Another approach is neither - no desuperheater use and no additional water heater.

http://www.ourcoolhouse.com/monitor/monitor.htm

While desuperheaters can provide a lot of hot water when your geothermal system runs a lot, it doesn't do much during shoulder months (when outdoor temperatures are mild). Will your PV system be able to provide enough capacity to keep you in hot water during these shoulder months?

Our Marathon works great. It does a great job of retaining heat (rated for 5f heat loss per 24 hours I believe). Expensive, but it has a lifetime tank warranty (and no steel tank to rust out.) If you get one, make sure the installer reads and follows the installation manual to avoid damaging the unit.

How about an integrated Heat Pump WaterHeater such as a GE Geospring fed by a preheat tank. Virginia has a fairly long cooling season during which a desuperheater harvests heat for essentially free hot water. I wouldn't give that up during the design process.

How did you determine that you need a 4 ton heat pump for a 3300sq.ft. zero-energy home? Seems kinda large. Humidity control might be a problem in the summer with a unit that large, even if it is variable speed. On low speed that GeoMax2 runs at 0.67 of rated, so you are looking at ~2.68 ton on low speed. Stills seems too large for a zero-energy home.

The calculated heating and cooling load for my planned well insulated ICF home with ~3600 sq ft (~2100 sq ft main floor w/9' ceilings, ~1500 sq ft conditioned walk-out basement w/8' ceilings) only shows a peak of ~2 ton heating or cooling load.

ditto arkie. 4 ton jumped out at me as well.
Also skeptical about loop (725'). This is underlooped or you are misinformed doc.
Solar hot water may be worth exploring here.
Joe

doctyphoon, do you mean 725' of trench? We have 750' of trench (250' x 3 trenches) with a 4 pipe layout in each trench totaling 3,000' of loop (of 3/4" HDPE) for 5 tons. Most installers quoted 500' of loop per ton. Our system is 600' of loop per ton. We're in SW VA.

I would have to agree that 750' of pipe is way under sized and 4 ton seems over sized if house is super-insulated. Typical would be 2 to 3 ton in N IL but man J load calculation will give proper sizing info.. if anything you should oversize the field and undersize equipment....Eric

4 tons may or may not be oversized for a 3300 SF supertight house in Richmond. Much will depend upon the amount of glass (windows), its orientation and shading. If windows are reasonable and the construction is as described I agree that a 3 ton two stage with a bit of aux strip is likely to suffice. Manuals J, D, and S calculations will reveal all - ask for them by name.

I'm less competent to guess on required loop length, but I have heard that in my neck of the woods, North Florida, we need 1000' of loop per ton for a horizontal system. That may be the result of a 'perfect storm' situation wherein lopsided cooling loads must be met with warm and potentially dry (during droughts) sandy ground, an extreme set of circumstances.

At any rate, competent local contractor(s) providing load calculations and loop design should trump internet advice, freely given and sometimes worth every penny.

First, I appreciate everyone graciously taking the time to respond. Second, I apologize for inadvertently posting some misleading information. It is challenging to communicate clearly about something that you are not an expert in. And I am sorry for the delay in responding back, but I wanted to be sure that I had fully accurate information this time. So, in the way of correction, as some of you suspected, we have 800' of trench and a total of 3200 linear feet of 3/4" pipe. Also, the PV array will be grid-connected.

Regarding the sizing of the unit, it was guided by the Manuals S, J & D calculations & report. It should be noted that the 3300 sq.ft figure is a bit misleading, as we have a large vaulted great room which increases the volume significantly. The options indicated were the 4 ton unit two-stage, variable speed we chose, or a 3.5 ton fixed capacity unit. Summer humidity in central Virginia is a concern, as some have noted, so we will be incorporating an Aprilaire 1750 to modulate humidity and air exchange.

We are looking for the best mechanism to complement the desuperheater generated hot water produced in our 4 months of summer and 2 months of winter. We do not have anymore south oriented roof space available for solar thermal, so that is out. The unit will be in conditioned space. The Marathon has gotten good reviews, but I too wonder whether the PV will be able to keep up with the load during the shoulder months? The upside would be that the geothermal system wont be drawing power then, so that may keep our dip into the grid minimal. The suggestion of the GE Geospring is an interesting alternative; does anyone know of it being used in conjunction with a desuperheater and whether there is any problem with higher intake water temperature? The Navien Condensing Gas Tankless is a sophisticated unit with very high efficiency and a recirculation loop, but is of unproven reliability. And as I said, our builder is being drawn to what he suspects will be surplus PV watts. I am open to other ideas, input and suggestions. Does anyone have a simple solution that is energy efficient and known to work?

I can't imagine that the Geospring (or any tank heater) cares about high incoming temperature. Geospring makes sense to me for anyone who has seasons of excess or cheap electricity (although is that really true if you are grid tied and can sell it to the utility?).

You may very well be right about high inlet temps, but I have learned in this process not to make assumptions. Virginia, unfortunately, will not pay for excess production; they "carry" it for up to 12 months, then is it has not been used, they thank you for your generosity. LOL.

I've never heard of the Navien; that doesn't mean it is no good but I've been dialed in pretty tight on most of the nifty ways of heating domestic water for several decades so I'd expect it to have popped up on my radar by now.

You have somewhat clarified your circumstances so I'm now more on board with your selection of a 4 ton 2 stage unit. It will almost certainly serve you well and at low cost in central VA.

If it has a desuperheater and you couple that to a preheat tank, don't drive yourself insane over the selection of the final element of your domestic hot water heating system. DOE figures, supported by my own measurements and calculations, suggest that a family of four in an average climate with average electric rates will pay about $500 per year to heat domestic water using a conventional, inexpensive storage electric water heater.

I take exception with your description of your climate's heating and cooling load - I submit that you'll have moderate heating loads in November, March and April, and moderate cooling loads in May and October, during all of which times a desuperheater coupled with its own preheat tank will significantly aid meeting your hot water needs. That plus the high heating and cooling loads in summer and winter should drive your net additional domestic hot water heating load to well below the national average, on the order of $150-200 using a conventional electric storage tank heater.

In summary, I think that you are likely to find that the best and cheapest way to meet your hot water needs is to plumb the desuperheater preheat tank outlet into the cold water inlet of a conventional, inexpensive storage electric water heater whose first hour rating matches or exceeds your household's hour of highest hot water use.

That final, finishing heater could be a Geospring Rheem, Bradford-White or Steibel Eltron HPWH, and I'd applaud your selection of any of those, but understand that the payback for that vs. a standard electric resistance storage water heater will likely run into decades, not merely years.

I doubt that a 4.2 kW PV array in Va will generate much in the way of excess kwh over a year's time. Such a system will be lucky to generate more than 15 kWh per day, a pittance compared to the typical use of a 3300 SF home outside of heating and cooling. Consider lights, cooking, ventilation, TV, computers, and the 100+ other doodads and gizmos we Americans plug in every day - they all add up fast.

Navien is made in Korea i believe design has been in use there for years according to local wholesaler. I have not installed any but I have talked to contractors that have and they give mixed reviews...I will probably stay away from them. quiteside boilers also is a brand that I've only had negative experience with, didn't install but serviced and found a lot of proprietary parts that are hard to acquire when needed. I would look at Rinnias condensing WH if you think u need the capacity. In Europe they tend to install Elect instant WH in each bathroom.....although I agree that at this there is not alot of energy to conserve.......depends on your priorities...........Eric

Not convinced on the size without a glimpse at the man J, but it certainly won't hurt you. Also not convinced of the need for dehumi if unit is properly sized.
I think you may be overly optimistic about surplus electricity from pv, particularly with electric heat and hot water.
j

Roughly, a Geospring should cost about $600 more (after rebate) and save about $100/year (over an electric tank heater) in your case (much more in other cases). But if electricity is "free", then there is no savings.

Posted By jonr on 28 Jul 2010 08:49 AM
Roughly, a Geospring should cost about $600 more (after rebate) and save about $100/year (over an electric tank heater) in your case (much more in other cases). But if electricity is "free", then there is no savings.

Energy tax credits are finite. If you use your $1,500 on insulation or windows, there is none left for the water heater.
Careful not to add 1 and 1 to make 3.
The $600 price difference balloons with no tax credit.
j

Thanks for everyone's observations. I have distilled a couple of thoughts from carefully considering them. One is that in aiming for a zero energy home, it is tempting, exciting and frustrating to chase that final little bit of efficiency. You do not want to drift into a situation where you are throwing thousands into an effort to save hundreds; at the same time the competitive nature is not to want to fall just those few watts short of zero and look back and say could have, should have, would have. Identifying the sweet spot of the cost-gain equation is an elusive goal.

Second, a zero energy home represents a complex set of interconnecting systems, with many uncertain variables. Is the geothermal unit sized spot on? What will the run time actually be & how much hot water will the desuperheater actually generate? How big are those "shoulder seasons" and to what degree will they be narrowed by the tightness, insulation level and extra thermal mass of the unique house shell? What will the output of that PV system actually be, and will there be extra watts to throw at an electric water heater during those shoulder seasons, or is a gas tankless the safer route? Manual J's, PV Watts, and other data based tools can help us make educated guesses, but in the end that is all they are. That's why I appreciate the addition of your experienced input.

Finally, I better recognize that there is not a clear answer to my initial question; or perhaps better said, there is no one answer, but hopefully multiple answers that will all lead successfully to the same end.

I welcome other opinions, and especially look for input from those who may have "solved" this particular equation before; I have never taken any satisfaction from reinventing the wheel.