Now I just need to decide on this solar thermal system.

Solar is employed worldwide for heating domestic hot water, so it seems like you should be able to make it work for you, right? Just consider for a moment that most of the world's hot water needs aren't quite as extensive as for us in the US.

From your posts, I would generalize and say that it looks like you are expecting a bit more output than is realistic during the winter months and you haven't touched much on what you do with the massive overproduction in the summer months.

I'm up at 49N and here is the strategy I employ.

I have a 160 gallon tank, holding water that is headed for my actual DHW tank (heated by a heat pump). All the water in the house that becomes hot water passes through this tank. That represents about two days normal DHW use for my family. The tank has two separate heat exchanger coils in it. One coil is dedicated to the heat fluid from the solar collectors. That is how the heat gets into the tank. Since we do freeze here, a glycol-containing closed system is simpler than a drain-back system.

During the winter, any solar heat at all that reaches the panels is circulated to the solar tank where it warms the water within. On a poor day, it might not warm it much, but it does raise it from the 50F it enters at. In any case, it is heat collected. When the water passes to the "finishing" tank, as it does under normal daily usage, it gets raised to usable temp, which is about 130F.

At some point during the transition from winter to summer, there is enough solar heat to raise the temperature of the water in the tank to 130F and there is no further need for additional energy to heat it before we use it for showering, washing, etc.

As even more solar becomes available toward high summer, the tank heats up to as much as 180F and the water that passes into the actual hot water tank comes under the control of a mixing valve which makes sure that nothing higher than 130F passes on.

Once the water hits 180F, a circulator loop (the second heat exchanger in the tank) has to kick in which dumps the excess heat to an outdoor radiator.

I am mentioning this system because, for me, it embodies several of the important aspects of solar;

1) In winter, there isn't enough production to even heat your hot water fully.
2) In summer, there is way too much and you have to deal with it.

Unfortunately as far as space heating goes, you need the heat during the winter, necessitating some kind of long-term storage schemes....

Very few heat load tools out there are set up to model high-R assemblies correctly. (The PassiveHouse spreadsheet tools do a much better job of it.) Your tool is CLEARLY not modeling an R27 whole-wall correctly. If it's 13KBTU/hr with R19 batts in a 2x6 frame, adding 2" of polyiso would move that to 7KBTU/hr or a bit less, at any framing fraction.

NO amount of R-value can make up for 24K of glazing losses PLUS 10K of infiltration, PLUS 3.5K of fireplace loss (the latter of which is a very squishy loss.)

Bottom line- you can't built a leaky glass house with 37-38K of losses from the glass, fireplace, and air leakage, slap some solar and fancy mechanicals on it and call it "green". You can pay a ton of money for tight German triple panes and cut that 24K of glazaing loss (17K windows + 7K glass doors) down to something like 6-7K if you insist on living in a glass house.

Using poly sheeting on the interior is not the most effective air barrier, since it only covers part of the framing, and needs special detailing at every electrical & plumbing penetration. Detailing the exterior structural sheathing as part of the continuous 6 sizes-of-the-cube continuous air barrier- it has fewer penetrations, and it less likely to be breached. Poly sheeting causes as many moisture problems as it ever solves in a climate zone 5 climate. If you want to use an interior side vapor retarder, use a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus, which will allow accumulated moisture in the assembly to dry toward the interior at a reasonable rate, but limits the rate of moisture migration into the cavity.

http://www.foursevenfive.com/index.php?main_page=product_info&cPath=70_76&products_id=219

http://www.certainteed.com/resources/30-28-080.pdf
http://www.certainteed.com/Locators/WhereToBuySearch.aspx

The 6-sides of the primary air-barrier need to be defined in the plan, executed by the contractors, and inspected/verified. The transition points from the sub-slab vapor barrier to the foundation wall, from the foundation wall to the foundation sill & band joist, band joist to sheathing (or interior side, if you're defining the vapor retarder & gypsum as the primary air barrier), the wall barrier to the window & doors, and the transition to the attic/roof air-barrrier all need to be spelled out. A sill gasket between the foundation and sill doesn't cut-it- they all leak. Foam sealing & insulating with expanding foam works though. Caulking between subfloor & bottom plate of the studs is mandatory, and caulking every stud to the exterior sheathing is also worthwhile, even if the interior vapor retarder and wallboard are being detailed as air barriers.

Instead of a fireplace, a small air tight EPA rated wood stove with a nice big viewing window and ducted-in combustion air would be far less lossy. Size it for the actual loads too- max fire no more than 2x the design heat load of the zone where it's located, or you'd have to fire it below it's efficient firing rate creating a local air pollution problem and the window will be more likely carbon-up. I'm partial to the Hearthstone soapstone stoves myself, which have both tradition box types as well as the euro-modern look, such as the Bari, Tula, or Lima, if that's more appropriate for your architectural feel goals:

http://www.hearthstonestoves.com/store/wood-products/wood-stoves/bari

http://www.hearthstonestoves.com/store/wood-products/wood-stoves/tula

http://www.hearthstonestoves.com/store/wood-products/wood-stoves/lima

They have gas versions too, if that's preferable: http://www.hearthstonestoves.com/store/gas-products/gas-stoves

Ignore any "heats up to xxxx feet" specifications- look only at max-fire BTU/hr.


AT 0.018 BTU per cubic foot per degree F, a 10,000 BTU/hr with in interior temp of 70F, exterior of 0F implies a leakage rate of 10,000/(0.018 x 70F)= 7937 cubic feet per hour, or 132 cfm. If you left a couple of bath fans and the range hood exhaust running or left some windows cracked you might hit those rates, but without active exhaust it's going to be well under half that. In a tight house even running the (no-science behind it and currently controversial) ASHRAE 62.2 rates you'd there, unless you used heat recovery ventilation (highly recommended) which brings the load down to about 1.5K BTU/hr from 10K. But since those rates are excessively drying and not really needed (unless you're smoking stogies and spraying insecticides & hair sprays daily), most people building tight houses with low-VOC materials can run it at 1/4 the ASHRAE 62.2 rate and still have pretty good indoor air quality. You have to get religious about running kitchen exhaust ventilation in tight houses though, even if cooking with electricity.

Ignore any "heats up to xxxx feet" specifications- look only at max-fire BTU/hr.

My wood fireplace says it heats up to 60,000 square feet.

Or, is that 60,000 BTU at max-fire? Probably with well seasoned Unobtainawood, in any case.

Most likely 60,000 BTU/hour at max fire...which seems like a lot of BTUs for a well insulated energy efficient home. If you had a 100% efficient woodstove, you would have to burn 6 1/2 pounds of dry Douglas Fir per hour to generate that heat gain.

Some energy efficient homes may only require 3,000 to 8,000 BTUs of heat gain per hour. For most conventional woodstoves, this is well below their critical burn rate for operating cleanly and they will start to smolder. If you operate most conventional woodstoves at or above their critical burn rate, you may easily overheat an energy efficient home. The only solution for this dilemma is to have many small firings, which is not very convenient. Therefore, wood burning and energy efficient homes are not normally compatible unless you have some way to burn the wood at or above the critical burn rate to allow operating cleanly and you have a way to store the excess heat that is created and release it as needed without having to accomplish frequent, inconvenient firings.

We prefer masonry heaters which provide the solution to this problem and have actually been the most efficient way to heat a home with wood for over a hundred years. Unlike fireplaces or woodstoves, there is very little heat loss because the exhaust gases are circulated through the masonry heater several times before going up the chimney. There is very little pollution because masonry heaters burn the wood very quickly and operate at about 1700 degrees so as to fully burn what even certified woodstoves cannot burn. Masonry heaters store and slowly release radiant heat over a 24 hour period accomplished by only one or two firings per day. Masonry heater surfaces never get extremely hot like stoves and do not overheat and excessively dehumidify your home which your sinuses will greatly appreciate. A masonry heater can also be located to absorb solar radiation and store this form of heat energy too. Therefore, masonry heaters are similar and compatible with hydronic radiant floor heating and passive solar heating. As a side benefit, you can have a nice masonry oven that is available for energy free baking duties perhaps 10 hours per day and you can also have heated benches for you, your guests, and your pets to enjoy all day. Constructing a masonry heater is a relatively simple DIY project and there are many good kits available in the marketplace to do this. Please be sure to fully research and comply with all your local building code requirements.

Posted By ICFHybrid on 11 Sep 2013 09:30 AM

Ignore any "heats up to xxxx feet" specifications- look only at max-fire BTU/hr.

My wood fireplace says it heats up to 60,000 square feet.

Or, is that 60,000 BTU at max-fire? Probably with well seasoned Unobtainawood, in any case.

I'd hate to see the fireplace that heats up to 60,000 square feet- it would probably be capable of smelting iron ore if you really cranked it up!

A wood stove rated for 60,000 BTU/hr really CAN be fired at that rate, and usually substantially higher, but it violates the warranty to constantly crank it at 80-100KBTU/hr, since that seriously shortens the lifespan of all metal components.  I suspect a fireplace rated that way has the same issue.

Non-catalytic EPA rated woodstoves NEED to be fired at above 1/3 the BTU rating in order to keep the secondary-burners lit.  Catalytic stoves need to be initially fired at above 50% of max to light off the catalytic converter, but can then usually be throttled back to about 1/4 max and still burn cleanly. From a maintenance & longevity point of view the non-catalytic stoves are better, but test somewhat higher on soot emissions than catalytic stoves (with fresh catalyst).  I'm no convinced that on a lifecycle basis the catalytic stoves are cleaner, and I suspect (but have no data that) in the real-world "as operated" they're slightly dirtier over a decade of use.

Masonry heaters can be great for low-load homes, but commercially installed they're quite expensive.  Soapstone stoves generally have enough thermal mass to handle intermittent hot-firing for handling low-loads cleanly, but it's still important not to grossly oversize them for the zone they're heating. It's definitely more futzing around than with truly massive masonry heaters, but they're also more responsive to the load, making it easier to track the daily/hourly swings in load than with masonry heaters. (But a 50K design heat load is NOT a low load home- you'll feel the chill pretty quickly when the temp drops if you don't bump up the fire.)