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
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.