Hot room in Green room

[byQ]

Well, I'm building two hot rooms to collect the sun's radiation - passive solar construction technique. I built up 2 rows of narrow cinder blocks. I got to work on my new found masonry skills. I only did 2 rows for now so I can get the insulation ready for the main slab concrete pour. I'll finish these rooms when the house has a roof and walls. The white styrofoam blocks seen are insulated concrete forms (for outer wall construction).

These hot rooms are about 5'x15' and 5'x13' or 140 sq feet in a ~1520 sq foot house (40.5' x 32.5'). I hope they work. By work I mean I hope they create enough btu's to cut my wood usage by half. I'm installing five 3'x6' south facing windows, and painting the insides of these hot rooms black for maximum heat absorption. But I don't know how much heat they will collect on a sunny winter's day.

It will be interesting to see how well they work. I didn't do any calculations but am going by feel. The cinder blocks are 3.5 inches wide. After putting on a stucco coat, inner and outer, they should be about 4+ inches thick. Too thin and the wall won't have enough thermal mass to hold much of the sun's radiation. Too thick and the wall will hold the radiation but won't pass it on into the outer wall (and thus into the living space).

I got a good deal on the 3'x6' windows before I found out they weren't shatter proof (tempered glass). I learned that non-tempered glass must be 18" above the floor, oops. I planned on installing windows 4" to 6" above the floor. What to do? I lowered the floors in these hot rooms to compensate. The hot room slab's will be 10" to 12" lower than the main house slab (which will be a couple inches below the top cinder block in the picture). Well do you think it will work? And how well?

 

[woodgeek]

I'd suggest you do a little more research. A lot of passive solar work done by 'feel' in the 70s really didn't work well at all. You don't want to repeat the same mistakes from 40 years ago.

1. Superinsulation (and airsealing) with a small amount of passive solar gain is great (and does not require a huge dedicated thermal mass).

http://www.greenbuildingadvisor.com...lar-versus-superinsulation-30-year-old-debate

2. Passive collectors (windows) are great at shedding heat at night, so you need to think on a 'gain/loss or profit basis'. Either you need fancy expensive windows, or thermal curtains that you are now on the hook to open and close every day you live there, or you can separate the collector from the storage. You might want to consider building an unheated sunroom/sunporch with a LOW thermal mass, and insulation between the room and the house. That way it will heat up quick in the winter on sunny days, you can open the doors to let the heat into the house, and then when the sun goes down, just close the door. Any heat out there when the doors are closed will be stranded and lost (so low thermal mass is better).

This sort of solution can be improved by:
--adding a thermostat controlled fan to blow the air into the house when the space heats up, automatically.
--you can avoid overheating your house in the spring and fall by leaving the door closed or fan off....control!
--make enough openable windows in the sunspace so you can keep it from being an oven in the summer, and you have a great space for starting plants, sitting in the sun, etc.

Also, surfaces don't have to be black black. Anything dark will be fine.

Gary Reysa at http://www.builditsolar.com/ is a member here. He can tell you anything you need to know.

 

[btuser]

Go for the super-insulation. It's the best thing ever.

 

[Seasoned Oak]

The sun provides a tremendous amount of heat during the day ,but those big windows give most of it back during the long night. I have partially solved this by allowing the sun heated air into the main house during the day through my front door which is between the sun room and the main house. At night i simply close the door once the sun room get cool. It works so well i can go about 12+ hours on solar alone on a sunny winter day. If my house were better insulated it may go much more.

 

[Seasoned Oak]

Seems the right recipe is some solar,lots of insulation and a wood stove.

 

[GaryGary]

Hi,
Looks like a really interesting project.

I'm a big fan of low thermal mass sunspaces (LTMS) for space heating.

The basic idea is that you you have an attached sunspace that is optimized for collecting heat and sending it to the house. During the day when the sun is shining, the sunspace makes a lot of heat and a blower moves the heat to the house. At night when the heat loss would be high, the sunspace is shut off from the house and just goes cold.
Optimizing the sunspace for producing house heat consists of: 1) Lots of glazing angled for good winter heat collection - preferably double glazed; good absorbing (dark) surfaces for the walls and floor; 2) all walls, floors and ceilings that are not glazed are insulated to reduce heat loss, 3) all the surfaces that the sun shines on are low mass so that the solar heat goes into heating the air and then the house, not into heating the sunspace mass, and 4) a good fan or blower and duct system to move the heated air from the peak of the sunspace into the house as fast as the heat is produced.

As long as the fan/duct system that moves heat from the sunspace to the house is sized correctly, the sunspace can be comfortable to be in (that is, not overheated) because the heat is being removed as fast as the sunspace makes it. This takes a pretty good fan/duct system -- it needs to move air about about 3 cfm per sqft of glazing to keep up with the sunspace heat production.

Good summer ventilation is also needed if you want to use the space in the summer -- probably along with some shade cloth.

To me, the LTMS is a really nice combination of adding useful space to the house that can be used for all sorts of things and at the same time getting a lot of good space heating.
Downsides are that you can't grow plants in an LTMS as it goes cold at night, and if the sunspace is large and the house is really well insulated, the sunspace can overheat the house -- this can be addressed by adding storage, but for most cases, the house will have enough mass to store the heat the sunspace can produce.

If designed correctly, they are as efficient at space heating as good commercial solar collectors with the same glazed area.

I did a whole section on this, including a pretty careful test of heat output of a test low thermal mass sunspace. Also several folks contributed detailed descriptions of their low thermal mass sunspaces.
http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#LowMassSS

We are doing an article for Home Power that will be out this winter on low thermal mass sunspaces -- it basically has most of the stuff in the section linked to above in a nice, easy to digest format.

Gary

 

[byQ]

Thanks for the feedback. No fan or ductwork will be used just natural air cyclying from cold to hot. The house is small and open. I think a combination of good insulation and solar gain is what I'm after. And modern house construction should be using both of these concepts. I'm pretty well insulated - ICF walls and 5" rigid board under the slab. The slabs in the hotrooms are also insulated on every side but the top. I'm going to do some extra rigid board insulation in the ceiling, too. I will probably put 4 openable vents in each hot room - 2 high and 2 low to let natural air cycling occur.

Most thermal shutters seem to be to big and bulky. But pull-down tight fitting insulated curtains are reasonable - maybe 2 seperated by a 1" space per window. I don't mind dedicating 5 minutes a day to this chore. I've read some of those 70's book on passive solar gain, and some of the ideas are good. But ya some were carried to extremes.

I think I will put dark grey/black natural slate/rock on the walls and floors. It will look good and is functional. Seasoned Oak I'll never install a wood stove in any house/cabin I live in since I will have the skills and knowledge to build masonry heaters which are superior heating devices. Why wouldn't I want the best? Just because most people take the well-worn same beaten path as everyone else doesn't mean I have to. And I will be able to build these superior heating device(s) at the cost of a mid-priced wood stove.

 

[DevilsBrew]

I also have experience with a sunroom. The room evolved to become our living space. There were crank windows, french doors, and a fireplace to regulate the temperature. Another use for the room was to extend the season for plants. I loved the sunroom so much that I wish I had a passive solar glass house to live in.

 

[byQ]

David & Geek, thanks for the links - interesting reading.

 

[DevilsBrew]

What is the update on your masonry heater build?

 

[byQ]

What is the update on your masonry heater build?

DB, Still working on building the house. I'll probably be able to do something with the masonry heater next spring. I'm going to attend a masonry heater build in Las Vegas, NV, in the spring. And try to attend a masonry heater workshop in Northern Minnesota in the winter at Northhouse.org. Northhouse has a rocket heater workshop, too. Two skilled masonry heater builders are starting to explore building rocket heaters/stoves.

I've been trying to figure out what size contraflow masonry heater to build. The plans I have have 2 contraflows, a small one 36" x 24" x 6' (36 ft3), and a large one 48" x 36" x 7' (84 ft3). I'm going for something in the middle say 42" x 30" x 6' (52.5 ft3). I've learned that MH size isn't that important in terms of heating. If you have a big heater and want less heat you just burn less firewood. The MH doors I got such a great deal on are too big for a small MH. Oh well, I'ld rather have a firebox I can get 18" pieces of firewood in instead of 14" to 16" pieces.

I've altered my house design to accommodate a medium sized masonry heater by increasing the house volume - 1) cathedral ceiling, and 2) upstairs bedroom/loft (+325 sq ft). Might as well give the MH something to heat. And I've decided to go with a refined organic look - 1' green marble tile on the outside of the MH and maybe a natural brown field stone hearth (horizontal and vertical). The MH will look like a large bush - a burning bush!



I'm going for a MH about this size with a small bench and an oven, but it will be green instead of red brick. And mine will have a bigger double door. I'll use red brick, but on it's side (facing method). Than I'll mortar/mesh and use 1 foot marble tiles to finish.

 

[Seasoned Oak]

Seasoned Oak I'll never install a wood stove in any house/cabin I live in since I will have the skills and knowledge to build masonry heaters which are superior heating devices. Why wouldn't I want the best? Just because most people take the well-worn same beaten path as everyone else doesn't mean I have to. And I will be able to build these superior heating device(s) at the cost of a mid-priced wood stove.

Most people take the well worn path of a wood stove No1 because it works and No 2. Probably 99 of those wood stove users have neither the cash to buy one or the time and skills,space or desire to build one. Its the law of diminished returns, if a wood stove is 85 % efficient and your MH is 90% how much did you pay for that 5% and is it worth it? You could say a fuel cell car is a superior trans.device as well but you wont get 99.9% of the population to pay 100K to find out.

 

[byQ]

Most people take the well worn path of a wood stove No1 because it works and No 2. Probably 99 of those wood stove users have neither the cash to buy one or the time and skills,space or desire to build one. Its the law of diminished returns, if a wood stove is 85 % efficient and your MH is 90% how much did you pay for that 5% and is it worth it? You could say a fuel cell car is a superior trans.device as well but you wont get 99.9% of the population to pay 100K to find out.

I guess sometimes I like to take a harder path just because I can. Even though cost-wise, a masonry heater won't be that much (~$1200) it will take a lot of personal time and learning. But I am fascinated by mass heaters and enjoy the challenge of attempting to build one.

 

[woodgeek]

The other issue, of course, is that MHs and superinsulated houses are not really compatible. A right-sized MH might be the 'ultimate' wood heating appliance for traditional construction, but with the lower heat loads of a 'modern' efficient house, even in Idaho, a MH might consistently overheat the place unless you make it so small that it works more like a stove anyway.

What I'm getting here is that you are building your own house, using your own plans, learning everything you need to know as you go. Great. It seems you have set a goal of heating entirely with wood, and using 'half' as much wood as a typical neighbor with the same sized house, through a combination of three technologies: passive solar technology, superinsulation and high eff masonry heater tech.

We are trying to tell you is that instead of learning about and juggling three complex technologies....solar, superinsulation and MH, you could instead just go for the superinsulation, skip the other two, and get wood usage that was 75% less than a typical neighbor, exceeding your goal. A reasonably superinsulated 1500 sq ft house, say to **half** the insulation levels of a Passive house, 2 BTU/sqft/HDD, would required 2*1500*8000 = 24 Million BTU per year, and much of that would come from appliances, so you would need <1.5 cords of softwood per year to heat the place, tops. The natural cooling time of the house (due to its thermal mass) would be longer than that of a masonry heater, so you would get equal (awesome) comfort from a MH and a small cheap stove doing ~2 burns a day, and the latter would get you better 'control' and faster warm up. If you got tired of kindling a couple fires a day from scratch, you could heat the place with two electric space heaters for <$1000/yr, or a single minisplit for <$400/yr. Or heck, a small cat stove you could throttle down and run continuously, like a Woodstock Fireview would save you a bundle on kindling.

So, from my point of view, you are not building an eff house that uses half as much wood as a house should. You are instead building a house with 2x the heating need of the current 2013 best-practice house, and thus creating a heating need that you will then try to bandaid over with two other things you are fascinated with, passive solar and MHs. For the same amount of learning/building effort and cost, you could go for superinsulated walls, either thick double stud and (cheap) cellulose or conventional 2x6 framing + exterior foam (with cheap recycled foambaord), with careful airsealing, using blower-door tests before the drywall goes in.

The basic idea of a low-cost, optimal superinsualted house is called the 'pretty good house' and there is a ton of info online:
http://www.greenbuildingadvisor.com...ng-blog/regional-variations-pretty-good-house

 

[Seasoned Oak]

Im fascinated by everything your doing there byQ so ill be following this thread and your progress. Keep in mind that as your solar gain and the insulation levels go up, the size of your MH and any other kind of heater may have to go down. As WG said if your house loses heat slower than the the MH is pumping it out you may have a problem. Plus you have the thermal storage to factor in as well,could be tricky. You may wind up getting most of you heat from the solar.

 

[GaryGary]

Here is a superinsulated home with an MH and some passive solar that appears to be working OK.

http://www.builditsolar.com/Projects/SolarHomes/SchoolHouseRetrofit/Main.htm

I'm sure Gordon would be happy to answer questions about how the combination is working for him.

Gary

 

[begreen]

Woodgeek brings up a very good point. It would be worthwhile investing in a plan review. Especially consider getting a heating and cooling load engineering analysis before casting decisions in stone and cement. Wise decisions made now could prevent expensive remedies later on.

 

[woodgeek]

Here is a superinsulated home with an MH and some passive solar that appears to be working OK.

http://www.builditsolar.com/Projects/SolarHomes/SchoolHouseRetrofit/Main.htm

I'm sure Gordon would be happy to answer questions about how the combination is working for him.

Gary

Nice project. I noted that he designed the MH for 1 fire per day, and an average output of ~10 kBTU/h, and he has a huge amount of thermal mass in the house (which helps avoid overheating).

 

[Seasoned Oak]

Im, tempted to try a little thermal mass in close proximity to my stove as im still finishing the stove room. It gets terribly hot in there in winter mid 90s,with the stove on low,makes the room pretty much uninhabitable. Other than checkin on the stove i dont spend much time in there. Perhaps something to siphon some of that heat off for later. Was also thinking of some type of disguised water container, but probably a lot of stone surround is the ticket.

 

[woodgeek]

the trick with thermal mass is either a modest amount of mass with a large temp swing, or a large mass with a small swing. And the rate of heat transerf in and out of the store has to be 'right'. Two things work well....(1) in a masonry heater a 'modest mass' heats up a good deal, and (2) in your house the entire house swings up and down a small amount and is VERY massive. In a SS stove, the mass is smaller than an MH, but the swing is a little bigger, and the effect is worthwhile, but not as great as with a MH. Water is tough, unless you are circulating to a store and some hydronic radiation.

 

[Seasoned Oak]

I have to build a finished wall around the back and sides of the stove so im thinking 4" cement block with stacked natural slate stone against the block.Not sure if i should fill the block voids or not. I figure about 1-2 ton + when finished. Could be more as the slate cube is 2 ton alone but i dont think ill need it all.
Would this qualify as the modest mass with the large temp swing?

 

[woodgeek]

I think it would be a modest mass with a small temp swing. How much do you expect the mass to heat up from a single burn? 20°F, 40°F? How long will it take to release that heat, 2 hours, 20 hours? If it is too massive, and the release is 20 hours, then it will basically equilibrate when you are doing a lot of burn cycles, and not do much good.

Unless you entomb the stove in masonry, most of the heat will still escape, and the mass will capture a fraction only. It should help some, but it won't be nearly as good as MH of the same mass.

If I were do it, and feeling experimental, I would be tempted to leave some voids in the masonry for channels (like a heatilator). If the mass is cooling off too slowly without the channels, open them. If they are cooling too fast with the channels, close them. If not enough heat is being captured, blow some heat in there. IOW, you'd have options.

 

[Seasoned Oak]

I think the stone would heat up nicely as the sides and back of the stove would be very close to the stone. I wont do a MH at my present location as i plan to move within 2 Yrs . But perhaps some kind of hybrid system may work.Id like to work water into the mix. Heat source is full solar today. 55 outside but sunny. You may be right that the stone may not capture enough of the heat but my burn times are about 12-15 hours load. At present i use the whole house as a heat sink,stove or solar heats it to around 80 ,takes half a day to get below 70 again after stove is out or overnight. So 1 load a day with no sun. Zero loads with sun. Very cold weather & no sun = 1-2 loads in the stove.

 

[byQ]

The other issue, of course, is that MHs and superinsulated houses are not really compatible. A right-sized MH might be the 'ultimate' wood heating appliance for traditional construction, but with the lower heat loads of a 'modern' efficient house, even in Idaho, a MH might consistently overheat the place unless you make it so small that it works more like a stove anyway.


What exactly is a masonry heater and exactly how does it work? It is a double walled thermal mass unit that is usually charged by a load of firewood. The load of firewood is burned rapidly in 1.5 to 3 hours. The heat energy from this chemical reaction is absorbed into the thermal mass. The outer surface of the thermal mass than radiates heat energy into the surroundings. If you want less radiant energy you charge the masonry mass with less energy. Which means the outside of the thermal mass will be at a lower temperature. Which means a good sized masonry heater won't overheat a superinsulated house if you charge the thermal mass with less energy - burn a smaller load of firewood. (So the outer wall of the masonry heater is say 120 F instead of 150 F).

In places where kachelofens and masonry heaters are common (Northern Europe) you will sometimes find large masonry heaters/kachelofens in small cottages. The cottage is heated by the owners burning small loads of wood like an armload of branches.

Superinsulated/supertight houses kind of remind me of being trapped underground in a coffin. And yes I've heard of an HRV. I would rather have a house that "breathes" a little bit and equates itself to the outer environment.

 

[Seasoned Oak]

byQ
WHat do you mean by double wall. Is there an air space inside? I was under the impression the masonry was solid between the interior flue area and the exterior . Also ,what would say the average weight of a small-medium one is.