wood stove assist for hydronic heat

[toddm]

I am building a passive solar house with high thermal mass in a part of the world where they don't really belong. (Winters are too grey; summers are too humid.) Happily, the rest of design is quite sensible. The house will be small and supertight with very good windows and good insulation. Multiple calculators put the heat loss at about 25k btu/hr at a winter design temperature of 10 degrees F, and roughly 40k btu at the worst-on-record temperature of 37 below. The calculators don't consider passive solar, which should be sufficient by itself until the ambient temperature drops below 50 and stays there.
My question: Is anyone using a wood-stove back boiler and hydronic heat to assist passive solar, in effect, supplementing heat to be released at night? My mostly concrete house would lose about a degree an hour at the design loss of 25k btu/hr. The trick would be to add enough heat during the day to carry the house overnight. Yes, a wood stove would perform that service with or without hydronic. But I already have the concrete, and it could modulate temperature swings to 10 degrees at most.
The stove would heat an indirect tank by thermosiphon. I'd use an electric boiler to provide backup heat as well as on-demand domestic hot water.
The rest gets murkier. The stove would have to be smallish to burn hot enough in a small house to meet boiler demands and maintain efficiency. Passive solar, if it works, would shrink the heating season to December and January. I can't add active solar to the tank because the house is in the deep woods. For two residents, domestic hot water needs are modest.
Should I size the tank for heating and divide the year into hot-tank and cold-tank periods, with the hot part being maintained by the electric boiler? (Needless to say, hydronic would be a closed loop system.) Should I commit to an extended hot tank season of four or five months, up from two or three, to garner savings by preheating domestic hot water? Should I shrink the tank and allow it to go cold at any point, while plumbing backup heat directly into the hydronic system? What can I do to preserve options if any part of this turns out to be much different than expectations?
All I can say in my defense is that the view justifies the windows.

 

[Martin Strand III]

My experience in 2003 building a tight house (cathedral window wall facing ESE, open design, 1950 SF main floor and loft) in a remote area with ample fire wood supply (22 acres) in NW MI:

1. Consider a centrally located masonry heater - you have the woodlot for fuel savings; long masonry heat retention time ideal for accessory hydronic heat; open floor plan is needed
2. Closed loop hydronic coils in heater wall is possible - I opted not to do this but I now wish I had
3. Windows allow sunlight (aka radiant heat) in morning to assist masonry heater; not quite as effective on a "grey day"; ideal position of heater would allow sunlight to contact dark exterior veneer for heat absorption
4. I put combo wood/coal stove in basement for redundant heat (LPG service in winter: nada) - very happy with this now basement is finished
5. If you are remote with trees near power lines, consider home standby generator (LPG)

Aye,
Marty

 

[toddm]

Firewood also falls from the sky around here, which is good and bad... Would you consider your house high or low thermal mass? Low mass would work better at this latitude, but we get regular warm spells. It hit 60 this week. I worry about overheating. I'll have about 280sf of glass facing SSW. High mass houses won't overheat, but solar barely registers in deep winter because there is too much to heat.

 

[Martin Strand III]

Sounds like your latitude is not PA.

And scratch the masonry heater recommendation. I don't think they burn coconuts.

Too much heat in winter? Open some windows or go for a swim.

Aye,
Marty

 

[jgoodnow]

Very interesting project. Just out of curiosity, what kind of windows and insulation are you using? How thick are your walls? What is your target for the number of air changes per hour? Do you have any plans to use a heat recovery system?

You might also want to post your message on Heating Help's "The Wall" message board:

http://www.heatinghelp.com
http://forums.invision.net/index.cf...fe091e5a-fdb2-45c0-a573-00a065eef1a5&CFApp=2&

 

[toddm]

Andersen's double pane Low-E4 windows have a u factor of .28 and a solar heat gain coefficient of .43. The choice for walls is eight inch or 12 inch block, and it's up to my engineer. My goal on fresh air is code: .35 changes/hr. I'd use an HRV to do it rather than an ERV.
I am going to live in the house before tackling the humidity question. With the house in constant shade, it probably will be comfortable even if it is on the wet side. If not the choices are whole-house dehumidifier or a small heat pump, if I also see that solar can't handle the odd cold snap in March. I'd do some ductwork anyway to circulate air from hot to cold, to distribute fresh air, to filter and perhaps humidify.
Theoretically the house should regulate itself for nine months of the year. The hottest summer month is July with a mean temperature of 78 and relative humidity of 68. The coldest winter month is December with a mean temperature of 30. The mean solar heat load on a horizontal surface that month translates to a piddling 5k btu/daylight hr on the outside glass surface of my house. January is less grey. November and February are sunny by northeast standards.
Trouble is, "mean" doesn't mean a whole lot around here. The highest horizontal radiation on record in December translates to 42k btu/hr on my windows, and the average high translates to about 30k btu/hr. You can see why I want to decide on a daily basis whether to fire up the stove.
Yes, you can also see that passive solar works best in spots where the sun actually shines.

 

[LeonMSPT]

"Mean" is known as "average" in common speak. Had a statistics professor who explained "average" very well.

"Sit in a chair, place one foot in a bucket of hot coals out of your woodstove, and one foot in a bucket of ice water."

On average, you're pretty comfortable aren't you?

 

[jgoodnow]

Toddm, thanks for the info. I have Andersen 400 Series Woodwright double-hung windows throughout my house. Most are replacement inserts, but some are new construction; all are under two years old. They are not 100% airtight. Most of the windows have small "micro-drafts" at the lower left and right corners of the lower sash. I think this is less a flaw with the Andersen windows and more a fact of life of the double-hung configuration. Casement windows are inherently more airtight because it is very difficult to get perfect air sealing when the surfaces to be sealed slide past each other (double-hung), as opposed to one surface closing on top of the other with a fair amount of pressure to hold them together (casement).

Anyone have double-hung windows which are perfectly airtight?

 

[sgcsalsero]

Andersen's double pane Low-E4 windows have a u factor of .28 and a solar heat gain coefficient of .43. The choice for walls is eight inch or 12 inch block, and it's up to my engineer.

My .02, is your architect taking you thru a software program like Energy-10, I would think your backup heat variables/options could be effectively explored. Also have they have utilized a solar pathfinder for placement of 280 sq. ft. of glass and necessary summer overhang.

http://books.google.com/books?id=im...&hl=en&sa=X&oi=book_result&resnum=1&ct=result

 

[toddm]

I ran EnergyPlus back when, but have pretty much stuck to UCLA's HEED and ClimateConsultant4 software and the design tools at builditsolar.com. I will revisit the website; I haven't explored the other tools there.
Gotta say I don't need software to tell me to burn wood. I can't walk 100 feet on the building site without tripping over the stuff. If it is doable, marrying wood heat and thermal mass should allow me to run the stove at peak operating efficiency in burns of five to 12 hours while keeping the house at a reasonably stable temperature. The more I think about it, the more I like the big-tank-big-stove answer. Even in spring and fall, a toasty evening fire should provide a couple days of domestic hot water.

 

[allhandsworking]

Toddm, thanks for the info. I have Andersen 400 Series Woodwright double-hung windows throughout my house. Most are replacement inserts, but some are new construction; all are under two years old. They are not 100% airtight. Most of the windows have small "micro-drafts" at the lower left and right corners of the lower sash. I think this is less a flaw with the Andersen windows and more a fact of life of the double-hung configuration. Casement windows are inherently more airtight because it is very difficult to get perfect air sealing when the surfaces to be sealed slide past each other (double-hung), as opposed to one surface closing on top of the other with a fair amount of pressure to hold them together (casement).

Anyone have double-hung windows which are perfectly airtight?

Other than the micro draft how do you like the windows I am considering them. (the Woodwrite relacements) Also how long have you had them? The low E with Sun Smart get a 30% tax rebate (Stimulus)

 

[Dune]

I think the idea of a masonry heater is a very good one especialy since you are building the home from scratch. I have a freind in Pa whose masonry heater is and has been their sole source of heat for many years and they are quite happy with it. As far as a woodstove backup boiler, that is what I use, except I haven't used the oil burner at all, wood only. I have major problems with the idea of using thermal syphoning to remove the heat from the wood fired boiler though. I can't imagine how that could work, unless you know something I don't, which is entirely possible, perhaps even probable. I use an aquastat to turn on a circulator pump as soon as the water temp in the stove reaches 160 degrees F. As far as hot water goes, I would think you could use solar domestic hot water for 6 or 7 months and use your solar hot water tank in the winter with a coil or a sidearm and heat from your wood fired boiler. Since you stated you already have a lot of thermal mass, a large water storage tank should not be needed, storage is storage regardless of the form it takes.

 

[toddm]

The British stove manufacturers want a thermosiphon open vented loop so the stove can shed heat in any situation. Happily the stove I bought on eBay www.eco-boiler.com has four 1 1/8 inch connections for a pair of cross flowed circuits. I'm connecting one directly to the hydronic pumps, and the second to the tank upstairs. I may have the tank shipped from England, too. http://heatweb.com/ Then all that's left for me is to connect the pipes. The British pound is depressed to the point that it still makes sense after shipping ($2300 for the stove, all in.)
A heat pump water heater is an intriguing possibility for backup hot water. If passive solar works well, it could turn excess heat during the day into hot water. It would also knock down the humidity in the summer. But I need more answers before deciding.