before the tax credits were deducted from the new heat pump. After tax credits the new system was 10K cheaper and had a lower operating cost than the old 1 stage unit. The man fixed his well.

    Temp drop in a few hours could be many degrees.
  5. DavidCraig

    DavidCraig Member

    It seems the increasing performance (min EWT) would be expoential. So you would need to double field size to cut drop in half. And double again to drop another half. (all else being equal)

    So $15,000 loop becomes $30,000 loop becomes $60,000 loop. Saving $100 / year between each interation two. At some point one has to stop

    Just in reading peoples problems throughout forums, often it would be better to spend the addtional money on the heat delivery (more ductwork, airflow, floor loops with aluminum plates, etc.)

    I get a little erked about "Oh, your loop shouldn't go below XXX". It is both unrealistic and often in conversation of blame shifting. Last year my heatpump was working only at 70% HE that it was supposed to be - due to bad TXV. So next year will tell. But with the long winter it was about 40 EWT. With properly working heatpump, then I'd expect lower EWT.

    But at some point the lower temp should speed heat transfer rate as with any heat exchanger. Just for Ha, Ha's. If drawing 16,000 btuh 24/7 dropped temp 12F, I'd expect 24,000 btuh 24/7 to actually be less that 18F drop (both a 50% change). All else being equal, although most of us hope next winter isn't as hard has last.

    Some systems might need to be designed such that LWT do not reach freezing or less. In areas that don't allow antifreeze, etc. That could be a concern with standing column wells (another thread)
  6. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    A running geo heat pump system can not be nailed down. The heat transfer is a moving target.

    So it is all guess work or a detailed design that takes all the variables into the system and hope for the best.

    You might find good refrigeration training at the local vo-ed school or Community College.

    I may soon go there to teach, if my knee keeps up.

  7. DavidCraig

    DavidCraig Member

    That is where the observations of successful installers is of value (not just simple testimonies) as well as courses. Not knocking engineering.

    Seek to learn more from my first installation ... watching EWT changes. Was somewhat surprised to see the EWT go up rapidly during off-time - which didn't match what had been told about 30-day latency, etc.

    Actually, keep getting spurred on because of finding inaccuracies (some gross) in what was told, even at the start. Had no real thought about installing such a system, of which I had no knowledge. But the proposal had serious flaws that I could see without training. So was both alarmed and intrigued.

    So ... read, ask, learn ... read, ask, learn ...

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