Desuperheater - what to expect

Discussion in 'General Discussions' started by Stickman, Jun 16, 2014.

  1. woodbutcher

    woodbutcher Member

    I've been following these posts because I have a similar installation. I have a single 50 gal. tank, but I've been wanting to add a buffer tank since I first read about them. An earlier post mentioned that the buffer tank can be smaller. So, I'm considering downsizing the powered tank to a 30 gal. and using a 20 gal. tank for a buffer. Our hot water lines are laid out in a circuit with a recirculating pump so it gives us a higher capacity than the tank itself. By downsizing the main tank, I can free up enough space for the buffer. Does anyone foresee any problems?
    Butch
     
  2. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Butch, the problem with downsizing is the slow recovery offered by electric storage tanks. The DSH will not always be producing. Also be careful with your circulation piping not to mix the buffer tank in there or you will cancel it's usefulness.
     
  3. woodbutcher

    woodbutcher Member

    With only two people in this house most of the time, we don't often exceed 30 gallon demand. We do host our kids and grandkids from time to time, though. That will usually be around Christmas and July 4, so I expect the DSH to provide part of the demand for hot water during that time. Otherwise, if I stay with the 50 gal. heater I have now, I will be limited to a buffer tank no bigger than 30 gallons. Is that big enough?
     
  4. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Yes, it will take advantage of your DSH while not compromising your capacity when the fam is in town.
     
  5. Stickman

    Stickman Active Member Forum Leader

    I've had several days of run time since installing the buffer tank. The HW out line from the heat pump to the drain of the buffer tank still feels only luke warm to the touch, although definitely warmer than incoming cold water. Is this normal or should the line feel like the 120 degree setting on my HP? The HP has been running in cooling mode the whole time, and there has been normal demand for hot water from the occupants of my house.
     
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Depending on your cooling load and ground loop, and ground temps you get certain DSH contribution in cooling mode.

    http://welserver.com/WEL0663/

    Example of Buffer temp right now around 68F which is only 15F more than found. All normal, where are you located?
     
  7. Stickman

    Stickman Active Member Forum Leader

    I'm located on Long Island, NY, zip 11793. I think I better understand the concept now. But where does the 120/150 degree selector switch on the DSH come into play?
     
  8. geoxne

    geoxne Active Member Forum Leader

    The 120/150 degree setting is a High Limit safety to prevent any possible scalding hazard. The higher setting should only be used with a thermostatic mixing valve on DHW systems. Many installations will rarely if ever see those temperatures

    A DSH (DeSuperHeater) or HWG (Hot Water Generator) is NOT a demand hot water system (It doesn't make hot water because you want it). The operation is coincidental to the heat pumps primary function. Output and resulting buffer tank temperatures are determined by the following factors -

    -compressor runtime
    -stage of operation
    -AC or Heating
    -source EWT
    -refrigerant hot gas temperatures
    -DSH pump control logic
    -DSH load EWT or buffer tank temperature
    -buffer tank size
    -hot water usage

    Basically, "What you see is what you get", assuming everything else is working properly.
     
  9. gnick

    gnick New Member

    We have a two person household with a 50 gallon Marathon tank and a 40 gallon buffer tank. DSH provides hot water to buffer from a 4T Waterfurnace in Dallas, Tx. During cooling season, the 4T runs 11-14 hours/day, yet the buffer tank rarely exceeds 115F. To answer your question about using a smaller buffer tank, you will be disappointed. ONE load of whites in the clothes washer will deplete the buffer and trigger the Marathon to turn on. It will take all day to reheat the buffer. Graphs can be viewed at welserver.com/WEL0222.
     
  10. moey

    moey Member

    Something to always remember the unit has to be running to make hot water. Sounds stupid I know. We rarely run our A/C it just does not get hot enough and our house is heavily shaded so even on hot days the house doesn't get much over 70F.
     
  11. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    a smaller buffer tank is better than no buffer tank and no dsh. when space is limited compromises are made.
     
  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Long Island climate has a fair amount of heating contribution, usually is heating dominated. I would stress that a small buffer tank gets up to temp quicker, thus reducing the second tank contribution, which is the purpose of the desuperheater.

    It is all about runtime, load of the house, and hot water needs. A larger tank might provide more, but colder water, causing the second tank to turn on more.
     
  13. gnick

    gnick New Member

    A large buffer tank filled with warm water could contain the same Btu of stored energy as a small tank of hot water, but the small tank will deplete faster and be quenched with cold incoming water. My point in a previous post was, it doesn't take much hot water usage to deplete even a 40 gallon tank. My data for a two person household suggests that a DSH feeding even a 40 gallon buffer tank will not repay its original cost for many years -- if ever. My WEL graph #11 shows I'm saving LESS THAN 50 khr per month on hot water during peak usage months (that's $6/mo) and nothing during off season months -- so annual savings is what, $36/yr. Not a great return on a $700 option.
     
  14. Stickman

    Stickman Active Member Forum Leader

    So can it be expected that in a household such as mine of 6 people with year-round usage that the savings should be greater?
     
  15. gnick

    gnick New Member

    If by "year round usage" you mean the heat pump is running many hours per day during most months, then yes. But having more people using hot water will not save you money unless your HP and DSH are able to keep up -- more people will just deplete the buffer tank faster and your powered water heater will run more often. As someone said earlier, it's all about how much the heap pump runs but don't be fooled by advertising. It may not work as well as you hope. The attached graph taken just a few minutes ago shows four consecutive days where my HP ran continuously for at least 7 hours and was typically producing only a 20 deg rise in the buffer tank. See highlights in yellow boxes. I don't know if my setup is typical of all DSH, but this data is normal for mine. Maybe someone else who tracks their system performance can offer more data.
     

    Attached Files:

  16. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Yes, but the small tank being hotter will cause the heat element in the second tank to turn on less, unless you are using a large amount of hot water relatively continue sly. And your graph #11 shows that you are using between 50-120 kWh/month for your hot water using an electric tank, which is at $0.12/kwh anywhere between $6-12/month. The rest is generated by your DSH, which is very very good. Your 4 ton unit only ran 3.7 hours in 1st stage in the last 12.8 hours, and your buffer tank is still at 110F, suggesting a good DSH contribution.

    It appears you are interpreting your WEL data incorrectly.
     
  17. Stickman

    Stickman Active Member Forum Leader

    Attached is a PDF with pictures of my buffer tank installed. Sorry for the delay in posting.
     

    Attached Files:

  18. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Your HP is mostly cycling in first stage, your DSH brings up your temp nicely when continuously running, but then it starts cycling again and does not get above 115-120, most of the time it feeds the final tank with 115 hot water. Then it drops down in the morning (presumably after showering) to 85F. Then your heatpump is cycling in the morning, which brings up your buffer tank again to about 100F, and starts to run full either in 1st or second stage for about 7 hours, which results in 115-120F. This seems to be the pattern during weekdays, weekend has lesser hot water usage in the morning. Overall it runs ideally and appears to save you a lot of energy for DHW. If you claim the opposite, why don't you shut off the DSH pump next month and see where your hot water energy consumption goes? I bet you will be surprised to find out how much the DSH actually saves you.
     
  19. gnick

    gnick New Member

    I have not performed any serious analysis of my data. I'm just saying that the difference between my high months and low months is not great, and even in low HVAC use my water heater only uses 120-130 kwh so the potential savings cannot be very great. I'm not saying the DSH isn't worthwhile. The attached graph shows DSH performance and WH runtime during four days last Nov that began with a period of mild weather (65 F outdoors) and then a 34 degree cold front. The difference in buffer tank temperature and WH runtime is significant -- it just doesn't result in a large cost difference.
     

    Attached Files:

  20. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    As folks say YMMV
     

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