Next Step on Sizing a Garn

[mlougheed]

Been lurking around the site for awhile, finally signed up a few weeks ago and now have my first question. My wife and I are going to be building a house in a few years. She is handling the design work and I'm looking into the heating aspect. It will be a single story, 3500sq ft slab on grade house with a 600 sq garage and a 1000sq attached secondary garage. I would like to do all in floor heat and DHW the 6-7 months the Garn is running. The Garn would be in a corner of the secondary garage and the supply/return line would run to a centrally located utility room where the water heater and in floor manifolds would be.

I entered our preliminary info into a heat loss calculator on builditsolar.com and it came up with a 106kbtu/hr loss for the house, not including what the water heater needs. We are planning on having an electric water heater so if it doesn't get what it needs it can heat its own water. I live in central MN and that high heat loss would be on the few times a year when it gets down to -30. Is my next step to look at the various storage BTU ratings of the units and divide that by my loss to figure out how often I would need to load it? It doesn't seem that simple but that's why I'm here.

I'm very new to using wood as a method to heat a house so I would very much appreciate any and all advice you can give me. Thanks ahead of time!

 

[Buzz Saw]

I'll be the first to tell you to start cutting splitting and stacking your firewood now. Cut as much as possible now so you are ready for later.

Good luck!

 

[jebatty]

Exciting project, I'm sure. Ditto cut, split and stack to have on hand two years wood supply when you want to first start heating with wood, and maintain your wood supply to always burn wood that has dried for two full summer seasons. Use mixed wood species, especially not only oak. You won't be disappointed in the results.

First reactions, regardless of the heating source:
1) insulate the perimeter around heated space down at least 4' below grade and insulate under the entire floor of all heated space. Most recommendations are minimum 2" foam, I would consider 4" around perimeter and do some research on whether 4" under part or all of the floor is appropriate.
2) insulate the entire house, 6" walls minimum, foam would be my first choice; dense pack second choice.
3) put in the highest quality, lowest e-value, windows you can possibly afford.
4) install the in-floor pex oxygen barrier tubing no more than 2" (maybe 1-1/2" is OK too) below the finished concrete surface. Concrete responds to temp changes very slowly, and you want the heat to go up, not down.

Second reactions:
5) site for passive and active solar, your forever free source of energy for heating, cooling, and electrical energy.
6) provide for excellent water drainage away from the structure.
7) use LED lighting fixtures or fixtures that can take LED bulbs as well as other standard bulbs.

Now, more to your question:
8) 106kbtu/hr is a lot of heat energy. Temps to -30F are quite uncommon and when they occur don't last long. A primary heating system planned for worst case may be overkill. Consider a heating system designed for -10F or so, and then a secondary heat source to cover the short, really cold periods. If you have off-peak or dual rate electric available, maybe a rate 1/2 of the general service rate, a booster electric boiler, strategically placed space heating, or similar alternatives may provide a very good solution. I live north of you, temps into the -35F range and colder are the design temp in my area, 35kbtu/hr is design heat load for my shop, but the most btu/hr I ever have reached in a 24 hour period is a little less than 20k with -35F temps, although I tolerate a lower interior temp during these periods of about 5-8F.
9) the Garn is good and so long as you can supply needed hot water on a batch burn basis (allow at least one period every day or two during the coldest periods (probably longer periods during milder weather) when the fire is out and you can clean the ash and coals from the Garn), you should be OK. Plan your radiant in-floor to use the lowest possible water temperature. In my shop I mix down to 100F. This gives you maximum heating draw down capacity from Garn storage and increases the time between needed firings.
10) If you consistently need water hotter than 140F, I recommend considering a gasification boiler with external storage of 1,000 gallons minimum, more would be better for your install. My favorite is the Froling, in part because I have personal experience with the Froling and it is a very sophisticated and high quality boiler, but there also are other quality brands. A gasification boiler is well designed to provide 140F+ hot water on a continuous basis when needed. With external storage it also will function similarly to a Garn.

And for a completely different take on your situation:
11) Supplement for cool weather (down to about 25-30F) mini-splits for heat, which also would provide your AC during hot weather, and if your utility provides a reduced rate for these, I think this is a no-brainer for a new build. The result is no need to burn wood for a substantial portion of the heating season, saving time and $, and the energy cost, especially if reduced rate, is very low, and easily could be less to much less than the cost of wood. And finally, with a solar PV install or plan for future PV, your energy cost could be about $0 for the mini-split heating and cooling.

Lastly, make this a fun family adventure.

 

[Karl_northwind]

You have an ideal GARN situation. everything Jebatty above said holds true, with the additional: GARN has the option for built in electric elements.
the low temperatures you need for in-floor radiant will be perfect, allowing the most heat usage from the GARN before re-firing.

you can use a differential temperature control to only deliver heat to the DHW tank when the boiler is hotter than the tank,and not strip it back off once the boiler has cooled down. you'll probably need a heat exchanger and antifreeze in the floors for the garages, to avoid freeze concerns. that will complicate things a bit.

you'll probably be looking at a GARN 2000, (same price as the 1500) but you won't be disappointed in your purchase. it'll keep you warm for the next 30-40 years.

cheers,
karl

 

[jebatty]

Correction:

3) put in the highest quality, lowest e-value u-value, windows you can possibly afford.

 

[Buzz Saw]

The 2000's are also in stock while the 1500's are special order

Can a Garn be oversized? Or any system with storage?

 

[Karl_northwind]

The 2000's are also in stock while the 1500's are special order

Can a Garn be oversized? Or any system with storage?

not really, but most of the time that involves spending more than necessary across the board, for boiler and tanks.

 

[mlougheed]

Lots of good advice coming through here. I had been thinking about using the electric backup heat option with the Garn. It looks to be easy to integrate and I wouldn't have to worry about my slab cooling down much if we left for a long weekend. This is going to be the house we grow old in so we aren't planning on cutting corners with insulation, windows, etc. Maybe the slab will stay warm enough so the recovery isn't too long. I'm sure it will be hit or miss that first winter while I get it dialed in.

You have an ideal GARN situation. everything Jebatty above said holds true, with the additional: GARN has the option for built in electric elements.
the low temperatures you need for in-floor radiant will be perfect, allowing the most heat usage from the GARN before re-firing.

you can use a differential temperature control to only deliver heat to the DHW tank when the boiler is hotter than the tank,and not strip it back off once the boiler has cooled down. you'll probably need a heat exchanger and antifreeze in the floors for the garages, to avoid freeze concerns. that will complicate things a bit.

you'll probably be looking at a GARN 2000, (same price as the 1500) but you won't be disappointed in your purchase. it'll keep you warm for the next 30-40 years.

cheers,
karl

Few questions for you. What dictates the need for something other than water running through the floors? If, and could be a big if, I never let the temp of the slab get down to below freezing, is having water a concern? Second question, I redid my load calc for -10 let Jebatty suggested and it came to around 89k btuh. I want to do this right the first time but I don't want to be firing it every 6 hours either. Is there a specific reason why you think the 2000 is the right one, not considering it's the same cost as the 1500?

 

[jebatty]

Would you have fun with a hydronics lesson? Answers are below.
How much does a gallon of water weigh?
How many btu's does it take to raise the temp of one gallon of water 1F?
How many gallons of water in a Garn 2000?
How hot can you regularly expect the water to get in a Garn 2000 wood burn?
How low of temperature of hot water can you still use to get the heat you need?
How many btu's would be in that hot water from the maximum to the minimum temp?
With a heat load of 89k btu/hr, how many hours of heat can you get before you need to burn again?
Same Q but with a heat load of 50k btu?

A: gallon of water weight = 8.33 lb
A: btu's to raise 1 gal of water 1F = 8.33 (1 btu/lb of water)
A: Garn 2000 has 1,830 gallons of water (Garn specs)
A: Garn can be heated to 195F (Garn specs, I would use a slightly lower number as a practical matter)
A: Lowest usable water temp = 120F (Garn specs, but your system may allow for a lower usable temp)
A: Btu's in 1,830 gal of water with a temp difference from 195F to 120F = 1,143,292 (8.33 x 1830 x 75 = 1,143,292 (Garn specs show 1,170,000 btu's and that number may include the heat value in the Garn itself and/or other factors not identified, but the difference is not significant)
A: Hours of heat at 89k btu\hr = 1,143,292 / 89 = 12 hr 51 minutes
A: Hours of heat at 50k btu\hr = 1,143,292 / 50 = 22 hr 52 minutes

This of course is laboratory stuff, does not account for heat load consumed during a burn or other variables. But you can use this info to run your own calculations on whatever assumptions you want to make. And you can do the same scenario with the Garn 1500 to answer your question on the 2000 vs the 1500.

BTW, the same info can be applied to a non-Garn gasification boiler with storage, using the volume of storage rather than the Garn volume.

And, as an approximation as to how much wood in lbs it takes to generate all of this heat, you can use 6,050 btu/lb for 20% moisture content wood. So, the 1,143,292 stored btu's in the Garn would take 189 lbs of wood to obtain those btu's. Again, a laboratory-kind of computation, many variables in the real world.

Keep in mind that hydronics is fun, and so is the path to knowledge and wisdom.

 

[Buzz Saw]

Would you have fun with a hydronics lesson? Answers are below.
How much does a gallon of water weigh?
How many btu's does it take to raise the temp of one gallon of water 1F?
How many gallons of water in a Garn 2000?
How hot can you regularly expect the water to get in a Garn 2000 wood burn?
How low of temperature of hot water can you still use to get the heat you need?
How many btu's would be in that hot water from the maximum to the minimum temp?
With a heat load of 89k btu/hr, how many hours of heat can you get before you need to burn again?
Same Q but with a heat load of 50k btu?

A: gallon of water weight = 8.33 lb
A: btu's to raise 1 gal of water 1F = 8.33 (1 btu/lb of water)
A: Garn 2000 has 1,830 gallons of water (Garn specs)
A: Garn can be heated to 195F (Garn specs, I would use a slightly lower number as a practical matter)
A: Lowest usable water temp = 120F (Garn specs, but your system may allow for a lower usable temp)
A: Btu's in 1,830 gal of water with a temp difference from 195F to 120F = 1,143,292 (8.33 x 1830 x 75 = 1,143,292 (Garn specs show 1,170,000 btu's and that number may include the heat value in the Garn itself and/or other factors not identified, but the difference is not significant)
A: Hours of heat at 89k btu\hr = 1,143,292 / 89 = 12 hr 51 minutes
A: Hours of heat at 50k btu\hr = 1,143,292 / 50 = 22 hr 52 minutes

This of course is laboratory stuff, does not account for heat load consumed during a burn or other variables. But you can use this info to run your own calculations on whatever assumptions you want to make. And you can do the same scenario with the Garn 1500 to answer your question on the 2000 vs the 1500.

BTW, the same info can be applied to a non-Garn gasification boiler with storage, using the volume of storage rather than the Garn volume.

And, as an approximation as to how much wood in lbs it takes to generate all of this heat, you can use 6,050 btu/lb for 20% moisture content wood. So, the 1,143,292 stored btu's in the Garn would take 189 lbs of wood to obtain those btu's. Again, a laboratory-kind of computation, many variables in the real world.

Keep in mind that hydronics is fun, and so is the path to knowledge and wisdom.

Jebatty, I've always enjoyed reading your posts. I'm not a huge math guy, but I like how your explainations are easy to follow and your logical, everyday use with numbers.

I'm following this thread closely because my heat load is similar. With you doing the math I can better explain things the the wifey. Thank you.

Is there a known value on how much a piece of wood weighs? For example: ash 6" diameter cut @20" with 20% moisture. Is there a chart out there?

Thanks again

 

[Buzz Saw]

Or a known weight of a cord of ash,hickory,etc? I know some guys weigh thier burn loads but can't remember who.

Thanks again

 

[Karl_northwind]

Or a known weight of a cord of ash,hickory,etc? I know some guys weigh thier burn loads but can't remember who.

Thanks again

That would be Jim.

also keep in mind that the useable heat for in-floor could well be below 120F, although getting the GARN below about 100 F will result in *some* flue gas condensation, which is to be avoided, so I would set the electric backup at 110 or so.
on antifreeze: for outbuildings the risk is too great that you'll freeze something up, so you really should antifreeze them. the plus side is that if you're only doing a single building the HX and such can be sized smaller (and therefore cheaper) than accommodating the entire system. you obviously don't want to fill the entire Garn with antifreeze, but in-floor tubing etc doesn't take that much (tables available online) and it lets you shut off a building (say when you're gone for an extended period so as to not use electric to heat your garage etc)

do you have trees around there? I have friends up the road and don't remember any.....

 

[heaterman]

There is no sense in going with a 1500 vs a 2000 at this point because they are the same price. The only real choices are the 1000 and the 2000. If you can swing the roughly $5,000 more for the WHS2000 it's a "no brainer".

 

[Buzz Saw]

There is no sense in going with a 1500 vs a 2000 at this point because they are the same price. The only real choices are the 1000 and the 2000. If you can swing the roughly $5,000 more for the WHS2000 it's a "no brainer".

Don't you mean swing the wife $5,000 more?! [emoji3]

 

[eauzonedan]

I agree with Heaterman unless the space is tight. Allow a minimum of 3+ feet behind to get at your your plumbing and insulation as well as 5+ feet in front to feed the beast. My 20 foot deep building is about perfect for a 1500. Allow more for a 2000

Also note you can stretch the time between burns a bunch if you watch your use of any higher temp emitters or indirect Dhw if mixed with in-floor. Shifting shower time /Dhw recharge and any use of rads helps. I also put the in-floor in the shop on a timer to time shift things a bit due to the thermal mass of the concrete floor out there you don't even notice it. At -30 I'll burn twice and above about zero a single burn with +/- the same loads as you and running a 1500. You will love the pretty much hands off the Garn and its storage gives you.
Dan

 

[Leeky]

Been lurking around the site for awhile, finally signed up a few weeks ago and now have my first question. My wife and I are going to be building a house in a few years. She is handling the design work and I'm looking into the heating aspect...

I'm very new to using wood as a method to heat a house so I would very much appreciate any and all advice you can give me. Thanks ahead of time!

hello,

Just finished building my new home in the Yukon.

This is my take on heating with my Garn jr

Design the building with energy efficient in mind . My design heat loss at -45 was 32000 btu, Slab on grade 4" hs40 high density foam u/s slab, R-60 walls. r-70 attic.,
Triple glazed low E argon windows. for example. The building is 2100 sq ft w 9 ' ceilings and a 1000 sq ft garage, and garn room attached.

Now to heating . Keep it simple low temp low flow heating system. Do not use the garn for DHW! I have a small elect back up boiler That I do not use. Elect HWT. Looking at solar options as well.
I fire the garn once a day regardless, down to about -15 then ever second or third day. I rarely fire it to design temp 190 deg. as long as I see a bit of dribble once in a while from the over flow i figure the fluids have expanded to fill the tank ( corrosion protected)
The system is Techmar controlled and I do not have a mixing valve in the system. None Needed. I also have fan coil forced air. Sized the tonage on the fan coil to put out 30000 btu at 120 drg. The whole system will heat down to 80 deg or so. Dont want to go lower ( condensation to avoid)
I treated the garage the same as the house as far as design for heat loss and heat it as well and never had to load the Garn more than once a day. Even at -35. Haven't had -40 , -45 temps yet as i started using it i in Feb? or so and it was a mild winter.
I have been in the const industry for 35 yrs with cold climate experience always open to new suggestions and ideas.
So far I am very satisfied with my Garn

Still learning

 

[heaterman]

Awesome and well thought out system design there Leeky. You have engineered a perfect match for a Garn which allows you to take 100% advantage of the storage principle.
Nice! Real nice design!