Not enough BTUs from my stove?

[nola mike]

I've taken to monitoring the output of my insert by clipping a meat thermometer in the exhaust outlet, since it's so hard to measure stove top temps. Seems that it gives a better overall indicator of the heat output of the stove as well, since it's an average of the heat output of the stove. Stovetop temps, as best as I can measure them, peak at about 550-600'. The rated heat output of the insert is 56k BTUs EPA test, with the max listed as 65k. So what does the stove actually put out?

Well, the blower is listed at 130 CFUs/min. The max temp I can get out of the thermometer is about 280', though 260' is more realistic.

So using this equation: BTU= 1.1*delta temp*CFU, and assuming room temp of 60', my BTU output is about 31k. The wood that I'm using at the moment may not be top-notch, but to get the claimed BTUs from this stove means that I'd have exhaust temps of 480'. Not sure what that translates into as far as stove top temps, but it seems like that would be well over 700'. Or is it possible that I'm losing a ton of heat somewhere? Or what?

 

[jimbom]

How much radiant are you getting?

 

[nola mike]

Good point, hadn't thought about the radiant heat. Can't imagine that I'm getting 1/2 my heat through the front glass though...

 

[certified106]

Is there a block off plate in the chimney and if so how well sealed is it?

Edit: I should have completed my full thought but my ADD got the best of me. It's not as simple as how much heat is blowing out of the front of your stove. I am positive that the hearth around the stove is also absorbing heat and radiating it out to the room. It might not feel like much but it takes a lot of btu's to bring the a large mass up a couple of degrees and sustain that temperature. Heat Transfer/Thermodynamics can be a very touchy and it's not always cut and dry.

 

[madison]

I maybe totally off. But my understanding is that the calculated BTU outputs of stoves is conceived by the firebox volume, total btu's the wood stuffed in x.x cubic feet of firebox, and then looking at btu loss up the stack and subtracting that from the theoretical btu load in the firebox....

But your method, should give a fair estimate as well.

I would guess that the max btu's when calculated by the mfg are with the stove choked all the way down, so the stack temp (loss) would be at a minimum. Not necessarily real world burning conditions.

Thus the rule of thumb: assume at best 70% of the mfg calculated btu output.

Happy Holidaze

 

[ddddddden]

+ some heat is being conducted outdoors by the masonry, if the stove is located in an exterior chimney. I never figured out how many BTUs I was losing this way, but the back(outside) of my exterior chimney was being kept warmish by the insert, which was installed with insulation on the back and sides. Masonry acts like a big heat sink. . .bad news if half of it is exposed to the outdoors. To keep more heat in the house, I went with a hearth-mount stove when upgrading to an EPA unit.

 

[madison]

But to the point, as mentioned above, BTU ratings for units do not account for BTU "losses" to the hearth and non living area surroundings.

 

[nola mike]

Well, the install is an insert with 6" liner installed in a ZC fireplace/interior chimney. There is a blockoff of questionable effectiveness. I kind of figured that the insert with the air blowing around it should minimize my losses to the environment, but I guess that's overly optimistic. I guess "usable" BTUs are what someone should consider when buying a stove, and of course go big when possible...

 

[ddddddden]

I think the blower reduces but does not eliminate heat transfer to the surrounding masonry. It's not like the convection jacket of the insert doesn't get warm, which then transfers to the masonry. If your chimney is inside of the house, then it's no big deal. The heat stays in the house, but you can't measure it coming out of your blower. It occurs to me that my case of heating exterior masonry was probably extreme. The ol' stove's hot smoke dragon exhaust was venting directly into the masonry("slammer" install), so I don't really know how much of the heat I felt in the masonry was from the back of the stove and how much was from the exhaust, but I could see no reason to install another insert. And yeah, mfr specs are lab values. . .the best indicator of how many BTUs you will get from a particular stove is probably how many lbs of wood it will hold (assume roughly the same efficiency for all EPA stoves.)

 

[summit]

Well, the install is an insert with 6" liner installed in a ZC fireplace/interior chimney. There is a blockoff of questionable effectiveness. I kind of figured that the insert with the air blowing around it should minimize my losses to the environment, but I guess that's overly optimistic. I guess "usable" BTUs are what someone should consider when buying a stove, and of course go big when possible...

How full are you stuffing it when taking the measurement? Those numbers are projections when fully loaded and runnin wide open. probably get 1/3rd of that when not loaded fully, or shut down/lower burns, per hr.... You are correct, however, go big when at all possible. You can always have a smaller fire in a big stove, can't have a bigger fire in a small stove.