Desuperheater seems to produce little hot water unless running constantly

Discussion in 'General Discussions' started by gnick, Aug 21, 2014.

  1. gnick

    gnick New Member

    Recent threads about DSH operations have prompted me to look more closely at my graphs, and I've found an interesting recurring issue. The four day graph (attached) shows a consistent daily pattern: the DSH produces a steady supply of increasingly hot water to the buffer tank during late evening (9pm-2am) when the 4T heat pump is running constantly to cool the MBR (note 9pm arrows marked on graph). During these periods the buffer tank reaches 105 to 120F -- not as high as I expected from the sales pitch.

    However, the DSH does not appear to be producing any additional hot water during normal HP cycling. Note the red boxes that show the DSH supply water temperature (black line) does not affect the warmer buffer tank temperature (green line) for typically five hours of heat pump cycling. I assume this is because the DSH water temperature is lower than the buffer tank, in which case this is normal and proper operation. This will be of interest to those expecting to get free hot water whenever the HP is running. Not so!

    Note the black boxes that show buffer tank temperature (green line) holding or in some cases FALLING while the HP is cycling during nighttime hours (when there is no hot water being drawn). This indicates to me that the DSH is pulling hot water out of the buffer tank. I would consider this to be a problem. Is there another explanation?

    The second attached graph shows typical HP cycle times. Note: only the 4T (blue line) has a DSH option.

    Attached Files:

  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Your 4 ton heatpump appears to be oversized, thus it cycles and then only runs in 1st stage. Thus the DSH contribution is The DSH temperature where your sensor is mounted is influenced by ambient air, and due to the high conductivity of copper, likely gets cooled quickly, meaning you have a large heat loss in the pipes. Now the DSH has to make up for the heat loss in the pipes first, before adding to the tank temperature. by the time it is adding to the tank temperature, it is already shutting off again. A 3 ton would run more constant, less cycling, and then also engage 2nd stage more, with more DSH contribution to the tank.
  3. gnick

    gnick New Member

    I appreciate your expertise, but you are making assumptions which are incorrect. My 4T covers 3000 sq ft in two zones (kitchen/living and MBR/bath), and Y2 is disabled to promote less cycling, longer runtimes, and better dehumidification. I find it to be perfectly sized for this application, drawing 2400 watts and easily maintaining the desired comfort level at minimal cost. Heat loss is minimal; attic temperature has run 74-85F this month (in the conditioned space where the HVAC is located). The fifteen foot DSH lines are insulated HDPE, not copper, and the WEL sensor is installed under the insulation at the buffer tank inlet. It is possible that some of our observations are skewed by the fact that buffer tank temperature is measured on the OD of the discharge pipe, not within the tank itself.
  4. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Doc's observation is one of the reasons oversized geos can cost more to operate in spite of aux. use in heating dominated climates, it is also why (again in heating dominated climates) a system produces more hot water in the winter.
  5. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    One confirmation that the 4 Ton is oversized was the indication that Y2 is disabled. While there is a 4 ton unit installed, you are only employing 2.5 of those tons.
  6. gnick

    gnick New Member

    When the geo system was retrofit into this 1989 home (which we purchased in 2009), the Man J said 4T was a good fit for 3000 sq ft of ground floor (replacing 6T of ASHP) although giving up two tons of cooling was a gamble and made me quite nervous (Dallas has a lot of 100 deg days in the summer). But after living with it for a year or two, it became evident that 2.6 tons would be adequate, so I installed a switch at the zone controller and I decide when to utilize it. I would never have agreed to install a 3T unit -- even if an experienced installer had recommended it (which he did not). I use the WEL to keep tabs on it, and its performance has been every bit as good as we hoped. Interestingly, the only time the 4T has been forced to work at full capacity was during a sub-20 degree period the first winter we were here. It ran continuously for hours on end, toggling between Y1 and Y2. So no -- it's not oversized.
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Why would you make a single stage heatpump out of a dual stage? No wonder your DSH does not get hotter! So your DSH line is HDPE? I guess the installer assumed it will never get up to temperature.
    PS: The fact that it cycled between 1st and 2nd stage on the coldest night of the year is a clear indicator that it is oversized. During the coldest days it should at least run full-time in 2nd stage, possibly dipping into supplement heat. So it is oversized for cooling and oversized for heating.
    Last edited: Aug 29, 2014
  8. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    2.6 tons is adequate, but "I would never have agreed to install a 3T unit".

    You can't make this stuff up...ya gotta come here and read it for yourself!

    DSH contributions are tricky to assay...IMO to really know what is going on takes 2 or more sensors at different elevations in the buffer tank, all insulated from ambient influence. An apparent drop at one sensor location may still indicate net addition of heat to the tank when mixing and destratification are considered.

    Short heat pump cycles (such as by an oversized system...there seems to be one around here...) can extract heat from the tank owing to pipe losses and time spent while the system attains steady state following each on-cycle.
  9. gnick

    gnick New Member

    I'm glad y'all are finding my attempts to contribute (and learn) to be so amusing! I've been lurking on this forum for years without submitting, so I know what kind of personalities you are. I do appreciate the experience and expertise that each of you brings to the table, but maybe we have some regional differences in play. I am a (retired) professional engineer, and I know physics is physics regardless of one's location. Nonetheless, I stand by everything I've said. "I would never have agreed to install a 3T unit" is a perfectly valid statement. As stated earlier, we removed 6T of ASHP from the 3000 sf ground floor (Are you familiar with the old rule of thumb using 500 sq ft/ton?) and replaced it with 4T. In Texas, that is a leap of faith and I was willing to take it, because that's what the Man J said and that's what my highly experienced installer recommended. I would NOT have agreed to a 3T before knowing that it was adequate, and it took two years of living with 4T/Y1 to prove to myself that it could handle both heating and cooling loads year 'round. BTW, it is common around here to have NO aux heat installed in the unit, given that cooling is the dominant load, so Y2 has to be able to keep up -- and usually does (99% of the time).

    So, yes -- now that I have five years of proof, we COULD consider a 3T unit next time, but I doubt that a 3T air handler could adequately cover 3000 sq ft (nine rooms, two closets, and an entry hall). As it was, we had to order the high capacity 1 hp blower option for the 4T. I don't remember seeing that option for 3T in their catalog. What do you think the delta T across the evaporator would be at 1600 cfm?
  10. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I'm both familiar with and highly contemptuous of any number of "X SF per ton rules of (th)dumb"

    I'm utterly mystified by the notion that a 3T AHU can't "cover" (whatever that means) 3000 SF. My own 3 ton package unit "covers" 3400 SF, 4 zones, 3 floors and 50 windows while locked in low stage 99.9% of the time. I hate to admit I oversized it...should have gone with the 026 instead of 038. I've owned it since 2008, and zoneboard hourmeters show 12000 hours in Y1 and about 70 hours in Y2. I engage Y2 for a day only upon returning home after a vacation during which system was off for several days. If I swapped in an internet thermostat I wouldn't even need Y2 then.

    We have a 4400 SF house coming out of the ground that looks likely able to run on a single 2 ton system, though we'll likely go with 3T to accommodate gatherings.

    Anyone needing a 1 HP blower on a 4 ton system is almost certainly pasting a big band aid atop really crappy ductwork.

    I can't overemphasize the value of a well-informed manual J. That often means authenticating the building's tighteness with a blower door rig. Man J is like any other modeling tool...garbage in = garbage out.
  11. kinglerch

    kinglerch Member

    I have a 2700sq foot house in which I went with the contractor suggested 4ton, but I have since only used it with stage 1 (essentially a 3ton) mode thus far. I have no intention of putting it in stage 2 mode, if possible. I am happy to save energy using just stage 1 as I am not in need of any faster performance than it is already providing.
  12. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    "the Man J said 4T was a good fit for 3000 sq ft of ground floor"

    Some think that if the load is 50MBH a 4 ton is a good fit, some think if the heat load is 38MBH a 4 ton is a good fit because in deep winter you only get about 3 tons out of a closed loop 4 ton. Some look at a 50MBH heat load in a heating dominated climate and notice a TEV38 can cover 98% of the heating with very little aux. use and call that a "good fit".

    So when you say "good fit" it means nothing without knowing the loads. When you say you scarcely need second stage even in an unusual weather event you are confirming that the system is grossly oversized. Yours are assumptions. You draw conclusions based a sample of one and the advice of an under-educated installer, disputing the comments of pros with many monitored systems in the field.

    We offer facts based on the evidence you provide and accumulated knowledge. Your DSH offers little contribution because of an unevolved design. You are otherwise happy with the performance of the system so enjoy.

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