Dana is dead on. If you are planning to use passive solar, you first need to work on having a high performance building envelope and minimizing your slab heat loss. Once you do that, you can accurately determine the building heat loss and the slab heat loss. With this information, you can then easily and properly design the passive solar heating system and the hydronic radiant floor heating system. Mike is dead on. For maximum efficiency, design the hydronic radiant floor heating system and controller so as to need the lowest hydronic fluid temp possible. Just tossing out arbitrary concepts and design numbers to a forum is no way to design a building. You might want to try your hand at using our free DIY heat loss analysis, hydronic radiant floor heating system design, and passive solar heating design software:

Borst DIY Building Design Software

I'll check out your design software interface. However, just to be clear, while we all have some passive solar in our homes, my home will not be designed to be a passive solar home. The solar thermal I am mentioning will be a set of four AET 4x10 collectors in addition to the solar PV array on the roof:

http://www.altestore.com/store/Solar-Water-Heaters/Solar-Collectors-and-Mounts/AET-Collectors-Racks-Mounts/4-X-10-AE-Series-Solar-Thermal-Collector/p104/

Back to your original question: Solar hot water for heating as your contractor envisions rarely makes sense. You need heat when panels are least able to produce it by virtue of short days, cloud cover, ambient temperature and so on. Ask your solar guy for tables that estimate how many btus his system can capture in IL in the winter months, I am betting you will be surprised at how small the number is, Worse, if you install an array of meaningful size, then you need a way to dump heat in the summer when the panels work very well. If you want a swimming pool there is an argument for using those panels year round. Otherwise...

The system will "drain" so the panels have no fluid in them in the summer. For the winter I have requested the list and will share once I get it. The solar guy is quoting these two systems to me:

(4) Four 4' x 10' AET Solar Collectors
(2) Two 4' x 8' AET Solar Collectors

the (4) large panels will supposedly get me radiant heat during the winter for a 1600 SF basement and a 3 car garage (garage at 55deg)

OK, drainback solves the heat dump problem at some penalty in efficiency. But the math is still awful. Look up Chicago in this solar company's literature. Dec. Jan and Feb account for 11 percent of the annual solar energy likely to strike fixed flat plate collectors in Chicago. http://www.sunpeakusa.com/insolation/insolation.htm Some things are cheap enough to use at 11 percent of capacity. Solar isn't one of them.

Seriously, in a Chicago climate, the better bang/buck deal isn't in any type of active solar-thermal running an annualized collector efficiency of <50%, and a wintertime efficiency <25%. Spending the same money on a ~$4000-7000 of mini-split with the rest of the $ spent on standard efficiency PV garners more wintertime heat per square foot of incident solar area than active solar thermal, and rather than having to spend power and hardware heat-dumping in the summer, the same system would be delivering phenomenal-efficiency air conditioning during those high-sensible-cooling load high-PV-output days. For hot water, a high-end AirTap (preferred, if you duct the cool air output where needed) or Stiebel-Eltron heat pump water heater can be a very reasonable way to go for a PV-clad house heated with high-efficiency air source heat pumps.

But there's even better bang/buck (and more comfort) to be had spending the entire solar-wad on appropriate efficiency improvements on the building envelope, taking it to 1.5-2x code min performance or better. Design the roof lines (and even wire it) for PV, and when the installed $/watt-peak numbers hit <$2 (which is likely to occur before 2025, possibly by 2020, with or without subsidy) you'll have a house that can actually make it to Net-Zero Energy with rooftop solar that actually fits on the house. If you're only building to code-min on the house, the any spending on solar hardware of any type is wasted capital. (That's very different calculus in new construction vs. retrofits.)

Ok so I haven't heard back from the solar guy. I plan to definitely move forward with the PV system. I plan to get an electric car soon so why not..? BTW, the installed price (after fed rebates, etc) is just slightly over $2/watt.

For my BTU load. I used HVAC-Calc software:

Total Home Heat Gain: 41k BTUs
Total Home Heat Loss: 56k BTUs

Some individual #s:
Window Heat Gain: 21k BTUs
Window Heat Loss: 17k BTUs

Glass Door Gain: 10k BTUs
Glass Door Loss: 7k BTUs

Infiltration (I used tightest setting available):
Gain: 3k BTU
Loss: 10k BTU

Funny I just got an email from the solar guy here's what he says: [quote]The average clear day performance is 1000 btu's per sq. ft. per day. In the winter it is 75% to 80% of that amount and in the summer it is about 120% of average. Keep in mind, the coldest day of the year is the sunniest. Also, assuming your conventional heating equipment has a seasonal energy efficient factor (SEER) of 80%, a solar BTU is worth 25% more than a natural gas BTU.[/quote]

I just got an email from the solar guy

So, what does all that mean with respect to heating your hot water with solar or trying to boost your radiant with solar? Have you figured out what temperature your radiant system will need to operate?

Posted By Surfsup on 26 Aug 2013 07:10 PM
Funny I just got an email from the solar guy here's what he says: [quote]The average clear day performance is 1000 btu's per sq. ft. per day. In the winter it is 75% to 80% of that amount and in the summer it is about 120% of average. Keep in mind, the coldest day of the year is the sunniest. Also, assuming your conventional heating equipment has a seasonal energy efficient factor (SEER) of 80%, a solar BTU is worth 25% more than a natural gas BTU.[/quote]

The collection efficiency of solar thermal changes pretty dramatically with the temperature difference between the outdoors and the working fluid in the collector. To run with any efficiency requires as low an operating temp as possible, to keep the delat-T bounded:
 
http://www.solarthermal.com/wp-cont...eb2210.GIF

The clever use of the AVERAGE is misleading in a space-heating context, since the number and intensity of the summertime clear days when you DON'T need the heat (and have to spend power heat dumping) raises the average.

Snow cover also has it's impact on total amount of heat collected.

Don't forget to calculate the energy cost of the pumping.

PV's efficiency is not negatively impacted with lower temps (but it has to be derated for higher temps).

A heat load of 56K  at 0F or whatever you used as a design temp is a ridiculously high number for a high-performance house that is to be heated primarily with solar.  Upgrading the building envelope to where it's under 30K (or under 25K) and you'll have a shot of doing most of your heating with solar. Otherwise, not.  At 800BTUs per square feet per day it takes a heluva lot of collector to cover the average daily load.   Assuming an average winter temp of 30F (December-February) your average daily heat load is about half that 56K number, or 28,000BTU/hr, or 24 x 28,000= 672,000BTU/day.  To cover the whole heat load on an average winter day would take 672,000/800= 840 square feet of collector. To cover even half the heat load would take 420' of collector, more than TEN 4x10' collectors (not four.)  And that's only half the heat load, on an average winter day, not a peak-cold day.

But if you can cut your heat load to 25KBTU/hr your four 4x10' collectors would cover about half the average sunny-winter-day heat load.

Dana, thanks again, I will work on the HVAC-Calc some more and see if/where I made a mistake and adjust to get that down, if at least just for everyday energy's sake. I am planning to have a boiler as well to make up the difference. This is not going to be a full "net zero" home. I realize that.

Solar manufacturers must submit panels for rating and approval in order for purchasers to claim tax credits. Which means you aren't obliged to accept your contractor's offhand characterizations of performance -- or for that matter to deal with contractors who won't show you documentation and walk you through the math.

Posted By Surfsup on 28 Aug 2013 09:10 AM
Dana, thanks again, I will work on the HVAC-Calc some more and see if/where I made a mistake and adjust to get that down, if at least just for everyday energy's sake. I am planning to have a boiler as well to make up the difference. This is not going to be a full "net zero" home. I realize that.

Energy use avoided with building upgrades is generally preferable to energy harvested by solar, since building upgrades usually have a much longer lifecycle and lower maintenance than solar. 

The problem isn't likely to be a data error of how you used HVAC-Calc, but more likely to be in the whole-wall R values of your house as-designed.  As I stated previously, you need to get the whole-wall R north of R30 (with other insulation and windows similarly upgraded) to be able to really get much out of the meager amounts of heat garnered by the active solar.  Those upgrades are likely to be cheaper in dollars per avoided-BTUs/year than upfront cost of active solar gained-&-used BTUs/year.

An R30 wall would be 2x6 studs 24" o.c. with R20 cavity fill (say, cellulose or open cell foam) and ~2.5" of exterior polyiso, which is sufficient exterior R to avoid the need to install any interior vapor retarders. A mere 1.25" of iso would be enough for dew point control, but the upcharge for the extra isn't that much compared to the cost of the active solar that the reduced heat load represents.

At that R-value the windows will usually dominate the heat load. Fewer windows, and higher performance windows are the way to go.  U0.30 is probably current code-min(?), but don't even think about using anything higher than U0.25 on anything but south facing windows that are sized for optimal passive solar gain.

In new construction using active solar for anything other than ~75% of the domestic hot water ever makes sense, and if you're building to code-minimums that money is usually better spent on the building, not the hot water system.  The arguments for PV are just now becoming favorable, and where subsidised and net-metered at full retail, a pretty good deal.  Your ~$2/watt post-subsidy price point is pretty good, but in terms of payoff, but it may still be not quite as good as bumping your whole-wall R values to 1.5x code min.

In new construction using active solar for anything other than ~75% of the domestic hot water ever makes sense, and if you're building to code-minimums that money is usually better spent on the building, not the hot water system. The arguments for PV are just now becoming favorable, and where subsidised and net-metered at full retail, a pretty good deal. Your ~$2/watt post-subsidy price point is pretty good, but in terms of payoff, but it may still be not quite as good as bumping your whole-wall R values to 1.5x code min.

Agree. I think I will be skipping the solar hot water thermal system and taking that $12k and putting it into more insulation for the walls. Losing windows will be tough though. I don't want to live in a cave.

An R30 wall would be 2x6 studs 24" o.c. with R20 cavity fill (say, cellulose or open cell foam) and ~2.5" of exterior polyiso, which is sufficient exterior R to avoid the need to install any interior vapor retarders. A mere 1.25" of iso would be enough for dew point control, but the upcharge for the extra isn't that much compared to the cost of the active solar that the reduced heat load represents.

I could have sworn Chicago (Z5) was 2" exterior polyiso. I will have to double check. But yes, more Coleman cooler effect, less natural resources. This exercise has made that abundantly clear.

Solar manufacturers must submit panels for rating and approval in order for purchasers to claim tax credits. Which means you aren't obliged to accept your contractor's offhand characterizations of performance -- or for that matter to deal with contractors who won't show you documentation and walk you through the math.

The two 4x8 AET panel system is $5000 after rebates, etc. So lets figure 800BTUs per SF per day for just hot water purposes (not whole house radiant). That's 32x800 = 32000 BTUs per day. It takes 1,100 BTUs to raise 1 gal of water from 40 to 170 degrees. (8.34BTU/gal/degree)

32000 BTUs per panel is 64000BTUs per day which is approximately 60 gallons of hot water per day. That's nearly free hot water for life of the system.

What is the cost of natural gas to heat 64k BTU per day, 23.4M BTU per year?

Nicor Gas shows a therm is $0.4 (I'm sure there are fees, delivery, etc with this so lets say it is $0.45)

1 therm = 100067 BTU

23400000 / 100067 = 234 * $0.45 = $106 per year

$5000 initial cost at a savings of $106/yr is a ROI of 47 years. Can that be right? I fear I messed up somewhere.

I fear I messed up somewhere.

Looks to me like you are getting back on track. So far, you've improved your envelope, saved $25K worth of questionable solar hot water and are willing to work those windows some.

Next thing to look at is the ducting, although I am worried that will be difficult to work with because of your site issues. How much will conventional ducting cost?

You might also consider a copper colored roof. It will be a much more pleasing appearance than light gray and still qualifies as a cool roof.

Posted By Surfsup on 30 Aug 2013 07:49 AM

In new construction using active solar for anything other than ~75% of the domestic hot water ever makes sense, and if you're building to code-minimums that money is usually better spent on the building, not the hot water system. The arguments for PV are just now becoming favorable, and where subsidised and net-metered at full retail, a pretty good deal. Your ~$2/watt post-subsidy price point is pretty good, but in terms of payoff, but it may still be not quite as good as bumping your whole-wall R values to 1.5x code min.

Agree. I think I will be skipping the solar hot water thermal system and taking that $12k and putting it into more insulation for the walls. Losing windows will be tough though. I don't want to live in a cave.

An R30 wall would be 2x6 studs 24" o.c. with R20 cavity fill (say, cellulose or open cell foam) and ~2.5" of exterior polyiso, which is sufficient exterior R to avoid the need to install any interior vapor retarders. A mere 1.25" of iso would be enough for dew point control, but the upcharge for the extra isn't that much compared to the cost of the active solar that the reduced heat load represents.

I could have sworn Chicago (Z5) was 2" exterior polyiso. I will have to double check. But yes, more Coleman cooler effect, less natural resources. This exercise has made that abundantly clear.

You don't have to live in a lightless cave to have a high performance house, but you DO have to consider both their solar gain and thermal losses relative to the site's shading factors and which side of the house they're on.  Most certified PassiveHouse have more than the average amount of glazed surface area.  What you can't get away with is a whole lot of glazed area at code-max U-factors- expect to spend some money on U0.18-U0.25 windows.

In zone 5 you only need R7.5 for dew point control on 2x6 framing with air-permeable cavity fill, and latex paint as the vapor retarder.

http://publicecodes.cyberregs.com/i...par025.htm

With polyiso it takes only 1.25", with EPS it would take 1.8", XPS 1.5". (But go with polyiso or EPS to avoid the HFC134a blowing agent issue.)

If you're going to run it all from purely an investment perspective you need to look at the internal rate of return (and consider that the return is in after tax dollars) and the net-present-value of that return using a reasonable discount rate, and don't forget to include lifecycle & maintenance/replacement cost issues.

Figure the PV panels pretty much good for 40+ years with some degradation of output, replacing the inverter at year 20, and otherwise very low maintenance. (Annual pollen & grit washing may be necessary in some areas to keep output at rating.)

Active solar will have pump swaps on a 15-20 year schedule, and anti-freeze & other system checks at least every 3 years.

Rigid insulation and dense-packed fiber is good for 50-100 years+, lower density blown fiber gets topped off in 25 years, then again 50 years after that.

BTW: The very definition of "therm" is 100,000 BTU. There are ~102,300 BTU/ccf, I'm not sure what comes in units of 100,067 BTU. In most US markets the delivery charges are about as much, sometimes more than fuel charges per therm or ccf or decatherm (mmbtu). Take a real mid-winter heating bill, divide the bottom line by the fuel use to find out what it's really costing. The average residiential retail price paid for gas in IL for 2012 was $8.22/mcf, or about 80 cents/therm, delivered. http://www.eia.gov/dnav/ng/ng_pri_sum_dcu_sil_a.htm

Mind you, the fuel portion of the price is near the absolute historical lows- betting that the current low spot price of natural will remain that low for 25 years is a highly speculative bet, a bet that most utilities with long-term gas contracts aren't taking (they're contracting long term at much higher rates than the spot price.) Nobody has a crystal ball, but neither should we be drinking the frack-water kool-aid and assume that gas will continue to be sold for less money than it costs to produce it forever. The only thing keeping shale gas profitable is the liquid fractions, and wells with insufficient liquids get capped, even if they could (at a loss) get the gas out. Factoring in at least SOME fuel price inflation into your net-present-value calc would be important.

At the subsidized price of ~$2/watt, if net metered at retail the PV is almost a no-brainer. A 2kw array would produce something like 2400-2600 kwh/year. Residential retail electricity in IL is running about 10 cents/kwh (http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a), so that ~$4K investment is returning ~$250/year, or a very safe ~5% after taxes return for the next 20 years (the first inverter bites the dust, at which time you'll have to do the NPV math on a new inverter at the costs/prices then), assuming electricity prices don't rise (or fall, which is remotely conceivable.) That's almost twice as good as the ~3%/year US home values do over time, even ignoring the maintenance costs of the house.

Still a 23 year payback. If ng prices double, that's a 12 yr payback, not exactly solid for the solar thermal. Now if prices tripled, that would be interesting. I thing the maintenance of solar thermal crosses this one off my list. Will do a 5k or 6k PV system, though, and use the solar thermal $ for more insulation for the exterior of the walls...