Wood gasifier + solar + tankless water heater to feed radiant floor?

Swamp_Yankee

I'm looking at the possibility of doing a 40' x 70' pole building home in the next five to six years and am sold on the idea of radiant floor (in the slab) but the question is what to feed it with. A friend of mine simply uses a tankless propane water heater to power his, and his home is very well insulated (R-30 walls, etc...), but I'd love to primarily utilize a wood gasifier (a Garn or Tarm, etc...) and solar and really only rely on the propane for when we're away for extended periods, etc... Obviously storage is key to this proposition, but how complex does the system have to be (electronics, controllers, etc...) in order to be relatively "hands off?" Ideally I'd like to only think about it in terms of firing the boiler every couple of days or so during the heating season in order to top up the storage. Obviously the solar will be dumping heat into the storage whenever the sun is shining and the propane will be there for whenever the storage is not adequate to heat the house and/or the DHW. Does anyone have any good links to schematics of a system that utilizes all three? I realize that there's a lot more to it in terms of sizing, etc... I'm just trying to get a handle on how to get each heat source to work in harmony at this point.

hot_rod

We have covered this a few times in Idronics, both from a solar perspective, and a wood fired. Explanation, piping, and control ladder diagrams of several systems.

Not a lot of solar available from a cold winter sky, short days, etc. You end up with a large array for not much thermal harvest. Then you need to tame that array in summer. Drainback makes the most sense for that lop-side load. At least 6- 10 large collectors to even put a dent in the load. 500 gallons or more storage to get by a day or so of non burn, depending on load of course.

You first need the heat load number. Then piece off how much each energy source could or should add.
If you have free wood and are young and willing, that is my preferred fuel. Burned via gasification for clean efficient transfer. It does get complicated if you want to walk away for more than what a burn cycle is.

Also, how much do you want the LP to contribute? 20%, 50%?

The tighter and more efficient the building, the less the load that has to be accounted for. Insulating that slab is a critical detail. An 80 degree slab outside on a 0 degree day is big temperature ∆. The edge and perimeter insulation is most challenging.

All this is estimable with some number crunching.

These are free download journals at www.caleffi.com

Larry Weingarten

Hello, I've got a few thoughts for you. First is to skip using a tankless water heater for space heating. There are lots of reasons which have been discussed on this site. It just isn't a good long-term way to go.

Now, the pole barn idea can be used effectively to greatly reduce heating/cooling loads. I have a friend in Boulder Colorado, who built an earth-coupled pole barn and it needs almost no energy for space conditioning, even when it's snowing. By being coupled to the huge mass of the earth under the building, the space remains nicely tempered. He insulated down four feet along the perimeter walls to minimize heat losses. He also went overboard insulating the structure. By combining that mass with a very well insulated and sealed building, you get temperatures than will not drift up or down very far. Of course, I don't know where you are or what your heating degree days are, but if the concept can work in Boulder, you probably have a good shot at it. With such an efficient structure, you can probably heat it with an incandescent light bulb.

Yours, Larry

Jamie Hall

There are a lot of moving pieces here, and they can either fit together well and do a great job, or... not so much.

First, orientation of the structure. If you can orient it to that the long dimension is east to west, you'll be much better off. Second, south wall: this could make a lot of difference. 10 foot high all glass, or almost all glass, and you will have 70 square meters of glass. On a sunny day, that will pick up 200,000 BTUh for the 3 hours around noon -- more heat than you will likely need, assuming the rest of the structure is heavily insulated. Third, then you have two problems: getting it to the back of the house (there's a lot to be said for clerestory windows for the back), and storing it. This can be a bit difficult, but some form of really heavy concrete slag and a hypocaust with a high volume but low velocity fan works. Fourth, heat loss. If at all possible, bury the north wall, and insulate the dickens out of all the walls except the south and also, perhaps particularly, the roof.

I'm very much of two minds about earth coupling. If you are in a setting where the earth is dry down at least eight feet, the extra mass below that hypocaust floor can be helpful. However -- from bitter experience -- if the is any ground water in that depth, forget it. Much better to increase the slab structure mass and insulate that, too, perimeter and underneath.

At this point you will have something which, as @Larry Weingarten said, you can just about heat with a light bulb. Your storage is in the mass of the slab and hypocaust. There is no moving or stored water to freeze. No pumps. You will want a nice high efficiency wood stove, though, or in that big a footprint possibly two - kitchen area and living area.

Or you could go with your gasifier boiler and a piped radiant floor and storage and a wet solar collector on the roof. As has been said, your away from home backup should be an LP boiler, NOT a water heater. The best control strategy probably will be sort of divide and conquer. The heart of the system will be the storage, of course. The volume of storage will be set by the heat loss of the house, how hot you want the stored water to be, and the temperature drop you can accept in the storage when the heat sources aren't available. The radiant floor will run with a mixing valve controlled by outdoor reset, probably with an interior trim, and come directly off the storage. The other heat sources -- drainback solar, your wood boiler, and the LP boiler -- should be controlled by the temperature in the storage tank. Simple aquastats will do for the wood boiler and the LP boiler, with priority as you like.

Hot_water_fan

The gasifier + propane can work great. I’d skip the solar thermal, solar thermal has lost out to solar PV so if you go solar, I’d use it to generate electricity. The radiant floor is actually the part I’d like you to think hard about: if you insulate and air seal well enough, then you’ll have an extremely comfortable, low heat loss home which is the gold standard. However, that will mean the floor temperature needed will be pretty low. Just want to make sure that’s clear before you proceed so you don’t wake up one chilly morning and put your toes on something that’s 75 degrees when you were expecting 95. 

Jamie Hall

A comment to add to @Hot_water_fan 's comment. And, perhaps, to clarify mine a bit. It you do decide to skip passive solar thermal, as I suggested (a lot of south glass and a mass floor/hypocaust for storage), you again need to be very careful about orientation and glazing -- this time not to collect heat, but to reject it. Significant amounts of south facing glazing in a heavily insulated house, if there is no storage which it illuminates or a way to move the heat, can result in real problems with overheating in those spaces. From experience with a number of houses, anything much over 30 to 40 percent glass in a south facing wall, unless there is a way to move the heat out of the space with the glass, WILL overheat the space, particularly in January and February.

I've never quite managed to figure out why folks don't make use of the heat -- it's free -- but many don't, and have to resort to drapes or shades...

hot_rod

And shading or over hangs to prevent summer over-heating on passive designs.

Jamie Hall

Overhangs work best -- they don't have to be outrageous, either. Awnings or movable shades are a pain...

leonz

A Garn unit is not available for sale unless it is used commercially now for heating.
A Garn unit has to be in its own space and wrapped in insulations to hold the heat in.
Are you going to keep all the fuel wood in the pole barn to keep it dry and 15-20% moisture?
Is this barn going to be placed in a shelter belt out of the prevaling winds or is it going to be totally exposed?? If it is exposed, will you be planting a shelterbelt to reduce the prevaling wind load exposure.

Do you have a source of fuel wood you can depend on? The Garn units can be installed with electric heating elements if desired. The slab under the Garn boiler has to be strong enough to support the weight of the boiler and the water supply that can range from 1,000-3,500 gallons.

There are many details to work out with this as the 2,800-foot square area of the barn is one thing but the barns interior height, number of windows, man doors, overhead doors and a possible second floor are things you have to settle on as well as the thickness of the wall and ceiling insulation and the type of insulation whether it's fiberglass batts, fire resistant cellulose, soy based foam or other foam.

The slab design with the proper foil back insulation and proper vapor barrier are a must have to help keep the heat in. Deciding on what size of pex tubing you need or want is one more item.
Deciding on how many heating zones you will need or make is one more thing or whether the entire floor is going to be one zone.

Deciding on what spacing of the pex tubing is one more thing 6", 12", 18" 24"? 6 inch spacing on approach aprons should be used to prevent ice and snow build up.

How many concrete aprons will you have? They may need to be heated as well to prevent ice build up.
Every time an overhead door or man door is opened the heat in the air will escape the building and have to be replaced.

Until you decide what you can spend or want to spend or can afford to spend we are whistling past
a graveyard here.

After all is said and done you may find that using steam to air garage forced air heaters will be better for heating this building only as you use it as dry steam can heat an area like this with a drop header and several ceiling header pipes feeding dry steam to the steam to forced air heaters and only heating the
building when you intend to be there and keeping a single thermostat set at 50 degrees when you are not there.

For the amount of money you wish to spend a coal stoker boiler used to make steam heat feeding several header pipes and garage heaters will cost you less money.

hot_rod

What are the diminishing return on underslab insulation? Based on pocketbooks I suspect.
The hydronic code in western Canada required 4" underslab insulation. For a short period of time! It brought the radiant slab business to a near halt. Homeowners, contractors and wholesalers all protested

The HD near me has 2" FOAMULAR at 46 bucks today. R-10. Would you spend another 46 bucks per 32 sq feet for R20? Not an easy question to answer.

Unfortunately the largest heat loss of a slab on grade is also the toughest to detail. The edge is exposed to outdoor temperature always. That is where you can see a 100° or more delta. That is where the R20, heck R50 would be nice. R5 per inch of foam makes a good R on slab edge hard to detail. I think perimeter, flat foam around the building has value. Beaver Plastics modeled that concept 30 years ago. Probably still find that data online.
Healthyheating.com takes a deep dive into slab insulation.

Solid_Fuel_Man

I built 3200 square feet of heated space (home and shop) in zone 7 at the top of Maine. I have heated the radiant slab on grade structure with wood since 2009. I used 1.5" Polyiso over standard R19 and also 1.5" Polyiso below R38 fiberglass. Did 2" on the north wall. No thermal bridging anywhere in the house. 

I did 4" of vertical Blue Board on the perimeter of the monolithic slab. And 2" under and 3' horizontally a foot deep around the perimeter. 

Less than 3 cord a year with a gasification boiler which also heats 100% of my DHW for a family of 4. 

I have a ModCon boiler I installed for backup which has been valved out accept for wood boiler preheat once in the fall. 

I did burn propane exclusively one winter here, and used 800 gallons. 

These numbers are well under half of what any other home here uses. 

Average homes here burn 1000 gallons of oil annually, we have 6 months of heating season. 

If you want any details I can share them with you. 

hot_rod

Solid_Fuel_Man said:

I built 3200 square feet of heated space (home and shop) in zone 7 at the top of Maine. I have heated the radiant slab on grade structure with wood since 2009. I used 1.5" Polyiso over standard R19 and also 1.5" Polyiso below R38 fiberglass. Did 2" on the north wall. No thermal bridging anywhere in the house. 

I did 4" of vertical Blue Board on the perimeter of the monolithic slab. And 2" under and 3' horizontally a foot deep around the perimeter. 

Less than 3 cord a year with a gasification boiler which also heats 100% of my DHW for a family of 4. 

I have a ModCon boiler I installed for backup which has been valved out accept for wood boiler preheat once in the fall. 

I did burn propane exclusively one winter here, and used 800 gallons. 

These numbers are well under half of what any other home here uses. 

Average homes here burn 1000 gallons of oil annually, we have 6 months of heating season. 

If you want any details I can share them with you. 

When you form the monolithic pour, do you form just around the slab and fill the trench below? I've never come up with a way to insulate the side of a mono pour. The horizontal insulation I'm sure helps with that detail.