Why desuperheater?

Discussion in 'General Discussions' started by bjl, Aug 18, 2014.

  1. bjl

    bjl Member

    I am in the process of building a 3600 Sq ft. Home (not including ext. Walls.) The hvac system called for a 3 ton and a 2 ton unit. My installer is recommending miami heat pump for the equipment. I sent the manuel j to them and they are recommending steping up the two units 1/2 ton each. If I do so I would need to redo the duct sizes to accommodate more air flow. I am building with efficiency in mind with the radiant barriers and high r value insulation. I think I am going to start from scratch and have a new Manuel j done from a different company and compare the two. Any suggestions on this? While checking into this I found a lot of installers use the marathon water heater with the desuperheater. I am planning on two 50 gal tanks on each unit. This may be a stupid question but why install a desuperheater with a second circulation pump when one could pipe the hot water output to the closed loop, to the heat exchange in the two tanks with a temperature control valve that would bypass the tanks when they come to a preset temperature. This is assuming that the Geo ports in the tank are a heat exchange (I don't know for sure). In fact, it seems like one could hook the same hot water loop side to the drain of an ordinary water heater and then t off the top to the loop field. Of course have temp. Bypass valves. All in all instead of having two or three water systems, there could be one. Suggestions and opinions will be greatly appreciated.
     
  2. bjl

    bjl Member

    i think i answered my own question. the incoming loop water would be a constant temperature and you are only going to get so much in a temp. rise. the desuperheater allows for the water to get warm and then circulate the warm water till it gets hot. now thinking about it, it was a stupid question. another question. i see rheem marathon series has several different configurations for the storage tank. can anyone direct me in the best route to choose a tank. should i just get a regular water heater or the heat exchange setup. thanks in advance.
     
  3. geoxne

    geoxne Active Member Forum Leader

    You don't need two water systems. DSH pumps could be piped in parallel (parallel pumps require check valves) to a single water system (DSH buffer and Finishing Tank). The water through the DSH is potable and is pumped directly to the DSH Buffer Tank. All hydronic rules apply and piping should be sized accordingly.

    Two Systems could be considered if -
    -you have a requirement for more hot water flow than one can flow (you can only squeeze so much water through the water tank dip tube).
    -the house is spread out enough that it might be cost effective to have the systems and plumbing separate at opposite ends of the house.
    -redundancy

    Do not think about connecting the DSH to the loop field in any way. These 2 loops have nothing in common, nor can they help each other in any way. Most manufacturers have an internal pump that comes with the DSH option.
     
  4. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Why spend money pumping heat from the loop field the send it back to the loop field. I have feed a single buffering tank from two heat pumps with the aid of a pair of swing check valves.
     
  5. gnick

    gnick New Member

    As a standalone water heater, the Marathon is hard to beat -- lifetime warranty to the original buyer and all fiberglas tank means no rust ever. Sizing it is simply a matter of determining how much hot water you need and how often. Whether the extra cost of adding a DSH unit is worthwhile is a bit harder to calculate. Having a buffer tank filled with preheated water (at low or no extra operating cost) allows the Marathon to run less but the initial cost of the DSH option is not insignificant.
     
  6. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    And my application with two Heat Pumps feeding a common buffer has worked well (thanks Mark.)
     
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The price for marathon tank is usually giving a very small ROI. What is the reason for 2 heat pumps, instead of a single one which would be zoned?
     
  8. bjl

    bjl Member

    The company I had that designed the system called for a three ton to cover about 2100 Sq ft and a 2 ton to cover a 1500 Sq ft area. These areas are divided by use. The bedrooms, the smaller of the two and the living room and kitchen areas. The two units are located pretty far apart, about 70' but the water lines would be over 100'. This is the reason I was thinking of two separate dhw systems. For the kitchen area two 50 gal tanks and for the bathrooms two 85 gal tanks. The bathtub my wife chose operates at 90 gal. I can't pull myself to pay the extra amount for the marathon series. Any suggestions on which heater to choose.
     
  9. geoxne

    geoxne Active Member Forum Leader

    The first issue with the two DHW system setup as described above, the smallest HP (bedrooms) would be assisting the largest DHW load (Bathrooms) and the larger HP would be assisting the smallest DHW load (kitchen).

    The price of 2 tanks could cover a lot of interconnecting pipe. Another choice would to locate a single buffer (feed by both dsh's) centrally then tee off to two finishing tanks appropriately sized for usage. Locating finishing tanks near point of use will also diminish the need for hot water recirc with such distance between points of use.

    Unless you have an nearly up to temp DHW buffer tank or you over heat and mix down your 85gal finishing tank, there is no way you are going to fill a 90gal tub with 100-105f water. If the tub gets used alot consider an electric instantaneous point of use water heater (fed by 85gal finishing tank) for tub only to cover that load when the DSH system is not producing rather than designing the whole DHW system around it.

    The choice of water heater is yours. I only use marathons because local water conditions kill a conventional tank at about 6 yrs., anodes here last about 2 years, I have not seen anything near their insulation value and my back appreciates their lightweight while getting them into the basement along with the peace of mind that I will not have to carry it out later.
     
  10. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    270 gallon total domestic hot water storage sounds like extreme overkill for a 3600 SF home. 5-6 tons of HVAC for a 3600 SF home that is striving to be energy efficient is suspect as well, but my opinion of that is devoid of knowledge of project location and glass area.
     
  11. Jerry_NJ

    Jerry_NJ Member

    Jumping in here late, but my current problem is deciding between DSH on a new/replacement Water Furnace - likely a 4 ton unit, and instead installing a hybrid electric water heater, a heatpump WH. These seem to offer most of the hot water at usable temperatures of 125 degrees or more from a heating process with a COP of about 2.5. Given the DSH is lower temperature and somewhat of a wimpy - what can it deliver the water heart, a couple thousand BTUs? I believe the HPWH dedicated compressor delivers considerably more BTUs per hour...and will use resistive for back up, I assume.
     
  12. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Jerry - we'd prefer you start your own thread out of fairness to OP, BJL
     

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