Biomass grower said:
First just let me say that I greatly appreciate the new Garn website. It is much easier to use and info was easier to find. I also like that it did not seem too commercial in the sense that "my unit is better than their's because it has XYZ widgets and theirs doesn't. I have to say that so far I'm leaning in it's direction for a number of reasons.
My question involves the stated BTU storage capabilities of the units. The smallest unit says something to the effect of 920,000BTU's. Obviously this is a best case senario, but I have calculated that my current nat. gas INPUT per month is ~150 therms which I calculate to be at 500,000btu's per day ON AVERAGE (please correct me if I am wrong as the rest depends on this). According to this, if I fired up the garn to top temp (200F), if I had no standby losses (i.e. no heat loss to environment) I would only have to fire it once every other day. Is this really possible or am I terribly wrong?
So if I built a very efficient storage shed with some wood storage inside and placed it close to my house (<75ft) do you think I could get away with firing it once per day?
And for those who have a unit, how much wood/how fast does it take to be all that water up to temp?
The statement is true. It's math so simple even I can understand it. BTU's transfered is simply the weight of the stored water x temperature drop. The formula used is for the WHS 1500 which has a nominal water capacity of 1500 gallons.
So here goes........1500(gallons) x 8.33(pounds per gl) x 80*(temperature drop 200-120) = 999,600 BTU's
Now for the variables because that is a pure math illustration only.
The first and biggest variable that throws a wrench in the works of that equation is this; what is the minimum usable temperature your heating system requires? For example: If you have a radiant floor system that is designed to work with water temps all the way down to the 90-100* range you can easily apply that figure to your application. If on the other hand you have a coil in your forced air furnace that needs a minimum of 140* you have 20* less drop to work along with a corresponding loss of usable BTU's.
(do the equation with 60 instead of 80* temp drop and you end up with about 750K)
The second variable is the weather obviously. As you already noted based on your therm usage, 500,000 BTU's is the average. At design conditions it wouldn't be abnormal to use 2-3 times that per day mandating at least a couple good fires per day. The flip side is that in shoulder season type weather, your heating demand may only be 150-200,000 btu's per day.
Thirdly, as you have already noted there will be standby losses and circulation losses. On the Garn installations that we have done I would say we average 5-7% over 24 hours provided no heating demand . Someone said 25% and that number could easily be the case in a poorly installed and designed system. I would say that would not be typical by any means and it should never be the case in a good installation. Certainly possible, I've seen lot's of installs that would go past that number.
The thing to remember is that there are a lot of variables that affect heat loss in your home. There is no way any responsible person would or should ever tell you that you'll only have to fire it once a day. If you run across such a person or salesman, politely say thank you, turn your back and walk away. He is either a liar or he doesn't have a clue what he's talking about.
As far as how fast the Garn will raise the water temp you can use the same equation only slightly reversed. The 1500 has a firing rate of approximately 325,000 btu/hr. ......again........depending on a few factors, but we can use that as an average with good wood. So if you drain approximately 900K out of the tank and recharge it a 325K/hr x .75( avg. efficiency) the math says about 4 hours. The 1500 and the 200 models are both sized to hold enough wood to last for about a 2 1/2 to 3 hour burn. You can figure it out from there.
Based on our experience with sizing jobs and installing the boilers and related items our customers with Garns usually wind up firing one full load plus another light load in a 24 hour period. The nice thing about the Garn is that tending the fire is not needed. When starting with a few coals (very few are needed with decent wood) you just fill it, crank 2-3 hours on the timer and walk away.
So as we heating guys are fond of saying........."It depends"..............
