Posted by Jack Finn on February 10, 2000 at 13:14:21:
In Reply to: upgrading a hot water gas residential boiler posted by jack finn on January 29, 2000 at 15:23:33:
: SUBJECT: Improving the efficiency of gas converted coal hot water gravity residential boiler.
Long Story:
(is this the right place to post these questions or is there a better site)
I am attempting to increase the efficiency of an Apx 70-year-old coal hot water gravity residential boiler, which has been converted to gas Apx 40 years ago.
The boiler is in Kansas City; two-story Apx 1800 sq. ft wood frame un-insulated house. The house is in good condition but not tight. This is my daughter’s house. The heating bills for the last 2 months have been Apr $600 per month.
I am an engineer but not a mechanical engineer and I do not have my 35-year-old books but this is what I have determined or guessed.
The furnace and hot water radiators produce reasonably acceptable heat to the house. The house is not drafty by but not draft free. On the coldest day in Jan Apx 5*F. The furnace consistently ran for 11 minutes and off for 3 minutes. The boiler temp was about 145*F, the shut off is set at 165* and did not seem to be being hit (maybe it was). My daughter likes the simplicity of the gravity system and the dust free heat of radiators. They may not stay in house long enough to justify a new boiler’s expense and, they like the old system. A neighbor installed a new 97% system and has had 4 fire department runs for Co2. My daughter states “they have a lot of power outages.”
The basis of my whole theory is the original design of the coal furnace (which was most likely good) was a big flame thus the 8” pipe to get the coal burning then it was dampered down to a low flame with very slow moving air past the heat exchanger. This is the opposite of what is present. The present unit is a 200,000 Btu conversion burner. I wonder if the burner size was reduced to say 100,000 Btu and the draft adjusted for it, would the actual output of heat into the water be the same as it is now.
The Conversion Burner has an existing Apx 15” diameter burner with multiple flame orifices Apx ¼” in diameter going upward. These burner rings are in two circles going all the way around. The flame-spread plate is Apx 13” in diameter.
This is way too small for the inside diameter of Apx 21” (at this point of the boiler). My plan is to push the fire and heat out to “touch” the inside of the cast iron boiler. Does this make sense?
(Does anyone know if in new boiler design does the flame actually hit the cast iron and if it does, does it “eat out”.)
The problem is:
1.) The interior of the heat exchanger has way too much volume. The interior is Apr 20-21 inch in diameter and Apx 30" in height. This is Apx 320 square inches (a horizontal slice across it). The area of the 8" flue pipe is Apx 50 sq. in. Installing a Big "PLUG" to take up the center of the fire camber to force the heat to wash the interior wall of the heater exchange (boiler) and slow down the flow or rush of air. A "PLUG" Apx 20" in diameter by 30" high would reduce the effective cross sectional area to the 1" ring between the "PLUG" and the inside of the heat exchanger. The new area would be Apx 100 sq. inches or 1/3 of previous. This washing of the heat exchange rather than letting the heat go up the center of the unit should produce better heat transfer therefore better efficiency.
My thinking is reducing the Btu of the input of the furnace has a compound effect.
A.) It reduces the fuel used and thus the combustion air.
B.) This reduces the speed the air must travel though the heat exchange thus increases its heat transfer.
C.) The increase in heat transfer transfer further reduces the need for input Btu.
2.) This lower Btu input could reduce the “Rush” through the boiler and thus increases the effecincy. This reduction in airflow will also reduce the cold air drawn into the house.
3.) The burners are too big 200,000 Btu. The burner is on 11 minutes and off 3 minutes (on the coldest day of Jan this year which was Apx 5*F)
Therefore using the 11minutes and the 3 minutes which is 11/14 or
11/14 = 78.5% of 200,000 is 157,000 Btu
100%-utilized boiler would need to be 157,000 Btu. (She never turns down the heat. It stays the same temp all the time, so the heat up time is not needed)
Assuming the drum gives 20% better efficiency the new burner should be sized.
157,000 * 4/5 =125,600 Btu
4.) The chimney is too big. The chimney should be relined with a pipe or flue liner Apx 5”or 6” which would reduce the need for such hot exhaust gas to create a draft.
This entire revised system could be then thought of as a “new” not a converter unit. It would not need such as a big pipe and thus a smaller “draft diameter” and thus again less infiltration.
5.) All the piping in the basement (this is a gravity system) has had the insulation removal, (it must have been asbestos covered); I should replace this. Although this will reduce the basements temp (and create colder floors, which is not good for my grandchild). The insulation will keep the water hotter and increase the flow rate of the water though there will be some “effiencency” benefit because less heat will be “wasted in the basement”.
6.) The house has almost full radiator covers. They are solid back, sides and top; the front is a decorative screen with 50% open area. My daughter insists on keeping then. If I raise the top Apx ¼” it will create a ¼” gap all around the top to spill out the heat. This will increase the heat transfer thus decrease the temp of the returning water to the boiler thus giving more temp difference in and out of the boiler. Thus another minor gain is effecincy.
My hope is that the above creates a 33% improvement or changes the $600/ month to $400.
Do you agree?
I plan to make the “drum” out of then stainless and fill the center of the existing furnace.
The Furnaces is:
Capital Windchester
Size 44
Serial 174718
Box = 34-210
The Burner is:
Security Manufacturing Co
Kansas City, MO
Input 200,000 Btu
Natural Gas
Min Grate Denmension 18
Model No. EX 216
The Conversion Burner has an existing Apx 15” diameter burner with multiple flame orifices Apx ¼” diameter going upward. These burner rings are in two circles going all the way around. The flame-spread is Apx 13” in diameter.
I think “the conversion burners” were a “quick and dirty” solution to eliminating coal and the installation had to be idiot proof thus a “big flame” and bolt it in no thought. They were not designed for efficiency.
Does anyone know of an original design manual on these, Americans Standard had to have done some design on these?
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