BTU rating confusion

[Jason Knapp]

I'm trying to figure out why the Englander 25 EP has so many different BTU ratings. Some reviews say 27,000 BTU, some say 55,000 BTU.
Then throw in the square foot heating abilities ( based on many factors, of course) and this really skews numbers around. Does anyone know for sure how many BTU's the 25 EP is capable of producing?

 

[SmokeyTheBear]

I'm trying to figure out why the Englander 25 EP has so many different BTU ratings. Some reviews say 27,000 BTU, some say 55,000 BTU.
Then throw in the square foot heating abilities ( based on many factors, of course) and this really skews numbers around. Does anyone know for sure how many BTU's the 25 EP is capable of producing?

Yes it depends upon the pellet being burned and how clean you keep the stove as to what will come out into the room.

And that by the way is all that matters.

Everything else is a crock.

 

[Jason Knapp]

OK then. I was hoping to buy another stove and I was looking at the P61 Harman, 61000 btu sounded good.

 

[mchasal]

According to the manual (http://englanderstoves.com/manuals/25-EP-EPI_Combined.pdf) the output is 25,100 BTU max. So, I'd expect that to be correct. As other's have said, the pellets used and other factors can affect that number. Also keep in mind that some manufacturers (Harman included) specify input BTUs instead of output so you can't directly compare the numbers in that case.

You have to multiply input BTUs by the efficiency to get the output. I don't know the efficiency of the Harman P61, but if it's around the 78% that the Englander quotes, that's 47580 output BTUs (61000*.78)

 

[Jason Knapp]

Excellent! Thanks.
I was always trying to figure out why my stove would keep up with my friends Quadrafire and my brothers Breckwell.
My stove isn't as fancy as some of the higher end stoves, so my previous statement may not be valid, but it kicks butt and keeps my family nice and warm for a lot less money than oil!

 

[smwilliamson]

Then there is also the low heat value and the high heat value.

The heating value is the amount of heat produced by combustion of a unit quantity of a fuel
We differentiate between gross and net heating values:

Gross (or high, upper) Heating Value
The gross or high heating value is the amount of heat produced by the complete combustion of a unit quantity of fuel.

The gross heating value is obtained when

• all products of the combustion are cooled down to the temperature before the combustion
• the water vapor formed during combustion is condensed

Net (or lower) Heating Value
The net or lower heating value is obtained by

• subtracting the latent heat of vaporization of the water vapor formed by the combustion

from the gross or higher heating value.

No useful heat can be generated until the water vapor in solid fuel is expelled. Manufacturers like to use the HHV on their units cause it looks better on the hang tag, yet what you get into your house is according to the LHV. This delta is between 3%-19%, though with pellet stoves the average is 8%.

 

[mchasal]

The gross heating value is obtained when

• all products of the combustion are cooled down to the temperature before the combustion
• the water vapor formed during combustion is condensed

Interesting. So since for the HHV you have to let any water vapor produced condense, thus giving back (some) of it's heat, does that mean that HHV is obtained in a closed system? In actually application any water vapor would be vented with the exhaust and so recovery of that heat isn't possible. Am I understanding that correctly?

 

[smwilliamson]

Yes, that heat is lost and not recoverable, unless you have a condensing unit...which you do not. So the actual BTU is usually much lower than advertised once you factor in efficency and the LHV calculation

 

[SmokeyTheBear]

And that is only under ideal conditions since that so called efficiency is actually a varying figure over the course of a burn.

Funny thing about burning things that produce ash is that the efficiency is not a constant. You can loose a large fraction of the heat due to that one thing alone.

 

[smwilliamson]

Speaking of BTU's, NH just passed into law (Massachusetts just voted today but I don't know the outcome) that the REC funds (Renewable Energy Credits) which states receive as $$$$ from the offsets of BIG utility companies have to now start making payments to folks that use small scale biomass to heat their home. Indeed, in the future, once a reliable metering system is adopted, residents in the States that approve the measure, will start to receive annual payments for using their stoves...just like solar, wind and geothermal enjoy. Just a little piece of info for ya. Wouldn't you like a couple hundred bucks a year?

 

[Harvey Schneider]

But don't the same considerations exist for all fuel burning heaters? The water produced by an oil burner as vapor is lost to the stack and the heat value is not recovered. The only system I know of that recovers the heat is a condensing gas fired system. There may be others.
When it comes down to it, the BTU statements are only meaningful relative to each other. They give you a feel for relative capability of each appliance, but all are only approximations at best.

 

[smwilliamson]

There are condeensing pellet boilers too

 

[SmokeyTheBear]

There are condensing oil burners as well.

 

[Jason Knapp]

This is starting to sound eerily familiar to Thermodynamics when I was in school. Yuck!
But had I been a little more diligent in class, I could have answered my own question.

 

[Jason Knapp]

These responses to my post are very intriguing. I am impressed at the amount of knowledge on this forum!
Some of the responses remind me of the steam cycle that is used in the power plant that I work in. We use extraction steam to heat Condensate by means of multiple feed water heaters. We also use higher temp and pressure drains from one stage manager of the feed water heater to help heat feed water in lower temp and pressure heaters. We try to recover as many BTU's as possible by reusing saturated water to heat our feed water so our reactor doesn't have to add a lot of heat.