Replies

1. 
   NRTRob | Jun 28, 2006 06:09am | #1

   If you want to do solar, low water temperatures is the name of the game, as low as you can go. Chances are with a joist install of any kind you'll be out of luck, but you *might* do something useful if you have heavy gauge extruded plates in the joists. Flashing grade light plates will not cut it.

   Also, for the love of all that is holy, do NOT install an open system. That is plainly a very bad idea for a whole lot of reasons. Pay the one time cost for a heat exchanger and one additional pump, use barrier tubing, and then no matter what happens down the road you have all your options open to you. And you, your system, and your home will be safer as well. And your "very efficient" Hot water heater is not quite what you think if it's the Polaris.

   Rather than dumping money into solar or radiant off the bat, I recommend increasing your insulation first if that's at all possible. R13 is pretty substandard these days. Of course that is much easier said than done if this house is existing which it sounds like it is. But it's worth considering at least. Sounds like your ceiling was done well, now look for any other deficiencies you might have.

   If you do go solar, generally speaking evacuated tubes are generally better for us here in the northeast. Focus on DHW first (cheapest, fastest payback, useful year-round), and if you don't choke on that, then investigating its usage for radiant might be worthwhile. Depending on your water temperature requirements, of course.

   -------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com
1. 
   mzahorik | Jun 28, 2006 07:48am | #2

   > If you want to do solar, low water temperatures is the name of the game, as low as you can go.What kind of temperature can I expect from the panels? Anyone have any graphs?I realize that staple-up is less efficient and has to run warmer to deliver a sufficiently warm floor. It's unfortunate, but in a retrofit like this its the only option. I don't want to pursue raising the floors, much less adding the additional static load.Relative to the existing two-furnace 20 year old probably <80% efficient leaky duct gas hot air system _anything_ is more efficient.> heavy gauge extruded plates in the joists. Flashing grade light plates will not cut it.I might be barking up the wrong tree here, but it sounds like if I need to add mass to a system to compensate for lower water temperatures using conductive metal is not the right answer. I should be using serious mass like sand/gypcrete/concrete or resign myself to running warmer.> Also, for the love of all that is holy, do NOT install an open system.Care to elaborate?Things swaying my decision either way:Open:Pro - One heating source; possible initial cost advantage and efficiency gain due to running one source.
   Open:Pro - No oxy barrier needed
   Open:Con - Need expensive stainless pumps
   Open:Con - Most tank heaters don't recover heat quickly or are inefficient in that mode.
   Open:Con - Unknown long term risk of mixing potable and heating water.Closed:Pro - Eliminates any unknown risk of mixing potable & heating water.
   Closed:Pro - Variable condensing boilers excel at heat recovery and are very efficient
   Closed:Pro - Can use cheaper iron pumps
   Closed:Pro - Can mix in antifreeze to guard against freezeup at a loss in efficiency.
   Closed:Con - Still got to run a water heater AND boiler
   Closed:Con - All tubing has to have the more expensive oxy barrierThe list is by no means exhaustive, just off the top of my head.> Rather than dumping money into solar or radiant off the bat, I recommend increasing your insulation first if that's at all possible.Attic was R-19 and had lots of unsealed openings. Fixed. I've been sealing behind door/window trim as I repaint rooms. Windows are all already double paned. This is about the best I can do without completely gutting the house and putting in foam. The 1st floor above the basement doesn't have insulation but I'll be adding that when I put in the staple up.> Focus on DHW firstIf I have it, I'll want to leverage it wherever its cost effective to maximize payback. (pool and DHW are obvious wins, radiant sounds iffy hence my original questions)Thanks for the input!- Matt

1. 
   NRTRob | Jun 28, 2006 04:31pm | #3

   Hi Matt,Mass is not the issue, conductivity is. Heavy gauge aluminum plates do a far better job than the lightweights at providing good heat transfer at lower temperatures. I don't have the necessary physics to really explain why, but I do have many systems of both configurations in the field which would indicate the manufacturer's claims are correct; heavy plates can drop your water temps a good 30 degrees under a lightweight system.Depending on your loads, subfloor and floor coverings, that might get you into the low 100's for a water temp which solar could help with. Or it might not.You've got a pretty good handle on the pros and cons of an open system vs a closed system, however you can do a closed system with one heat source.. use a heat exchanger. I will say, however, that it's rare for a full sized home here in the northeast to be best served by a hot water heater and a mod/con boiler would most likely be a far better choice both for efficiency and for comfort, and a storage tank with a coil could be used for domestic.Elaboration on the rest of the pros and cons though; you can't antifreeze a system without an oxygen barrier with confidence; oxygen can break down the corrosion inhibitors. So if you ever, ever want to have the option of antifreezing the system, you need O2 barrier pipe. If you ever, ever want to have the option of using a high efficiency boiler, you need 02 barrier pipe. Since you should have o2 barrier pipe in any configuration, adding a heat exchanger is the only true "added" expense to a closed system. It will protect your system against whatever comes in with your water (and comes in, and comes in, and comes in... forever). It will protect you from whatever grows in your heating system.. and it will. It's simply a bad idea to continually run fresh water through a heating system. Some people get lucky. Some people don't. It's a lot of money and work to gamble, I think.You'll get your maximum payback from a simple DHW solar system. It's cheaper to put in and it works year-round. Scaling up for heating adds initial expense that may take much longer to pay back (naturally, there are a whole lot of questions involved with payback that are basically just guesses, but still). If you just wanted to be ready to tie it into heating, say if fuel prices really go nuts, get a dual coil storage tank for the solar.-------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com

1. 
   ronbudgell | Jun 28, 2006 07:26pm | #4

   NRT Rob,

   I was talking about a roof installed DHW solar system a while ago with somebody who knew a thing or two. He told me to avoid the evacuated tube collectors for that installation because they are so efficient there is not enough heat leakage to melt snowfall off.

   Any opinion?

   Ron

1. 
   NRTRob | Jun 28, 2006 07:31pm | #5

   Perhaps he's right, I'm not sure. All I am basing my preference on is that vacuum tubes do a much better job at gathering heat on cloudy days and days of low solar gain, which we have an awful lot of up here in the northeast. As you get into better solar climates, I believe the difference between Evacs and flat plates shrinks.Maybe the price for that increased collection power is some panel dusting after snow storms?-------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com

2. 
   SamT | Jul 01, 2006 07:25am | #7

   necessary physics to really explain why

   The heat conductivity of metal is standardised along a 1" square bar.

   It's also only 1" long.

   If it's standard or Unit conductivity is X, then its' measured conductivity, if it was 1" wide and 0.1" tall, would be 0.1X.

   If it is thinner, say 0.05", then it would be 0.05X.SamT

1. 
   NRTRob | Jul 02, 2006 05:45pm | #9

   That was the simplest and most clearly understandable attempt at explaining that I have ever seen. Thank you very much sam!! Now I can go play with numbers with a good point of reference :D-------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com

2. 
   SamT | Jul 02, 2006 11:27pm | #10

   Maybe it was square centimeters?

   Or something with mills?

   let me know.

   The process is right, anyway.SamT

2. 
   GaryGary | Jul 01, 2006 06:16am | #6

   Hi,

   As has been mentioned, any solar system (evac tube or FP) will be more efficient if you can keep the water temperature down. An outdoor reset helps solar systems a lot.

   On the evac tube vs Flat Plate, I decided on the FP for my radiant heating system. I ran simulations, and (for my system) the evac tubes gained very little in performance, and cost a lot more. This might turn out differently if you need high temperatures.

   The SRCC has tested the performance of many evac tube and FP collectors -- you can download the "Directory of SRCC Certified Collectors" here:
   http://www.solar-rating.org/RATINGS/RATINGS.HTM
   and have a look at the differences yourself.

   Basically, good evac tubes (the brands do vary) are a bit more efficient than good FP collectors for the high end of the temperature range and a bit less efficient at the medium and low end. The less efficient at medium and low temps is puzzling, but happens (I think) because the evac tube rigs have quite a bit of empty space between tubes that does not collect anything, whereas the FP's are nearly all effective collection area.

   Gary
   http://www.BuildItSolar.com

3. 
   constantin | Jul 01, 2006 03:56pm | #8

   Do it right, or don't bother. Pairing an open system with evac tubes is like putting a Ferrari engine in a Yugo. Yes, it might work, but what's the point?

   Open systems are shunned by professionals for many reasons, including massive corrosion problems, legionella, etc. Look up articles by David Yates, you'll see what I mean. The only proponents of open systems tend to be internet supply houses that claim to offer all sorts of design, etc. services which I consider to be of questionable value.

   If you really want to do radiant with a solar supplement, I suggest you go with the best there is in terms of house infrastructure and add on to that as time (and budget) allows.

   If this is a new home, I'd go with Warmboard throughout the home, it has the best thermal properties for sub-floor systems. If it's an existing home, put the tubing over the subfloor. Otherwise, your required water temperatures are likely to cook tubing and house alike and be of NO use to a solar-heated system

   I'd also insulate the home to the point where the heating and cooling systems can be minimized, i.e. use 2x6 wall construction with Corbond or thicker wall systems with less-effective insulation. RFH is only good up to about 25BTU/ft^2 for residential temperatures, and the less energy you loose, the lower the water temperatures can be. The lower the water temperatures, the more your solar system can do for you.

   Couple that with smart siting, good window placement, and insolation control (i.e. letting the winter sun in and keeping the summer sun out) and you'll recoup the additional investment when you want to add the solar heating part, because those systems are expensive. You can always put the pipes and wiring ducts in place to plan for the future.

   Window choices are another big area for potential improvement. Some of them go to R8 these days, a vast improvement over many windows today which are barely better than holes in the wall.

   I suggest more reading at ourcoulhouse.com and buildingscience.com
   Both resources helped me when the time came to renovate my home. Cheers.