Efficiency calculations?

[James Reimer]

I posted this on another forum, but I thought I would put it here as well to generate some controversy.

This is a quick test you can do with a boiler that contains a fair amount of water such as the Optimizer 250. Ok, I wouldn't say it's particularly quick, but no good (accurate) experiments are.

With the fire out, drop the water temperature to 140, then weigh and burn an amount of wood (for the optimizer 250 I found that 20 lbs. was a good amount) and see what the increase in temperature is. Do this with the heat load off of the stove but leave the circulator on to make sure the water is being evenly mixed. I have found that around 20 lbs. of wood would raise the 240 gallons of water approximately 40 degrees, from 140 to 180.

Use softwood for this experiment so the wood burns up fairly quickly. Don't worry about the last bit of small hot coals as they don't hold much BTU compared to the rest of the wood. You can also use this type of test to see what the true BTU output of your stove is by timing how quickly it burns the wood.

So here are my calculations:

BTU's to heat the water: 240 gallons X 8.35 lbs. X 40 degrees = 80160 BTU.
BTU's to heat the metal and masonry in the stove: 1500 lbs. X .12 BTU per pound (specific heat of steel) X 40 degrees = 7200 BTU.
Total BTU's required: 80160 + 7200 = 87360.

20 lbs. of wood has 6191 BTU per pound at 20% moisture content, so it has 6191 BTU X 20 lbs. = 123820 raw BTU.

So the efficiency calculation using 20 lbs. is 87360/123820 = .7055

So the efficiency is around 70%.

You could run this experiment multiple times to get a pretty accurate rating for your stove. Overall I think I comfortably get around 70% honest efficiency average from the Optimizer 250. It may be as high as 75% under some conditions.

If you don't have a large water capacity stove, you could run the same experiment drawing off a set amount of heat if you measure the flow and delta T of your load.

Off topic, but for the REAL nerds in the group, using molecular weights I have calculated that burning 1 lb. of wood (at 0% moisture) will create 0.6622 lbs. of H2O and 1.70 lbs. of CO2. (Obviously using 1.3622 lbs. of Oxygen from the atmosphere.)
That's a lot of water generated for potential condensation corrosion!

Also, when it is tuned for efficient operation I have found the true output of the stove to be 125,000 BTU absolute maximum. It is officially rated for 250,000 BTU but I would suspect that rating to be wood consumption when running wide open, not actual BTU's into the water.

Still enough BTU's for my purpose, but I'm guessing all manufacturers are guilty of this practice.

FYI, I'm not bashing P&M in any of my posts. I think they are one of the best out there and would absolutely recommend them over others.

Thoughts? Am I doing this wrong?

 

[ewdudley]

Looks like a nice solid measurement. It would be nice to have more storage so you could see what the efficiency of a longer steady-state burn would be, but demonstrating a no-nonsense 70% efficiency with just 20 lbs of wood is very encouraging I would say.

This is known at the BBMCP (Ballenthin Batch Mode Calorimetry Protocol):

-Start with a know quantity of wood with a known moisture content.
-Instrument system with a few temperature measuring devices. Boiler return and supply, plus tank top and bottom should be sufficient.
-Account for the water volume of the boiler, plus the water volume of storage. Optionally account for the steel in the tanks and the boiler.
-Override the storage circulator and bypass return temperature protection device for a while until all temperature sensor readings are reasonably consistent. Average the starting temperatures and record as beginning system temperature.
-Burn the wood with no load being drawn from storage.
-Again override the storage circulator to mix the system. Average the ending temperatures and record as ending system temperature.
-Calculate the heat gained by the system and divide by the LHV of the fuel supply, to yield efficiency.
-Optionally estimate heat loss rate from storage and account for it.

It's simplified some in your case with no external storage tank to worry about, but so much the better. Jim himself will be along in a couple hours to compare notes with the tests he has done at Deep Portage and with his shop system.

 

[jebatty]

BBMCP? Thanks ew!

Here is a link to the efficiency test I did which matches pretty well the protocol of OP JR's test.

https://www.hearth.com/talk/threads/storage-charging-stratification-efficiency.79688/

 

[jebatty]

Are you sure your calculation of maximum output at 125,000 btuh is close to correct for your 250,000 btuh rated boiler? My Tarm is rated at 140,000 btuh, and the data in the above link shows average output of 127,500 btuh over the 6.5 hour burn period. The Tarm will hit the 140,000 btuh rated output and exceed it by a moderate amount at a very high burn rate, which also means stack temps into the high 400F range.

 

[tom in maine]

Depending on the plumbing of the storage, you should have multiple storage tank sensors. Most tanks, once drawn down have some level of stratification.
This must be in the equation. In a usual tank, an upper temperature can be significantly higher than a lower temp, especially if you draw DHW off the tank.
70% is pretty decent considering losses off the boiler shell which usually are contributory for an indoor boiler this time of year.

 

[ewdudley]

Depending on the plumbing of the storage, you should have multiple storage tank sensors. Most tanks, once drawn down have some level of stratification.

To conform to the protocol the boiler and tanks should be mixed before and after the burn. Multiple sensors is good because it can show that the discrepancies between various sensors remains about the same before and after, and to show that the mixing is the same before and after, but the mixing is still needed to help eliminate the stratification factor.

 

[James Reimer]

I think 125K output for this boiler is fairly accurate. I know there is no way I can get 250K actual heat output continuous burns from it. The firebox size and airflow just aren't there. There is no way I could burn 63 lbs. of wood an hour.

 

[Karl_northwind]

I'd like to see the corollary to that test, which would be: when the stove is up to 180F or whatever, and the firewood load is exhausted, shut it down(except maybe that mixing circulator), and see how long it takes to drop 50 degrees. Of course it would take a chilly or cold day (normal heating season outdoor temperature) and see what the standby heat loss of the outdoor burner is. not including the underground piping of course.
5000 btu? 10,000 btu?

karl

 

[flyingcow]

Got to find s set of scales for my wood. Already have a frig by the boiler, a nice chair...... sounds like fun?
The Garn Jr took water from 47f to 180f in 3hr 40 mins.
I didn't measure the weight of my wood. But the MC was low 20's.

 

[Karl_northwind]

Got to find s set of scales for my wood. Already have a frig by the boiler, a nice chair...... sounds like fun?
The Garn Jr took water from 47f to 180f in 3hr 40 mins.
I didn't measure the weight of my wood. But the MC was low 20's.

just got 'er fired up eh? did you get the momentary oh S#!t... when the condensate started draining out of the flue upon first fire up?
I really like the junior and what it can do, and the price point.
karl

 

[maple1]

Got to find s set of scales for my wood. Already have a frig by the boiler, a nice chair...... sounds like fun?
The Garn Jr took water from 47f to 180f in 3hr 40 mins.
I didn't measure the weight of my wood. But the MC was low 20's.

Do you have a guess at cu.ft.?

 

[Karl_northwind]

Got to find s set of scales for my wood. Already have a frig by the boiler, a nice chair...... sounds like fun?
The Garn Jr took water from 47f to 180f in 3hr 40 mins.
I didn't measure the weight of my wood. But the MC was low 20's.

that's waaaaayyyy higher than the advertised 180,000 btu firing rate. if you figure 10,000 lbs of stove and water, and a 133 deg rise, that's like 1.3mmbtu, and the firing rate and 3.66 hours should give you like 660,000 btu. were you mixing? and how many batches of wood?

 

[jebatty]

Without weighing wood consumed during a burn, it is nearly impossible to be objective in evaluating boiler performance. Of course, other things also need to be known objectively. And a protocol needs to be described and followed which can be duplicated. As others have stated, multiple sensors and thorough mixing of the water being heated is necessary. A plus is temperature data logging and graphing of the storage vessel and the flue temperature.

 

[flyingcow]

No mixing. No not sure of cu/ft of wood. But I think it was at least 3 loads of wood. And yeah the water dumped out of stack until it got above 100f-ish.

I PM'd heaterman because it seemed like quite a jump. But he allowed with the water so cold at first the efficiency was very high. Chris Holly agreed also. But keep in mind I kept the fire box full. As soon ad it dropped down to a half a load I refilled it. When the Garn burns pretty much every stick is on fire. The Jr was shaking the door at times.

 

[Karl_northwind]

No mixing. No not sure of cu/ft of wood. But I think it was at least 3 loads of wood. And yeah the water dumped out of stack until it got above 100f-ish.

I PM'd heaterman because it seemed like quite a jump. But he allowed with the water so cold at first the efficiency was very high. Chris Holly agreed also. But keep in mind I kept the fire box full. As soon ad it dropped down to a half a load I refilled it. When the Garn burns pretty much every stick is on fire. The Jr was shaking the door at times.

sounds like fun. they really are batch-burn. I've seen strange things happen when loaded partially, and re-filled mid-burn.
and I glanced at the cut sheet again, the 180Kbtu is the rated over a full burn. the instantaneous output at low water temps and full burn must be much higher, (never mind condensing mode)

karl

 

[TCaldwell]

The starting temp was about as close to a mixed temp and probably pretty accurate, however using the face gage or strap on for finish high temp, probably not too accurate. You might try placing some sensors on the face and averaging the temp and using weighted burns for consistency. Reload mid burn will work with larger splits. Enjoy your boiler!