Hey all.... First-time poster, long-time lurker.

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 before they are considered "too big" (ie, inefficiencies overtake, either in desuper warmup time, surface area temp losses, etc)?


The information you need if you wish to help 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 glycol 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) at $400 each. 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 electrically if I can avoid it...)   I think 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 $650 each (versus $400 ea for the 50-gal 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 longer, and thus likely not have as "hot" of water entering the primary tank when needed?  Pun intended, but am I full of "hot water" with my concerns?

So, in conjunction with those question(s).... 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


Can some knowledgables offer me some solid advice on what might be the best (most efficient / cost-effective) option?  Thanks a bunch, guys.  Excellent forum; very professional and informative....
- Mike

Given the situation you describe I'd go with the 80-80 setup. That's what I have, on a 3 ton HP and it seems to be working well - total cost of hot water since moving into house in late May has been about $30, including both water heater and desuperheater pump operation (Hour meters on both).

My well water arrives at 70 F year round. I imagine your incoming potable water will be much colder much of the year, increasing your hot water energy requirement. Your 4 ton unit will have available more desuperheat than my 3 ton most of the time, so you might as well design to store that heat. More desuperheat is typically available (though at higher cost) in winter, when more is needed (lower incoming water temp, more tank and pipe losses).

Preheat tank should hold 1 day's worth of hot water use. A slightly oversized preheat tank should recover more, not less, superheat - reason for this is that average EWT into the desuper will be lower, increasing recovery. The extra capacity will bridge a bit between high load and milder days of heating and cooling (spring + fall). Undersizing the preheat tank will result in less total recovery and storage and occasionally subject the main heater to 'raw', unpreheated water even on high heating / cooling load days.

You'd get away with undersizing the main tank during high heating and cooling load days since preheat would bail you out as you wrote. Spring and Fall you'd fall short, and the soaking tub would be unuseable as well.

If you went with the 50 main tank you could deal with the shortage in either of two ways:

1) Raise water temp setpoint - bad idea as it wastes energy, increases risk of burns, especially with children, and reduces heater life (more scale)

2) Energize only the top element of the preheat tank, and set it low, maybe 90-100 F and only during spring / Fall. This could work fairly well, but might fail in execution if the preheat tank were left on during summer winter, then degrading superheat recovery on high load days.

You might ask 'em if they can ease up on the cost difference a bit. My 2 80s cost $700 total at HD, and they are easy to install.

I often offer my customer's the option to purchase their own tanks at depot to save a few bucks, as mine (due to greater warranty obligations) will cost them twice as much. While I'll install customer provided tanks, it is understood that any repairs are to be at customer's expense. You will save a few hundred dollars, but it is "not for nothing".
I often wire pre heat tanks for my customers for transitional seasons and when kids are home from college etc.
Also where nat. gas is available, I prefer that to electric for the downstream tanks due to recovery advantages.
Good luck
Joe

I thought the premium for the 80s was a bit high. I advocate that a typical HO get everything from a single contractor for warranty purposes and not nickel and dime with HoDepot stuff unless they can face the consequences. My thinking here was that if the contractor could come off the 80s $50-100 or so it would make the 'right' system a bit more palatable.

My 80s are 12 year warranty units - I certainly wouldn't suggest a HO cry over the difference in $ between the builder grade units at HoDepot and the better ones a good contractor will typically supply to provide a better system with fewer callbacks.

Thank you for the input guys. I opted for the 80-gal primary and a 50-gal preheat. 80-gal will give me plenty of hot water on the non-Geo months (and still support the soaking tub when necessary). Similarly, on my normal usage days when the Geo/Desuper is operating, the 50-gal ought to be sufficient for daily replenishment. I ended up thinking the 80-gal preheat was just too much - thinking about the daily normal ~50-gal replenishment into the primary tank that getting 100* water from the 50-gal preheat-tank was better than 90* water from the 80-gal preheat-tank (temperatures arbitrary just for this sake of conversation...). If my daily volume of water usage was closer to 80-gal then the option for the bigger tank would have been a no-brainer.... But since my calcs are showing normal usage around 55-60 gallons max, the 50-gal preheat was closer to me "daily usage of hot water" and thus should maximize my free hot-water into the primary tank.