Cat temp into BTU

[Wolves1]

Now I’m really geeking out here. Is there a formula to convert cat temperatures into BTU output or a chart? Might be a great project for an engineer student.

 

[bholler]

Now I’m really geeking out here. Is there a formula to convert cat temperatures into BTU output or a chart? Might be a great project for an engineer student.

It would need to be specific to each stove. And even then it is going to vary depending upon the install.

 

[jetsam]

Even if someone made an install-specific chart for your house, it would still vary with airflow through the stove, which is influenced by draft and stove settings. Draft is influenced by pressure and temperature differences between the stove's intake and top of the stack, and the stove's air settings. BTU output into the house varies with the stove's temperature and air movement over the hot parts of the stove.

All in all, even a chart made for your exact install in your exact house would be pretty inexact, and would at a minimum require also measuring stack temperatures and stovetop temperatures to see what percentage of generated heat was going up and what percentage was going into the house.

I think this would be a fun question to answer, but it's a project for someone with several thermal probes, a digital manometer, a working knowledge of at least one programming language, and a raspberry pi- not someone with a piece of graph paper and a calculator.

 

[Sawset]

Let us know when you close in on the 6th order Fourier approximation. We'll all hop in and help yah out.

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[Highbeam]

No. Since the output from a cat stove comes from both the cat and the actual firebox, there is no direct relation.

I have an actual cat thermometer with numbers and it usually just sits within a very narrow range , about 1300, whether I'm on low output settings or high. It's only the super low settings where the cat is beginning to stall that I see it come down.

A closer way to estimate might be flue temperatures.

 

[Mech e]

I think this would be a fun question to answer, but it's a project for someone with several thermal probes, a digital manometer, a working knowledge of at least one programming language, and a raspberry pi- not someone with a piece of graph paper and a calculator.

Hmm, BTU output calculations were around long before microprocessors.

 

[jetsam]

And they made

Hmm, BTU output calculations were around long before microprocessors.

But they were pretty crappy and approximate for the above reasons.

How are you going to sit down and figure out what your draft is going to be next January 17 st 4:15?

I'm not down on what can be done with a pencil - the mind-boggling design of the A-12 and its famous brother the SR-71 were both done by guys with slide rules and graph paper- but I the path to 'success'without accounting for dynamic variables here is to say "oh well it was only supposed to be approximate anyway".

 

[Wolves1]

Ok here’s some information, I filled the stove with 46 pounds of wood 15% moisture hard wood. Estimating 24,000 BTU an hour for 11 hours. Stove (regency f3500) air was half open, with the regency BTU numbers is also around 24,000 btu hr. Here are some charts.
This chart is cat temp top line, flue temp bottom line.

this chart is outside Temps.

this is inside temps. Between 73f and 71.3 the entire 11 hr.

Did a heat loss calculator for my house came up with 24,600 btu loss with the outside temps last night, consistent with all other btu numbers. Not sure if this would help with the btu and stove temp comparison but was fun doing.

 

[Mech e]

That is a fun approach to the problem.

And they made

But they were pretty crappy and approximate for the above reasons.

How are you going to sit down and figure out what your draft is going to be next January 17 st 4:15?

I'm not down on what can be done with a pencil - the mind-boggling design of the A-12 and its famous brother the SR-71 were both done by guys with slide rules and graph paper- but I the path to 'success'without accounting for dynamic variables here is to say "oh well it was only supposed to be approximate anyway".

Getting an "accurate" (need to define accurate ahead of time) answer to BTU output outside of a lab environment will definitely require some approximations. The actual calculations are well understood and we use them on a regular basis. The key to this problem is not draft, the fuel used, or the internal design of the stove. The key to solving this problem is the surface temperature of the stove and pipe exposed to the room. This will drive both the radiation and natural convection components of heat transfer into the house. If you have a blower, that is another set of calculations that has to be added to the problem.

The surface temperature of the stove and pipe are anything but homogeneous. This is one of the main issues that will affect the accuracy of the BTU output calc. Here is how I would approach this problem with the understanding that I am not trying to design a Swiss watch.

Get the stove to steady state and get an average temperature for each stove surface and the flu pipe. I would then run calculations for both radiation and natural convection of heat transfer into the room for each surface. Add them up and you have total BTU output. I would then correlate this to my cat temp and call it done.

We have developed many spreadsheets (a great engineering tool) for helping out with these types calculations. We also use very complex computational fluid dynamics software for these problems, but that would be overkill. If you guys are interested, I could put together a spreadsheet that would do the calcs for you based on your particular setup. I could even add forced convection (a blower) into the equations as well.

 

[brazilbl]

For those who have a decent level of geek in their bloodstream would be very appreciative of that kind offer...

 

[Wolves1]

That is a fun approach to the problem.


Getting an "accurate" (need to define accurate ahead of time) answer to BTU output outside of a lab environment will definitely require some approximations. The actual calculations are well understood and we use them on a regular basis. The key to this problem is not draft, the fuel used, or the internal design of the stove. The key to solving this problem is the surface temperature of the stove and pipe exposed to the room. This will drive both the radiation and natural convection components of heat transfer into the house. If you have a blower, that is another set of calculations that has to be added to the problem.

The surface temperature of the stove and pipe are anything but homogeneous. This is one of the main issues that will affect the accuracy of the BTU output calc. Here is how I would approach this problem with the understanding that I am not trying to design a Swiss watch.

Get the stove to steady state and get an average temperature for each stove surface and the flu pipe. I would then run calculations for both radiation and natural convection of heat transfer into the room for each surface. Add them up and you have total BTU output. I would then correlate this to my cat temp and call it done.

We have developed many spreadsheets (a great engineering tool) for helping out with these types calculations. We also use very complex computational fluid dynamics software for these problems, but that would be overkill. If you guys are interested, I could put together a spreadsheet that would do the calcs for you based on your particular setup. I could even add forced convection (a blower) into the equations as well.

Regency f3500, straight pipe 15 feet 7 feet is in the room, blower on low speed.

 

[EbS-P]

I think putting the stove on load sensors and measuring mass change would give you an a precise heat output. Using efficiency rating , temperature water content and heat loss would paint a clear picture. Really we could just average the burn rate out for an entire load. I’m just picking 5 # per hour. 20% water gives 4# x 8000 BYU’s per pound times 75% efficient. Gives 24,000 BTUs per hour. So I totally just guessed at the burn rate and it just happens to be the same as Wolves1.

Another way would be mass airflow and an oxygen sensor. We’re geeking out right. Burn

Side tracked a bit the cat just burns what’s leftover from the primary combustion process. So I second Highbeams’s point. Not a reliable path to take.

 

[thunderhead]

So outside of the lab, the three methods to calculate BTU/time, already mentioned, I can think of are:

1) Integrate the radiation and conduction/convection equations over the area of the stove and pipe(especially if single wall pipe). Given the significant stove and pipe surface temperature changes over its various surfaces, this sounds hard.

2) Mass burn rate * energy density * efficiency * (1-water content). This sounds like the simplest, and would be my first choice I think. You can weigh a load of fuel, energy density is known to a high order of accuracy, and efficiency and water content can also be determined to pretty high accuracy levels.

3) Exhaust mass * temperature / (1-efficiency). Or substitude oxygen meter and do the chemical equation to get burn rate. You would need to place an anemometer(for wind speed and thus exhaust mass) and a thermometer(or oxygen sensor) in your chimney pipe. But this sounds doable.

 

[EbS-P]

So outside of the lab, the three methods to calculate BTU/time, already mentioned, I can think of are:

1) Integrate the radiation and conduction/convection equations over the area of the stove and pipe(especially if single wall pipe). Given the significant stove and pipe surface temperature changes over its various surfaces, this sounds hard.

2) Mass burn rate * energy density * efficiency * (1-water content). This sounds like the simplest, and would be my first choice I think. You can weigh a load of fuel, energy density is known to a high order of accuracy, and efficiency and water content can also be determined to pretty high accuracy levels.

3) Exhaust mass * temperature / (1-efficiency). Or substitude oxygen meter and do the chemical equation to get burn rate. You would need to place an anemometer(for wind speed and thus exhaust mass) and a thermometer(or oxygen sensor) in your chimney pipe. But this sounds doable.

It would be simpler to measure intake flow. But I neglected to think that the cat completes the burning process with out oxygen ( that’s the idea right. I have unburned fuel and not enough air. Secondary combustion just adds extra air. ) So that kinda tosses out my run idea of calculating power based on air intake.
I would like to know how much extra heat (power) the cat adds to the system. The answer could be in here somewhere