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I am trying to undestand how the BTUs of a particular stove is rated. My thought is that I have a furnace rated at 170,000 BTUs (I assume per hour input). Does that mean that it will effectively burn 170,000 BTUs of coal in one hour or does that mean it has the ability to feed 170,000 BTUs of coal per hour (approximately 27.2 lbs)? I know that if I have my stoker set to full stroke, it will push a significant amount of still burning coal off of the grate. My manual states to keep approximately 1 inch of ash at full burn, so I'm assuming that the 170,000 BTU value is calculated based on the 1" of ash. Thoughts?

I'm not a HAVAC man, but I do a lot of reading. BTU output is normally lower than BTU input. Example: If 170,000 input, may be 160,000 output (this is only a guess). I would guess the furnace is capeable of 160,000 BTU output when having one inch of ash at the end of the grate. That just sounds logical. You can run it lower. Seems that someone once told me that BTU input/output differences do not apply to solid fuel heaters, and that BTU ratings are output only. Now you see why I'm guessing at this.

There may be a misunderstanding. I'm not trying to back-calculate efficiency or anything. I'm just interested in how the stoves are rated. Is it based on potential feed rate or is based on actual burn on the grate area?

pconn171 wrote:There may be a misunderstanding. I'm not trying to back-calculate efficiency or anything. I'm just interested in how the stoves are rated. Is it based on potential feed rate or is based on actual burn on the grate area?

It would be burn on the grate area. You can push all the coal you want. I can dump 80 pounds (all at once)in a hand fired stove, it wouldn't raise the BTU rating. I can push coal in your stove with a doser, and no BTU's. BTU's come from the fire. It's how many British Thermal Units per hour(rate of heat) the fire gives off. Bigger grate=bigger fire=more BTU's per hour comming off the fire. How you feed the fire has nothing to do with BTU"s. If the stove was sized by feed mechanism , it would be something like pounds per push/revolution/etc..Lets try another approach; Your stove is capable of 100,000 BTU's/hr with full grate. Now cut your grate in half. In theory, your fire will only be putting off 50,000BTU's/hour. So, now you'd ajust the pusher so you don't push the coal off the grate, because the grate is only capeable of burning 50,000 BTU's/hr. I know how your thinking. The stoves rating comes from the size of the fire. Actually, I would say the STOVE RATING would be more the surface area of the stove itself. So stoker rating & stove rating would be two different things. Lets see what others have to say.

The calculation I have for BTU output of a stoker is, for example on a Reading Tri-burner,

The grate area is 8" x 10" = 80 sq.in. or .55555 sq.ft. X .90 the efficiency of the burning of the coal X 13.6 the maximum amount of coal feeding per hour X 12,500 BTU's per pound of coal = 85,000 BTU's per hour per stoker. Your two stokers have the potential to burn 170,000 BTU's per hour.

The feed rate is going to determine the amount of BTU's per hour you get as long as you don't push the burning coal off the grate. The efficiency rating will change in relation to the amount of air or idle time the fire has on the grate also. So, that is one reason for full time combustion fans.

Take a look at this post on calculating the BTU output by surface temperature, it may help you better with designing your heat exchanger. Stove BTU Output Based on Temp and Surface Area? This may be more helpful in gaining more heat from the surface area of your stove, than trying to figure out the coal burning BTU rating. The two calculations should give you enough working knowledge to figure out what you want to do.

Hope this helps and I'm here if you need any assistance. DOUG

here's another thread on BTU Calculating.

How Much Coal/Hour?

pconn171 wrote:I am trying to undestand how the BTUs of a particular stove is rated. My thought is that I have a furnace rated at 170,000 BTUs (I assume per hour input). Does that mean that it will effectively burn 170,000 BTUs of coal in one hour or does that mean it has the ability to feed 170,000 BTUs of coal per hour (approximately 27.2 lbs)? I know that if I have my stoker set to full stroke, it will push a significant amount of still burning coal off of the grate. My manual states to keep approximately 1 inch of ash at full burn, so I'm assuming that the 170,000 BTU value is calculated based on the 1" of ash. Thoughts?

To the best of my knowledge, there is no "standard" for rating or testing of output solid fuel appliances. It is up to each manufacturer to rate their appliance. How they do that is sometimes more optimism that science. Good companies will not mislead you.

Ratings in BTU are usually per hour.

Most ratings for solid fuel devices are rated at output. Therefore a 170,000 BTU device would consume more than 170,000 BTU of coal depending on it's combustion efficiency.

Coal stokers are more steady output than hand fired devices. Hand fired devices are usually rated at peak burn and will produce less btuh at either end of the firing cycle.

The stoker must be adjusted to match the appliance output. If you are pushing unburned ash off the grate, then the feed rate is too high.

Stokers are often designed to be used in more than one rating of appliance. Stokers that I have researched are rated at maximum pounds per hour of coal and are usually capable of moving more coal than the appliance can efficiently burn, hence the need for adjustment.

I made these notes related to BTU's and my Harman VF3000, you may find them helpful.

Therm is 100,000 BTU's
7.7 lb's coal per therm

Btu's to heat water.
Takes 1 BTU to heat 1 lb 1 degree.

Boiler Recovery Rates:
1 lb coal to heat boiler from 140-180
320 BTU's to heat boiler 1 degree.
40 degree's 12,800 btu's
13000 BTU's in pound coal

Boil burn rates
If the boiler can burn 80 lb's in 24 hour's then thats a max of 3.3 lb's an hour. Or 43,000 BTU's an hour. Rate above 90K BTU's

It will also depened on How Much BTU's Per lb are in the Anthracite coal you are burning and how efficent the unit
you are using to burn the coal. Your Gross BTU Output and Net BTU Output what is going in is not what is coming out
Anthracite coal on the average will be 12.500 BTU's Per lb up to 14.000 BTU's Per lb Some can be as Low as 11.500 BTU per lb & as High or over 15.000 BTU per lb but I have yet to see any over 13.800 BTU per lb most in this area runs
between 12.900 to 13.800 BTU's per lb