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  1. #1

    Geo Desuperheater: Pre-heat tank and primary tank sizing help?

    Hey all.... First-time poster, been a lurker for a while...

    I suppose the easiest way to present my question is to ask it first (so you can have it in your mind while continuing to read), then present you with the background...


    Q: How big of a preheat tank and primary water-heater tank are recommended, before they are considered "too big" (ie, inefficiencies overtake, either in desuper water-warmup time, surface area temp losses, etc)?


    My background information to help in offering suggestions to my predicament....

    I am in process of building a home. 3300sqft 2-story, ~40 miles North of Detroit Michigan. 90* summers, 0* winters, and perfect Spring/Fall temps to have the windows open and no HVAC operational. Home is to have wood-cellulose insulation; caulked and sealed windows/corners/etc. My builder and I both expect the home to be very tightly sealed. After obtaining various closed-loop DX and conventional Geo quotations, my builder and I have chosen a closed-loop water Geo system, Waterfurnace Envision NDV049 dual-stage unit (4 ton), dual-zone. I am having the Geo company also add the Desuperheater option along with a pre-heat water-tank. Primary tank is electric. I have a family of four; two young boys. Master-bath shower will have 2 shower-heads (occasional usage of both at same-time), and we do have a 80-gal soaking tub that will *rarely* get used.

    My predicament focuses around what size water-heaters to purchase. As default in their quotation, the Geo company has as "standard" two 50-gallon AO Smith electric water heaters (one primary, one pre-heat). However, I am concerned that this is not large enough for our family. In the summers or winters when the Geo+Desuper are functional, my family would be fine with a 100-gal capacity of hot water. But, I worry about the ~4 months when we don't have the system 'on' (May/June, Sept/Oct), and my hot water essentially is reduced to only the 50-gal stored in the electric primary tank. (I do not wish to connect the preheat-tank electrically if I can avoid it...) I am certain there will be occasions where we will run out of the 50-gal hot water during those months, and ideally I'd like to spec/install a system that *doesn't* put us in that "no hot water" situation.

    Despite the 80-gal size of the soaking tub, I have determined that our family-of-4 would need a 65-gal hot-water tank for 99% of the time. The other 1% comes into play when we actually use the soaking tub... So, I asked my Geo company to increase the size of their offered primary and preheat tanks to 65-gal units. This would then provide me with adequate hot-water on those days when the Geo system is 'off', and of course a solid pre-heat volume to replace the primary tank's usage on the days the DSH is functional.

    Unfortunately, my Geo source (for whatever reason) is unable to offer a cost-effective 65-gallon upgrade for their hot-water tanks. Instead, they have offered me 80-gallon electric tanks for only slightly more than their 50-gal "standard" units. Herein is where my questions and concerns come into the scene.

    Given what my predicted demand is going to be for my hot-water, I think going with an 80-gallon primary tank and an 80-gallon preheat tank is overkill, for both non-DSH heating days and DSH-heating days. I actually fear that its going to be "too much", and that I may be losing some efficiency by not actually "using" the hot-water volume that my primary & preheat tanks are heating & storing. Similarly, asking the DSH to heat-up 80-gal in the preheat tank versus 50 or 65-gal means that my warm-up times are going to be much longer, and thus likely not have as hot of water entering the primary tank when needed...?

    So, in conjunction with those questions (concerns).... Is there such a thing as going "too big" for primary and preheat tanks? Given what I've stated above, which of these would you consider to be my "best" option for efficiency? Keep the preheat volume matched to the primary tank, or go smaller/larger with one or the other?

    Tanks (size in gallons):
    A) 50-preheat -- 50-primary
    B) 80-preheat -- 50-primary
    C) 50-preheat -- 80-primary
    D) 80-preheat -- 80-primary

    ...or...

    E) Find a pair of 65-gal tanks and use those for preheat & primary? (most difficult option since I'd have to venture out on my own versus letting the Geo Company do this all turnkey)


    Can some knowledgables offer me some solid advice on what might be the best (most efficient / cost-effective) option? Thanks a bunch, guys.

    - Mike

  2. #2
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    I think you are over analyzing this. Upgrade your electric from 50 gal to 80 gal. The loss difference between the two is minuscule, so no real efficiency loss. It will give you some extra capacity that comes in nice with guests visiting, etc. No real downside except for the initial cost.

    As for the geo pre-heat tank, 50 I have found is large enough. The only time you would need a bigger tank is if you charged up the preheat tank to max temp and the pump is cut off by the max temp stat. I can see this happening in the dead of winter, and no one is home for a couple of days. With normal winter weather and usage, the preheat tank will run around 90 deg. As you use hot water it gets diluted with cold water. Since you are putting BTUs in and taking BTUs out, the capacity of the tank is hit only at 120+ deg when the geo HWT stat kicks off. In my own home, 4 tons total, 4 adults, this rarely if ever happens.

    paul

  3. #3
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    Quote Originally Posted by MKutchey View Post

    Home is to have wood-cellulose insulation; caulked and sealed windows/corners/etc. My builder and I both expect the home to be very tightly sealed.

    - Mike
    Not quite the question you are asking - but if you really want to tightly seal your house, spend some time on the top floor ceiling.

    Most problems with sealing a house are in the attic. I reccomend you either go with a conditioned attic, or look into the air tight drywall approach.

    And, don't forget the fresh air intake.

  4. #4
    Join Date
    Jun 2006
    Location
    Lititz, PA (Lit' itz) not ( Li' titz)
    Posts
    122
    In short put in a 80 electric with a 52 gal preheat tank. or put in a 80 gal preheat this would not be a total waste if the preheat tank is absorbing heat from the basement doing nothing. It would end up being warmer than the water from your source (well/city) and in the winter with long run times you would maximize the hot water generator.

  5. #5
    Join Date
    Jan 2007
    Posts
    115
    It would seem the larger pre-heat tank you have the better. More ability/store to convert cheap BTU to hot water

    I'm 3200' finished, Illinois, on a 5 ton waterfurnace. Not as tight or well insulated as I' like, but quite close. I'm quite sure I'd not be happy w/4 ton.

    I used 75G for pre-heat, (2) 50G gas, in series, for final. I keep the middle heater @ a medium heat setting and the final as a hot, but not quite scaling, temp. Why 2 gas? Same thing you wondered, what about when the Geo ain't running? I built to consider the GEO a cost saver , not a needed water heater.

    Electric Water heater? Wow.. I'd be using a LOT of pre-heat.

  6. #6
    Quote Originally Posted by danf58 View Post
    It would seem the larger pre-heat tank you have the better. More ability/store to convert cheap BTU to hot water
    Electric Water heater? Wow.. I'd be using a LOT of pre-heat.
    Heh. My other alternative to electric is propane.


    Thanks to all for the replies. Biggest concern I have with the larger preheat is its ability to heat the water - that is, time duration for that 80-gal to reach a ~100* preheat temp, versus a smaller 50-gal that effectively has more cycles through the DSH & thus heats faster (or given the same time duration reaches a warmer temp).

  7. #7
    Join Date
    Feb 2003
    Posts
    168
    Isn't the domestic water heating capability for the 4 ton unit around 12 gallons per hr? If so the 80 gallon tank could be heated in just 6.66 hours. My feeling is the bigger the better.......

  8. #8
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    Quote Originally Posted by cliebold View Post
    Isn't the domestic water heating capability for the 4 ton unit around 12 gallons per hr? If so the 80 gallon tank could be heated in just 6.66 hours. My feeling is the bigger the better.......
    The capacity of the desuperheater is in BTU, not GPM. The temperature you achieve is a function of the hot gas temperature (heating season is best) and the volume of water. With my own 4 tons worth of geo heat pumps, I might see a 25 to 30 deg rise over night, in a 50 gallon tank, on the coldest of nights. Most typical nights are a 10-15 deg rise and the preheat tank hovers around 85 degrees in the winter (70 in the summer0

    If you use an 80 gallon tank, you will have less rise, but over a greater volume of water. A 50 gallon gives a greater rise but to fewer gallons. As soon a s you run hot water, you dilute the tank temperature, but require fewer BTUs in the HWH. In the end, it really does not matter what size the tank is, unless you empty the preheat tank of its warm water. The BTUs you generate are in the water, subject only to radiant losses.

    I prefer 50 gallon preheats since they cost less, are smaller, and never hit capacity issues. Either tank will work. The 80 may give a bit more capacity in the most extreme situation, but it will never pay for the additional cost of the tank.

    paul

  9. #9
    Join Date
    Mar 2008
    Location
    northern Indiana
    Posts
    114
    I pick answer C

  10. #10
    Join Date
    Jan 2007
    Posts
    115
    Tough call. Water use patterns come into play here. Use it all at one time vs heavy off and on. one favors big tank plenty of reserver, the other favors fast recovery, (so small tank)

    electric recovers SLOW....

    Remember the pre-heat tank can be anything. I used a 75G scratch and dent gas. Not like it was gonna get hooked up.

    And in a perfect world I'd go with "D". I do think you'll still have some cool water times, but I'll bet not too easily.

    Did you notice the rwo 50's I used give nearly unlimited hot water? That's a lot of BTU heating water when both burners are going. I'll bet two electric 40's ( as final heat) are close to an 80 in price, would give you better recovery, and etc..

  11. #11
    Join Date
    Dec 2007
    Location
    oregon
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    885
    Quote Originally Posted by danf58 View Post
    Tough call. Water use patterns come into play here. Use it all at one time vs heavy off and on. one favors big tank plenty of reserver, the other favors fast recovery, (so small tank)

    electric recovers SLOW....

    Remember the pre-heat tank can be anything. I used a 75G scratch and dent gas. Not like it was gonna get hooked up.

    And in a perfect world I'd go with "D". I do think you'll still have some cool water times, but I'll bet not too easily.

    Did you notice the rwo 50's I used give nearly unlimited hot water? That's a lot of BTU heating water when both burners are going. I'll bet two electric 40's ( as final heat) are close to an 80 in price, would give you better recovery, and etc..
    Do not use a gas water heater for a preheat or desuperheater storage tank! the hole up the middle of the tank ( the flue) will create an updraft ( convection) and rob your tank of stored heat. use a scratch and dent electric.
    You could use a gas fired tank, pull out the burners, stuff the cavity with spray foam to stop the draft. The electric tank would a much simpler and effcient method.
    Genius = The guy who can do anything...except make a living!

  12. #12
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    The larger the home, the larger the preheat tank should be. You will have longer run times with a larger home so a larger preheat tank would make sense. we have installed 80 gal. as a norm on homes over 3000 sq ft.
    My own home has two 50's in series. One is the preheat ( 1st one) the second is a high effciency electric. works great, and I get more hot water in the winter then the summer. Longer run cycles. use any type of water heater after the preheat ( desuper tank). IF you use a on demand type be sure it will function with elevated input temps. Some of the cheaper models will lock out at in put temps over 70 degree far.
    Genius = The guy who can do anything...except make a living!

  13. #13
    Join Date
    Mar 2008
    Posts
    21

    Thumbs up

    MKutchey, I built two years ago and used a different approach. We are in a 2500 ft. ranch with 4 ton WaterFurnace. I have a 50 gallon Rheem Marathon(storage only no juice to it) that feeds a Rinnai 8.5 gpm tankless propane...This combo works great. The Rinnai senses the incoming geo heat and it winds the gas down to compensate saving $$$. This way I have unlimited use if needed. I also max out the gpm on the Rinnai because of the low degree rise with preheated water. Mine now tops at 8.5 gpm which would run your two shower heads and at least another shower all at once. The Rinnai also allows me to push a button and shut it off. Lately with the AC on alot I will turn off the water heater all together and get evening kids' bath time and a shower out of storage from the days geo use without burning any gas at all. I am in SW Ohio and am currently storing 93-102 degrees in the evening give or take. I shut the Rinnai off and flow from the tank and check with digital thermometer. To further save $ the Rinnai can be set to the temp you actually use not 120 as standard. Unlike a tank heater, there is no reason to fire the BTU's needed to heat 120 when you just add cold to get in it. We leave ours at 104 unless running dishwasher. This combined with the geo preheat has signifigantly dropped my propane use which was already extremely low for a family of four. One other thing is that I valved it so that I can isolate the Rinnai if a problem and can flip a breaker on, and use the storage tank as primary if necessary. Good luck

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