I posted this in the goe forum with no luck, maybe someone here can get me pointed in the right direction.....

I am trying to put together a system for my DHW. The house in new construction ICF, with open-cell foam in the roof (hopefully). Total load 42kbtu done by manual J. I am located in RI (southern New England).

Geo unit is a 4-ton Waterfurnace with desuperheater and 80 gal hot water tank.

The solar hot water unit consists of two banks of evacuated tubes and a 119gal tank.

From looking at the other posts I don't think the geo alone will provide me with the hot water I need. It may be important to mention that we will be in a rural area, so if the weather allows the windows will be open and the A/C off.

First off is the solar hot water really worth it? Should I just use electric backup (no gas)?

If it is a good idea, and I feel that it mot likely is, then how is the best way to get it all to work together. One tank, two tanks, series or paralell.

I have the stuff spec'd out but haven't bought anything yet, so I'm flexable.

JBA
The desuperheater can provide the hot water you need but it really would work better as a preheater for another form of DWH. The desuperheater is pretty much waste heat from running the compressor in the geo unit. So, if the unit is not running then no waste heat. Where you are located you will probably have a few months where the unit may not turn on.
Most set ups would run the desuperheater to a storage tank (50 gal electric water heater) and use that water as make up water to a primary water heater. Several of the manufactures web sites show how to pipe this arrangement. The idea is to minimize the amount of energy you need to add to the water to raise it up to the proper temp for use.
For the solar water heater are you trying to get that little bit extra heat out of the sun by using evacuated tubes? I am not sure the cost difference between plate collectors and evac tubes would be worth the extra cost over the lifetime of the products. Something to consider though. Still, you will need some kind of backup water heater for those week long rain storms. You could use excess heat from the solar to warm a small part of the house in the winter.
Do a little calculation on your water requirements and paybacks including the tax credit.

JBACC1103,

Why the open cell roof?  Why not SIP or closed cell?  I just completed my house in Douglas, MA with closed cell.

Our heating load was quite low this past winter.  Unless the outside was nearing design temp, we never made much hot water with our geo units.  I am a bit disappointed about that, but that is another story.

Which evacuated tube system are you going with?  Have you seen http://www.sunmaxxsolar.com?  They have the best prices I could find.  I will be buying mine in the next few weeks.

I bring my water to final with a propane fired on demand system.  It works great, and is really efficient, but I was really hoping it would run less.

You can see my project at www.GouinGreen.com.  I am always happy to help.  Since you are so close, feel free to contact me though my website and I'd be glad to give you a tour.

Regards,
Ed 

egouin,

I considered open cell mostly on price, closed cell is much more expensive they say. The rafters are plenty deep so that is not a concern. 

I really did want to do a SIP roof but the design proved to complicated to be cost effective. I just couldn't justify the extra cost to get the same performance. 

For the solar hot water I am not sure of the brand but I will look into it. They are good systems and work well no matter the outside temps.

I am glad to hear of a project close by, I'll have to check out your web site.

I am sorry that you geo unit didn't make much HW, but was something I was concerned about.

You tend to get more out of a desuperheater in climates with heavy AC loads (where the desuperheater is extracting waste heat as a freebie while improving the thermal efficiency of the heat pump) than in areas with light/moderate heating loads (where the desuperheater is an addition to the primary load, running at the same COP.)

But in either AC or heating scenarios, the more you reduce the primary load (by better building envelopes), the less you will get out of the desuperheater, unless it's controlled as a separate "zone" for hot water heating. Odds are recovery cycles on an 80 gallon tank wouldn't be an efficiency robbing severe "short cycle" for a mere 4 ton heat pump, but the details might get messy (not sure how often this is done.) For a 6 ton system or bigger it might be like swatting a fly with a sledgehammer, whereas with a 4 ton system it's only swatting it with a 2lb maul. ;-)

You may/may not get better performance out of evacuated tubes for DHW than a flat plate in RI. Evacuated tubes aren't generally necessary unless you have very low ambient air temps during sunny days (like northern Saskatchewan), high water temp requirements (like 180F hydronic space heating), or hazy/cloudy winter climates with minimal direct sun (like Seattle/Vancouver "bright clouds".) Flat plates in temperate climes like southern New England will almost always be cheaper on a performance basis, and lower-maintenance to boot, but for sure evacuated tube can deliver the goods too. If the evacuated tube DHW system is oversized you may need to set up a heat-dumping loop to keep it under boiling temps in summer though. Flat plates can usually operate as their own heat-dump, pumping to radiate heat away all night to bring the storage tanks down to temp when needed, pumping during the daylight hours to keep the collector from boiling. This doesn't work for evacuated tubes- they don't re-radiate well even at high temps- err to the small side ( or get a hot tub/swimming pool to use as a heat dump in summer! :-) ).

If you're looking for high solar fraction even in winter, go with flat panels and higher than standard panel area, then tilt them more toward the vertical- latitude plus 15 or 20 degrees rather than the typical latitude minus 15 used with water heaters. If you're high enough and inland enough that tend to have snow cover in January, even vertical can work as-well or better than tilted, and your summertime heat dumping will be much less (near-zero, with some well designed overhangs to provide partial shading.) There may be issues getting subsidy money if you're outside a specified tilt angle range though- check the fine print. But it may be cheaper/better to use 30-50% more flat panel and a better winter-gain angle than evacuated tubes angled to max out the annualized gain.