Manufacturer's BTU vs EPA BTU

[kalevi]

Hi Folks:

I have a Pacific Energy Vista insert. The PE brochure says it has a heat output of 56,000 BTU and the EPA test was 26,300. I don't have any calibration in terms of standing in front of a heat source of x btu to tell how much heat I get but I'm pretty sure I'm not getting 56,000BTU = 12 standard baseboard heaters on at the same time. How do the manufacturers measure this? EPA test talks about 4x4 oak cribs. What is an oak crib? What is the typical heat output of a stove/insert compared to specs when burning a few good hardwood splits at lower air intake settings?

Thanks,

 

[webbie]

They are 2x4 and 4x4 douglas fir with spacers nailed around the ends- that is what they use for testing.

Here at Hearth.com, we always suggest that folks NOT take EITHER the epa figures or the man. rating(s) into account, but go by the cubic foot capacity of the firebox.

As an example, if your stove could fit 30 lbs of hardwood in your stove (based on cubic firebox), that would be about 240,000 BTU. We have to adjust that figure down by about 20% to make up for the wood moisture content - so 200,000 BTU worth of wood may fit into your box. Now we have to adjust for efficiency, so using 70%, that would be 140K OUTPUT. Note that an insert may do worse because of the cold masonry mass and part of the unit behind the panels.

Also, a wood load is rarely burned down to nothing, so the effective load is even smaller.

So what that all means is a 4-5 hour burn at the mid-20K BTU level. If you could max it out to 56, it would only burn slightly over 2 hours, and they would probably accuse you of over firing!

To answer the 2nd part, I would guess that most stoves such as yours are run at 15K to 20K BTU, about the size of a Kerosene heater, or three plus electric heaters.

 

[stoveguy2esw]

EPA ratings for BTU output comes from the data from the test burns of which there are several , the average is taken from that and listed as the "epa rated output" , this test is not performed with oak , but douglas fir , dried to a specific moisture content , then weighed. the size of the charge (or load) put in the stove is determined by the measured cubic ft capacity of the firebox. this charge is set up to allow virtually complete combustion, as the weight of the charge is measured during the burn (the unit is burned while sitting on a scale) and the finished weight is factored into efficiency rating

this test is not a clear determination of the output the stove will give in real world conditions. but the charge that is used is used to determine not only the efficiency of the unit as far as percentage of efficiency , but also to determine the "GPH" of particulate matter released by the unit. this is important as the unit must come in under a ceratin standard (based on method of recombustion) to carry an epa certification. the listed data which the stove performs at in the lab , must by law be posted on the stove in plain view (most use a hang tag or stick it to the window) this label is even standardized by epa , and must be printed in a certain format. (which is why all epa hang tags look virtually identical)the BTU listed on that tag is what the unit did burning that charge of douglas fir. not a normal "jouse load" of your favorite wood. so they always come in very low.

the data listed showing this is unfortunate but necessary, but does not reflect real world conditions. we at ESW have a lab virtually identical to the one at intertek (warnock hersey) given the expense of testing , we run the same battery of tests prior to taking a unit to the lab to ensure that the unit will be able to pass, im sure most if not all other manufacturers do likewise as if the unit fails , its a large loss, and the cost to retest is the same , once the unit is taken off the test stand , the test is over and the manufacturer must pay again to test the same unit again as many times as necessary to obtain the certification.

in doing our testing , we also burn test loads of "real wood " to determine output that a customer would be more able to judge by, we clearly list that these figures (whil accurate as far as we can test to) are "unofficial" as the testing is done in our in house lab. we do try to be as accurate as possible with these numbers. the reason for that is clear, if the unit doesnt get that hot (as we say it will) obviously we would be questioned by the consumer about it.

i hope you find this explanation understandable and gives you a better insight into what the tags mean as well as the reason behind manufacturers claims and listed numbers. if you would like a more detailed explanation of how this data is collected , (at least in as far as how we do it) , i'd be happy to share it with you in greater detail. its not a "trade secret" or somthing that the industry tries to hide. (if it is , they didnt tell me so i'll spit it right out if asked)

 

[nshif]

Mike Id like to see you to write up what could be a wiki entry on this subject as I think it would be interesting to alot of us and educational. maybe you and Corrie could colaberate on it. might be good info for newbies trying to pic out a stove,

 

[webbie]

but the charge that is used is used to determine not only the efficiency of the unit as far as percentage of efficiency ,

its not a "trade secret" or something that the industry tries to hide. (if it is , they didn't tell me so I'll spit it right out if asked)

Mike, two questions....

1. You say "the efficiency of the unit", yet EPA says on their own web site:
"Efficiencies shown are default efficiencies. These stoves have not been laboratory tested for efficiency."

Are you saying that the ACTUAL operating efficiency is tested and available to the customer from the hang tag or EPA?

2. The enclosed table shows three EPA tested stoves - all non-cats with similar technology. All have similar size fireboxes (within 10%) and similar low burns. But the high burn varies vastly....and so does the GPH.

One reason is that the manufacturer and the Test Lab technicians are allowed to manipulate certain parts of the testing procedure. I have listed those below......anyway, as to the "trade secrets", it would appear that there are many, and that these have been used by some manufacturers so that they appear to have an advantage, while there actually may be none at all (or the other way around, the higher GPH stove may burn better or cleaner with oak cordwood).....


I have listed some of the ways that Omni (the test lab) states that the tests can be manipulated - totally legally and within spec.

Anyway, let me know if your info is different on the efficiency testing. EPA says:
"There is currently no strong impetus to improve or test wood stove efficiency. There is a well documented efficiency test method published as a proposed method by the U.S. EPA" (but it is neither required nor used).


-------list of legal ways to change the EPA "numbers"

Using higher or lower average moisture content fuel loads ranging from 19% to 25% (dry basis) to increase or decrease burn rates and/or emissions rates,

Placing higher or lower moisture content fuel pieces at different locations within
the firebox (e.g., bottom/top or back/front) to control the timing, location, and
temperature regimes of pyrolysis products and volatile gas releases into secondary
combustion zones,

Placing higher or lower fuel density pieces at different locations within the firebox
to control the timing, location, and temperature regimes of pyrolysis products and
volatile gas releases into secondary combustion zones. There are no Method 28
fuel density specifications and Douglas fir wood densities vary up to 60% from
low to high density fuel pieces,

Starting the test at the high or low end of the allowed coal-bed size range (i.e.,
20% to 25% of the test fuel load weight) to affect the start-up pattern and the
ultimate average burn rate and emissions characteristics for the fuel load,

Starting the test at high or low average firebox/stove temperatures which also
affects the start-up pattern and ultimately the average burn rate for the fuel load.
The desired relative firebox/stove temperature can be obtained by both managing
how much of the coal bed is present at test start-up and managing how the stove is
operated before the required one-hour, no-adjustment pre-burn period is started,
and,

Using fuel load weights at the high or low end of the Method-28-allowed fuel
weight limits (i.e., 7 pounds per cubic foot of firebox volume, plus or minus 10%).
Fuel load weight differences of 10% can affect burn rates and measured emissions
rates. It is well known that smaller fuel loads produce lower emissions rates at any
burn rate.

Although the effects of these factors have not been quantified, the overall concern is that EPA
NSPS testing can be manipulated and a practiced technician can prepare custom results

 

[kalevi]

Thanks for the answers. Merry Christmas and All the Best for the New Year!