# Calculating btu's- how does this pertain to boiler ratings.

Status
Not open for further replies.

#### Scotty2

##### Member
Hearth Supporter
My understanding is that it takes 8.33 btus to raise 1 gal of water 1 degree f.
If so, an 1800 gallon tank needs (at least) 8.33 x 1800 = 15,000 btu's?
If the mfg. states 1 firing will raise the water temp approx. 30 degrees (with no load)...then 1 firing is 450,000 btu's.
This unit is rated 300,000 btu...how does this equate to the above info?
Am I way off here?

Your understanding is pretty good on a theoretical basis, assuming every potential output btu reaches and is stored in the tank with no losses. Continuing with theory, if your wood is well seasoned (20% MC) and your average stack temperature is 400F, then one pound of wood can deliver 6050 btus. To get 450,000 btus, you will need 74 pounds of wood in the boiler for that one firing. And I assume your boiler can take 74 pounds of wood in a single load. The boiler rating of 300,000 btus is maximum output at high burn; average output over a burn, depending on your boiler, probably is around 195,000 btus, which means a firing length is close to 2-1/2 hours. End of theory.

Real world? Probably none of the above. Need a lot more info about your boiler, components, plumbing and install to get a better estimate based on reality.

Actual testing will show wood burning appliances running into the 80% efficiency range, and some of the OWF into the 40% range.

I'd guess low 80% is possible for a well fired gasification boiler with top quality fuel.

So a 300,000 BTU/hr input boiler X 80%= 240,000 output. Then as Jim mentioned you need to consider jacket and distribution loss to calculate how much gets to the actual load.

When considering the efficiency, at least three things come to mind, assuming other variables are constant: 1) efficiency at high burn; 2) efficiency over the course of a single burn (batch burn); and, 3) seasonal efficiency. IMO 1) will result in the highest efficiency, then 2) and finally 3), And then, when other variables are not constant, those efficiency numbers change again.

On a seasonal basis, I doubt any quality gasification boiler with storage achieves overall efficiency greater than 70%. At the same time, on my tests on efficiency at high burn (Garn WHS3200, Wood Gun E500 and Tarm Solo Plus 40) I find mid to high 80% range efficiency. And by efficiency, I mean btus computed on the basis of 6050 btu/lb for well seasoned wood input btus and output btus measured by increase in fully mixed tank storage temperature and/or logging delta-T boiler output/return at a known gpm and computing the btus. What's missing from this and which would or might increase usable efficiency, depending on the installation, are btus lost through the boiler shell and plumbing before the point of measuring.

So, if:
Temperature from Mixing Valve toward slab is 110 degrees
1800 gallon tank is at 160...and firing is complete.
When the tank temp drops 10 degrees, that would (theoretically), mean the slab used 150,000 btu to maintain temp?
And this would be consistent through each 10 degree drop until it gets to 120 or so?

When considering the efficiency, at least three things come to mind, assuming other variables are constant: 1) efficiency at high burn; 2) efficiency over the course of a single burn (batch burn); and, 3) seasonal efficiency. IMO 1) will result in the highest efficiency, then 2) and finally 3), And then, when other variables are not constant, those efficiency numbers change again.

On a seasonal basis, I doubt any quality gasification boiler with storage achieves overall efficiency greater than 70%. At the same time, on my tests on efficiency at high burn (Garn WHS3200, Wood Gun E500 and Tarm Solo Plus 40) I find mid to high 80% range efficiency. And by efficiency, I mean btus computed on the basis of 6050 btu/lb for well seasoned wood input btus and output btus measured by increase in fully mixed tank storage temperature and/or logging delta-T boiler output/return at a known gpm and computing the btus. What's missing from this and which would or might increase usable efficiency, depending on the installation, are btus lost through the boiler shell and plumbing before the point of measuring.

Efficiency at high burn. If instead of doing a single burn, I continue to add wood, am I lessening the efficiency of a 'high burn'?
bye the bye...thanks to you two, I've got to haul a scale out to the wood shop and weight a load of wood...What fun

What brand/model is the 300,000 btuh boiler? What's the plumbing layout? What distances and size of pipe are used? Is the slab the only zone you are heating? If not, what other emitters, etc. There can be a huge difference between theory and application in a particular install.

So, if:
Temperature from Mixing Valve toward slab is 110 degrees
1800 gallon tank is at 160...and firing is complete.
When the tank temp drops 10 degrees, that would (theoretically), mean the slab used 150,000 btu to maintain temp?
And this would be consistent through each 10 degree drop until it gets to 120 or so?

Theoretically, yes.

But you won't see that action in practice, unless your tank is constantly mixed. What you should see, with 110 water returning to storage, is a 160/110 stratification line that starts out at the bottom of the tank, then rises as the hot gets used up - so your water temp at top of storage will suddenly drop like a rock as that line rises close to the top.

Or is this something like open storage that uses a coil as HX so therefore does get mixed?

Efficiency at high burn. If instead of doing a single burn, I continue to add wood, am I lessening the efficiency of a 'high burn'?
bye the bye...thanks to you two, I've got to haul a scale out to the wood shop and weight a load of wood...What fun

Generally speaking, you will get better efficiency (heat transfer efficiency), the longer you can keep the coolest water possible (140) coming into the boiler return. Which usually means only charging storage when it is depleted. So then say after one lap through storage, when your return gets to 160, efficiency will drop a bit. Then drop a bit again if another lap starts.

How much of a drop it is - I don't know. Might not amount to much. But I think I noticed less wood consumption when I started fully depleting storage, and not heating it as high.

So, possibly a complete burn, then restart may be more efficient than a continuous burn?
GARN 2000...well insulated and well insulated to the slab...then terrible at the slab and warehouse...thus the Idea is to figure a very rough heat loss calc based on the reduction of tank temperature over a period of time when there is no fire in the box.

I'm hoping that will give me some sort of baseline to compare to when I insulate the warehouse walls.

Status
Not open for further replies.