# Thread: Commercial Boiler Venting Questions

1. Boiler design/ flue design has gotten easier with newer controls and vfd's. Controlling flue fans and makeup air. Had a job 20 years ago three 250 hp steam boilers low pressure common flue. A nightmare ,two boilers running at one time rotating. Five years ago 7 steam boilers 6- 100 hp & a 50 hp common flue with flue fans, makeup fans and vfd's. 54" flue . Didn't matter what boiler was running or not efficiency of burners didn't vary. All design by boiler manufacturer.

2. Originally Posted by TechmanTerry
I am talking 100k btu "in" and about 80K btu out.

Not sure about the "two CuFt a minute of gas thing"
Natural draft appliances need 30 cfm of combustion air (15 to burn and 15 for dilution air) per 1 cuft of natural gas burned, as previously mentioned.

Example: 100,000 btuh input appliance (always based on input).

Burns 100 cuft per hour divided by 60 mins = 1.6666 cuft per min (cfm).

1.6666 cfm natural gas x 30 cfm of combustion air = 50 cfm of combustion air (not 60 as "superheatmaster" listed) .

It is just easier to calculate required combustion air if you divided the cubic feet per hour of natural gas input by 2 - 100/2 = 50 cfm

Should these natural draft appliances have a barometric damper instead of a draft hood, the combustion air can be reduced to 20 cfm per cuft of natural gas burned (15 cuft for the fuel to burn and 5 cuft for the barometric).

And in this case, just divide the cubic feet per hour of natural gas burned by 3 - 100/3 = 33.3cfm.

3. In 2000 ASHRAE did a combustion air study and stated that the 50 cu. ft. of air per 1000 btus was just made up and not based on any actual combustion knowledge. They also stated that passive combustion air did not work in the field under many field conditions and wasn't reliable. Nothing in the Code books, manufacturers specs on combustion air is based on facts, just opinions.

gravity - you have every thing correct except there is no such thing as an oversized flue. If this was true we could not vent 6 pieces of equipment in the same flue on a three story apartment building. If you have three furnaces and three waters heaters the flue is 12 to 14 times bigger than a single water heater flue running by itself.

rundawg and gravity are both using combustion air numbers based on real combustion. When combustion air is sized it is based on equipment running 60 minutes every hour. If this is the case something might be wrong with the equipment unless it is used for process. Also, combustion air numbers are based on a room that is 100% air tight with no leaks (a safe, submarine, coffin?).

Draft pressure combined with combustion analyzer measurements are the only true determinations if there is plenty of combustion air in an area and if it is actually getting to the equipment. The only place this can be determined is in the field, on the job, under the infinite conditions that exist in the field, not a book based on opinions.

How do we know we have never been in a room without enough combustion air? WE ARE ALIVE!!!

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Originally Posted by rundawg
Natural draft appliances need 30 cfm of combustion air (15 to burn and 15 for dilution air) per 1 cuft of natural gas burned, as previously mentioned.

Example: 100,000 btuh input appliance (always based on input).

Burns 100 cuft per hour divided by 60 mins = 1.6666 cuft per min (cfm).

1.6666 cfm natural gas x 30 cfm of combustion air = 50 cfm of combustion air (not 60 as "superheatmaster" listed) .

It is just easier to calculate required combustion air if you divided the cubic feet per hour of natural gas input by 2 - 100/2 = 50 cfm

Should these natural draft appliances have a barometric damper instead of a draft hood, the combustion air can be reduced to 20 cfm per cuft of natural gas burned (15 cuft for the fuel to burn and 5 cuft for the barometric).

And in this case, just divide the cubic feet per hour of natural gas burned by 3 - 100/3 = 33.3cfm.
Well said.

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What is the required CuFt of combustion air needed for a 1 Gal oil boiler? I was told at a Beckett Class to use 4200 CuFt for 1 Gal.

6. Think about what we are taught or what is listed in most manuals and the Code.

1 sq in. area per 4000 btus if you get the air from a grill to outside.
1 sq.in. area per 1000 btus if you get the air from a inside room of the same floor
2 sq.in. area per 1000 btus if you get the air from a floor on a different level
1 sq.in. area per 2000 btus if you use horizontal duct to outdoors
2 sq.in. area per 2000 btus if you use vertical duct to outdoors
or .35 cfm per 1000 btus Also the air changes in the building must be at least .40 ACH but not more than .60 ACH

If this isn't the most fabricated bunch of whatever, I don't know what is! gravity and rundawg have it correct. Not sure how holes in the wall know what to do?

7. Originally Posted by TechmanTerry
What is the required CuFt of combustion air needed for a 1 Gal oil boiler? I was told at a Beckett Class to use 4200 CuFt for 1 Gal.
Using Beckets own combustion numbers the amount that I have calculated is 3500 cu.ft.. per gallon, assuming the room is air tight.

8. Originally Posted by TechmanTerry
What is the required CuFt of combustion air needed for a 1 Gal oil boiler? I was told at a Beckett Class to use 4200 CuFt for 1 Gal.
The only way I can come up with that number is this:

A gallon of #2 heating oil has a heat content of approximately 140,000 btu’s.

If you use the same formula as before - 30 cfm per 1000 btus (natural draft), and your furnace/boiler burned 1 gallon an hour, that would equate to 70 cfm of combustion air (140 x .5 ).

or

70 cfm x 60 mins = 4200 cuft hour/gallon (matches what Beckett told you).

Either way, these numbers are really irrelevant (as noted by Jim Davis), as a draft test and combustion analysis are the only way to tell if any unit is getting enough combustion air.

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The biggest venting mistakes I see are always materials used, and excessive uncontrolled draft.

For high efficiency, its generally undersize, or too long of runs. On occasion, oversize.

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