Yesterday I brought home a used 1000 gal milk tank. My wife thinks I'v gone overboard. My plumer helper thinks a 500gal is better. but admits no data research. I probley can take it back to the supplier in exchange for a 500gal & get some $ back for other stuff to buy. My house has in floor heat with a lp 140 Munchkin, a recent heat loss calc by a geothermal guy said about 57 to 60M heat load. I just bought a used small wood boiler with a 10" x 10" door with a 20" front to back firebox, forced draft brick lined with a half way the sides & top saddle shaped water tank . Anyone got an opinon on size for a 60M heat load. I'm fine for the gas picking up the load if we leave for an weekend. Thanks
Buffer tank size
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I agree that one can go overboard with buffer tank sizing. They serve a few functions really. One is to serve as a "parking lot" for your thermal energy, BTUs.
The other is to handle small, or micro loads on a system. You can never eliminate the loss from a warm tank to the space so the insulation value has a bit to do wit how much energy escapes. The cost of piping, pumping and insulating neede to be considered, it all costs you in terms of $ and losses.
The size you chose depends on what you want the tank to do for you.
Even the smallest of wood boilers have an output @ 100,000 btu/hr. How does that match your load at design day?
Here is a buffer sizing formula Volume of tank = amount of wood X efficiency of burn X density of wood X BTU/ lb of wood at 20% moisture content (7000 btu/lb) minus building load x burn time
divided by 8.33 x delta T
Lets say 3 cubic feet of oak burned at 70% with a load of 40,000 btu/hr. for 3 hours
You heat the tank to 190F and draw it down to 160F
So 3X.7X36X7000 - 40,000X3
divided by 8.33X (190-160) = 802 gallons
Draw down time with the fire out would be 8.33 X 802 X 30 divided by 40,000 (load) + 5 hours of heat without fire burning.
Now if you have a radiant floor system, for example, and can run from 190F down to 100F do the math. 400 gallons of storage and a 7 1/2 hour storage at 40,000 load.
So ideally you would have a low temperature heating system, like radiant floor heat, and of course a low as possible load.
hr
The other is to handle small, or micro loads on a system. You can never eliminate the loss from a warm tank to the space so the insulation value has a bit to do wit how much energy escapes. The cost of piping, pumping and insulating neede to be considered, it all costs you in terms of $ and losses.
The size you chose depends on what you want the tank to do for you.
Even the smallest of wood boilers have an output @ 100,000 btu/hr. How does that match your load at design day?
Here is a buffer sizing formula Volume of tank = amount of wood X efficiency of burn X density of wood X BTU/ lb of wood at 20% moisture content (7000 btu/lb) minus building load x burn time
divided by 8.33 x delta T
Lets say 3 cubic feet of oak burned at 70% with a load of 40,000 btu/hr. for 3 hours
You heat the tank to 190F and draw it down to 160F
So 3X.7X36X7000 - 40,000X3
divided by 8.33X (190-160) = 802 gallons
Draw down time with the fire out would be 8.33 X 802 X 30 divided by 40,000 (load) + 5 hours of heat without fire burning.
Now if you have a radiant floor system, for example, and can run from 190F down to 100F do the math. 400 gallons of storage and a 7 1/2 hour storage at 40,000 load.
So ideally you would have a low temperature heating system, like radiant floor heat, and of course a low as possible load.
hr
Nofossil
Moderator Emeritus
From my experience with a milk tank, I'd suggest that a maximum average temperature of 170 is more reasonable. Because an unpressurized tank is heated through a heat exchanger, it's very hard to get it close to the boiler outlet temperature. On the other hand, you can get usable heat out of baseboards down to 140 degrees.
Bottom line: You want enough storage so that you can build a fire at a convenient time of the day, burn enough wood to proviude a day's worth of heat, and not have to worry about it until the next day. In my case, I need about seven hours of boiler burn time on the average winter day. That means my storage has to carry me the other 17 hours.
Bottom line: You want enough storage so that you can build a fire at a convenient time of the day, burn enough wood to proviude a day's worth of heat, and not have to worry about it until the next day. In my case, I need about seven hours of boiler burn time on the average winter day. That means my storage has to carry me the other 17 hours.
My infloor antifreeze from the Munchkin is seldom above 105f, in the coldest winter nights it has never gotten to 120f. The dealer (a local Maple syrup equipment place) said they will take back the tank & sell me what ever I want from their inventory, I'm a regular Maple equp customer for them. My Wife wants a smaller tank so Ill go pick out a used 500gal insulated milk tank & run the system on that this winter. If I don't like it, I'll move it to the surgar house & buy a bigger tank for the house heating system. Around here we Maple sugar makers can find milk tanks. Neither of you jumped up & down against a smaller tank, the math thing looks like it will work for us since we arn't away from home that long & if so our house stays warm for a really long time during powr outages. Thanks for the help.
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